i

TITLE PAGE

IMPROVING SEPTIC TANK PERFORMANCE BY A NEW

RATIONAL DESIGN APPROACH

Ph.D THESIS

BY

NNAJI, CHIDOZIE CHARLES

(Reg. No.: PG/Ph.D/08/49126)

SUPERVISOR:

ENGR. PROF. J. C. AGUNWAMBA

MARCH, 2011

ii

CERTIFICATION

This is to certify that Nnaji, Chidozie Charles, a postgraduate student in the

Department of Civil Engineering with registration number PG/Ph.D/08/49126

has successfully fulfilled the requirements for the award of Doctor of

Philosophy (Ph.D) in Civil Engineering (Water Resources and Environmental

Engineering option). This work is original and has not been submitted in part or

full for the award of certificate in any other institution, University, referred

journals, book or any publication.

…………………………………………………….

Engr. Prof. J. C. Agunwamba

(Supervisor)

………………………………………….................

Engr. J. C. Ezeokonkwo

(Head of Department)

………………………………………………………..

Engr. Prof. Ify L. Nwaogazie

(External Examiner)

March, 2011

iii

DEDICATION

To my lovely wife, Favour Nnenna, Nnaji; my precious little daughter,

Chigozirim Hephzibah Nnaji and my parents Pastor and Mrs. Nathaniel Nnaji

for believing in me.

iv

ACKNOWLEDGMENT

First and foremost, I am grateful to the Almighty God for His Eternal Mercies.

I am mostly indebted to my indefatigable supervisor, Engr. Prof. Jonah

Chukwuemeka Agunwamba, the Dean of Faculty of Engineering, UNN. His

blazing trail of intellectual and scholarly versatility has provided for me a sure

take-off base.

I wish to Acknowledge Mr. Ted Kulongosky of Orenco Systems Incorporated,

USA for providing me with some materials on sludge accumulation audit. I

express my gratitude to all the staff of Civil Engineering, University of Nigeria

Nsukka. I remain grateful to the following: Engr. Prof. N. N. Osadebe, former

Dean of Engineering, Prof. J. O. Ademiluyi, Engr. Dr. C. U. Nwoji, Dr. F. O.

Okafor, Engr. J. C. Ezeokonkwo, Dr. B. O. Mama, Dr. H. N. Onah, Engr. A. J.

Anyaegbunam, Arch A. Adamou, and Engr. Mrs. C. N. Mama for their positive

impacts on me. I thank Dr. O. O. Ugwu for the useful tips he provided. I

appreciate Engr. Ifeanyi Obeta for being a good friend and colleague who is

always willing to help.

I am grateful to the staff of the Public Health Laboratory of Civil Engineering

Department, especially Engr. Chinedu Anyanwu and Mrs. Eze, for their

assistance in laboratory analysis. I wish to thank the Head of Agric and

Bioresources Engineering Department, Dr. B. O. Ugwuishiwu for technical

support. I also thank my students: Ikenna Ezeugwu and Friday Oligie for taking

the time to do dirty job of sewage sample collection with me at the sewage

treatment plant. I thank Mr. Barnabas Eze for helping me out with pumping

sewage into the reservoir and maintaining the pilot scale tanks. Not forgotten

are all the staff of the sewage treatment plant for their friendliness.

I will not forget my lovely wife, Favour Nnenna Nnaji who was very empathic

with me and supportive throughout the period that this research lasted. My

infant daughter, Chigozirim Hephzibah Nnaji was also supportive in her own

v

little way – struggling with my fingers or pen or even the laptop while I worked

made the whole effort worthwhile. I remain eternally indebted to my parents

for being such wonderful parents; and my siblings (Ndidiamaka, Chinedu,

Uzochukwu, Miracle, Grateful and Grace) for their loving support. My sincere

gratitude also goes to my parents In-law, Mr. and Mrs. David Ogbonna for their

prayers and moral support.

Finally, I would like to mention my friends who, by virtue of being my friends,

have touched my life in one way or the other. They are Mr. Peter Nick

Chineke, Obinna Ezeja, Damian Itumo, Pastor Samuel Ezeh, Engr. Dr.

Matthew Aho, Engr. Joseph Utsev, Engr. Ifeanyi Nweke, Igwebuike Udeh, Dr.

Charles Dike, Dr. Chris Afangideh, Dr. Timothy Adibe and others.

May God bless you all.

vi

ABSTRACT

This study was aimed at developing a rational approach to septic tank design in

order to reduce health risks associated with improperly treated effluent

especially in developing countries. To this end, several research tools including

questionnaires, pilot scale study and model formulation were employed.

Questionnaires were used to conduct a preliminary study with a view to

ascertaining people‟s perception with regard to septic tank design, use and

maintenance. This preliminary study revealed that the septic tank is a poorly

designed and grossly overlooked but indispensable waste management facility.

Pilot scale studies were conducted to monitor physicochemical and microbial

parameters. A sludge accumulation model was formulated from first principles

by applying material balance to a model septic tank. The model was calibrated

using data from three different septic tank audits spanning 3 years, 5 years and

8 years respectively and involving over 1000 septic tanks. A correlation

coefficient of R = 0.98 was obtained between measured and calculated sludge

accumulation data. The sludge accumulation model showed that sludge does

not accumulate at a constant rate as is usually assumed but rather at a reduced

rate over time. The sludge accumulation model was compared with two

existing but purely empirical models namely: Weibel‟s model derived in 1955

for the US Public Health Service and Bound‟s (1995) model. Finally a rational

approach to septic tank design was developed. Design charts and a Microsoft

Excel based design programme were produced to aid the unlearned designer

and the computer literate designer respectively.

vii

LIST OF FIGURES

Figure 2.1 Relationship between Air Space and Sludge Accumulation

Figure 2.2 Efficiency of Suspended Solids Removal between Compartments

(Kamel and Hgazy, 2006)

Figure 2.3 Efficiency of BOD Removal between Compartments (Kamel and

Hgazy, 2006)

Figure 2.4 Efficiency of Treatment for Different Modifications of the Septic

Tank (Nguyen et al., 2007)

Figure 2.5 Efficiency of Treatment versus Number of Baffled Reactors

(Koottatep et al., 2004)

Figure 2.6 Efficiency of COD Removal for Different Modifications of the

Septic Tank at Various Detention Times (Koottatep et al., 2004)

Figure 2.7 Efficiency of Treatment versus Wastewater Composition

(Washington et al., 1998)

Figure 3.1 Generalized Sketch of Experimental Set up

Figure 3.2 Picture of Experimental Set up

Figure 3.3 Mass Balance of Solids in the Septic Tank

Figure 3.5 Generalized Relationship for Accumulated Sludge versus Solids

Removal Efficiency

Figure 3.4 Accumulation of Sludge versus Efficiency of SS Removal Plotted

from Data Obtained by Heinss et al. (1999)

Figure 4.1 Compliance to Basic Septic Tank Tests (Questionnaire result)

Figure 4.2 Kinds of Construction Problems Encountered

(Questionnaire result)

Figure 4.3 Design Issues (Questionnaire result)

Figure 4.4 Flushing of Non-biodegradable Materials into the Septic Tanks

(Questionnaire result)

Figure 4.5 Is Your Septic Tank Malfunctioning? (Questionnaire result)

Figure 4.6 Temperature Variation Tanks

Figure 4.7 Temperature Variation Tanks (Different Inlet Types)

Figure 4.8 Change in pH between Inlet and Outlet

viii

Figure 4.9 Change in pH between Inlet and Tank Midpoint

Figure 4.10 Change in pH between Inlet and Outlet for Different Types of

Baffles

Figure 4.11 Change in pH between Inlet and Tank Midpoint for Different

Types of Baffles

Figure4.12 Outlet BOD Removal Efficiency for Different Baffle Types

Figure 4.13 Tank Midpoint BOD Removal Efficiency for Different Baffle

Types

Figure 4.14 Outlet COD Removal Efficiency for Different Baffle Types

Figure 4.15 Tank Midpoint COD Removal Efficiency for Different Baffle

Types

Figure 4.16 Tank Midpoint E-coli Removal Efficiency for Different Baffle

Types

Figure 4.17 Tank Midpoint Suspended Solids Removal Efficiency for

Different Baffle Types

Figure 4.18 Outlet Suspended Solids Removal Efficiency for Different Baffle

Types

Figure 4.19 Effluent E-coli Removal Efficiency

Figure 4.20 Outlet BOD Removal Efficiency

Figure 4.21 Tank Midpoint BOD Removal Efficiency

Figure 4.22 Effluent Suspended Solids Removal Efficiency

Figure 4.23 Tank Midpoint Suspended Solids Removal Efficiency

Figure 4.24 Plots of Model and Measured Sludge Accumulation versus Time

Figure 4.25 Comparison of Model with Bounds‟ and Weibel‟s Models

Figure 4.26 Decline of detention time with sludge accumulation

Figure 4.27 Decline of Detention Time for House Connection, Simple

Plumbing (Typical wastewater flow = 0.064m3/day)

Figure 4.28 Decline of Detention Time for Urban House with Full Water

Connection and Garden (Typical wastewater flow =

0.275m3/day)

ix

Figure 4.29 Decline of Detention Time for Basic Water Requirement

(Typical wastewater flow = 0.04m3/day)

Figure 4.30 Decline of Detention time for Average Nigerian House

(Typical wastewater flow = 0.03m3/day)

Figure 4.31 Chart for Determining Volume of Sludge for a Chosen

Desludging Interval

Figure 4.32 Residual Depth per Occupant (hre) versus Number of Occupants

Figure 4.33 Tank Dimensions and Residual Depth for Simple House

Connection, pour flush (Q=0.064m3/capita/day) and L = 2W

Figure 4.34 Tank Dimensions and Residual Depth for Full Simple House

Connection, Pour Flush (Q=0.064m3/capita/day) and L = 3W

Figure 1.35 Tank Dimensions and Residual Depth for Simple House

Connection, Pour Flush (Q=0.064m3/capita/day) and L = W

Figure 4.36 Tank Dimensions and Residual Depth for Full house connection,

urban with garden (Q= 0.22) and L = 2W

Figure 4.37 Tank Dimensions and Residual Depth for Full house connection,

urban with garden (Q= 0.22) and L = 3W

Figure 4.38 Tank Dimensions and Residual Depth for Full house connection,

urban with garden (Q= 0.22) and L = W

Figure 4.39 Tank Dimensions and Residual Depth for Nigerian Average,

Urban Areas without Pipe Borne Water (Q=0.03) and L = 2W

Figure 4.40 Tank Dimensions and Residual Depth for Nigerian Average,

Urban Areas without Pipe Borne Water (Q=0.03) and L = 3W

Figure 4.41 Tank Dimensions and Residual Depth for Nigerian Average,

Urban Areas without Pipe Borne Water (Q=0.03) and L = W

Figure 4.42 Tank Dimensions and Residual Depth for Basic Water

Requirement (Q=0.04) and L = 2W

Figure 4.43 Tank Dimensions and Residual Depth for Basic Water

Requirement (Q=0.04) and L = 3W

Figure 4.44 Tank Dimensions and Residual Depth for Basic Water

Requirement (Q=0.04) and L = W

x

Figure 4.45 Sample Design using Excel Codes

Figure 4.46 Determination of Residual Depth per Occupant Using Charts

Figure 4.47 Determination of Tank Dimensions Using Charts

Figure 4.48 Tank Design Using Excel Codes

xi

LIST OF TABLES

Table 2.1 Soil Limitation Ratings Used by NRCS for Wastewater

Absorption Fields

Table 2.2 Desludging Intervals as Recommended by Bounds (1995)

Table 3.1 Description of Pilot Scale Units

Table 3.2 Hydraulic Characteristics of Tanks

Table 3.3 Hydraulic Characteristics of Inlet Pipes

Table 4.1 Dissolved Oxygen Values (mg/l)

Table 4.2 Sludge Accumulation Data

Table 4.3 Water consumption under different supply conditions

Table 4.4 Schedule of Septic Tank Sizing and Dimensions (PWD, 1943)

Table 4.5 Septic Tank Volumes (Crites and Tchobanoglous, 1997)

xii

TABLE OF CONTENT

Title page………………………………………………………………….i

Certification……………………………………………………………….ii

Dedication…………………………………………………………………iii

Acknowledgment………………………………………………………….iv

Abstract……………………………………………………………………vi

List of figures……………………………………………………………...vii

List of tables……………………………………………………………….xi

Table of Content.………………………………………………………….xii

CHAPTER ONE: INTRODUCTION

1.1 Background………………………………………………………...1

1.2 Statement of problem……………………………………………...2

1.3 Objectives of the study…………………………………………….3

1.4 Scope of work……………………………………………………...4

1.5 Justification of the study…………………………………………...4

1.6 Limitations of the study……………………………………………4

CHAPTER TWO: LITERATURE REVIEW

2.1 Origin of the septic tank…………………………………………..5

2.2 Septic tank construction and material…………………………….5

2.2.1 Septic tank construction…………………………………...5

2.2.2 Septic tank material………………………………………..7

2.3 Domestic wastewater……………………………………………..9

2.4 Operation and performance of the septic tank system………….10

2.5 The drain field……………………………………………………13

2.6 Septic tank failure………………………………………………..17

2.7 Sludge accumulation……………………………………………..19

2.8 Contributions of previous researchers………………………….....21

xiii

CHAPTER THREE: METHODOLOGY

3.1 Data collection…………………………………………………...42

3.1.1 Preliminary study (questionnaires)…………………….....42

3.1.2 Pilot scale septic tanks……………………………………42

3.1.3 Laboratory analysis……………………………………….46

3.1.4 Sludge accumulation data acquisition…………………….47

3.2 Model formulation…………………………………………….....47

3.2.1 Sludge accumulation model……………………………...47

3.2.2 Initial conditions……………….………………………….52

3.2.3 Assumptions………………………………………………53

3.2.4 Solution of model…………………………………………54

3.2.5 Depreciation of detention time and rate of settling………56

3.2.6 Residual depth…………………………………………....59

3.2.7 Reserve space…………………………………………….60

CHAPTER FOUR: RESULTS AND DISCUSSION

4.1 Preliminary observations…………………………………………62

4.2 Result of pilot scale study………………………………………...66

4.2.1 Dissolved oxygen and temperature………………………..66

4.2.2 pH variation……………………………………………......68

4.2.3 Effect of baffle on treatment efficiency...........................70

4.2.4 Effect of detention time on treatment efficiency..............75

4.3 Model calibration………………………………………………...78

4.3.1 Comparison of model with existing sludge accumulation

models…………………………………………………….81

4.4 Basis for the new design approach………………………………82

4.5 The new design approach……………………………………......90

4.6 Design example…………………………………………………...109

4.7 Caution For Users…………………………………………………115

xiv

CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion……………………………………………………….115

5.2 Recommendations………………………………………………116

REFERENCES…………………………………………………………...118

APPENDIX I……………………………………………………………..125

APPENDIX II……………………………………………………………..133

1

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND

The septic tank system is the most widely used onsite treatment system for

domestic wastewater. In fact, most developing countries (Nigeria inclusive)

lack the technology and economic power to construct and operate sewerage

systems for conveyance of domestic wastewater to central sewage treatment

facilities, so a greater population rely on the septic tank system for sewage

treatment. It is an enclosed receptacle designed to collect wastewater, segregate

settleable and floatable solids (sludge and scum), accumulate, consolidate and

store solids, digest organic matter and discharge treated effluent (Bounds,

1997). In the United States only, over 50 million people use the septic system

(Collick et al., 2006). According to Fidelia (2004, in Burubai et al., 2007),

over 46% of the Nigerian population use the septic tank system. The septic

tank system was once thought to be a temporary solution to domestic

wastewater treatment and disposal. This was true until 1997 when the United

States Environmental Protection Agency and Congress officially recognized

the system as a sustainable, long-term solution for treating wastewater.

The septic tank is an anaerobic reactor due to the insufficiency of oxygen

concentration to act as electron acceptor. The wastewater is degraded by

micro-organisms aerobically while the C, CO2 SO4 act as electron acceptors to

form CO2, H2, CH4 and S2-

(sulphides). At the same time, most of the organic

N is converted to NH+

4 (inorganic). The effluent flows into the drain field

where aerobic degradation occurs due to abundance of oxygen in the

unsaturated soil layer. The C in the wastewater is now oxidized to CO2 while

NH4+ is oxidized to NO2

- thus raising the nitrate level of the sewage to about

seven times the limit acceptable for dumping water (10mg/l). The H+ released

from the oxidation of NH4+ now reduces the pH of the effluent.

2

A properly functioning septic tank system should be able to reduce the

pollutional level of wastewater to such a level as is within local and

international standards for wastewater disposal. The septic tank system

consists of a water tight tank for removal of solids and partial digestion of

organic matter, and a drain field which is a secondary treatment system. The

tank is an anaerobic system while the drain field is mostly aerobic which

further treats the effluent before channeling it to the groundwater. In some

cases, the drain field could be a gravity type or a dosing type.

All things being equal, the septic tank system does not pose much problem and

requires little maintenance. However when the system is not working properly,

it merely serves as a route for recycling pathogens and deadly chemicals

through the ecosystem. According to Cogger (1988), nearly 40% of

groundwater attributed disease outbreaks can be traced to the failure of onsite

disposal systems. Weissman et al. (1976), Bidgman et al. (1995) and Taylor et

al. (1981) among others, reported cases of disease outbreak resulting from

groundwater contamination due to septic tank failure. In Africa where most

people depend on streams, shallow wells and boreholes, the case is even more

severe.

1.2 STATEMENT OF PROBLEM

If wastewater flowing into the septic tank does not receive adequate treatment,

it is simply passed on to the groundwater unnoticed thus wreaking havoc on

public health. Researchers have shown that most septic tanks especially in

developing countries do not even attain an average performance throughout

their lifetime. The result is that most septic tanks only act as a conduit for

conveying raw / under treated sewage into the soil leading to massive fouling

of our groundwater. And because the groundwater is the main source of potable

water in most communities, man constantly stands the risk of water borne and

water related diseases. Most times, the groundwater is used without treatment

on the common assumption that it is “always clean”. The menace of such

3

diseases as typhoid fever, diarrhea, giardiasis, gastroenteritis, hepatitis,

methemoglobinamia, samonellosis, dysentery, etc will continue to plague

humanity until a systematic approach to the design, construction and

maintenance of the septic tank system is adopted.

The foregoing indicates that the septic tank system requires proper design,

construction, use and maintenance. The cardinal aspect of septic tank

maintenance which is of interest in this research is desludging. The absence of

a deterministic equation for the prediction of desludging interval has usually

led to too frequent desludging or excessive accumulation of sludge in the septic

tank. Too frequent desludging increases cost of operation while excessive

accumulation of sludge drastically reduces the efficiency of the septic tanks.

The problem at the heart of this research is to develop a systematic and rational

approach to the design of septic tanks and also to provide suitable guidelines

for the maintenance of the septic tank system in order to ensure the protection

of public and environmental health.

1.3 OBJECTIVES OF THE STUDY

Most of the existing methods of septic tank design are not based on extensive

scientific research and have so far proved inadequate. Most times what is

referred to as design is mere lumped sizing instead of systematic and rational

design. Therefore, the main objective of this research is to develop a systematic

approach to the design and maintenance of septic tanks.

Hence, the specific objectives of this research are:

(i) To derive a model to predict the rate of sludge accumulation in

septic tanks;

(ii) To calibrate the model using field data;

(iii) To predict the desludging interval of septic tanks by relating sludge

accumulation to reduction in detention time;

(iv) To compare the sludge accumulation model to existing models

4

(v) To expose the unreliability of prevailing design methods and

maintenance; and

(vi) To present a step by step procedure of how to design a functional

septic tank using facts from the research.

1.4 SCOPE OF WORK

The core of this research shall concentrate on only the septic tank and not any

of its secondary complements such as the soil absorption field, mound system,

wetland, waste stabilization pond, etc. The reason is that, under normal

circumstances, the tank itself is the limiting factor of performance in the septic

tank system. Because most homes in developing and even developed countries

still use the conventional septic tanks, this study will not extend to modified

septic tanks. However, some of these systems will be highlighted during

literature review for the sake of completeness.

1.5 JUSTIFICATION OF THE STUDY

The septic tank is pivotal to public health and yet one of the most overlooked

and least maintained waste treatment facilities. Research has shown that most

outbreaks of water borne epidemics result from fecal contamination. In most

developing countries, people are not aware of the crucial role of the septic tank

as they merely view it as a sewage pit that needs no special design, construction

and maintenance considerations. The result is the ubiquity of malfunctioning

septic tanks. This is why this subject deserves a serious intellectual attention.

1.6 LIMITATIONS OF THE STUDY

Several researches conducted on the septic tank system have shown that

sewage is very difficult to work with. Characteristics of sewage vary from

place to place and from septic tank to septic tank depending on the activities of

users. Sewage is very inhomogeneous, consisting of a liquid phase, settled and

partly settled solids, scum, dissolved solids such that it is difficult to obtain a

representative sample (Heinss et al., 1999)

5

CHAPTER TWO

LITERATURE REVIEW

2.1 ORIGIN OF THE SEPTIC TANK

Louis M. Mouras was acclaimed to be the inventor of the septic tank in 1860.

He called it the Mouras pit or automatic scavenger; however, it was not

patented until 1881. Around the same period that Mouras invented his septic

tank, Dr. Tracey and Dr. Featherton of the Lying – In Hospital, Carlton,

Melbourne had been operating what they described as an inoffensive system for

disposal of sewage. This was in 1861. In 1871, a Brisbane architect, Andrea

Stombuco invented a new kind of closet which he tested in his Royal Oak

Hotel for two years. Though Stombuco was fined by the Board of Health for

keeping an unauthorized system, the same board later considered the invention

in 1883. However, the term septic tank was not used until 1885 when Donald

Cameron patented his version.

In some places, it was thought that the septic tank did not need to be covered

because the scum layer provided the necessary cover. It was also common

belief that dung worms could be used to reduce the scum layer and that a piece

of meat should be used to kick start a new septic tank.

2.2 SEPTIC TANK CONSTRUCTION AND MATERIAL

2.2.1 Septic Tank Construction

Early septic tanks were cast in situ while there are now available precast septic

tanks which can be made of materials ranging from concrete to fibre glass. The

usual configuration is the rectangular type. All the wastewater from the home

is routed to the septic tank through pipes that merge into one that conveys

wastewater to the tank. Originally, a concrete splash baffle was used to slow

down the influent wastewater to avoid disturbance of the settled solids. The

concrete splash baffles are fast disappearing in favour of tee pipes that serve as

both inlet and baffle. Tee pipes are more efficient in reducing the kinetic and

6

potential energy of the flow thus providing quiescent conditions for effective

settling (Burubai et al., 2007). According to Bounds (1997), the change in

direction of flow dissipates its incoming velocity reducing mixing action as the

influent rushes into tank. The settleable solids retention is improved by starting

the settling at the clear zone level, near the bottom and sludge larger rather than

at the surface. It has also been found that the aggressive wastewater attacks

concrete baffles so that they deteriorate over time. This causes the effluent

quality to reduce drastically.

The tank is usually provided with a concrete cover with air vents to allow the

escape of gaseous products of anaerobic digestion. There are also manholes on

the cover, near the inlet and outlet for inspection and desludging of the tank. In

some case, the tank could have two or more compartments to enhance the

treatment efficiency of the tank. The bottom of the tank may be flat, or it may

slope towards inlet to enhance pumping of sludge. The tank is usually designed

based on the daily wastewater follow with a detention time of between 24 hours

and 72 hours. The length of the tank is usually more than the width or the

depth in order to increase the travel path of particles to enhance solids removal

and biodegradation. The conventional septic tank is constructed in such a way

as to leave some air space above the scum layer.

Septic tank effluent is usually discharged by gravity or with the aid of pumps.

Experience has shown that most outlet baffles vanish with time thus allowing

the scum layer to pass on to the drain field and clog it. The use of siphon or

pump for effluent discharge helps to reduce this problem. It is worthy of note

that most septic tanks constructed in Nigeria and other developing countries

still make use of concrete baffles. Moreover, only very few individual can

afford the dosing septic tank because of cost.

The structural requirement of the septic tank is that the septic including all

extensions to the surface shall be watertight to prevent leakage into or out of

7

the tank (Bounds, 1997). It shall be structurally sound and made of materials

resistant to corrosion from soil and acids produced from septic tank gasses

(Kansas State Department of Health and Environment, 1997). Leaking tanks

are unacceptable and watertightness is a requirement that should be mandatory

for all onsite application. Even in the United States, leaking and structurally

unsound septic tanks abound, and regulatory bodies do not make any effort to

enforce those requirements (Bounds, 1997). It therefore calls for strict quality

control during the construction of septic tanks to ensure that they attain a

desirable level of structural integrity. The American Society for Testing of

Materials recommends a hydrostatic test which involves filling the newly

constructed tank with water and allowing it to stand for hours to allow the

concrete to absorb water. Then the tank is topped off and an initial

measurement made with a hook and gauge with veneer scale. Any loss of

water after an hour renders the tank unacceptable. Observation of the outside

of the tank can also give clues of leakage. Any trickle, ooze, or exterior wet

spot is a sign of leakage (KSDHE, 1997).

2.2.2 Septic Tank Material

Concrete

The most commonly used material for the construction of the septic tank is

reinforced concrete because of its relatively lower cost. Concrete septic tanks

can either be cast in situ or precast. In most developed countries, precast

concrete septic tanks are now favoured more than the in suit type because the

precast type is made in a more controlled environment whereas cast in situ

tanks are subject to the different skills of different workers. Because it is

impossible to cast a tank in a single pour, the risk of cold joints subject to

leaking are introduced. On-site casting of septic tanks introduces job delays

and environmental hazards with the excavation being open overnight waiting

for concrete to cure. On the other hand, a precast concrete tank can be

completely installed in a single day. In most developed countries, there are

8

now many companies that specialize in manufacturing and installing precast

septic tanks of different sizes and modifications to meet growing needs.

Polyethylene

Polyethylene tanks offer the cheapest solution for onsite water treatment and

disposal. They are easy to transport and install and no cranes are needed. One

big disadvantage of polyethylene tanks is that under saturated soil conditions,

they normally tend to float out of the ground. This problem can be eliminated

by the combination of high density polyethylene (HDPE) and a ribbed design.

Fiberglass Tanks

Fiberglass can be a good substitute for concrete in the construction of septic

tanks as it has been found reliable in underground gasoline storage. Even

though fiberglass tank is lighter than a concrete tank, it is costlier than an

equivalent concrete tank. However fiberglass tanks are easier to install and

require no cranes. One brand of fiberglass septic tank is made of fiberglass

reinforced polymer (FRP). The FRP is made from a liquid resin that hardens

when a catalyst is added. The fiberglass acts as the reinforcing agents. Ribs

are also provided to improve the structural integrity of the tank.

Copolymer Polypropylene Tanks

These are made of copolymer thermoplastic sheets. The synthesis of

copolymers consists of additional copolymerization of propylene and ethylene

in a secondary reactor. By varying the quantities of co-monomer used and the

conditions of the reactor, the copolymer properties can be tailored to meet

specific use and application as in the septic tank. Copolymers have good

impact resistance and excellent corrosion resistance over a wide range of

temperatures. They are very light in weight and have exceptionally high

strength-to-weight ratio. Though a copolymer septic tank weighs only one

tenth of the weight of an equivalent precast concrete tank, it is much stronger

9

than concrete. It easily recovers after being stressed and is environmentally

safe.

2.3 DOMESTIC WASTEWATER

Domestic wastewater is the wastewater resulting from domestic activities in the

home. It contains a relatively high concentration of organic compounds

containing reduced carbon (C) and nitrogen (N) (Wilhelm et al., 1994). It also

contains a large number of potentially harmful micro-organisms and chemical

compounds. Specifically, household wastewater contains bacteria, viruses,

household chemicals and excess nutrients such as nitrates and phosphates

(Rose and Gerba, 1991; Pang et al., 2003). Raw domestic wastewater poses a

potential health risk for transmission of a large number disease causing

organisms (Nirel and Revadier, 1999). Four types of pathogens potentially

present in domestic wastewater are viruses, bacteria, protozoa and helminthes

eggs. Some of the pathogens in domestic wastewater are not “frank” pathogens

but normal flora which reside in the human gut. Enteric bacterial pathogens

can cause a wide variety of diseases ranging from gastroenteritis to ulcer and

typhoid fever.

According to Tchobanoglous et al. (1991), wastewater contains between 0.2 g/l

to 0.6 g/l of organic matter mostly composed of protein, carbohydrates and

smaller amounts of lipids. Wastewater contains two classes of contaminants:

anthropogenic organic chemicals and microbial pathogens. Proteins and urea

contribute over 97% of the 20 to 70mg/l of nitrogen typically found in

wastewater (Lake 1974; Tchobanoglous et al., 1991). Organic nitrogen

contributes 90 to 320mg/l of domestic wastewater; organic sulphur contributes

5 to 10mg/l (Hypes, 1974) of the oxygen demand while organic carbon

contributes 200 to 1000 mg/l (Tchobanoglous et al., 1991). The pH of domestic

wastewater typically ranges between 6.5 and 8.0 (Canter and Knox, 1985;

Huder and Hcukelekian, 1965).

10

The strength of domestic wastewater can be measured by its biochemical

oxygen demand (BOD), suspended solids (SS), chemical oxygen demand

(COD) or microbial content (E.coli, fecal coliform, etc). The standard BOD test

measures only the oxygen demand for organic carbon. The characteristics of

domestic wastewater flowing into the septic tank depend on whether it is only

toilet wastewater (sewage) or a combination of sewage and grey water

(bathroom and kitchen waste water). Whereas toilet wastewater is composed

mainly of organic compounds, bathroom and kitchen wastewater can contain a

relatively high quantity of inorganic compounds which may not be amenable to

biodegradation. Surface runoff from roofs and paved areas, subsurface

drainage from drains and sewers pumps and cooling water are not domestic

wastewater and therefore should not be channeled into the septic tanks as they

will lead to overloading and subsequently reduce the efficiency of the tank.

The wastewater flow is usually taken as 80% of the water consumption of a

home. However, water consumption varies from place to place depending

mainly on the availability of water. The average wastewater flow reported by

various researchers are 160 lpcd, 24 lpcd, 71 lpcd, 51 lpcd, 29 lpcd (Watson et

al., 1967); 64 lpcd (Kriesel, 1971); 100 lpcd and 120 lpcd (Rahman et al.,

1999). The standard wastewater flow used for septic tank design in the United

States is 75 gallons/capital/day (283.9 lpcd); in Egypt it is 100 lpcd for a

population less than 5000 (Kamel and Hgazy, 2006); while in Nigeria, it is 114

lpcd (Aluko, 1978). The use of water softener in areas with hard water can

raise the wastewater flow by about 40 lpcd.

2.4 OPERATION AND PERFORMANCE OF THE SEPTIC TANK

SYSTEM

The septic tank is a primary settling tank as well as an anaerobic reactor. The

influent wastewater is interrupted by the concrete splash baffle and is scattered

11

on the surface of the tank‟s content disturbing the scum layer and settled

sludge. But in tanks using tee pipes as inlet, the part of the pipe pointing

downward is made to dip into the liquid at mid depth to provide minimal

disturbance of the content. Settling starts at the inlet of the tank. Given enough

detention time, the septic tank can achieve as much as 81% total suspended

solids removal, 68% BOD removal, 65% phosphate removal and 66% fecal

coliform removal (Seabloom et al., 1982; Rahman et al., 1999). These values

are not fixed; they could be more or less depending on design, construction,

maintenance and modification. A malfunctioning septic tank will cause

damage to the drain field if the issue is not addressed.

Ideally, the septic tank operates as a plug flow reactor (fluid and particles enter

and exit the tank is progressive sequence), so there is usually no mixing or

heating, particles ascend or descend and stratification develops (Bounds, 1997).

The septic tank is primarily a sedimentation tank. The low rate of

biodegradation in the septic tank is as a result of insufficient oxygen in the

tank. The tank consists of four zones viz:

1. The sludge zone – this is the lowest portion of the tank where particles

denser than water settle given a sufficient detention time.

2. The clear zone – this is just above the sludge zone where clarified

wastewater is retained for a while before discharge into the drain field.

Detention of waste tends to homogenize the flow of waste water to the

drain field (Baumann et al., 1978). Detention also provides some time

for biodegradation by anaerobic micro-organisms.

3. The scum layer - this is the top layer just above the clear wastewater

where materials lighter than water rise to form a thick layer of about

3cm. Trojan et al. (1985) and Winneberger (1984) estimated the rate of

sludge and scum accumulation at 40 l/c/yr. The scum layer has the

undesirable effect of hindering the diffusion of air into the septic tank

content. The dissolved oxygen concentrations in septic tanks have been

found to average 0.3mg/l (Winneberger, 1984).

12

4. Air / Reserve Space – this is an empty space above the sum layer. This

provides a factor of safety against clogging of septic tank pipes. It is

recommended that the air space be equivalent to one day detention time

to provide enough time for repairs before the tank fills up completely.

However if the septic tank is not functioning properly, there will be short

circuiting causing some particles to leave the tank in a period less than the

design detention time. There will also be dead zones where some particles

seem to lodge permanently thus reducing the effective volume of the tank and

hence reducing the detention time.

The particles that settle at the bottom of the tank form the sludge layer.

Anaerobic decomposition will normally reduce the volume of the accumulated

sludge by 40 to 50% producing methane (CH4), carbon IV oxide (CO2), water

(H2O) and hydrogen sulphide (H2S) gases (Seabloom et al., 1982; USEPA,

2000). Usually there are more microbial activities at the outlet than at the inlet

resulting in less sludge accumulation at the inlet. Hydrolysis and fermentation

are typically fully functional within 48 hours of operating a new septic tank.

(Jowett, 2007). That is why septic tanks are usually sized based on twice the

daily design flow. Sludge keeps on accumulating in the tank until the effluent

quality falls below certain limits and the tank is desludged. Some septic tanks

are emptied after a specified period of time while others emptied when they are

one third full or even completely full. The level of sludge and scum in the tank

can be measured using a sludge judge which typically consists of rod with pH

or light sensitive tips. The effluent from the septic tank is sent to a drain field,

waste stabilization pond, wet land, peat or sand filter, mound, upflow and

synthetic filters, pressure distribution system or nitrogen reduction system for

further polishing before discharge or reuse. A baffle is usually placed near the

outlet to prevent the exit of scum.

13

In order to prolong the life span of the septic tank, there should be strict control

over what is sent into it. Non-biodegradable materials will cause the tank to fill

up quickly thereby causing the tank to require more frequent desludging and

consequently raising the cost of operation. Non-degradable items such as rags,

wool, hair, plastics and polythene bags can also clog the plumbing system

leading to the back flow of sewage into the house

2.5 THE DRAIN FIELD

The drain field or soil absorption system is the most commonly used facility for

final treatment of septic tank effluent before discharge into the soil. The two

most common types are the trench type and the bed type. The effluent from the

septic tank flows to a distribution box from which the effluent is distributed to

a network of perforated pipes which then gradually release the effluent to the

soil. The soil just below the perforated pipes is a graded soil of porous

material. It is usually required that there be at least 4 feet (1.22m) of

unsaturated soil below the distribution pipes, in order to ensure that wastewater

undergoes a reasonable level of treatment by the soil before it joins the ground

water. While the septic tank removes most of the solid organics, the soil

absorption system is concerned with the removal of nitrates and pathogens.

Accumulation of organic matter and micro organisms just below the perforated

pipes results in the formation of a biological layer commonly called biomat.

The micro organisms act on the accumulated solids to degrade them. The

hydraulic and purification processes that occur when effluent passes through

the biomat and underlying unsaturated zone are closely linked. The relatively

long detention time in the unsaturated soil provides opportunity for treatment

processes such as oxidation, adsorption, pathogen die-off and ion exchange.

The drain field is an aerobic treatment system provided there is an adequate

depth of unsaturated soil below the field. The domestic wastewater undergoes

its most significant geochemical changes in the drain field, where it flows from

14

the biological mat to the water table (Wilhelm et al., 1994). When there is

adequate oxygen, the micro organisms can completely oxidize the reduced

wastewater components in the unsaturated zone. Anaerobic bacteria in the

septic tank transform the organic nitrogen in the wastewater to NH+

4- N

(ammonium nitrogen) while the aerobic bacteria in the drain field subsequently

oxidize the NH+

4-N to N0-3-N (nitrate nitrogen). Organic carbon is also

oxidized to CO2 and a certain fraction is removed by retention by sediments.

The most persistent contaminant in wastewater is the nitrate which is a

potential health hazard and can cause eutrophication in coastal, marine and

surface wasters. However, nitrates can be removed from wastewater by

denitrifying bacteria which are located deep down in the soil. These bacteria

require anoxic conditions as well as equal amounts of carbon and NO-3-N to

accomplish denitrification. In ground water settings, a lack of labile organic

carbon is the most common limitation to denitrification (Keeney, 1986).

Bouma et al. (1972) stated that not all soils are suitable for waste disposal.

Suitable soil should be reasonably permeable and well aerated (drained) so that

oxidation of the organic waste can take place (Canter and Knox, 1985).

Karathansis et al. (2006) noted that extremely fine soils and extremely coarse

textured soils are not ideal for the soil absorption system. This is because in

fine textured soil, solid particles may clog the soil pores making it difficult to

maintain adequate long term drainage, thus leading to system failure.

The soil absorption system can be limited in efficiency by number of factors.

i) Percolation Rate

Soils with percolation rates less than 5 minutes/inch or greater than 60

minutes/inch are not recommended for soils absorption system. Very high

percolation rates permit septic tank effluents to pass through the soil without

proper treatment. On the other hand, very low percolation rates can drastically

reduce the rate of wastewater movement which in turn can result in ponding of

septic tank effluent. This will create anaerobic conditions and a complete

15

failure of the entire system. The size of the drain field is usually dependent on

the percolation rate. The percolation rate is usually measured by digging about

six holes on the site where the drain field is to be located. The depth of the

holes should be the same as the depth of the distribution pipes (roughly 2 feet).

The holes are first filled with water and left for 24 hours to ensure that the soil

is saturated. Afterwards the hole is topped with water and the depth of water in

each hole is measured at a regular interval, usually thirty minutes. The

percolation rate is determined by dividing the time interval by the drop in water

level. Measurement is continued until each of any three consecutive calculated

rates varies by no more than 10 percent from the average of the three values

(Kansas State Department for Health and Environment, 1997)

ii) Depth to Bedrock or Water Table

High water table or shallow soil over rock is restrictive to the use of drain field.

A high water table will give rise to anaerobic conditions in the drain field and

allow pathogens to escape to ground water. Shallow soil over rock will not

provide sufficient soil for wastewater treatment. In addition, it will lead to

ponding which will give rise to anaerobic conditions. It is usually required that

the soil for drain field have at least four feet (1.22m) of suitable soil below the

distribution pipes. High water table and shallow soil over rock can lead to

hydraulic failure and treatment failure respectively. Treatment failure occurs

when contaminants are not fully removed from water because of an insufficient

depth of the aerated zone while hydraulic failure occurs when the water table

inundates the disposal pipes or reaches the ground surface, where overland

flow can transport the pollutants directly to the stream without adequate

treatment (Collick et al., 2006). Such soils can only be suitable for the mound

system of effluent treatment rather than the conventional drain field.

iii) Soil Surface Slope

Areas with slopes steeper than 20% will cause considerable difficulty during

construction and are not recommended for lateral field installation (KSDHE,

16

1997). Steep slopes can cause hydraulic failure when the septic effluent flows

laterally and surfaces downslope of the field (Collick et al., 2006). Drain fields

on very steep slopes can also be subject to erosion by high velocity runoff

which can lead to total destructions of the drain field.

Table 2.1: Soil Limitation Ratings Used by NRCS for Wastewater Absorption

Fields

PROPERTY

LIMITS

Slight Moderate Severe Restrictive feature

USDA Texture - - Ice Permafrost

Flooding None,

protected

Rare Severe Flood water inundates site

Depth to bedrock

(m)

> 72 in 40-72 in <40 in Bedrock or weathered

bedrock restricts water

movement or reduces

treatment capacity

Depth to

cemented pan (in)

> 72 in 40-72 in < 40 in Reduces water & air

movement

Permeability

(in/hr) 24-60 in

layer

less than 24 in

layer

2.0 – 6.0

-

0.6 – 2.0

-

< 0.6

> 0.6

Slow percolation rate, poor

drainage poor filter

Slope (%) 0-8 8-15 > 15 Difficult to construct and

hold in place

Layer stones

greater than 3 in

(% by wt)

< 25 25 – 50 > 50 Restricted water and air

movement results in

reduced treatment capacity

17

2.6 SEPTIC TANK FAILURE

Septic tank failure constitutes any situation that detracts from optimal

performance of the system. Septic tank failure is very common though

overlooked in many cases because the septic tank system is usually hidden

from sight. When septic tanks fail, they release nutrients and pathogens into

the environment (Geary and Gardner, 1998; Yates, 1985; Scalf et al., 1997)

such as groundwater, surface waters, swimming pools, farmlands etc. Jelliffe

(1995) reported septic tank failure rate as being higher than 40% in Australia.

Of the 48 septic tanks studied by Ahmed et al. (2005), 32(67%) needed

cleaning out, 23(48%) had soggy absorption fields, 4(8%) had structural

defects such as broken baffles or lids, 2(4%) had technical faults such as high

water table or the absorption system being too close to a water well, 3(6%) had

insufficient capacity, and only 7(15%) were well maintained.

Gordon (1989) identified causes of septic tank failure as: too small absorption

field, unsuitable depth or soil type, under sizing and improper design, high

water table, physical damage to plumbing works and lack of maintenance.

Another inevitable cause of septic tank failure with respect to effluent quality is

high density of septic tank systems in an area (Jelliffe, 1995).

With minimal maintenance and good practices, the septic tank can last very

long. Septic tank failure can be caused by the following:

i. Excessive accumulation of sludge and scum

Good practice requires that the septic tank should be desludged at intervals.

Failure to desludge the tank can cause excessive solids to be carried over to

the drain field and thus clogging it. When this happens, the drain field

becomes inundated with septic effluent leading to anaerobic conditions.

This usually gives rise to pungent smells indicating gaseous product of

anaerobic decomposition. Excessive sludge accumulation in the tank can

also cause back up of sewage into the house when the inlet pipes are

clogged.

18

ii Deterioration of Baffles

Sewage is corrosive to metal. The force of impact on the baffle gradually

eats away the concrete thereby exposing the steel to corrosion. Chemicals

and detergents present in domestic wastewater can also contribute to the

deterioration of concrete splash baffles especially when the mix is poor.

The destruction of the baffles is a very critical failure case because there

will be short circuiting of influent wastewater as well as unrestricted

discharge of scum into the drain field. Constant short-circuiting of sewage

to the drain field will lead to overloading and subsequent failure of the

whole system. The baffles should therefore be inspected from time to time

in order to ascertain its state.

iii Leakage

The basic structural requirement for septic tanks is that they are watertight.

Leakages in the septic tank can result from poor construction. Where

ground water levels are high, leaky tanks allow infiltration that causes

solids and greases to wash through the tank, lowering treatment efficiency

and leading to eventual failure of onsite disposal system (Bounds, 1997). In

areas without very high water table, untreated wastewater will just leak out

of the tank untreated and join the ground water. In addition, exfiltration

will lead to the lowering of water and scum level in the tank such that

floatable solids, fats, soaps, oils and grease can be washed through the

outlet assembly. Infiltration/inflow (I/I) in effluent sewers overloads both

collection and treatment capacities. Exfiltration also hinders segregation,

biological activities and proper development of a clear zone. Overall,

leaking septic tank has the detrimental effect of destroying the drain field

and short-circuiting raw sewage to the ground water, thereby posing a

serious environment and health risk. It is therefore necessary to test septic

tanks for water tightness before putting them to use.

19

iv Clogging and Plugging of Drains

Plugging of drains can result from excessive accumulation of solids in the

tank or flushing of non biodegradable materials such as sanitary towels,

rags, cotton buds and plastic materials into the tank. These materials can

clog the septic tank inlet or any other part of the sewer. Plugged pipes will

cause sewage to back up into the house and cause wastewater to drain

slowly. Sometimes plugging can occur if the plumbing pipes used are of

too small diameters.

v Overloading

Diversion of surface and roof runoff to the septic tank can cause occasional

hydraulic overloading which will cause wastewater to leave the tank before

the design detention time. The use of macerators and garbage grinders for

disposal of waste food can cause rapid overloading of the septic tank.

Discharge of pesticides, herbicides and materials with high concentration of

bleach or caustic soda can also hamper the functioning of the septic tank.

For properly functioning septic tanks, occasional overloading can occur at

weekends or during holidays or festivals. In addition, if there are leakages

in the plumbing or tank, runoff can find its way into the tank thus giving

rise to overloading. Hydraulic overloading can be avoided by ensuring that

only wastewater from the home enters the tanks. Laundry activities can

also be spread out over the days than doing them in one day.

2.7 SLUDGE ACCUMULATION

Sludge accumulation is an intrinsic aspect of septic tank operation. Sludge

accumulation results from the settling of solids on the bottom of the tank. The

treatment quality of the tank is greatly diminished when excess sludge and

scum accumulate in the tank so that they start to be carried over into the

absorption field. Excess solid particles leaving the septic tank plug up the

leaching pipes and then there is no adequate distribution of the effluent and no

20

proper treatment of the waste in the drain field. It is important to estimate the

scum and solids accumulation rates in the septic tank in order to predict the

septage removal intervals. The most popular equations (Equations 2.1 and 2.2)

for estimating sludge and scum accumulation in the septic tank were obtained

by Bounds (1988) and Weibel et al., (1955) respectively.

675.047tN (2.1)

86.5039.13 tN (2.2)

Where

N = volume of septage accumulated in tank in US gallons per capita.

t = number of years of operation.

These equations are purely empirical in nature and have a statistical confidence

level of 95%, and predict the gallons per capita accumulated after any time

given in years. However, Bounds‟ equation (Equation 2.1) gives slightly higher

values of septage accumulation.

Seabloom et al. (2004) recalled that in 1980 and 2002, the USEPA

recommended that if the systems are not regularly inspected, the septic tank

should be pumped out every 3 to 5 years, depending on the size of the tank, the

number of building occupants, and household appliances. Bounds (1995)

opposed this stand, stating that such pump-out intervals were not supported by

scientific evidence, and suggested much longer intervals (Table 2.2).

21

Table 2.2: Desludging Intervals as Recommended by Bounds (1995)

TANK CAPACITY SPECIFICATIONS

1000 (US gallons)

No of Occupants 2 3 4 5

Desludging interval (years) 22 11 7 4

1500 (US gallons)

No of Occupants 5 6 7 8

Desludging interval (years) 9 7 5 4

2.8 CONTRIBUTIONS OF PREVIOUS RESEARCHERS

Jowett (2007) carried out a research on septic. The aim was to verify the

significance of air space in the septic tanks. Two tanks were used; one being a

conventional 1500 gallon (6820 l) septic tank with air space and the other being

a long, narrow, shallow tank without air space but of the same volume as the

former. The tanks were dosed at half the effective volume i.e 750 gallons per

day (34.0 l/d) to simulate the full design daily flow. Dosing was done fifteen

times per day at different times to simulate the pattern of flow in actual septic

tanks.

His results showed that within the first three months of operation, the

conventional septic tank with air space had accumulated 52% by volume of

solids mostly as sludge while the long, shallow, narrow septic tank without air

space had accumulated only 15% by volume of solids with scum only at the

inlet space. Within the same period, the septic tank without air space

performed better than the conventional type with air space by 18% in terms of

BOD and TSS removal. His results also showed that both septic tanks

performed better in summer than in winter though the conventional septic tank

with air space still fared worse than its counterpart without air space. He also

found that alkalinity increase from inlet to outlet probably as a result of

methanogenesis. This result is in line with the findings of Paing et al. (1999)

who monitored sludge accumulation and digestion in a primary anaerobic

lagoon. Sludge was sampled at several points in the lagoon to determine

22

spatial variations. Their results show that more sludge accumulated at the inlet

than at the outlet due to higher methanogenic activities towards the outlet.

Wilhelm et al. (1994) reasoned that metanogenesis raises the alkalinity of

septic tank effluent. However Paing et al. (1999) and Jowett (2007) do not

agree on the spatial variation of volatile fatty acid in the tanks. While Jowett

(2007) reported that volatile fatty acids generally increase from inlet to outlet,

Paing et al. (1999) reported a decrease in volatile fatty acid from inlet to outlet.

Figure 2.1: Relationship between Air Space and Sludge Accumulation

(Source: Jowett, 2007)

Furthermore, Jowett (2007) concluded that the presence of air space is

disadvantageous to the general welfare of septic tanks (Figure 2.1). This

rubbishes the view of Baumann (1978) who was of the opinion that the air

space is a reservoir with two main functions: permanent storage of floating

scum and temporary storage of influent surges to decrease velocities through

the outlet pipe. Jowett (2007) & Dunbar (1907) countered by saying that the

air - water interface actually encourages vegetative moulds that trap sludge

particles rising on fermentation bubbles, creating a hard leathery scum layer

0%

10%

20%

30%

40%

50%

60%

Tank Air Space Tank Without Air Space

Slu

dge

Acc

um

ula

tio

n

Tank Modification

23

which could overturn and sink, causing resuspension and outflow of sludge.

They also stated that the scum can cause more nuisance by presenting

difficulties during pumping. However, it is counterproductive to recommend

that septic tanks be constructed without air space because the reserve space will

always be a functional component of the septic tank. Without the reserve space,

any blockage will result in the backup of sewage into the building. While

Jowett (2007) might have made an interesting discovery regarding air space,

the practicability of their findings remains elusive.

Rock and Boyer (1995) carried out a research on the effect of

compartmentalization and baffle type on the efficiency of septic tanks at the

University of Maine for a period of two years starting from 1992. First they

compared a single compartment tank with tee pipes acting as both inlet and

baffle, and another single compartment tank with a concrete splash baffle. The

tank with inlet-outlet baffle produced 20% better BOD removal than the tank

with concrete splash baffle while the tank with splash concrete baffle had a

better SS removal by 1% margin. The single compartment tank with inlet-

outlet baffle was next compared with a 2:1 double compartment tank with a

100mm elbow opening in the partition opening to serve as outlet for the first

compartment. The double compartment septic tank had the same inlet-outlet

baffles just like the single compartment tank. The single compartment tank

was found to produce effluent 23% better in BOD removal and 14% better in

TSS removal than the double compartment tank. Seabloom (1982, reported in

Seabloom et al., 2007) who also worked on compartmentalization obtained

similar result although of a bit different pattern. His own results showed that

the single compartment tank gave 17% better BOD and 69% better TSS

removal than the double compartment tank.

However, the double compartment tank improved when a larger opening was

provided in the partition wall. The slot was horizontal and covered 75% of the

width of the partition wall. The result was amazing: 11% better BOD removal

24

and 7% better TSS removal than the single compartment tank. If these results

are anything to go by, they should lay to rest the age-long controversy on

whether compartmentalization really does improve the quality of septic tank

effluent. Winneberger (1984) explains this phenomenon by saying that

velocities and turbulence affects the migration path of particles traveling

through the septic tank such that slow velocities yield the highest effluent

quality. He went on to say that the critical factor is the management of flow

through the septic tank not the geometric shape of the tank nor the size of the

second compartment. Bounds (1997) does not seem to be concerned about

compartmentalization hence he wrote: “regardless of number, size or shape of

supplemental compartments, the primary or first compartment‟s capacity

should be defined based on hydraulic loading, velocity through the tank,

reserve capacity, solids storage capacity and hydraulic retention time”. This

statement, sound as it may seem, is not in complete consonance with the more

recent research works of Jowett (2007) whose results suggest the elimination of

the reserve space being advocated by Bounds (1997).

Lay et al. (2005) in an attempt to contribute their own quota to the issue of air

space (reserve space) and compartmentalization, compared the performance of

four different septic tanks. The first was the conventional septic tank, (single

compartment with air space), the second was a 2:1 double compartment tank

with air space, the third was 2:1 double compartment tank without air space

while the fourth was 1:1 double compartment tank without air space. All the

tanks were of 4500l. Light expanded clay was used as surrogates for sludge

particles and dosing was done at the rate of 3.75 l/s. The result obtained show

that the conventional septic tank has the worst performance followed by the 2:1

double compartment tank with air space. The last two tanks without air space

did not allow short circuiting of solids even when the doing rate was increased.

According to Jowett and Lay (2005), the septic tank without air space provides

a „closed-conduit‟ flow similar to that in a flooded pipe or flooded cave, with

equal frictional drag on all sides of the tank: walls, ceiling and floor. This equal

25

frictional drag minimizes the velocity differentials between the center of the

tank and the sides thus prohibiting short circuiting. On the contrary, a tank

with air space is similar to an open channel which experiences unequal

frictional drag between the walls and floor, and the air space. This allows

greater velocity differential thus encouraging short circuiting.

Jowett and Lay (2005) used two interconnected narrow long and shallow tanks

to show that long and shallow configurations favour sludge particle capture and

thorough fermentation. Like Winneberger (1984), they concluded that flow

characteristics are more important than size alone. They reasoned that

differential flow velocities, causing unwanted higher velocity plume, increase

in tanks with shorter, wider and deeper aspects especially in those with point

source inlets and outlets like a septic tank. Higher velocity plumes produce

turbulent flow with eddies that resuspend solids and allow untreated sewage to

short-circuit to the outlet. This was also corroborated by Winneberger (1984)

using dye as a tracer in a short, partitioned model tank. Short circuiting was

witnessed in the short tank while no short circuiting was observed in a long,

meander model tank. Jowett and Lay were of the opinion that in order to

optimize separation of solids and to maximize retention time without short-

circuiting, the tank should encourage a well developed laminar flow regime.

In Egypt, Kamel and Hgazy (2006) worked with forty (40) modified septic

tanks in an attempt to improve the quality of septic tank effluents. The

modified septic tanks consisted of four sealed chambers arranged serially,

discharging into a fifth chamber filled with gravel. The first Chamber was 1m

long x 1m wide x 1.5m deep, the second chamber was 1m x 0.5m x 1.2m, the

third and fourth compartments were 1m x 0.75m x 1.2m each. Forty percent

(40%) of each of the third and fourth compartments was covered with gravel to

serve as filter beds. The compartment openings were such as to allow

alternating up flow and down flow discharge from compartment to

compartment. The fifth compartment was totally filled with gravel and acts as a

26

drain field from which effluent flows into the surrounding soil. Though the

inlet into the first compartment and outlet from the fourth compartment were

made of 3 inches tee pipes, the compartment openings were similar to that used

by Rock and Boyer (1995) in their compartmentalization studies.

The results obtained show that the best performing tank gave a total of 83.7%

SS removal in this order: 43.3% between the first and second compartment;

33.7% between the second and third compartments; and 6.77% between the

third and fourth compartments (Figure 2.2). The overall best BOD5 removal

was 81.7% in this order: 45.2% between the first and second compartments,

32.22% between the second and the third compartments and 4.29% between the

third and the fourth compartments (Figure 2.3). They also recorded very high

pathogen removal with an almost complete elimination of salmonellae.

Figure 2.2: Efficiency of Suspended Solids Removal between Compartments

(Source: Kamel and Hgazy, 2006)

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

45.00%

1st & 2nd Compartment

2nd & 3rd Compartment

3rd & 4th Compartment

Susp

end

ed S

olid

s R

emo

val

Compartment

27

Figure 2.3: Efficiency of BOD Removal between Compartments

(Source: Kamel and Hgazy, 2006)

Though their study was impressive and gave outstanding results, Kamel and

Hgazy did not look at the impact of the rate of sludge accumulation on the

septic tanks. This omission renders the study inconclusive because a closer

look at the work shows that the first two compartments have a total volume of

1.8m3 and a detention time of 1.8days. However, according to the Egyptian

Cod Pluming (ECP), the rate of sludge accumulation is 50 l/capita/year

implying that within one year, the sludge accumulation in the two

compartments could reach 500l for a population of ten used in the study.

Because the flow rate of wastewater was 100l/day, the detention time will

reduce from 1.8 days at the start of operation to 1.3 days in just one year or 0.8

days in two years. This will result in carry over of more suspended solids and

BOD into the third and fourth compartments which have no space for sludge

accumulation. When this happens, there could be a clogging of the pore spaces

of the gravel in the third and fourth compartments which will result in the

failure of the entire system.

In addition, the researchers did not clearly indicate how long their studied

lasted but one can easily deduce from their presentation that sampling was done

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

45.00%

50.00%

1st & 2nd Compartment 2nd & 3rd Compartment 3rd & 4th Compartment

BO

D R

emo

val

Compartments

28

only once. If this was the case, then no valid conclusion can be drawn from

their results though they look good. This is because the septic tank is a

continuously operated system whose performance could be drastically affected

with the passage of time.

Rahman et al. (1999) investigated the influence of wastewater characteristics

on the performance of septic tanks. They used three different arrangements viz:

(i) Septic tank receiving only toilet wastewater,

(ii) Septic tank receiving toilet wastewater and kitchen waste

wastewater, and

(iii) Septic tank receiving toilet wastewater, kitchen wastewater and

bathroom wastewater (all purpose).

Their results show that the septic tank receiving the three kinds of wastewater

performed better than the other two, followed by the tank receiving toilet

wastewater and kitchen wastewater despite that the three arrangements have the

same volume. The reason is that discharging kitchen wastewater and bath water

into the septic tank dilutes its content. However, this will increase flow rate and

reduce the efficiency of fecal coliform, NO3- and PO4

- removal. Again, the

study period was too short and therefore insufficient to conclusively establish

any pattern. The researchers recommended a five day retention time for septic

tanks receiving only toilet wastewater, a three day retention time for septic

tanks receiving toilet wastewater and kitchen wastewater, and one day

detention time for an all purpose septic tank.

Büsser et al. (2006) reported in Nguyen et al. (2007) have a somewhat different

opinion about discharging grey water into septic tanks. They noted that grey

water can contain up to 50% of the COD of domestic wastewater. Nguyen et

al.(2007), based on their Vietnamese experience reported that discharging only

toilet waste (black water) into the septic tank can reduce the pollution load of a

household.

29

Nguyen et al. (2007) did a comparative study of the conventional septic tank

and its modified version in Vietnam between July 2004 and November 2005.

The modified septic tanks are baffled septic tank (BAST), baffled septic tank

with anaerobic filter (BASTAF) and septic tank with anaerobic filter (STAF).

The baffled septic tank is a type of compartmentalized septic tank with

partition openings located near the bottom of the tanks so that wastewater flows

into the second compartment in an up flow direction. This is to force more

contact between the wastewater and the sludge (biomass) in the tank to increase

the rate of biodegradation.

In their experiment, they used six plastic upright cylinders arranged serially so

that wastewater enters each one from the bottom and leaves through a tubing

connected near the top from which it then flows to the next cylinder. The up

flow velocity was so controlled as to avoid the wash out of sludge which can

cause the failure of the whole system. The baffled septic tank with anaerobic

filter is a further improvement on the septic tank by adding an anaerobic filter

chamber to the BAST. The filter materials were 60mm diameter plastic balls.

The study was aimed at determining the extent of improvement in effluent

quality provided by the modified septic tanks over the conventional type. For

the BAST, they found that the optimum number of up flow chambers is four

and that the optimum hydraulic retention time is 48 hours as increasing the

retention time beyond this will require more cost in terms of tank volume while

at the same time not producing any significant result above that observed for 48

hours hydraulic retention time. Typical results obtained are as follows: 55.7%

COD removal and 47.4 TSS removal for the conventional septic tank; 72%

COD removal and 70.4% TSS removal for the BAST system; 86.3% COD

removal and 90.8 TSS removal for the BASTAF system and 84.1% COD

removal and 84.7 TSS removal for the STAF system (Figure 2.4).

30

Figure 2.4: Efficiency of Treatment for Different Modifications of the Septic

Tank (Nguyen et al., 2007)

Nguyen et al.(2007) observed that adding an anaerobic filter chamber to either

the BAST or the conventional septic tank system gives an effluent quality

better than that of both the BAST and the conventional type. However, their

results indicate that the BASTAF system, though more complex, is not as cost

effective as the STAF system. For all its cost and complexity, the BASTAF

could only afford approximately 2% better COD removal, 5% better BOD

removal and 6% better TSS removal. In addition it was observed that effluent

quality started declining after two years. This prompted the researchers to

recommend a two-year desludging interval for the BASTAF.

It should be noted that Nguyen et al.(2007) carried out their research using both

laboratory scale models and full scales. However, they used black water (toilet

water) as influent into the laboratory scale models, while some of the full scales

received a combination of black water and grey water. In addition, they failed

to point out that the BAST and the BASTAF system could be subject to

resuspension of settled sludge if wastewater flow is not properly managed.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

COD TSS

BA

ST, 7

2%

BA

ST, 7

0.4

0%

BA

STA

F, 8

6.3

0%

BA

STA

F, 9

0.8

0%

STA

F, 8

4.1

0%

STA

F, 8

4.7

0%

Co

nve

nti

on

al T

ank,

55

.70

%

Co

nve

nti

on

al T

ank,

47

.40

%

Per

cen

tage

Rem

ova

l

31

Wilhelm et al. (1994) presented a conceptual model which borders on the

geochemical evolution of wastewater right from inflow into the septic tank to

the final stage when it joins the groundwater. They noted that the septic tank

itself is a zone of redox reactions catalyzed by micro-organisms. Because

domestic wastewater is composed mostly of organic matter, the micro-

organisms first hydrolyze the large organic compounds to simpler ones.

Carbohydrate is hydrolyzed to sinple sugar; protein is hydrolyzed to amino acid

while fat is hydrolyzed to fatty acid and glycerol.

The second stage is the conversion of simple sugar and amino acids into

organic acid acetate and H2. The fatty acids earlier produced by the hydrolysis

of fat and the intermediate organic acid produced from simple sugar and amino

acid undergo anaerobic oxidation in which protons accept electrons to form H2

in the presence of SO2-

4. The micro-organisms will use the So2-

4 to oxidize

organic carbon to produce CO2 and S2-

. Finally methanogenic bacteria then use

acetate or CO2 and H2 to produce CH4. It was also observed that most of the

nitrate-nitrogen (No-3 -N) in the influent are usually converted to ammonium-

nitrogen (NH+

4 –N) which is usually denitrified in the absorption field while

10% to 30% of the total organic nitrogen is removed by sludge storage (Laak

and Crates, 1978). They further noted that the production of organic acid by

fermentation and the formation of H2 by oxidation usually reduce the alkalinity

of the septic tank effluent while the reduction of SO2-

4 and the consumption of

acetate by methanogenesis usually raise alkalinity thus maintaining a balance.

If however, methanogenesis is inhibited by low pH, then the pH of the effluent

will drop precipitously (Grady and Lim, 1980).

Burubai et al. (2007) used modified septic tanks, similar to those used by

Kamel and Hgazy (2006), to improve septic tank effluent in Rivers State of

Nigeria which is a high water table area. The Kansas State Department of

Health and Environment (1997) recommends at least four (4) feet of

unsaturated aerated) soil below the bottom of the soil absorption field to ensure

32

adequate treatment. The set up consist of a three compartment septic tank: two

are for sedimentation while the third acts as a sand filter from which partially

treated wastewater flows into the drain field. The aim was to achieve a high

level of treatment in the septic tank by augmenting with the sand filter so that

the more or less inefficient soil absorption field will have less work to do. This

is similar to those of Kamel and Hgazy (2006) in that, their own set up

consisted of two sedimentation chambers leading to two gravel filtration

chambers whose final effluent flowed into the drain field.

Burubai et al.(2007) monitored three of the full scale set up sited in an area

where the water table was less then 100mm (4 in) below the surface. The drain

field consisted of perforated pipes covered with graded sand and gravel under

gravity flow. The effluent from the soil absorption field was collected by

means of a horizontal screened outlet below the field.

The initial results obtained show that the effluent quality for BOD, COD, TSS

and fecal coliform fell within both local and international standards within the

first one year. Effluent quality started declining from around the 15th

month and

this was attributed to filter clogging. The result is interesting but the researchers

did not indicate whether they also collected samples from the septic tank

effluent. This would have enabled them estimate what percentage of the

treatment was contributed by the drain field. In fact taking effluent from each

unit would have helped them know which unit was the most efficient and

which one was the least viz-a-viz cost. However, intuitively, one can easily

come to the conclusion that the drain field was inefficient as 4 inches of soil

will easily be overwhelmed by the effluent. Moreover, Burubai et al.(2007) did

not say how to maintain or revive the failed filter after fifteen months. Their

cost estimate also shows that the cost of the modified septic tank was 57%

higher than that of the conventional septic tank and yet it failed in fifteen

months. If satisfactory answers can be given to the issues raised so far, then this

modified septic tank can be extended to cover other areas where such

33

restrictive soil factors as high percolation rate, very low percolation rate and

very steep slopes render the septic tank system inefficient.

Koottatep et al. (2004) conducted a research similar to that of Nguyen et al.

(2007) using a slightly different experimental set up. While Nguyen et

al.(2007) used a number of standing cylindrical units to emulate baffled septic

tanks with anaerobic reactors, Koottatep et al.(2004) used a set up which better

approximates an actual septic tank. The set up of Nguyen et al. seem over

idealized as it is not yet clear how their set up can be executed on site.

Koottatep et al.(2004) used four laboratory scale models of dimensions (64 cm

long x 25cm wide x 40 cm high) with vertical standing and hanging baffles to

imitate an anaerobic baffled reactor (ABR). Just like Nguyen et al.(2007), their

reason for this was to ensure more contact between wastewater and biomass in

the sludge zone. The description of the four experimental set up are as follows:

reactor A - two baffles; reactor B - three baffles; reactor C – two baffles with

anaerobic filter and reactor D – conventional septic tank with two

compartments serving as a control unit. The reactors were subjected to strong

wastewater being a combination of septage from Bangkok and wastewater from

the Asian Institute of Technology (AIT). The flow rate of wastewater was such

as to give 24 hours and 48 hours detention times, respectively.

Their results show that at 24 hours detention time, there was no significant

difference between three baffled reactor the two baffled reactor in BOD, COD

and TS removal (Figures 2.5 & 2.6). But as the detention time was increased to

48hours, the performance of the two baffle reactor with anaerobic filter

improved over the three baffled reactor, while the performance of the three

baffled reactor remained fairly constant. In fact, a closer look at their results

revealed that the improved septic tanks performed much better than the

conventional septic tanks at low detention times such as 24hrs. However, as the

detention time increased the performance of the conventional septic tanks

improved and even recorded a better performance in SS removal than the three

34

baffled reactor. They however, noted that the three-baffled reactor is more

robust than all the others. When subjected to both shock loading and fluctuating

loading, the other reactors responded by a fluttering performance while the

three baffled rectors maintained a fairly constant performance irrespective of

loading.

Their results also showed that the two-baffled reactor performed better than the

two-baffled reactor with anaerobic filter. This is not in order because the

anaerobic filter is meant to effect more treatment in addition to the work done

by the baffles. The researchers did not point out this anomaly in their result but

one can deduce that this incongruence is due to experimental error.

Figure 2.5: Efficiency of Treatment Versus Number of Baffled Reactors

(Source: Koottatep et al., 2004)

The researches however, jumped to a hasty conclusion about the three-baffled

reactor by stating that “the shorter the hydraulic retention time (HRT), the

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

COD BOD TS SS TKN TP

2-B

affl

e

2-B

affl

e

2-B

affl

e

2-B

affl

e

2-B

affl

e

2-B

affl

e

3-B

affl

e

3-B

affl

e

3-B

affl

e

3-B

affl

e

3-B

affl

e

3-B

affl

e

2-B

affl

e +

Filt

er

2-B

affl

e +

Filt

er

2-B

affl

e +

Filt

er

2-B

affl

e +

Filt

er

2-B

affl

e +

Filt

er

2-B

affl

e +

Filt

er

Co

ntr

ol

Co

ntr

ol

Co

ntr

ol

Co

ntr

ol

Co

ntr

ol

Co

ntr

ol

Rem

ova

l Eff

icie

ncy

35

smaller the size of the reactor required”. This cannot be correct. There is

certainly an optimum detention time below which the reactor cannot attain the

desired performance and above which, efficiency may not increase

considerably. In fact Nguyen (2007) reported an optimum detention time of 48

hours.

Figure 2.6: Efficiency of COD Removal for Different Modifications of the

Septic Tank at Various Detention Times (Source: Koottatep et al., 2004)

The discharge of chemical such as water softener and disinfectants into the

septic tanks has been a source of worry to septic tank experts. It has been

reported that water softeners resuspend solids and cause them to be washed out

of the tank with effluents while disinfectants adversely affect the microbiology

of the tank resulting in reduced treatment efficiency or total failure.

Washington et al. (1998) performed an experiment with a view to determining

the fate of absorbable organic halide from household bleach in a septic tank and

assessing the effect of bleached laundering on septic tanks. They noted that the

COD removal fell from 40-50% for a septic tank receiving toilet wastewater to

25-35% when laundry water was added (Figure 2.7). Gross (1987) determined

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

24hr Retention Time 48hr Retention Time

3 B

affl

es A

BR

3 B

affl

es A

BR

2 B

affl

es A

BR

wit

h A

F

2 B

affl

es A

BR

wit

h A

F

\Tw

o C

om

par

tmen

t Se

pti

c Ta

nk

Two

Co

mp

artm

ent

Sep

tic

Tan

k

Per

cen

tage

CO

D R

emo

val

36

the effects of liquid chlorine bleach, high test hypochlorite (HTH), Lysol and

Drano crystal on the performance of the septic tanks. All four chemicals used

are common household disinfectants. He found that the discharge of these

chemicals into the septic tanks led to a drastic reduction in the microbial

population in one septic tank. Slug concentrations of these disinfectants were

found to be more detrimental to the septic tank than a gradual dosage.

However, the septic tank recovered quickly after the use of the disinfectants

was discontinued. This is similar to the observation of Novac et al. (1990) who

is however of the opinion that the septic tank will not recover completely if the

household disinfectants are dosed beyond a certain critical concentration.

Below this dosage the septic tank would recover as the chemical will be flushed

out by new wastewater without chemical.

Vaishar and McCabe (1996) investigated the effects of household chemical on

three parametric indicators of the efficiency of the septic tank namely:

microbial activities, settling of solids and the adsorption of these chemicals to

the septic tank sludge. Using an anaerobic sludge respiration test as measure of

the level of microbial activities taking place in the tank, they found that the 96-

hour no - effect concentration (NOEC) is 625ml. They concluded that dry

bleach in septic system is acceptable so long as solid settling is not adversely

affected.

37

Figure 2.7: Efficiency of Treatment versus Wastewater Composition

(Source: Washington et al., 1998)

Ignatius and Jowett (2004) investigated the effect of two household

disinfectants on the performance of the septic tank. One is a two-tablet sanitary

bleach puck system which is a chlorine based disinfectant while the other is

granular laundry detergent, an oxygen based bleach. Four laboratory scale

septic tanks were used in this research. The first was control receiving only

wastewater, the second received wastewater and the two-tablet sanitary bleach

puck, the third received wastewater and the granular laundry detergent while

the fourth received wastewater and both chemicals. Results show that a

combination of both chemicals has more effect on the performance of the septic

tank followed by the granular laundry detergent. Surprisingly, it was found that

the septic tank receiving wastewater and the chlorine based bleach performed

better than the other system. This better performance was however not

significant when compared with the control. The reason the chlorine based

bleach did not have any noticeable effect on the performance of the septic tank

was explained by the researchers. Raw sewage contains a high concentration of

ammonia which will normally react with the chlorine in the bleach. The

oxidizing power of free chlorine is inactivated by reaction with ammonia

(Washington et al., 1997).

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

Sewage Only Sewage & Laundry Water

Ave

rage

CO

D R

emo

val

38

The researchers used only two parameters in their research viz: biochemical

oxygen demand (BOD) and fecal coliform. This is not commendable as the

primary function of the septic tank is suspended solids removal. The work

would have been more realistic if suspended solids had been included in the

parameters of interest. Moreover, the wastewater used was not strictly raw

wastewater. They diverted effluent from a biofilter which has already reduced

the characteristics of the wastewater. For instance, the wastewater used has a

BOD ranging from 13-79mg/l which in some cases is even lower than that from

properly functioning septic tanks.

The most critical effect of failure of the septic tank to effectively treat

wastewater can be witnessed in its impact on ground water quality. A failed

septic tank is most likely to result in failure in the drain field which will then

respond by transmitting improperly treated wastewater to the groundwater. As

mentioned earlier, nitrate (NO3-) is the most persistent pollutant in wastewater

because of its conservative nature. Sinton (1982) and Close et al. (1989) found

elevated NO3- levels and locally high level of fecal coliform in groundwater as

a result of carryover of pollutants from the drain field. Wilhelm et al. (1994)

observed that denitrifying bacteria need labile carbon in order to effectively

reduce NO3- to nitrogen. They however, noted that by the time the wastewater

gets to the denitrifying zone, there will be little or no carbon left to act as

electron acceptor. The result of this is that dentrification will be grossly

hampered resulting in the discharge of elevated levels of NO3- into

groundwater.

Still on the impact of improperly treated wastewater on groundwater, Pang et

al. (2006) developed a ground water transport model using HYDRUS 2D

which is a Microsoft Window based finite element model. The model was used

to evaluate the effect of clustered septic tanks on the quality of ground water in

Yaldhurst area of Christ Church, New Zealand. Their results show that

39

clustered septic tanks resulted in an increase in the NO3- levels of groundwater

in the area under investigation. The model showed that a distance of 2.9km is

needed between septic tanks in order to affectively attenuate the NO3- level in

ground water. This distance was found sufficient to bring nitrate concentrations

to background levels. They were quick, however to observe that this separation

distance is impracticable and recommended more efficient treatment in the

septic tank system. They also found that depth of 10m below the ground water

level was needed for effective dilution of the nitrate level. Hence they

recommended that water wells should penetrate 10m below the ground water

level.

The same model was applied to fecal coliform transport in ground water.

Results obtained show that fecal coliform had only a localized impact. No

cumulative effect as a result of clustering was observed. They stated that there

is no correlation between microbial contamination and distance in the direction

of ground water flow. The reason for this is that unlike nitrates, fecal coliform

was easily removed by straining, attachment and die-off in the unsaturated

zone. Moreover, the hydraulic conductivity, initial concentration of NO3- and

discharge rate were found to have the much more significant influence on NO3-

transport than nitrification rate coefficient, longitudinal dispersivity and

absorption coefficient of NH4+. Nitrate concentration is directly proportional to

flow rate and initial nitrate concentration but inversely proportional to

hydraulic conductivity.

Ahmed et al., (2005) used a biochemical fingerprinting method to assess septic

tank failures with specific emphasis on surface water contamination in

Australia. They collected effluents from 39 septic tanks as well as water

samples from creeks upstream and downstream of the septic tanks. The

samples were tested for E.coli and m-enterococcus and the fecal indicators

were typed using PhPlate; PhPlate AB fingerprinting method. For

enterococcus, they found a total of 100 biochemical phenotypes with 31 being

40

common to some of the septic tanks and 79 being unique to individual septic

tanks. For E.coli, they found a total of 114 biochemical phenotypes with 27

being common to some of the septic tanks and 87 being unique to individual

septic tanks. When the number of fecal indicators in the septic tank effluents

was compared with that of the creek waters in the septic tank environment, it

was found that the creek water samples had higher numbers of fecal indicators

as well as more biochemical phenotypes. This was attributed to the fact that the

creek waters received bacteria from diffuse sources such as animal farms or

industrial processes via runoff, in addition to septic tank effluent. They also

suggested that not all fecal indicators that entered the septic tanks survived the

septic tank activity, implying that certain species of fecal indicators are more

resistant to the septic tank processes. They also discovered that in cases where

identical biochemical phenotypes were found in water samples and septic tank

effluents, the number of enterococcus was more than that of E.coli indicating

that enterococcus is more resistant than E.coli. This position was also shared by

Baudisöva (1997). Furthermore, they found that the strains of fecal indicators

present in the properly maintained septic tanks were absent from the creek

water samples while this was not the case for the failing septic tanks. Hence,

they concluded that well maintained septic tanks do not contribute fecal

bacteria to surface waters.

Nicosia et al. (2001) studied the removal of bromide and bacteriophage in

septic tank drain field. They found that rainfall and the presence of organic

matter reduce the rate of virus removal from septic tank effluent in the drain

field. Inactivation was found to be the major mechanism of virus removal.

Removal by absorption was not so significant as rainfall results in the

desorption of adsorbed bacteriophage. Moreover organic matters compete with

virus for adsorption sites thus resulting in the decrease in virus removal

efficiency with time.

41

Collick et al. (2006) developed a model for predicting the failure of septic tank

drain field in sloping terrains. The model revealed that failure may not occur in

the drain field itself but the flux of septic effluent to a portion down slope can

lead to failure of that portion which may not be a part of the drain field. When

the soil depth and conductivity of a soil portion up slope of the drain field are

both low, the failure rate in the drain field itself will reduce. This is because

this decrease in soil depth and hydraulic conductivity will result in increased

runoff being generated from the field upslope. This runoff will infiltrate in the

septic effluent disposal field (without causing the water table to rise above the

perforated pipe), and the overall system drains faster due to high saturated

conductivity of the septic field resulting in a generally lower water table. In

addition, a conductivity of less than 1m/day and a slope greater than 4% gave

rise to a 100% failure rate in the drain field. The reason is that these conditions

encourage plugging which will prevent fast drainage of water and a subsequent

rise in water table.

Al-Layla and Al-Rawi (1989) studied the performance of several septic tanks in

the Mosul City of Iraq. They found that a number of the septic tanks were

performing poorly as a result of high water table, absence of inlet tees or

elbows and poor soil types. They also observed that some of these tanks

experienced rapid filling with sludge, thus necessitating desludging within

short intervals ranging from two weeks to six months. They suggested that the

poor performance of the septic tank system was due to poor design especially

with respect to inlet and outlet structures. It was, however, noted that some of

the tanks achieved high levels of pollutant removal due to their long detention

times (approximately 10 days).

42

CHAPTER THREE

METHODOLOGY

3.1 DATA COLLECTION

Data for this research was collected from questionnaires, pilot scale septic

tanks and other sources.

3.1.1 Preliminary Study (Questionnaires)

In order to have an overview of the condition of septic tanks as well as

practices common among septic tank users in Nigeria precisely, using Nsukka

as a case study, two hundred questionnaires were distributed in the following

proportions:

Contractors – 50

Landlords – 50

Tenants – 100

Respondents were randomly selected and questionnaires administered to them.

Out of the 200 questionnaires distributed to contractors, landlords and the

tenants, 144 were completed and returned, while a total of 56 questionnaires

were not retrieved.

3.1.2 Pilot Scale Septic Tanks

Five rectangular pilot scale septic tanks were constructed with metal sheets and

painted on the inside to check the rate of corrosion given that raw sewage is

known to be corrosive. All the tanks had the same dimensions (78cm long,

50cm wide and 50cm deep) but different dosing rates. Table 3.1 is a summary

of the description of the pilot scale tanks. Four of the tanks were dosed with

sewage at different rates to simulate one, two, three and four days detention

times. The fifth one was dosed to simulate one day detention time but its inlet

pipe was positioned at the edge instead of centrally like the other four. All the

inlets consisted of one inch PVC pipe entering the tanks at 0.4m above the

43

bottom and bent downwards to serve as a baffle hence directing the influent

downwards. The effluent pipes also were 25.4mm (one inch) in diameter. Each

of the tanks was perforated midway on the top so that samples could also be

collected from the middle of the tanks in order to monitor treatment progress

along the tank. The perforations were fitted with a one inch PVC cork to keep

away oxygen. The inlet of each tank was fitted with a stop cork and connected

to a concrete reservoir through a central one inch pipe. Raw sewage was then

pumped from the inlet of the Imhoff tank of the University of Nigeria, Nsukka

central waste treatment plant to the reservoir using a model 30WPX Techno

water pump fitted with a three inch hose. Samples were scheduled to be

collected twice a week from August to December 2010, however, this schedule

could not be followed religiously because of inadequate personnel at the

laboratory. While this part of the research was in progress, final year

undergraduate students commenced their projects which caused facilities at the

laboratory to be overstretched.

Samples were collected from the outlets and the midpoints of these tanks and

immediately sent to the Civil Engineering Laboratory, University of Nigeria,

Nsukka for physicochemical and microbial analyses. Sample were collected

from the midpoint of the tanks using a 10ml pipette fitted with a rubber pipette

filler and subsequently transferred to a 1 litre plastic can. This was repeated

severally until the can was filled up. Samples collected were analyzed for

biochemical oxygen demand (BOD), suspended solids (SS), chemical oxygen

demand (COD) pH, dissolved oxygen (DO) and E.coli according to the

Standard Methods (1992). Dissolved oxygen was measured in situ using a

dissolved oxygen meter by inserting the electrode in the tank through the

midpoint perforations. Temperature measurements were also made in situ

during sampling.

44

Table 3.1: Description of Pilot Scale Units

Tank Detention

Time(days)

Volume(m3) Inlet Position Baffle

A 2 0.156

Skewed to edge Metal splash baffle

B 2 0.156 Centred PVC tee

C 1 0.156 Centred PVC tee

D 3 0.156 Centred PVC tee

E 4 0.156 Centred PVC tee

Fig 3.1: Generalized Sketch of Experimental Set up

45

Fig 3.2: Picture of Experimental Set up

Flow Characteristics

The septic tank operates optimally under quiescent conditions. The presence of

turbulence or hydraulic jumps will result in the resuspension of settled solids.

Hence, it was ensured that the pilot scale tanks conformed to the same

operational conditions as the full scale tank. The hydraulic characteristics of the

pilot scale septic tanks have been tabulated in Table 3.2. The flow

characteristics of concern are the Froude number (Fr) and the Reynolds number

(Re). In actual practice, the septic tank system is not a continuous flow system

but rather an intermittent flow system. Hence, two kinds of hydraulic

characteristics were considered. The Reynolds number and Froude number for

intermittent flow are designated as Re(int) and Fr(int) respectively while those

for continuous flow are designated as Re(con) and Fr(con) respectively. For the

continuous flow, the velocity is obtained by spreading out the flow throughout

46

the day while, for the intermittent flow, the discharge is obtained by dividing

the total volume of flow by the actual time of flow, hence ignoring idle times.

The hydraulic characteristics of the inlets have been tabulated in Table 3.3.

Table 3.2: Hydraulic Characteristics of Tanks

Tank Fr(con) Fr(int) Re(con) Re(int) Condition

A 0.000002 0.000110 0.000731 0.035073 Subcritical Laminar

B 0.000002 0.000110 0.000731 0.035073 Subcritical Laminar

C 0.000005 0.000220 0.001462 0.070164 Subcritical Laminar

D 0.000002 0.000073 0.000487 0.023352 Subcritical Laminar

E 0.000001 0.000055 0.000363 0.017447 Subcritical Laminar

Table 3.3: Hydraulic Characteristics of Inlet Pipes

Tank Fr(con) Fr(int) Re(con) Re(int) Condition

A 0.000009 0.000440 0.000046 0.002192 Subcritical Laminar

B 0.000009 0.000440 0.000046 0.002192 Subcritical Laminar

C 0.000018 0.000880 0.000091 0.004385 Subcritical Laminar

D 0.000006 0.000293 0.000030 0.001460 Subcritical Laminar

E 0.000005 0.000219 0.000023 0.001090 Subcritical Laminar

3.1.3 Laboratory Analysis

All samples collected for laboratory analysis were analysed immediately they

were brought into the sanitary laboratory. Sample which could not be analysed

on the collection day were preserved in the refrigerator and analysed the

following day. The laboratory analyses of samples were done in accordance

with the Standard methods (1992). Owing to the sensitivity of the dissolved

oxygen level, it was determined in-situ. The test was done using dissolved

oxygen (DO) meter. The DO meter was switched on and left to acclimatize.

After which the electrode of the DO meter was dipped inside the tank until a

steady value was obtained from the meter. The steady value which is given in

mg/l was recorded as the dissolved oxygen level of the pilot scale tank. The

47

temperature of the contents of the tanks was also taken with the use of

thermometer during the DO test. The E.coli test was carried out first before

other tests to avoid deterioration of the sample with time. E.coli determination

was done using standard total coliform Most Probable Number(MPN) while

COD (Chemical Oxygen Demand) test and suspended solid (SS) test were

performed using the dichromate reflux method and gravimetric method

respectively. The pH test was determined using glass electrode method.

3.1.4 Sludge Accumulation Data Acquisition

Sludge accumulation data covering several years were needed for the

calibration of the sludge accumulation model. Sludge accumulation data used

for model calibration was obtained from Orenco Systems Incorporated,

Oregon, USA, a septic tank manufacturing and servicing company with over

sixty years of experience; as well as other sources in literature.

3.2 MODEL FORMULATION

3.2.1 Sludge Accumulation Model

A model for predicting sludge accumulation in the septic tank was formulated

using material balance. Consider a septic tank receiving an influent of fairly

constant settleable solids concentration (QC0) but gives an effluent of variable

concentration of settleable solids (QCt). The idea is that as sludge accumulates

in the tank, the detention time reduces such that effluent concentration of solids

increases with time.

QC0

QC(t)

he

Figure 3.3: Mass Balance of Solids in the Septic Tank

48

The mass of sludge dM accumulated in the tank in time dt can be expressed as

follows.

tQCQCdt

dM 0 (3.1)

If we consider the fact anaerobic decomposition will normally reduce the

volume of the accumulated sludge by 40 to 50% producing methane (CH4),

carbon IV oxide (CO2), water(H2O) and hydrogen sulphide (H2S) gases

(Seabloom et al., 1982; USEPA, 2000), then Equation (3.1) translates to

)( 00 tQCQCktQCQCdt

dM (3.2)

M = mass of sludge accumulated at time t

Q = Flow rate (m3/s)

C0 = Influent concentration of settleable solids (mg/l)

C = Effluent concentration of settleable solids (mg/l)

k = rate constant of degradation by bacteria(day-1

)

However, though the bacteria converts some sludge to gas thus reducing the

quantity of sludge in the tank, the bacteria will multiply as they consume the

sludge so that the net quantity of sludge actually converted to sludge is given

by

Net decrease in sludge mass = Mass of sludge consumed by bacteria -

Increase in bacteria mass.

But increase in bacteria mass is proportional to mass of sludge consumed that

is:

49

tQCQCkdt

dX 0 (3.3)

Where = Yield coefficient (no unit)

Therefore

kktQCQCmasssludgeindecreaseNet (0 (3.4)

Rewriting Equation (3.2) to incorporate bacteria mass contributing to sludge

mass, Equation (3.2) becomes:

tQCQCktQCQCktQCQCdt

dM 000 (3.5)

Because the concentration of effluent solids increases as sludge accumulates in

the tank ie increase in effluent concentration of solids is proportional to sludge

accumulation. This can be expressed mathematically as

dt

dM

dt

dC (3.6)

Where is a proportionality coefficient.

Equation (3.6) follows from the result obtained by Heins et al. (1999) as shown

in Figure 3.4.

50

The graph can be generalized as shown in Figure 3.5

Acc

um

ula

ted

Slu

dge

Efficiency of Solids Removal

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Effi

cien

cy o

f Su

spen

ded

So

lids

Re

mo

val

Accumulated Solids (t SS/m2)

C1

Fig 3.4: Accumulation of Sludge Versus Efficiency of SS Removal Plotted from Data

Obtained by Heinss et al. (1999)

Fig 3.5: Generalized Relationship for Accumulated Sludge versus Solids Removal Efficiency

51

From the graph, it follows that

(3.6b)

M = mass of sludge

= intercept (having a dimension of mass)

n = efficiency of solids removal (no unit)

= slope (having a unit of mass)

0

0

C

CCM (3.6c)

C0 = Influent concentration of settleable solids (mg/l)

C = Effluent concentration of settleable solids (mg/l)

CC

M0

(3.6d)

Recalling that dt

dM

dt

dC then

0

1

CdC

dM

This implies

0C (3.6e)

Knowing C0, can be determined from the graph M versus C and hence can

also be determined. The unit of is m-3

Hence dt

dMC

dt

dC

0 (3.7)

Differentiating Equation (3.5) yields Equation (3.8):

dt

dCkkQ

dt

Md1

2

2

(3.8)

Substituting Equation (3.7) in Equation (3.8) gives Equation (3.9) as follows:

nM

52

dt

dMkk

CQ

dt

Md)1(0

2

2

(3.9)

Let

)1(0 kkC

Q (3.10)

Hence dt

dM

dt

Md

2

2

(3.11)

3.2.2 Initial Conditions

At the start of operation, the septic tank does not produce effluent until after

some days depending on the flow rate and the effective volume of the tank.

Before effluent is produced the rate of change of the tank content with time is

equal to the flow rate. Hence

QdtdVQdt

dV (3.12)

V = volume of tank (m3)

t = time (days)

Q = flow rate of sewage (m3/s)

Because the plan area of the tank is roughly constant, Equation (3.12) becomes

dtA

Qdy (3.13)

A = plan area of tank (m2)

Integrating Equation (3.13) between the limits y(0) = 0 and y(te) = he, that is, te

is the time the contents (both liquid and solids) of the tank reach the height (he)

of the effluent pipe. This is equal to the time it takes the tank to produce

effluent. Hence

Q

Ahtt

A

Qh e

eee (3.14)

53

At time te, the amount of sludge in the tank (assuming that before effluent is

produced, all the solids in the tank would have settled) is given by

eee AhCtQCtM 00)( .

This is the first initial condition, that is, when t = te, M =C0Ahe

For the second initial condition, we refer to Equation (3.5).

Neglecting the action of bacteria at the time the tank is just about to start

producing effluent; Equation (3.5) reduces to

tQCQCdt

dM 0 (3.15)

At time te QC(t) is equal to zero, therefore

)( 0 e

t

CCQdt

dM

e

This is the second initial condition, that is, when t = te,

dt

dM = QC0 - Ce where Ce is the initial concentration of suspended solids in the

effluent just as it starts producing effluent. At this stage, the tank should be

performing at its optimum.

Summary of initial conditions

(1) M(te) = C0Ahe

(2) M’(te)= Q(C0 - Ce)

3.2.3 Assumptions

1. Redissolution and resuspension are negligible;

2. Uniform bacteria activity is assumed throughout the body of

accumulated sludge;

54

3. The possible variation in the concentration of settleable solids in the

septic tank influent is negligible;

4. Since the septic tank is usually sealed and water tight evaporation

and seepage losses are negligible;

5. The construction material does not absorb water;

6. The rates of bacteria growth and sludge decomposition are governed

by first order kinetics;

7. The tank is in continuous use;

8. Only biodegradable solids are flushed into the tank; and

9. The plan area of the tank remains constant.

3.2.4 Solution of Model

dt

dM

dt

Md

2

2

is a second order ordinary differential equation with a

straightforward solution.

"" MM (3.16)

Hence 02 DD (3.17)

tDtDBeAeM 21 (3.18)

But D1 = 0 and D2 = β

Thus tBeAM (3.19)

Differentiating, we obtain

tBeM ' (3.20)

Applying the initial conditions

(1) ee AhCtM 0

Substituting initial condition (1) in Equation (3.18), we have

et

e BeAAhC

0 (3.21)

(2) )()(' 0 ee CCQtM

Substituting initial condition (2) in Equation (3.20), we have

55

et

e eBCCQ )( 0 (3.22)

Substituting (3.22) in (3.21), we have

)( 00 ee CCQ

AhCA

(3.23)

Also et

e eCCQ

B

)( 0 (3.24)

Substituting the expressions for A and B in Equation (3.18), we have

tt

eee eeCCQ

CCQ

AhCM e

)()( 000

ett

eee eCCQ

CQQ

AhCM

)()( 00 (3.25)

If the initial efficiency of the septic tank is given by 0

0

C

CC e , then

ett

eee eCCQ

CQQ

AhCM

)()( 00

)1(0

0 et

t

ee

eCQAhCM

(3.26)

But Q

Vte = initial detention time (θi) of the tank. We refer to “initial”

detention time because the tank has a maximum detention time at the start of

operation. However, there is a reduction in this maximum value as sludge

accumulates in the tank. Hence

)1(0

0 ie

eCQAhCM

t

e

(3.27)

Because not all the settled solids are biodegradable, a term shall be introduced

to take care of the accumulation of this non-biodegradable fraction. Equation

(3.27) is therefore modified as follows:

)1(0

00 ie

eCQAhCtQCM

t

e

(3.28)

Where is the fraction of settled solids that are non-biodegradable which has

a value of about 0.1.

56

Equation (3.28) above shows the rate of sludge accumulation with time in the

septic tank. But our interest lies in knowing the sludge level or volume of

sludge at any future time from the start of operation. Hence, the equation shall

be rewritten in terms of y. Because the sludge is oversaturated with water, the

density of the mixed liquor is given as follows:

)1( wSG wml (3.29)

Where

ml = density of mixed liquor (Kg/m3)

w = density of water = 1000Kg/m3

SG = specific gravity of sludge (no unit)

w = water content of mixed liquor (no unit)

Hence )1(1000

)1()(0

00

wSG

eCQ

AhCtQC

V

it

e

(3.30)

Or in terms of sludge depth AwSG

eCQ

AhCtQC

y

it

e

)1(1000

)1()(0

00

(3.31)

Where all parameters remain as previously defined.

3.2.5 Depreciation of Detention Time and Rate of Settling

As the mass of sludge in the tank increases, the detention time decreases. Thus

dt

d

dt

dM (3.32)

dt

dK

dt

dMs

(3.33)

Ks = proportionality coefficient (having the dimension of mass)

57

= detention time of tank at any time t

Recall that VwSGM )1(1000 where V is the total volume occupied by the

mixed liquor of sludge and water. The above equation can be rewritten as

follows:

))(1(1000 lt VVwSGM (3.34)

Vt = design volume of tank and Vl (residual volume) = volume of tank above

sludge layer.

But QVl , hence

))(1(1000 QblhwSGM (3.35)

Differentiating Equation (3.35) yields

dwSGQdM )1(1000 (3.36)

Comparing Equation (3.32) and Equation (3.36) shows that

)1(1000 wSGQK s where Ks represents a form of settling rate having a

dimension of MT-1

.

Integrating Equation (3.36) under the boundary condition M(0) = 0 and (0) =

i , where i is the initial (design) detention time of the tank.

)())(1(1000 isi KwSGQM (3.37)

Recall that )1(1000 wSGVM ml and note that )( iQyblV

V = volume occupied by sludge (m3)

y = depth of sludge (m)

b = width of tank (m)

l = length of tank (m)

All other parameters have previously been defined.

Hence:

58

)( ioSy (3.38)

y = depth of sludge and So = Q/bl which is the overflow rate

Equation (3.38) relates loss of effective detention time to sludge accumulation.

Equation (3.38) can be rewritten as follows:

)(

i

re

bl

blHy (3.39)

Hre = residual depth above sludge layer (m)

irere HHy

y + Hre = total effective height of tank (he) which is the same as the height to

the effluent pipe, hence,

i

eh

y

1 (3.40)

By substituting Equation (3.31) in Equation (3.40), we obtain a relationship

between sludge depth and residual detention time.

i

e

t

e

AhwSG

eCQ

AhCtQC i

)1(1000

)1(

1

)(0

00

(3.41)

Equation (3.41) represents decay of detention time and will enable anyone to

assess the decline of detention time as sludge accumulates in the tank. Sludge

accumulation reduces the effective volume of the tank so that the detention

time available for solids separation is reduced in turn. Equation 3.41 can be re-

written as follows:

i

i

t

e

QwSG

eCQ

AhCtQC i

)1(1000

)1(

1

)(0

00

(3.42)

59

This is because Ahe is the initial effective volume of the tank which is equal to

the product of wastewater flow and initial detention time. Hence:

QwSG

eCQ

AhCtQC it

e

i)1(1000

)1()(0

00

(3.43)

Equation 3.43 relates residual detention time to time of tank operation

3.2.6 Residual Depth

As sludge accumulates in the septic tank, the depth of the tank decreases. In the

design of septic tank, it is necessary to specify a minimum residual depth and a

minimum residual detention time in order to maintain the efficiency of the tank

above a threshold limit. These conditions should be the prevailing conditions at

desludging. Considering a septic tank of plan area A, receiving wastewater at

the rate of Q m3/s per capita which is required to maintain a minimum residual

detention time, ϴre days and a minimum residual depth per capita hre (m), the

following relationship holds.

A

Qh re

re

(3.44)

For the sake of economy, it is necessary to choose plan dimensions that will

yield the minimum plan perimeter for a given plan area. This will ensure that

the minimum amount of materials is used for construction. Tank perimeter, P is

given as:

lwP 22 (3.45)

ll

AP 2

2 (3.46)

For minimum perimeter, we differentiate with respect to l and equate to zero.

02

22

l

A

dl

dP (3.47)

Hence l = w for the most economic plan area. This implies that the most

economic plan should be a square. However, researchers have been advocating

for tanks with narrow plan for higher efficiency, hence three classes of plan

60

specifications will be included in the evolving design approach. The cases are: l

=w for economy; and l = 2w and l = 3w for laminar conditions (Jowett and

Lay, 2005).

Case 1: l = w

Substituting for A = l2 in Equation (3.44), we obtain,

2l

Qh re

re

(3.48)

Most standards recommend that a tank must maintain a minimum detention

time of 24 hours (1 day) at desludging. Hence,

2l

Qhre (3.49)

Case 2: l = 2w

For this case, we obtain an expression for minimum residual depth per capita

observing a minimum residual detention time of 24hrs. Hence,

2

2

l

Qhre (3.50)

Case 3: l = 3w

For this case, we obtain an expression for minimum residual depth per capita

observing a minimum residual detention time of 24hrs. Hence,

2

3

l

Qhre (3.51)

3.2.7 Reserve Space

The reserve space refers to the empty space above the liquid level of the septic

tank or above the effluent pipe. The reserve space is an additional space that

takes care of such malfunctions as blockage of the effluent pipe or clogging of

the drain field. When blockage of effluent pipe or clogging of drain field

occurs, the additional space in the tank accommodates influent until the defect

is corrected. If the defect is not corrected then sewage will back up into the

61

house. The reserve space should be such that it will take the tank about one full

day to fill up the extra space so that there will be adequate time to effect

repairs. Considering that, one day detention time is also allowed for the

residual depth, hence, volume of the reserve space is equal to the residual

volume.

hre = hrev (3.52)

62

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1 PRELIMINARY OBSERVATIONS

Figures 4.1 to 4.5 show results of the preliminary investigation which sought to

ascertain the level of compliance to standard practices in the construction and

use of the septic tank system. Figure 4.1 reveals that most contractors who may

or may not be engineers do not perform the standard tests required before siting

and construction of the septic tank. Most of the few who claimed that they

usually perform the standard tests could not give an answer when asked to

mention the kind of test they perform.

The fact that these tests are neglected is brought to the fore by Figure 4.2 which

shows that most of the contractors encounter hard rocks and high water table,

in a few cases, during construction of the septic tank. These problems could

have been avoided had they conducted the standard tests first. It is also obvious

that none of the contractors encountered perform water tightness test after

septic tank construction. This is a bad practice because the structural

requirement of the septic tank is that the septic tank including all extensions to

the surface shall be watertight to prevent leakage into or out of the tank

(Bounds, 1997).

Leaking tanks will cause untreated sewage to short-circuit to the groundwater

leading to pollution and possible outbreak of epidemics. Moreover, raw sewage

can escape from leaking septic tanks and pollute surface waters such as ponds,

streams, springs, lakes and even swimming pools. Several studies have found

the presence of Hepatitis A, Shigella spp, E coli O157, Giardia,

Cryptosporidium, etc. resulting from fecal contamination (Blostein, 1991;

Cransberg et al, 1996; Greensmith et al, 1988; Galmes et al, 2003).

63

Most of the contractors claimed that they design each septic tank before

construction, but some others were honest enough to admit that they use the

same specification for all the tanks they construct (Figure 4.3). However,

almost all of them admitted that they do not recommend desludging intervals to

owners after construction. This implies that occupants do not even know when

to expect their septic tank to be desludged. This usually makes occupants to

wait until the tank malfunctions or fills up completely before thinking about

desludging. Figure 4.4 shows that some septic tank users flush non-

biodegradable materials such as sanitary pad, condom, hair, wool and

polythene into the septic tank. These materials have the potential to clog

plumbing connections and cause back flow of sewage into the house. They also

cause the tank to be filled up too soon due to the accumulation of inorganic

matter.

The result of practices with regard to the design and construction as well as

maintenance/use of septic tanks is poor performance of the septic tank and even

total failure in some cases. This fact is highlighted in Figure 4.5 which shows

that about 60% of the tenants believe that their septic tanks are having one kind

of problem or the other. However, only about 24% of the land lords think that

their septic tanks are malfunctioning. This discrepancy in response could have

stemmed from the fact that most landlords live in other people‟s houses; and in

addition they would like to protect their image by claiming that their septic

tanks do not have any problems. The discrepancies notwithstanding, both

tenants and landlords agree that septic tanks disorder is a common occurrence

which needs to be addressed.

64

Figure 4.1: Compliance to Basic Septic Tank Tests

Figure 4.2: Kinds of Construction Problems Encountered

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Do you check depth of

ground water or bed rock?

Do you perform water

tightness test?

Do you consider

location/distance of water wells, or boreholes?

Per

cen

tage

com

plia

nce

Yes

No

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

45.00%

High water table Hard rock Too loose soil Sloppy soil None

Perc

enta

ge

65

Figure 4.3: Design Issues

Figure 4.4: Flushing of Non-biodegradable Materials into the Septic Tanks

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Do you carry out septic tank design

before construction?

Do you recommend desludging intervals?

Perc

enta

ge co

mpl

ianc

e

Yes

No

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

Sanitary pad Condoms Polythene Wool/hair Clothes None

Perc

enta

ge R

espo

nse

66

Figure 4.5: Is Your Septic Tank Malfunctioning?

4.2 RESULT OF PILOT SCALE STUDY

4.2.1 Dissolved Oxygen and Temperature

Table 4.1 shows the results of dissolved oxygen values for the pilot scale septic

tanks. Obviously, the septic tank system is not entirely free of dissolved

oxygen; rather it is an anoxic system. When fresh sewage flows in the septic

tank, it is almost saturated with oxygen which aerobic organisms immediately

begin to utilize for biodegradation. However, because there is no means of

replenishing the oxygen consumed, the dissolved oxygen level drops greatly

giving rise to anaerobic conditions. The dissolved oxygen levels varied

between 0mg/l and 0.8mg/l with an average value of 0.39mg/l. This value is

close to the 0.3mg/l observed by Winneberger (1984). As can be seen from

Table 4.1, this level of dissolved oxygen level was established within a few

days of commencing operation of the tanks

Table 4.1: Dissolved Oxygen Values (mg/l)

0%

10%

20%

30%

40%

50%

60%

70%

80%

Landlords Tenants

Perc

enta

ge R

espo

nse

Yes

No

Date Tank A Tank B Tank C Tank D

17/08/2010 0.5 0.4 0.5 0.4

19/08/2010 0.5 0.4 0.5 0.4

24/08/2010 0.1 0.1 0.1 0.3

26/08/2010 - - - -

02/09/2010 0.7 0.6 0.6 0.8

67

The temperature levels inside the tanks are shown in Figures 4.6 and 4.7. The

lowest temperature recorded was 240C while the highest was 33

0C. The

temperature levels had an average value of 260C within the first two months of

operation but increased to an average value of 300C in the third month

(November). This can be attributed to the microbial activities in the tanks as

well as increased ambient temperature at the end of the rainy season.

Generally, there was a gradual upward trend in temperature in all the tanks.

However, there were no marked differences in the temperature levels of

different pilot scale tanks. Though temperature levels indicate the level of

microbial activities in a treatment plant and consequently the efficiency, this is

not entirely the case for the septic tank which is first and foremost a settling

system accompanied by some levels of microbial degradation. The highest

level of activity in the septic tank occurs in the sludge layer where anaerobic

bacteria consume settled sludge.

14/09/2010 0.3 0.2 0.5 0.3

28/09/2010 0.7 1.2 0 0

12/10/2010 0 0.1 0.6 0.7

19/10/2010 0 0 0 0

28/10/2010 0 0 0.6 0.7

11/11/2010 0.8 0.7 0.6 0.5

18/11/2010 0.5 0.6 0.4 0.4

25/11/2010 0 0.5 0.5 0.8

Average 0.31 0.33 0.45 0.48

Overall

Average 0.39

68

Figure 4.6: Temperature Variation in Tanks

Figure 4.7: Temperature Variation in Tanks (Different Inlet Types)

4.2.2 pH Variation

One of the most active groups of micro-organisms in the septic tank are the

methanogens (methane formers) which convert acetic acid to methane

(Wilhelm et al., 1994). This conversion usually leads to an increase in the

20

22

24

26

28

30

32

34

0 20 40 60 80 100 120

Tem

per

atu

re (

0C

)

Time (Days)

2 Days 1 Day 3 Days

23

25

27

29

31

33

35

Tem

p (0

C)

Time (Days)

Splash Baffle Inlet Tee

69

alkalinity of the septic tank content and hence increased pH. This is

corroborated by Figures 4.8 to 4.10. An increase in pH between inlet and outlet

is a sign that the septic tank is performing well. The effect of detention on pH is

not very clear as results obtained show that 2 days detention time achieved a

higher pH removal than both 1 day and 3 days detention time. This is not

reasonable because it is expected that the longer the sewage stays in the septic

tank the more time methanogens will have to act on it and hence the more the

increase in pH levels.

Figure 4.8: Change in pH between Inlet and Outlet

6.6

6.8

7

7.2

7.4

7.6

7.8

8

0 20 40 60 80 100 120

pH

Time (Days)

Influent 2 Days 1 Day 3 Days

70

Figure 4.9: Change in pH between Inlet and Tank Midpoint

4.2.3 Effect of Baffle on Treatment Efficiency

Tanks A and B were used to monitor the effects of baffle types on the

efficiency of the septic tank system. Figures 4.10 and 4.15 show that the use of

inlet tees in the place of the commonly used splash baffles could lead to higher

efficiency. This is indicated by higher pH values both in samples taken from

the midpoint of the tanks and those taken from tank outlets. The same trend

was observed for BOD removal, COD removal and E-coli removal both at the

outlets and midpoints. The only exception was suspended solids removal where

there was no marked difference between splash baffle and inlet tee removal

efficiencies. The essence of baffles in septic tanks is to retard the flow rate as

sewage enters into the tank. The slowing down ensures that settled sewage is

not resuspended by turbulence from fast flowing influent. Baffles also prevent

short circuiting. In Nigeria and most developing countries, the most commonly

used baffle type is the concrete splash baffle which consists of a concrete beam

placed directly in front of the inlet pipe to intercept incoming sewage. This

baffle type has the disadvantage of being attacked by sewage so that it is eaten

away over time. The results of this research have revealed another disadvantage

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8

0 20 40 60 80 100 120

pH

Time (Days)

Influent 2 Days 1 Day 3 Days

71

which is reduced efficiency. One of the reasons for this reduction in efficiency

could be because the impact between the incoming sewage and the splash

baffle caused sewage particles to be broken into smaller particles some of

which may not settle easily. Secondly, while inlet tees direct incoming sewage

into the sludge layer, the splash baffled septic tank experiences disturbance of

the scum layer causing particles trapped in the scum layer to be resuspended. A

third possible reason could be that the inlet tee directs incoming wastewater

down to the sludge layer so that there are more chances of decomposition by

micro-organisms in the sludge layer. In the case of the inlet tee, there is a

higher possibility of stratification so that inflow and outflow of sewage occurs

only in the upper layers of the tank. This will not allow the wastewater to have

adequate contact with the sludge layer.

Figure 4.10: Change in pH between Inlet and Outlet for Different Types of

Baffles

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8

pH

Time (Days)

Influent Splash Baffle Inlet tee

72

Figure 4.11: Change in pH between Inlet and Tank Midpoint for Different

Types of Baffles

Figure4.12: Outlet BOD Removal Efficiency for Different Baffle Types

6

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

7.8

8p

H

Time (Days)

Influent Splash Baffle Inlet Tee

82

84

86

88

90

92

94

96

98

100

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

Splash baffle Inlet tee

73

Figure 4.13: Tank Midpoint BOD Removal Efficiency for Different Baffle

Types

Figure 4.14: Outlet COD Removal Efficiency for Different Baffle Types

70

75

80

85

90

95

100

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

Inlet tee Splash baffle

70

75

80

85

90

95

100

0 20 40 60 80 100 120

Eff

icie

ncy

(%

)

Time (Days)

Splash Baffle Inlet Tee

74

Figure 4.15: Tank Midpoint COD Removal Efficiency for Different Baffle

Types

Figure 4.16: Tank Midpoint E-coli Removal Efficiency for Different Baffle

Types

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Eff

icie

ncy

(%

)

Time (Days)

Splash Baffle Inlet Tee

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80

Eff

icie

ncy

(%

)

Time (Days)

Splash Baffle Inlet Tee

75

Figure 4.17: Tank Midpoint Suspended Solids Removal Efficiency for

Different Baffle Types

Figure 4.18: Outlet Suspended Solids Removal Efficiency for Different Baffle

Types

4.2.4 Effect of Detention Time on Treatment Efficiency

Tanks B, C, D and E were used to monitor the effects of detention time on the

efficiency of the septic tank system. Tank E started leaking profusely soon after

the commencement of operation and could not be repaired immediately.

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

Splash Baffle Inlet Tee

60

65

70

75

80

85

90

95

100

105

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

Splash Baffle Inlet Tee

76

Figures 4.19 to 4.23 show the results obtained from tanks A, C and D. No

significant difference was observed between the efficiencies of the three tanks.

It seemed that even a detention time of 1 day is sufficient to achieve acceptable

levels of treatment.

Figure 4.19: Effluent E-coli Removal Efficiency

Figure 4.20: Outlet BOD Removal Efficiency

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80

Eff

icie

ncy

(%

)

Time (Days)

2 Days 1 Day 3 Days

70

75

80

85

90

95

100

0 20 40 60 80 100 120

Eff

icie

ncy

(%

)

Time (Days)

2 Days 1 Day 3 Days

77

Figure 4.21: Tank Midpoint BOD Removal Efficiency

Figure 4.22: Effluent Suspended Solids Removal Efficiency

84

86

88

90

92

94

96

98

0 20 40 60 80 100 120

Eff

icie

ncy

(%

)

Time (Days)

2 Days 1 Day 3 Days

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

2 Days 1 Day 3 Days

78

Figure 4.23: Tank Midpoint Suspended Solids Removal Efficiency

4.3 MODEL CALIBRATION

The sludge accumulation model (Equation 3.30) was calibrated using the

sludge accumulation data obtained from Ted Kulongosky of Orenco Systems

Inc. Oregon, USA as well as other sources in literature. Table 4.2 is a summary

of data sources used and the time length of septic tank sludge accumulation

monitoring. The water content w and specific gravity SG of sewage sludge is

0.88 and 1.03, respectively (Saqqar and Pescod, 1995). The calibration was

done by filling a column in Microsoft Excel with time values ranging from half

a year to 9 years which roughly covers the duration of the sludge accumulation

study. The next column was filled with an arbitrary constant value for β. The

third column was programmed to use the corresponding time value of the first

column and the arbitrary value of β to evaluate the generalized sludge

accumulation model of Equation 3.30. The fourth column was filled with the

sludge accumulated per capita corresponding to the time of measurement in the

30

40

50

60

70

80

90

100

110

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Time (Days)

2 Days 1 Day 3 Days

79

first column. Scatter plots of the measured sludge accumulation versus time

and that of the calculated sludge accumulation versus time were made. The

value of β was manipulated until the two curves came the closest. The final

curve is shown in Figure 4.24. Hence Equation (4.1) was obtained with a

correlation coefficient of 0.985.

)1(56.0011.000832.0 11.0 t

sludge etV (4.1)

Where Vsludge is the volume of sludge in m3/capita accumulated in time, t years.

Table 4.2: Sludge Accumulation Data

Time

(Years)

Volume of Sludge

Accumulated (m3/capita)

Period of

Monitoring

No of

Septic

Tanks

Source

0.5 0.046 5 years 28 Gary (1995)

1 0.047 3 years 727 Bounds (1994)

2.8 0.174 8 years 486 Bound (1990)

4.8 0.29 8 years 486 Bound (1990)

5 0.325 5 years 28 Gary (1995)

8 0.378 8 years 486 Bound (1990)

80

Figure 4.24: Plots of Model and Measured Sludge Accumulation versus Time

In order to obtain the total volume of septage (sludge and scum) in the tank at

any time, Equation (4.1) was modified to include a scum accumulation term.

The Douglas County Audit found that, the scum accumulation was 12 US

gallons/capita/year (0.0454m3/capita/year) in the first year and a constant rate

of 2.6 US gallons/capita/year (0.0098m3/capita/year) in subsequent years. This

rate of scum accumulation was corroborated by a 3-year audit of 727 septic

tanks by the Montesano Community, Washington at 95% confidence level

(Bounds, 1994). Hence, the rate of scum accumulation can be given as:

tVscum 01.0035.0 (4.2)

Where Vscum is the volume of scum in m3 per capita and t is time in years.

Equation (4.1) is then merged with Equation (4.2) to yield Equation (4.3) for

the total volume of solids (septage) accumulated in the tank in time, t (years).

)1(56.0021.0043.0 11.0 t

septage etV (4.3)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10 12

Vo

lum

e o

f S

lud

ge

per

Ca

pit

a(m

3/c

ap

ita

)

Time (years)

Model Measured

R = 0.985

81

The total depth occupied by solids is given by Equation 4.4.

A

ety

t

septage

)1(56.0021.0043.0 11.0

(4.4)

Hence, Equation (3.40) which relates detention time to time after

commencement of operation becomes:

Q

et t

i

)1(56.0021.0043.0 11.0

(4.5)

4.3.1 Comparison of Model with Existing Sludge Accumulation Models

It is necessary to compare the model (Equation 4.3) for solids accumulation

derived in this study with the existing models. The most popular models for

solids accumulation are those of Bounds (1995) and Weibel et al. (1955)

derived for the US Public Health Service.

Figure 4.25: Comparison of Model with Bounds‟ and Weibel‟s Models

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14 16 18 20

Vo

lum

e o

f S

epta

ge

(m3/c

ap

ita

)

Time (Years)

Bounds Weibel Model

82

Figure 4.25 shows that the new model gives lower estimates than the models of

Bounds and Weibel et al. But Seablom et al. (2004) noted that Bound‟s

equation gives high estimates of septage accumulation. Weibel‟s equation has

the disadvantage of assuming that sludge accumulation is a linear function.

Sludge accumulation studies have shown that this is not the case as a result of

consolidation and decomposition of accumulated sludge by micro-organisms.

The aspect of solids accumulation that can be reasonably assumed to have a

constant rate is scum accumulation and the accumulation of non-biodegradable

fractions of sewage.

4.4 BASIS FOR THE NEW DESIGN APPROACH

Many codes and designers recommend that the septic tank should be desludged

when sludge has reached one third of the tank height. But this has no rational

basis. The most critical parameter in septic tank design and operation is the

detention time. At any point in time, the detention time must be sufficient to

allow solid particles to settle, otherwise, its performance will be impaired.

Desludging interval of the septic tank should rather be based on the parameter

introduced earlier in this research which was referred to as the minimum

residual detention time. This was defined as the minimum allowable detention

time in the septic tank. It is the detention time which should be specified by the

designer such that once it is attained, the tank must be desludged. Research has

shown that detention time between 12 hours and 24 hours are close to the

threshold of acceptable solids removal in the septic tank. Equation 3.40 relates

residual detention time to the ratio of sludge depth to tank effective height. In

order to expose the arbitrariness in the recommendation that septic tanks be

desludged when sludge depth reaches one third of the effective height, a plot of

residual detention time and the ratio of sludge depth to tank height is presented

in Figure 4.26 (based on Equation 3.40).

83

Figure 4.26: Decline of detention time with sludge accumulation

From the figure, it can be seen that if the initial detention time of a septic tank

is 2 days, at a sludge depth to effective height ratio of 0.3 the tank will still

have adequate detention time of 1.4 days. Desludging the tank at this stage will

be tantamount to waste of time and money. It will be more rational if the

designer sets a minimum residual detention time of 1 day, then the tank will be

due for desludging at a sludge depth to effective height ratio of 0.76 which will

take a longer time to attain. This approach has the advantage of both economy

and efficiency in that it ensures that the tank is not desludged prematurely

resulting in waste of resources. It also ensures that the tank is still performing

within acceptable limits at the time of desludging. The other extreme of

irrationality in septic tank issues is waiting for the tank to fill up and refuse to

take in more sewage or even result in the back up of sewage into the house.

While the practice previously mentioned leans to the side of safety though

uneconomical, this particular practice is neither safe nor economical because

0

0.5

1

1.5

2

2.5

3

3.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Res

idu

al d

ten

tio

n t

ime

(day

s)

Ratio of sludge depth to effective height

Initial 3days detention time

Initial 2,5days detention time

Initial 2days detention time

Initial 1.5days detention time

Initial 1day detention time

84

this can shorten the life span of the septic tank as well as result in the clogging

of the drainfield or soak pit.

Another anomaly in the prevailing design approach of septic tanks is that most

times, consideration is not given to water use and availability in the house. It‟s

not just enough to know the number of people that are likely to live in the

house, it is important to know the rate of water consumption. Table 4.3 shows

the variation in water consumption per capita for different water supply

conditions.

Table 4.3: Water consumption under different supply conditions

Water Supply Condition Water Use (lpcd) Source

Public standpipe farther than 1 Km ≤ 10 Gleick (1996)

Public standpipe closer than 1 Km 20 Gleick (1996)

House connection, simple plumbing,

pour, flush toilet

80*

Gleick (1996)

Urban house connection with garden 275*

Gleick (1996)

Nigerian Average water use 36 UNDP (2006)

Basic water requirement 50 Gleick (1996)

* represents average value.

Water consumption will determine the capacity of septic tank to use. Though

water consumption varies from house to house, the raw sewage flow per person

into the tank will not vary much. A superficial reasoning will lead to the

conclusion that a house with low water consumption will require a septic tank

with low detention time. But this is not the case because a low detention time

coupled with very low wastewater flow rate will yield an insignificant design

volume that will be soon be filled up with solids. This is because, though water

use may vary widely from place to place, the average feaces input per person

into the tank does not vary significantly. Very small septic tank volumes lead to

frequent need for desludging and hence high cost of maintenance. Figures 4.27

85

to 4.30 produced from Equation (3.43) show how wastewater flow affects

residual detention time. It should be noted that the wastewater flow is taken as

80% of the total water consumption per person per day.

Figure 4.27: Decline of Detention Time for House Connection, Simple

Plumbing (Typical wastewater flow = 0.064m3/day)

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7 8 9 10

Det

enti

on

Tim

e (d

ay

s)

Time after commencement of operation (years)

ϴi =2days ϴi =3days ϴi =4days ϴi =5days ϴi =6days ϴi =7days

ϴi = 7days

ϴi = 6days

ϴi = 5days

ϴi = 4days

ϴi = 3days

ϴi = 2days

86

Figure 4.28: Decline of Detention Time for Urban House with Full Water

Connection and Garden (Typical wastewater flow = 0.275m3/day)

Figure 4.29: Decline of Detention Time for Basic Water Requirement

(Typical wastewater flow = 0.04m3/day)

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14 16 18 20 22 24

Det

enti

on

Tim

e (d

ay

s)

Time after commencement of operation (years)

ϴi =2days ϴi =3days ϴi =4days ϴi =5days ϴi =6days ϴi =7days

ϴi = 6days

ϴi = 7days

ϴi = 6days

ϴi = 5days

ϴi = 4days

ϴi = 3days

ϴi = 2days

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7

Det

enti

on

Tim

e (d

ay

s)

Time after commencement of operation (years)

ϴi =2days ϴi =3days ϴi =4days ϴi =5days ϴi =6days ϴi =7days ϴi =8days ϴi =9days

ϴi = 7days

ϴi = 6days

ϴi = 5days

ϴi = 4days

ϴi = 3days

ϴi = 2days

ϴi = 8days

ϴi = 9days

87

Figure 4.30: Decline of Detention time for Average Nigerian House

(Typical wastewater flow = 0.03m3/day)

These figures demonstrate vividly that houses with high water consumption or

wastewater flow can have tanks with moderate to low detention times and still

operate for about twenty years without requiring desludging. Figure 4.28 shows

that a septic tank designed for a house with full water connections and having

an initial detention time of three days can operate for nine years and still

maintain the minimum 24 hours residual detention time introduced earlier on.

On the other hand, Figure 4.30 shows that for the average Nigerian water

consumption, a septic tank sized for an initial detention time of three days for

the same number of occupants will attain the minimum 24 hours residual

retention time in only nine months and will, in fact, get filled up in less than

one and half years. This explains why some people have frequent need to

desludge their septic tanks while others have not had the need to desludge

theirs for about a decade or more. This justifies the approach developed in this

research which starts design by fixing a desired desludging interval and a

minimum residual detention time of 24 hours.

0

1

2

3

4

5

6

7

8

9

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Det

enti

on

Tim

e (d

ay

s)

Time after commencement of operation (years)

ϴi =2days ϴi =3days ϴi =4days ϴi =5days ϴi =6days ϴi =7days ϴi =8days ϴi =9days

ϴi = 7days

ϴi = 6days

ϴi = 5days

ϴi = 4days

ϴi = 3days

ϴi = 2days

ϴi = 8days

ϴi = 9days

88

In Nigeria, septic tanks are rarely designed, rather, most contractors resort to

arbitrary sizing or adopt the specifications of the Public Works Department

(PWD, 1943) shown in Table 4.4 or other Local Government specifications.

The specifications on this table are based on 1 day detention time and a

wastewater flow of 0.114m3/capita/day. This specification is not realistic as a

septic tank sized for 1 day detention time will need desludging frequently.

Furthermore, the wastewater flow of 0.114m3/capita/day is unrealistic (see

Table 4.3).

Table 4.4: Schedule of Septic Tank Sizing and Dimensions (PWD, 1943)

Tank Size Dimensions No. of

Users

Length(m) Width(m) Depth(m) Capacity(m3)

I 2.032 0.457 1.220 1.134 10

II 2.286 0.534 1.220 1.448 13

III 2.286 0.610 1.220 1.700 15

IV 2.540 0.686 1.220 2.125 18

V 3.048 0.762 1.220 2.832 25

The code recommended that a septic tank serving 10 people should have a

dimension of 2.032m (length), 0.457m (width) and 1.22m (depth) giving a total

volume of 1.13m3. Even if the usual constant sludge accumulation rate of

0.04m3/capita/year (Agunwamba, 2001; Winneberger, 1984) is assumed, in

three years the tank will be overflowing with sludge (1.2m3). This implies that

the tank will need desludging about every two years. Compare this with the

recommendations of Crites and Tchobanoglous (1997). For instance, they

recommended a tank of 2000 US gallons (7.57m3) for a four bedroom house

(see Table 4.5). Obviously this is a very long shot from the meager 1.13m3

recommended by PWD for a septic tank serving 10 people.

89

Table 4.5: Septic Tank Volumes (Crites and Tchobanoglous, 1997)

No of Bedrooms Tank Capacity (US gallons) Tank Capacity (m3)

One or two bedrooms 1,000 3.785

Three bedrooms 1,500 5.678

Four bedrooms 2,000 7.570

The septic tank is not just meant for sewage treatment, it is also meant for

sludge storage and decomposition. For effective operation, the septic tank

should have both adequate detention time for solids separation and enough

volume for long term storage of sludge. Longer storage periods for sludge

(desludging interval) allows enough time for maximum biodegradation. Gary

(1995) is of the opinion that increasing the desludging interval significantly

reduces the volume of sludge produced, and so the operational cost of the unit

to the owner. He further stated that longer sludge ages result in much more

stabilized sludges which do not need to be disposed of to sewage treatment

works. Another anomaly in the PWD specifications is the constant depth

maintained for all sizes of septic tanks. Aluko (1978) stated that the reason for

this is not known and queried why bigger tanks should not have bigger depths.

This method developed in this study has following advantages.

Desludging will not be frequent and hence the cost of maintenance will

be reduced.

Occupants will have an idea when to expect the tank to require

desludging.

At desludging, the septic tank will still be performing within acceptable

limits by maintaining a minimum residual detention time of 24 hours.

Under sizing, which is very critical, will be averted.

The soakpit or drain field (whichever is applicable) will be protected

because the carryover of sludge into these units will be reduced.

The life span of the whole septic tank system will be prolonged.

90

It is our opinion that the septic tank is a very vital aspect of waste management

and public health that merits more than casual sizing. Every septic tank is

unique and must be designed to maintain minimum conditions. In this regard,

Poe (2001) noted that the key to effective sewage treatment is proper design,

installation, periodic maintenance and responsible operation.

4.5 THE NEW DESIGN APPROACH

The need and basis for a new and rational approach to the design of septic

tanks, especially in developing countries where outbreak of fecal-related

epidemics is rampant, have been set out in previous sections. The new design

approach developed in this research is such that even unlearned contractors can

use it to perform an accurate sizing of the septic tank. The new design approach

has been presented in three different packages to suit the designer‟s fancy and

level of education. The three aspects are

Use of Equations 3.44, 3.52 and 4.3 (for the learned designer),

Use of charts which will be presented shortly (for anyone),

Use of simple Microsoft Excel based programme (for the computer

literate designer).

(i) Use of Equations 3.42, 3.52 and 4.3

It has been previously stated that a good septic tank design must fix a

minimum residual detention time at which it becomes necessary to

desludge the tank. Many standards usually specify 24 hours. Also,

based on Equation 3.44, a minimum residual depth per occupant (hre)

corresponding to the chosen residual detention time should also be

specified. The overall residual depth (Hre) is the product of the

residual depth per occupant and the number of occupants. For good

design and for practical purposes, the value of the overall residual

depth should not be less than 10cm and not more than 75cm. This is

because too low residual depth will cause the wash out of sludge and

also interfere with inlet and outlet fittings while too high residual

91

depth will result in a tank with very low length to depth ratio which

will be inefficient. Narrow tanks have been found to provide

quiescent hydraulic conditions which favour settling and thus solids

removal. Then using Equation 3.44, the plan area of the tank is

determined. A desired desludging interval is chosen and then

Equation 4.1 is used to determine the volume of sludge that will

accumulate in that period of time. The depth of sludge in the tank at

this time is then obtained by dividing the volume of accumulated

sludge with the plan area obtained as described above. The total

depth of tank is obtained as the sum of sludge depth, the residual

depth and the depth of the reserve space. The reserve space should

correspond to a volume of 24 hours detention time. Finally a ratio of

length to width is chosen and hence, the length and the width can be

determined. The length should always be longer than the width to

provide for quiescent conditions.

(ii) Use of Charts

The steps described above have been translated into a series of charts

using the relevant equations and covering as many scenarios as

possible (see Figures 4.31 to 4.44). The overall residual depth is

chosen and the residual depth per occupant (horizontal axis)

corresponding to the number of occupants to use the septic tank is

read off on the vertical axis of Figure 4.32. The residual depth per

occupant obtained is located on the vertical axis of Figures 4.33 to

4.44 depending on the length to width ratio chosen. Figures 4.33 to

4.43 have been produced to cover cases where length to width ratio

is equal to 1, 2 and 3 as well as different water availability

conditions. Figures 4.33 to 4.35 are for simple house connections

where toilet is flushed by pouring with buckets. Figures 4.36 to 4.38

are for full house connection with adequate water supply conditions.

In this case, the shower, the sink taps, the water closet and kitchen

92

connections are in full service. Figures 4.39 to 4.41 are for an

average Nigerian house located in an urban area where water is

purchased from commercial water suppliers. Figures 4.42 to 4.44 are

for the basic water requirement. Here, water is not in abundance but

is sufficient to meet basic needs. The residual depth per occupant is

then traced horizontally to meet the residual depth per occupant

curve. From this point, the line is produced vertically to cut the

length and width curves as well as the horizontal axis which

represents the area of the tank. The length and width are noted. The

volume of sludge corresponding to the desired (chosen) desludging

interval is obtained from Figure 4.31. The depth of sludge is obtained

by dividing the volume of sludge by the plane area read off from

Figures 4.33 to 4.44. The total depth of tank becomes the sum of

sludge depth, overall residual depth and depth of reserve volume.

The depth of the reserve space should be equal to the residual depth

because it is based on 24 hours detention time. If the overall depth of

the tank is much higher than the length, a lower overall residual

depth should be chosen and the design repeated.

93

Figure 4.31: Chart for Determining Volume of Sludge for a Chosen

Desludging Interval

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12 14 16 18 20

Vo

lum

e o

f S

ep

tag

e (m

3/c

ap

ita

)

Time (Years)

94

Figure 4.32: Residual Depth per Occupant (hre) versus Number of Occupants

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80

Res

idu

al D

epth

per

Occ

up

an

t (cm

)

Number of Occupants

Hre (10cm) Hre(20cm) Hre(30cm) Hre(40cm) Hre(50cm)

95

Figure 4.33: Tank Dimensions and Residual Depth for Simple House Connection, pour flush (Q=0.064m

3/capita/day) and L = 2w

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30 35 40 45

Tank

Lin

ear D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

NB: Residual depth obtained should be multiplied by the number of occupants

96

Figure 4.34:Tank Dimensions and Residual Depth for Full Simple House Connection, Pour Flush (Q=0.064m3/capita/day) and

L = 3w

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70

Tan

k L

inea

r D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

97

Figure 2.35: Tank Dimensions and Residual Depth for Simple House Connection, Pour Flush (Q=0.064m

3/capita/day) and L = w

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30

Ta

nk

Lin

ear

Dim

ensi

on

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Residual Depth per occupant (cm)

98

Figure 4.36: Tank Dimensions and Residual Depth for Full house connection, urban with garden (Q= 0.22) and L = 2w

0

2

4

6

8

10

12

14

1 3 5 7 9 11 13 15

Tan

k L

inea

r D

imen

sion

(m

)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

99

Figure 4.37: Tank Dimensions and Residual Depth for Full house connection, urban with garden (Q= 0.22) and L = 3w

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20

Tan

k L

inea

r D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

100

Figure 4.38: Tank Dimensions and Residual Depth for Full house connection, urban with garden (Q= 0.22) and L = w

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45

Tan

k L

inea

r D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Residual Depth per occupant (cm)

101

Figure 4.39: Tank Dimensions and Residual Depth for Nigerian Average, Urban Areas without Pipe Borne Water (Q=0.03) and

L = 2w

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14

Tan

k L

inea

r D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

102

Figure 4.40: Tank Dimensions and Residual Depth for Nigerian Average, Urban Areas without Pipe Borne Water (Q=0.03) and

L = 3w

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

Tan

k L

inea

r D

imen

sion

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

103

Figure 4.41: Tank Dimensions and Residual Depth for Nigerian Average, Urban Areas without Pipe Borne Water (Q=0.03) and

L = w

0

1

2

3

4

5

6

0 5 10 15 20 25 30

Tan

k L

inea

r D

imen

sion

(m

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Residual Depth per occupant (cm)

104

Figure 4.42: Tank Dimensions and Residual Depth for Basic Water Requirement (Q=0.04) and L = 2w

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14

Ta

nk

Lin

ear

Dim

ensi

on

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

105

Figure 4.43: Tank Dimensions and Residual Depth for Basic Water Requirement (Q=0.04) and L = 3w

0

1

2

3

4

5

6

7

1 3 5 7 9 11 13 15

Ta

nk

Lin

ear

Dim

ensi

on

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Tank Width (m)

Residual Depth per occupant (cm)

106

Figure 4.44 Tank Dimensions and Residual Depth for Basic Water Requirement (Q=0.04) and L = w

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40

Ta

nk

Lin

ear

Dim

ensi

on

(m)

Tank Plan Area (m2)

Tank Width (m) Residual Depth (cm) Tank Length (m)

Tank Length (m)

Residual Depth per occupant (cm)

107

(iii) Use of simple MS. Excel based programme

A simple Microsoft Excel programme has been written to aid quick

and easy sizing of the septic tank. All the relevant mathematical

relationships have been coded into cells in MS Excel worksheet; and

all the user needs to do is to enter the desired desludging interval,

wastewater flow per capita per day and number of occupants.

Immediately this is done Excel will automatically produce a series of

tank sizes corresponding to the chosen desludging interval and

number of occupants based on different residual depths. The

designer does not need to write a fresh programme, neither can he

modify the codes in this programme because the cells containing

formulae have been protected to prevent modification. However, the

cells for receiving input have been clearly distinguished and are not

protected.

All the designer needs to do is to use engineering judgement to select

the appropriate dimensions. However, in order to aid the designer

who might get confused as to which dimension to choose, a

conditional formatting has been performed on the depth row such

that Excel highlights all the tank dimensions whose depths are less

than the length but greater than the width (Figure 4.45). It should be

noted that all the tanks whose dimensions are generated by this

programme for a specified desludging interval, flow rate and number

of occupants will have the same volume and require desludging at

the same time. However, it has been established that narrower tanks

enhance solids removal.

108

Figure 4.45: Sample Design using Excel Codes

109

4.6 DESIGN EXAMPLE

In order to demonstrate the use of the design approach developed in this

research, a design example shall be presented using the three packages.

Consider a building that will accommodate 15 people in a typical Nigerian

middle class city, say Enugu. It is required to construct a septic tank that will

require desludging once every five years.

Solution

(i) Using Equations (3.44), (3.51) and (4.3)

Q = 0.03 m3/capita/per day (based on UNDP average Nigerian water

use)

t = 5 years.

First determine the volume of septage accumulated in five years

using Equation (4.1): )1(56.0021.0043.0 11.0 t

septage etV where t =

5

Substituting t = 5 in the equation, we obtain V = 0.384m3 per capita.

Hence the total volume of sludge accumulated is 0.384 X 15 =

5.76m3.

Next, we use Equation (3.44) (A

Qh re

re

) to obtain the plan area of

the tank. Equation (3.44) is for one occupant, so for N occupants, the

overall residual depth (Hre) = A

NQNh re

re

. Using a minimum

residual detention time of 24 hours (1 day) and an overall residual

depth of 15cm, and substituting in the above equation, we obtain the

plan area as:

20.315.0

103.015m

XXA

Hence the depth of sludge is obtained as

mA

Vy 92.1

3

76.5

110

Total depth of tank = depth of sludge + residual depth (hre) + depth

of reserve space ( hrev). But previously, it has been shown that hre =

hrev. Hence total depth (D) of tank = y + 2hre = 1.92+ 2 x 0.15 =

2.22.

Finally, a suitable length to width ratio is chosen. For this design, let

L/W = 2. Hence

mA

W 22.12

3

2

and L = 2.5m

The tank dimension is 2.5m(length) x 1.2m(width) x 2.2m(depth)

for a desludging interval of 5 years, a minimum residual

detention time of 24 hours and a minimum residual depth of

0.15m.

For a large population, the design dimensions may become

excessive. When this is the case, two septic tanks or more should be

designed or the desludging interval may be reduced.

(ii) Using of charts

First, an overall residual depth per occupant is chosen. In the

preceding solution, the overall residual depth was taken as 15cm

(0.15m). The population (15) is located on the horizontal axis of

Figure 4.32. From here, a vertical line is drawn to meet the curve for

0.15m overall depth (see Figure 4.46). The residual depth per

occupant (hre) is 1cm.

111

Figure 4.46: Determination of Residual Depth per Occupant Using Charts

Next choose a suitable length to width ratio, and as before let L/w =

2. Hence we locate 1cm on Figure 4.39 (chart for Nigerian average

water use and L = 2w) and draw a horizontal line to meet the residual

depth per occupant curve. From this point, the line is extended

vertically upwards and downwards to meet the length and depth

curves as well as the area (horizontal) axis (see Figure 4.47).

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40

Res

idu

al D

epth

per

Occ

up

an

t (c

m)

Number of Occupants

Hre (10cm) Hre(15cm) Hre(20cm) Hre(30cm) Hre(40cm) Hre(50cm)

112

Figure 4.47: Determination of Tank Dimensions Using Charts

From Figure 4.47, L =2.4m, w =1.2m and A = 3m2. The depth of sludge

is obtained by dividing the volume of sludge obtained from Figure 4.31

with the area obtained. The total volume of sludge = 0.265 x 15 =

5.76m3, hence the depth of sludge (y) = 5.76/3 =1.92m. Finally, the total

depth of tank (D) = y + 2 x 0.15= 1.92 + 0.3 = 2.22m.

The tank dimension is 2.4m (length) x 1.2m (width) x 2.2m (depth) for

a desludging interval of 5 years, a minimum residual detention time

of 24 hours and a minimum residual depth of 0.15m. The slight

differences between these dimensions and those previously used

stemmed from the interpolation and reading errors inherent in the use of

graphs. However, for all intents and uses, the two tanks are practically

the same.

(iii) Using Excel Code

This is the simplest and most straightforward of the three

approaches. The only values required to be entered are wastewater

flow Q (m3/capita/day), suitable desludging interval in years (Excel

converts it to days before using it to compute), the number of

113

occupants and the desired length to width ratio. As can be seen from

Figure 4.48, tank dimensions corresponding to overall residual

depths from 0.1m to 0.75m will automatically be generated so that

the designer can make a pick. For a residual depth of 0.15m (used in

two previous approaches), the corresponding tank dimensions are

2.45m (length), 1.22m (width) and 2.07m(depth) to two decimal

places. Any other suitable dimensions can also be chosen from the

array of results.

Figure 4.48: Tank Design Using Excel Codes

Hence the tank dimension is 2.5m (length) x 1.3m (width) x 2.1m (depth) for

a desludging interval of 5 years, a minimum residual detention time of 24

hours and a minimum residual depth of 0.15m

4.7 CAUTION FOR USERS

The septic tank system is an on-site wastewater treatment facility and is

therefore limited in capacity. The septic tank system is most suitable for

individual buildings and as water use and number of users increase, the design

size can become unwieldy, making the system uneconomical and difficult to

114

maintain. For several dwelling units or housing estates, the option of a central

wastewater treatment unit such as waste stabilization pond is preferable.

115

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

5.1 CONCLUSION

The septic tank is pivotal to wastewater treatment as well as public health

especially in developing countries where central treatment plants are not

affordable. Just like every other waste management facility, the septic tank

deserves a rational design approach rather than the current haphazard sizing

method currently being employed. A preliminary investigation showed that

septic tank malfunctioning is common as a result of poor design, construction

and maintenance. In order to address this anomaly, a new design approach was

developed in this research. This method is based on specifying a desired

desludging interval, a minimum residual detention time and a residual depth. A

sludge accumulation model was developed and calibrated for the purpose of

estimating sludge accumulation per capita. This model shows that sludge

accumulation in septic tank is not constant as is commonly assumed.

It has also been shown that because the septic tank is also a storage system, the

detention time reduces as sludge accumulates. Charts that show the decline of

detention time with sludge accumulation have been produced. These charts

show that the usual recommendation that septic tanks be desludged when they

are one-third full could be irrational most of the times. It was shown that a tank

that is one-third full may still have enough residual detention time for optimal

performance. A more rational approach is that septic tanks should be desludged

when they no longer have enough detention time for efficient performance. In

order to simplify the new design approach, charts have been produced to aid

the designer who may not be learned enough to use the equations presented.

The reason for this is that preliminary investigation showed most people who

undertake the construction of buildings have no formal education in

engineering. This set of people will be more at ease with charts. The steps for

the new design approach have also been coded into cells in a Microsoft Excel

worksheet to facilitate design for the computer literate designer. All the

116

designer needs to supply are the desired desludging interval, wastewater

discharge, number of users and a desired length to width ratio. Several septic

tank dimensions appropriate for these conditions will be generated and designer

can make a choice by applying engineering judgment.

In summary:

Every septic tank is unique and therefore should be designed taking

cognisance of the number of users, desired desludging interval and

expected wastewater flow which is a function of water availability.

Users should always know when to expect to desludge their tanks. This

should be an intrinsic aspect of the design. Septic tanks should have

enough initial volume for long term storage of sludge to avoid frequent

desludging. Tanks with small initial volumes soon get silted up with

sludge thus requiring frequent desludging;

Concrete splash baffles should be completely phased out. Inlet tees

should be used instead;

All septic tanks should not be designed for a detention time of 24 hours

and desludging must not always necessary follow the one third tank

volume sludge accumulation specification. The designer should be able

to know whether his chosen detention time will maintain the desired

efficiency with regard to suspended solids removal efficiency and for

how long.

5.2 RECOMMENDATIONS

Based on the outcome of this research, the following recommendations are

necessary:

A general awareness campaign on the indispensable role of septic tanks

in municipal wastewater management and public health should be

mounted by the government and non-governmental organizations. In this

117

campaign, the place of proper design, construction and maintenance

should be emphasized.

People should not wait for their septic tanks to be overflowing with

sludge before desludging as this reduces the life span of the whole

system and also reduces the efficiency of the drain field or soak pit.

The Federal Ministry of Environment should support an extensive test of

this design approach and then incorporate it into the Nigerian septic

tanks design standard.

118

REFERENCES

Agunwamba, J. C. (2001). Waste Engineering and Management Tool.

Immaculate Publications Ltd., Enugu.

Ahmed, W., Neller, R. and Katouli, M. (2005). Evidence of septic system

failure determined by a bacterial biochemical fingerprinting method.

Journal of Applied Microbiology, 98, pp 910 – 920.

Al-Layla, A. M. and Al-Rawi, S. M. (1989). Evaluation of septic tank

performance in some parts of Mosul City in Iraq. Journal of

Environmental Science and Health, Part A, 24: 5, pp 543 – 556.

Aluko, T. M. (1978). The septic tank in nigerian building practice. Nigerian

Journal of Engineering and Technology Vol. 1 July/August 1978, pp.

33-42.

Baudisöva, D. (1997). Evaluation of Escherichia coli as the main indicator in

fecal pollution. Water Science and Technology.

Baumann, E. R., Jones, E. E, Jakubowski, W. M. and Nottingham, M. C,

(1978). Septic tanks in home sewage treatment: Proceedings of the

Second International Home Treatment Symposium. American Society of

Agricultural Engineers, St. Joseph, MI, pp 38-53

Blostein, J. (1991). Shigellosis from swimming in a park in Michigan. Public

Health Reports, 106: pp 317–322.

Bouma, J., Ziebell, W. A., Walker, W. G., Olocott, P. G., McCoy, E. and Hole,

F. D. (1972). Soil absorption of septic tank effluent: A field study of

some major soils in Wisconsin. Information Circular no. 20.

University of Wisconsin Extension Geological and Natural History

Survey, Madison, WI.

Bounds, T. R. (1997). Design and performance of septic tanks. Conference of

the American Society for Testing and Materials, Philadelphia,

Pennsylvania.

Bounds, T. R. (1995). Septic tank septage pumping intervals, Proceedings 8th

Northwest On-Site Wastewater Treatment Short Course and Equipment

Exhibition, University of Washington, Seattle, WA.

119

Bounds, T. R. (1994). Septic tank septage pumping intervals. Conference of

the American Society of Agricultural Engineers, in Atlanta, Georgia.

Bounds, T. R. (1990). Glide Audit 1986-1987 summary of sludge and scum

accumulation rates. Conference sponsored by the Rural Community

Assistance Corporation (RCAC), in San Francisco, California.

Bridgman, S.A., Robertson, R. M. P., Syed, Q., Speed, N., Andrews, N. and

Hunter, P. R.(1995). Outbreak of cryptosporidosis associated with a

disinfected groundwater supply. Epidemiology and Infection. Vol.

115, pp 555-556.

Burubai, W., Akor, A. J., Lilly, M. T. (2007). performance evaluation of a

septic system for high water-table areas. American Eurasian Journal

of Scientific Research 2 (2), pp112-116.

Canter, L. and Knox, R. (1985). Septic tank system effects on groundwater

quality. Lewis Publishers, Chelsea, MI, USA.

Close, M. E., Hodgson, L. R. and Tod, G. (1989). Field evaluation of

fluorescent whitening agents and Sodium Tripolyphosphates as

indicators of septic tank contamination in domestic wells. N.Z.J.

Mar. Freshwater Res. 23, pp 563-568

Cogger, C. (1988). On-site septic systems: the risk of groundwater

contamination. Journal of Environmental Health, Vol. 51, No.1, pp 12-

16.

Collick, A.S., Eaton, M.Z. Montalto, A. F, Gao, B., Kim, Y., Day, L. and

Steenhuis, S. T. (2006). Hydrological evaluation of septic disposal

field design in sloping terrain. Journal of Environmental Engineering

(ASCE) 0733-9372 (2006) 132:10, pp. 1289 - 1297

Cransberg, K., Van den Kerkhof, J. H., Banffer, J. R., Stijnen, C., Wernors, K.,

Van de Kar, N. C., Nauta, J., and Wolff, E. D. (1996). Four cases of

haemolytic uremic syndrome – source contaminated swimming water?

Clinical Nephrology, 46, pp 45–49.

Crites, R. W. and Tchobanoglous, G. (1997). Small and Decentralized

Wastewater Management Systems, McGraw-Hill, New York.

120

Dunbar, W. (1907). Principles of Sewage Treatment. Translated in 1908 by

H.T. Calvert. Griffin, London

Galmes, A., Nicolau A., Arbona, G., Gomis, E., Guma, M., Smith-Palmer, A.,

Hernandez-Pezzi, G., Soler, P. (2003). Cryptosporidiosis outbreak in

British tourists who stayed at a hotel in Majorca, Spain.

Eurosurveillance Weekly, 7(33).

Gary, F. N. (1995). The influence of sludge accumulation rate on septic tank

design. Environmental Technology ISSN 0959-3330, vol. 16, No.8,

pp. 795 – 800.

Geary, P. M. and Gardner, E. A. (1998). Sustainable on-site treatment systems,

individual and small community sewage systems. St. Joseph, Michigan.

American Society of Agricultural Engineers.

Gleick, P. H. (1996). Basic water requirement for human activities: meeting

basic needs. Water International, 21, pp 83 – 92.

Gordon, D. G. (1989). Managing Non-point Pollution. An Action Plan

Handbook for Puget Sound Watershed, Seattle, W. A: Puget Sound

Water Quality Authority.

Greensmith, C. T., Stanwick, R. S., Elliot, B. E., Fast, M. V. (1988). Giardiasis

associated with the use of a water slide. Pediatric Infectious

Diseases Journal, 7, pp 91–94.

Gross, M.A (1987). Assessment of the effects of household chemicals upon

individual septic tank performances. University of Arkansas at Little

Rock, Graduate Institute of Technology, Little Rock, AR 72203,

Publication No. 131

Heinss, U., Larmie, S. A. and Strauss, M. (1999). Characteristics of Fecal

Sludges and their Solids-Liquid Separation. SANDEC/EWAG, January

1999.

Ignatius, I. and Jowett, C. (2004). The Effect of household chemicals on

septic tank performance. Presented at the 2004 ASAE Conference,

Sacramento CA.

121

Jelliffe, P. A. (1995). Managing of On-site Effluent Disposal: Conclusions

from a Study on the Performance of 101 Systems in Maroochy Shire,

Queensland, Australia. Report Prepared for Maroochy Shire Council.

Jowett, E. C. and Lay, R. (2005). Comparative performance of closed-conduct,

laminar flow septic tanks. Submitted to Small Flow Quarterly,

University of West Virginia (In press).

Jowett, E. C., (2007). Comparing the performance of prescribed septic tank to

long, narrow, flooded design. Presented at 2007 WEFTEC Technical

Program 16. San Diego CA.

Kamel,. M. and Hgazy, B. E. (2006). A septic tank system: on site disposal.

Journal of Applied Sciences 6 (10), pp 2269-2274.

Kansas State Department of Health and Environment (1997). Minimum

Standards for Design and Construction of Onsite Wastewater System.

Bulletin 4-2, March.

Karathanasis, A. D., Mueller, T. G., Boone, B. and Thompson, Y. L. (2006).

Effects of soil depth and texture on fecal bacteria removal from septic

effluents. Journal of Water and Health, 04.3, pp 395 – 404.

Keeny, D. (1986). Sources of nitrate to ground water. CRC Critical Reviews

in Environmental Control. Vol. 16, pp. 257-304

Koottatep, T., Wanasen, A. and Schertenleib, R. (2004). Potential of the

anaerobic baffled reactor as decentralized wastewater treatment

system in the tropics. Presented at the 1st International Conference on

Onsite Wastewater Treatment and Recycling in Perth, Australia.

February 2004.

Kriessl, J.F. (1971). Waste treatment for small flows. Paper Presented to the

1971 Annual Meeting, American Society of Agricultural Engineers.

Washington State University, Pullman, Washington, June 27-30, 1971.

Lay, R., Weiss, M., Pataky, K and Jowett, E. C. (2005). Rethinking hydraulic

flow in septic tanks. Environmental Science & Engineering V. 18, No.

1 (March), pp. 50-52

122

Nguyen, V. A., Pham, T. N., Nguyen, H. T., Morel, A. and Tonderski, K. S.

(2007) Proceedings of the 2007 IWA-NOWRA International

Conference. March, Baltimore Maryland, USA.

Nicosia, L. A., Rose, J. B., Stark, L. and Stewart, M. T. (2001). A field study

of virus removal in septic tank drain fields. Journal of Environmental

Quality 30, pp 1933-1939.

Nirel, P. M. and Revaclier, R. (1999). Assessment of sewage treatment plants

effluents impact on river water quality using dissolved Rb/Sr ratio.

Environ. Sci. Technol., 33, pp 1996 - 2000.

Novac, J. T., McDaniel, C. R and Howard, S. C. (1990). The effect of boat

holding tank chemicals on treatment plant performance. Res J. Water

Pollut. Control Fed. 62 (288): 288-295.

Paing, J., Picot, B., Sambuco J. P. and Rambaud A., (1999). Sludge

accumulation and methanogenic activity in anaerobic lagoon. Water

Science and Technology.

Pang, L., Close, M., Goltz, M., Sinton, L., Davies, H., Hall, C and Santon, G.

(2003). Estimation of septic tank setback distance based on E.Coli and

F. RNA phages. Environ. Intl., 29, pp 907-921.

Pang, L., Nokes, C., Simunek, J., Kikkert, H. and Hector, R. (2006). Modeling

the impact of clustered septic tank system on groundwater quality.

Vadoze Zone J.5, pp 599-609.

Poe, K. F. (2001). Antibacterial products in septic systems. problem

environments for septic systems and communal treatment options,

Conference Proceedings. Waterloo Centre for Groundwater Research,

University of Waterloo.

Public Works Department (1943). Information Book, Sanitary Structures, 19 – 22.

Lagos, Nigeria.

Rahman, M., Shahidullah, A. H. M. and Ali, A. (1999). An Evaluation of

septic tank performance. 25th

WEDC Conference, Addis Ababa,

Ethiopia, 1999.

123

Rock, C. A. and Boyer, J. A., (1995). Influence of design on septic tank

effluent quality: in Proceedings, 8th

On-site Wastewater Treatment Short

Course and Equipment Exhibition, Seattle W.A. pp 45-62.

Rose, J. B. and Gerba, C. P. (1991). Use of risk assessment for development of

microbial standards. Water Sci. Technol., 24, pp 29-34.

Saqqar, M. M. and Pescod, M. B. (1995). Modelling sludge accumulation in

anaerobic wastewater stabilization ponds. Wat. Sci. Tech. No 12, pp 185

– 190.

Scalf, M. R., Dulap, W. J. and Kreissel, J. P. (1997). Environmental effects of

septic tank systems. US Environmental Protection Agency 18, EPA-

600/3-77-096.

Seabloom, R. W., Bounds, T., and Loudon, T. (2004). Septic Tanks

http:/./www.onsiteconsortium.org./files/septictankspaper.pdf62p

Siegrist, R., Witt, M. and Boyle, W. (1976). Characteristics of rural household

wastewater .J. Env. Eng. ASCE Vol. 102, pp. 533 – 548.

Taylor, J. W., Gary, G. N. and Greenberg, H. B. (1981). Norwalk-related

viral gastroenteritis due to contaminated drinking water. American

Journal of Epidemiology. Vol. 114, No. 4, pp 584-592.

United Nations Development Programme (2006). Human Development Report.

www.data360.org.

Vaishnav, D. D. and McCabe, J. W. (1996). Development of an anaerobic

sludge respiration inhibition test and its use to assess septic tank

safety of consumer products. Environmental Toxicology and

Chemistry, 15 (5), pp 663-669.

Washington, B., Ong, S. K., Smith, W. L. and McCabe, J. W. (1997). Fate of

absorbable organic halides from bleaching laundering in septic tank

systems. Env. Toxic and Chem. 17 (3): 398-403.

Watson, K. S., Farrell, R. P. and Anderson, J. S. (1967). The contribution from

individual homes to the sewer system. Journal of the Water Pollution

Control Federation.

124

Weibel, S. R., Bendixen, T. W. and Coulter, J. B. (1955). Studies on household

sewage disposal systems, Part III,” U.S. Public Health Service

Publication No. 397.

Weissman, J. B., Graun, G. F., Lawrence, D. N., Pollard, R. A., Saslaw, M. S.

and Gangarosa, E.J. (1976). An epidemic of gastroenteritis traced to a

contaminated pupil water supply. American Journal of Epidemiology.

Vol. 103, No.4, pp 391-398.

Wilhelm S. R, Schiff, S. L and Cherry, J. A. (1994). Biochemical evolution of

domestic wastewater in septic system: a conceptual model. Ground

Water Vol 32, No. 6, November –December 1994.

Winneberger, J. H. T., (1984). “The Septic Tank”. In Septic Tank Systems, a

Consultant‟s Toolkit. Volume II. Butterworth, Boston, 123p.

Yates, M. (1985). Septic tank density and groundwater contamination.

Groundwater 23, pp 586 – 591.

125

APPENDIX I: EXPERIMENTAL RESULTS

Table A1: Readings Obtained on 17/08/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.2 7.6 7.5 7.7 7.5 7.3 7.2 7.5 7.4

SS(mg/l) 166 126 160 160 68 56 104 163 165

Coliform (MPN/100ml) 23000000 240000 240000 240000 110000 240000 110000 240000 240000

BOD(mg/l) 1020 144 66 60 30 150 78 48 66

COD(mg/l) 2360 192 200 240 212 268 240 192 168

E-Coli(MPN/100ml) 9000000 7500 700 700 700 1400 1400 1400 400

Table A2: Readings Obtained on 19/08/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.9 7.3 7.3 7.2 7.1 7.4 6.9 7.4 7.2

SS(mg/l) 166 94 44 166 164 128 166 130 102

Coliform (MPN/100ml) 30000000 430000 750000 2100000 460000 2100000 2100000 1500000 430000

BOD(mg/l) 525 105 173 105 110 98 103 93 83

COD(mg/l) 944 160 180 300 320 252 240 220 200

E-Coli(MPN/100ml) 3000000 30000 40000 210000 70000 70000 70000 40000 30000

126

Table A3: Readings Obtained on 24/08/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.7 7.2 7.1 7.1 7.1 7 7 7.3 7.2

SS(mg/l) 608 272 100 286 248 190 190 20 300

Coliform (MPN/100ml) 2100000 70000 70000 1500000 200000 30000 140000 930000 280000

BOD(mg/l) 960 108 168 120 88 118 115 65 100

COD(mg/l) 1584 280 260 272 268 280 240 144 120

E-Coli(MPN/100ml) 150000 110000 40000 70000 70000 30000 70000 90000 30000

Table A4: Readings Obtained on 26/08/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.9 7.1 7 7.3 7.1 7 6.9 7 7.1

SS(mg/l) 286 238 206 264 248 202 152 228 236

Coliform (MPN/100ml) 2400000 24000 9300 2800 1500 700 2000 300 900

BOD(mg/l) 285 127 130 111 183 102 119 107 156

COD(mg/l) 424 140 140 144 120 212 160 200 180

E-Coli(MPN/100ml) 30000 2100 900 700 300 300 300 300 300

127

Table A5: Readings Obtained on 02/09/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.8 6.9 6.9 6.9 7.1 6.9 6.8 6.9 6.9

SS(mg/l) 432 378 366 414 424 404 338 304 428

Coliform (MPN/100ml) 200000 700 700 1400 1100 12000 15000 12000 700

BOD(mg/l) 1035 183 177 175 188 180 210 185 189

COD(mg/l) 2624 320 356 368 440 416 360 312 296

E-Coli(MPN/100ml) 30000 700 400 300 900 700 700 300 300

Table A6: Readings Obtained on 14/09/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.8 7.1 7.1 7.2 7.3 7 7 7.2 7.2

SS(mg/l) 390000 21000 9300 4300 400 24000 2800 9300 9300

Coliform (MPN/100ml) 390000 21000 9300 4300 400 24000 2800 9300 9300

BOD(mg/l) 3300 1005 1020 930 1065 960 1005 930 973

COD(mg/l) 202 169 173 119 122 194 54 36 47

E-Coli(MPN/100ml) 30000 300 300 400 400 700 300 300 300

128

Table A7: Readings Obtained on 28/09/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 6.8 7.2 7 7.4 7.3 7.5 7.2 7.3 7.5

SS(mg/l) 974 689 752 776 832 795 674 680 702

Coliform (MPN/100ml) 200000 7500 15000 110000 24000 4300 2000 2800 21000

BOD(mg/l) 2850 1920 2100 1680 1740 2350 2200 2050 2350

COD(mg/l) 295 288 160 188 212 116 88 120 220

E-Coli(MPN/100ml) 30000 300 300 1500 300 900 1100 700 1100

Table A8: Readings Obtained on 12/10/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.3 7.5 7.4 7.5 7.5 7.4 7.3 7.5 7.4

SS(mg/l) 17062 15262 6840 9312 5406 1282 4830 4934 7764

Coliform (MPN/100ml) 24000000 2100000 24000000 4600000 11000000 11000000 24000000 1200000 2100000

BOD(mg/l) 3000 1920 1560 1860 1500 780 3120 1980 2040

COD(mg/l) 216 128 148 128 68 48 56 48 52

E-Coli(MPN/100ml) 90000 110000 30000 30000 30000 70000 30000 30000 30000

129

Table A9: Readings Obtained on 19/10/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.1 7.4 7.4 7.5 7.4 7.4 7.4 7.5 7.4

SS(mg/l) 452 186 284 300 104 240 174 274 282

Coliform (MPN/100ml) 24000000 11000000 4600000 11000000 4600000 230000 11000000 11000000 1500000

BOD(mg/l) 2920 2340 1740 1920 1560 2160 1560 1680 1860

COD(mg/l) 1220 32 40 25 61 54 61 86 95

E-Coli(MPN/100ml) 30000 30000 30000 30000 30000 30000 30000 40000 30000

Table A10: Readings Obtained on 28/10/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.4 7.46 7.3 7.7 7.8 7.3 7.1 7.1 7.1

SS(mg/l) 1234 328 401 398 429 547 521 644 572

Coliform (MPN/100ml) 150000 70000 90000 150000 150000 70000 30000 90000 150000

BOD(mg/l) 2700 2160 2640 2160 2340 1650 1900 1850 1900

COD(mg/l) 828 135 141 110 73 98 167 64 65

E-Coli(MPN/100ml) 110000 70000 90000 70000 30000 30000 30000 30000 30000

130

Table A11: Readings Obtained on 11/11/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.5 7.4 7.4 7.8 7.8 7.3 7.2 7.1 7.2

SS(mg/l) 874 510 392 400 438 453 374 560 448

Coliform (MPN/100ml) 24000000 11000000 24000000 15000000 24000000 700000 21000000 280000 24000000

BOD(mg/l) 2970 1860 1705 1793 1380 851 2030 2004 2049

COD(mg/l) 856 168 148 32 84 100 140 80 60

E-Coli(MPN/100ml) 70000 40000 30000 30000 70000 30000 40000 40000 30000

Table A12: Readings Obtained on 18/11/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.1 7.3 7 7.7 7 7.3 6.9 7.2 7.3

SS(mg/l) 406 144 326 100 164 130 694 702 516

Coliform (MPN/100ml) 11000000 210000 280000 280000 2100000 90000 11000000 1200000 140000

BOD(mg/l) 2280 1920 2280 2040 2160 1320 2220 2100 2100

COD(mg/l) 268 88 232 132 52 132 132 124 84

E-Coli(MPN/100ml) 40000 30000 30000 30000 40000 30000 40000 40000 40000

131

Table A13: Readings Obtained on 25/11/2010

Parameter Influent AM AO BM BO CM CO DM DO

pH 7.2 7.3 7.3 7.7 7.7 7.3 7.4 7.2 7.3

SS(mg/l) 2536 260 278 354 86 74 186 164 410

Coliform (MPN/100ml) 430000 430000 230000 230000 40000 90000 150000 110000 200000

BOD(mg/l) 2890 1840 1732 1755 1430 975 1980 2010 2086

COD(mg/l) 1784 152 152 80 63 144 120 96 172

E-Coli(MPN/100ml) 30000 30000 30000 30000 40000 40000 70000 90000 40000

132

Table A14: Temperature Values (0C)

Date A B C D

17/08/1010 28 26 26.7 27

19/08/2010 - - -

24/08/2010 24 24 25 24

26/08/2010 - - -

02/09/2010 25 25 26 26

14/09/2010 26 26 26 26

28/09/2010 25.5 26 25 25

12/10/2010 27.5 27 27.5 28

19/10/2010 27 27.5 27 27

28/10/2010 28 26 27 28

11/11/2010 - - - -

18/11/2010 31 33 30 31

25/11/2010 28.5 29 28.5 28

133

APPENDIX II: IMPLEMENTATION OF EXCEL PROGRAMME

Sludge Volume

Equation 4.3 was coded into cell C12 of Figure 4.45 as follows:

= 0.043+0.021*B12-0.56*(exp(1-0.11*B12)-1)

Overall Residual Depth

The overall residual depths for different possible dimensions were specified in

cells E3 to R3. These cells do not contain any formula.

Residual Depth

Cells E4 to R4 contain residual depth per occupant. It was obtained by dividing

the overall residual depth by the number of occupants. For instance, the

formula in cell E4 is:

= E3/$C3

Plan Area

Cells E5 to R5 contains formulae (Equation 3.44) for plan area. For E21, the

formula is

= $C4/E4

C4 contains the specified number of occupants while E4 contains the residual

depth per occupant. The notation $ was used before C4 in order to hold the

value of C4 constant.

Sludge Depth

Cells E6 to R6 contain sludge depth for different residual depths. The sludge

depth was obtained by dividing the product of sludge volume per capita (cell

C12) and population (cell C3) by the plane area. The formula in cell E6, for

instance is:

=$C3*$C12/E5

Tank Depth

The tank depth was obtained by adding the sludge depth to the overall residual

depth. The formulae are contained in cells E7 to R7:

=E6+E3

134

Tank Width

This was obtained from the plan area and the length to width ratio (L/W)

specified in cell C5. For cell E8, the formula is:

=(E5/$C5)^0.5

In the example of Figure 4.45, the value of L/W used was 2 hence the square

root.

Tank Length

The thank length (cells E9 to R9) was obtained by multiplying the tank width

by the length to width ratio. For cell E9, the formula is:

=2*E8

Tank Volume

The tank volume was obtained by multiplying the plan area by the tank depth.

Cells E10 to R10 contain the tank volume. For Cell E10, the formula is:

=E5*E7.