university of nairobi school of engineering...

UNIVERSITY OF NAIROBI

SCHOOL OF ENGINEERING

DEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS ENGINEERING

DESIGN OF RECYCLING SYSTEM FOR A WASTEWATER TREATMENT PLANT

EFFLUENT

CASE STUDY: KIPEVU WASTEWATER

PROJECT BY: MARIAN MAPENZI

STUDENT REG NO: F21/2490/2009

COURSE CODE: FEB 540

SUPERVISED BY: Dr. C. O. Omuto

DATE OF SUBMISSION: 31

Report submitted in partial fulfilment for the requirements for the degree

Science in Environmental and Biosystems Engineering at the University Of Nairobi.

UNIVERSITY OF NAIROBI

SCHOOL OF ENGINEERING



CASE STUDY: KIPEVU WASTEWATER TREATMENT PLANT

MARIAN MAPENZI

STUDENT REG NO: F21/2490/2009

ERVISED BY: Dr. C. O. Omuto

31ST MARCH, 2014.

Report submitted in partial fulfilment for the requirements for the degree of Bachelor of


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of Bachelor of


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TABLE OF CONTENTS

Contents

ACRONYNMS AND ABBREVIATIONS ............................................................................................ 4

1.0 INTRODUCTION ...................................................................................................................... 5

1.1 BACKGROUND OF THE PROBLEM ........................................................................................... 6

1.2 PROBLEM STATEMENT AND ANALYSIS ................................................................................... 6

1.3 PROBLEM JUSTIFICATION ....................................................................................................... 7

1.4 SITE ANALYSIS ......................................................................................................................... 8

1.4.1 Suitability of Kipevu ............................................................................................................... 8

1.5 BROAD OBJECTIVE ................................................................................................................. 10

1.5.1 Specific objectives: ............................................................................................................... 10

1.5.2 Scope of the project ............................................................................................................. 11

2.0 LITERATURE REVIEW ............................................................................................................... 12

2.1 SCREENING .................................................................................................................................. 12

2.2 SLOW SAND FILTRATION ............................................................................................................. 12

The drawbacks of slow sand filtration are: ................................................................................... 14

2.3 RAPID SAND FILTRATION ............................................................................................................ 15

2.4 NANOFILTRATION ....................................................................................................................... 17

2.5 ULTRAFITRATION ........................................................................................................................ 17

2.6 CARBON FILTERING ..................................................................................................................... 18

2.7 FLOCCULATION ........................................................................................................................... 19

2.8 COAGULATION ............................................................................................................................ 20

2.9 SEDIMENTATION ......................................................................................................................... 20

2.10 CHLORINATION ......................................................................................................................... 22

2.11 ULTRAVIOLET TREATMENT ....................................................................................................... 23

2.12 PH ADJUSTMENT ....................................................................................................................... 25

3.0 THEORETICAL FRAMEWORK .................................................................................................. 26

3.1 SLOW SAND FILTER ..................................................................................................................... 26

3.1.1 Flow Rate.............................................................................................................................. 26

3.1.3 Head Loss ............................................................................................................................. 27

3.2 CHLORINATION ........................................................................................................................... 31

3.3 COAGULATION ............................................................................................................................ 33

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3.4 SEDIMENTATION ......................................................................................................................... 37

3.4.1 Design Parameters ............................................................................................................... 38

4.0 PRELIMINARY DESIGN SKETCHES .................................................................................. 39

5.0 METHODOLOGY ......................................................................................................................... 40

5.1 SITE SELECTION ........................................................................................................................... 40

5.3 LABORATORY ANALYSIS OF WASTEWATER ................................................................................ 40

Effluent Report for 3rd

June to 30th

June 2013 .............................................................................. 41

6.0 ANALYSIS AND DESIGN............................................................................................................ 42

6.1 COAGULATION DESIGN ........................................................................................................ 42

6.2 SEDIMENTATION TANKS ...................................................................................................... 45

6.3 FILTRATION TANKS ............................................................................................................... 47

6.3.1 The required flow of water .................................................................................................. 48

6.3.2 Sand Depth ........................................................................................................................... 48

6.3.3 Gravel distribution. .............................................................................................................. 49

6.3.4 Under drainage system ........................................................................................................ 49

6.3.5 Filter Air Wash Rate ............................................................................................................. 50

6.4 CHLORINATION TANK .......................................................................................................... 50

6.5 CONTACT TANK ...................................................................................................................... 51

6.6 STORAGE TANK CAPACITY ................................................................................................. 51

7.0 CONCLUSION AND RECOMMENDATION .............................................................................. 52

8.0 REFERENCE .................................................................................................................................. 54

9.0 APPENDICES ................................................................................................................................ 56

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ACRONYNMS AND ABBREVIATIONS

Å Angstrom

BOD Biochemical Oxygen Demand

CBMWD Central Basin Municipal Water District

Cm Centimetre

COD Chemical Oxygen Demand

CWSB Coast Water Services Board

DNA Deoxyribonucleic acid

Ha Hectare

SSF Slow Sand Filtration

UF Ultrafiltration

UV Ultraviolet

WARMA Water Resource Management Authority

WHO World Health Organisation

WWTP Wastewater Treatment Plant

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1.0 INTRODUCTION

It is estimated that75% of the World is covered in water but only 1% of it is available as

fresh water to a growing global population of about 6.6 billion people. Owing to cases of

floods and drought this percent is getting depleted at an alarming rate. For instance in the

United States of America, Southern California 66 percent of their water supply is

imported from outside. (Central Basin Municipal Water District, 2013)

The Vision 2030 for water and sanitation is to ensure that improved water and sanitation

are available and accessible to all. The short term goal for this project is to increase both

access to safe water and sanitation beyond current values. Designing a system to treat and

supply water to the residents is in accordance to this goal of the Kenyan Vision 2030 and

would go a long way in assisting its achievement.

This further intensifies the value and importance of water in our country and the need to

find alternative sources of it. Water supply and sanitation in Kenya is characterised by

low levels of access, in particular in slums and in rural areas, as well as poor service

quality in the form of intermittent water supply. According to the Water Resource

Management Authority only 9 out of the 55 water service providers in Kenya produce a

continuous supply.

Although urban water tariffs are high by regional standards (US$0.46 cubic metres on

average in 2007) the level of cost recovery is low due to a high level of non-revenue

water. Costs are high due to the need to tap water from far away sources and high levels

of staffing, as is the case in Mombasa county being supplied by a source (Mzima springs)

located 220km away and having 11workers per 1000 connections. (WARMA Service

Charter, 2009)

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1.1 BACKGROUND OF THE PROBLEM

The great demand for water in the country as a whole brings about the need for alternative

sources of water to meet industrial and domestic demands. Water is easily contaminable and

once it is contaminated it is treated as waste (wastewater) and discarded. Just as we recycle

and manage solid waste the same principles of conservation should apply for water in order

to develop sustainable systems. This water can then be used for non-potable purposes which

accounts for 85 percent of the capacity used in a normal household.

1.2 PROBLEM STATEMENT AND ANALYSIS

Water recycling is a key component for delivery of sustainable water resource management

in the urban environment. There is a significant emphasis on substituting drinking water with

alternative water resources. One way of achieving this is to promote the use of dual pipe

networks in new residential developments to deliver recycled water for non-potable uses such

as garden watering, washing and toilet flushing. As the name implies, dual distribution

systems involve the use of water supplies from two different sources in two different

distribution networks. Recycled water has a great chance of reducing water shortage

problems in Mombasa and its neighbouring counties. It would also ensure that water demands

are met where there is now a major deficit of about 380,000 m3/day.

1.3 PROBLEM JUSTIFICATION

The cost of recycled/reclaimed water exceeds that of reclaimed water in many

world, where a fresh water supply is plentiful. However, reclaimed water is usually sold to

citizens at a cheaper rate to encourage its use. As fresh water supplies become limited due to

distribution costs, increased population demands the cos

recycled water for non-potable uses saves potable water for drinking, since less potable water

will be used for non-potable purposes. Usage of recycled water also decreases the pollution

sent to sensitive environments, in t

recycled water we are actually conserving our drinking water supplies.

Laundry

20%

Kitchen and

Drinking

15%

Domestic Water Use

PROBLEM JUSTIFICATION

The cost of recycled/reclaimed water exceeds that of reclaimed water in many



distribution costs, increased population demands the cost ratios will evolve also. Using

potable uses saves potable water for drinking, since less potable water

potable purposes. Usage of recycled water also decreases the pollution

sent to sensitive environments, in this case the Indian Ocean. By increasing our use of

recycled water we are actually conserving our drinking water supplies.

Shower and Bath

35%

Toilet Flushing

25%

Cleaning

5%

Domestic Water Use

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The cost of recycled/reclaimed water exceeds that of reclaimed water in many parts of the



t ratios will evolve also. Using

potable uses saves potable water for drinking, since less potable water

potable purposes. Usage of recycled water also decreases the pollution

his case the Indian Ocean. By increasing our use of

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1.4 SITE ANALYSIS

The Kipevu Treatment Plant is made up of 9.3km trunk sewer lines with diameters ranging

from 15mm to 1000mm. It receives waste water from two sources: tankers that collect waste

and a sanitary system that operates in Jomvu, Changamwe and its environs. It was

constructed in 1952 by the then Public Works Department to serve housing estates in

Changamwe and later extended in 1957 to serve rail-served Industrial Area. The system was

expanded between 1988 and 2000 to serve a population of 396,000 persons mostly from the

West Mainland. (Kipevu Treatment Plant Manual, 2001)

1.4.1 Suitability of Kipevu

This technology is suitable only in areas where a supply of raw water is readily available.

This type of system is generally used near the coast where sea water is abundant, or in places

where wastewater is readily available as a source of supply. It can also be utilized in area that

have rivers, streams or other sources of water but lack treatment facilities; in other words, in

areas supplied with public water but having access to additional water sources that would

otherwise go unutilised or underutilised. From previous reports of the plant it is evident that

the effluent has to be treated further in order to achieve levels that can even allow it to be

applied in non-potable domestic purposes.

Figure 1: Photograph of effluent (to be recycled) as it joins the ocean.

Source: (Kipevu WWTP)

Photograph of effluent (to be recycled) as it joins the ocean.

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1.5 BROAD OBJECTIVE

To design a system for treating effluent from Kipevu Wastewater Plant for non-potable

domestic use.

1.5.1 Specific objectives:

• To establish quality of the water by testing for specific parameters

• To design a slow sand filter

• To compute dimensions of a sedimentation tank

• To design a storage facility for the treated effluent

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1.5.2 Scope of the project

The total water bulk supplied by the Coast Water Services Board to the region is 110,000

m3/day against a demand of 490,000m3/day. This translates to a deficit of approximately

380,000 m3/day. Demand has surpassed supply owing to the rapid increase in population in

the region; hence designing a system for recycling wastewater effluent from the treatment

plant would solve problems of water shortage in the Mainland as well as the Island of

Mombasa. With an inlet flow rate of 262,000m3/day the treatment plant is capable of catering

for about 70% of this deficit. Due to limitations in time and resources detailed designing of

this project will not be possible. It encompasses designing a slow sand filtration system,

sedimentation tanks, chlorination and storage tank for the treated effluent.

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2.0 LITERATURE REVIEW

For the water to be safe and accepted for any use it has to undergo different processes to alter

its composition as well as get rid of any harmful microorganisms. Some treatment usually

applied depending on the quality and quantity of the water may include but not limited to:

2.1 SCREENING

Screening is the first operation at any wastewater treatment works. This process essentially

involves the removal of large non-biodegradable and floating solids that frequently enter a

wastewater works. Efficient removal of these constituents will protect the downstream plant

and equipment from any possible damage, unnecessary wear and tear, pipe blockages and the

accumulation of unwanted material that will interfere with the required wastewater treatment

processes. Since the wastewater has undergone primary treatment, screening will not be

necessary in this case.

2.2 SLOW SAND FILTRATION

The first recorded use of sand filtration for a citywide water supply was back in 1804 by John

Gibb in Paisley, Scotland. The filter provided water for Gibb’s bleaching business and for

public purchase. The model for present practice, however, was a one-acre slow sand filter

designed by James Simpson for the Chelsea Water Company in London and Completed in

1829. The London filter was one of its kind and was a technological breakthrough in that the

design laid the foundation for a widespread practice that continues today. Simpson based his

filter design upon information he gained during a 2000 mile study tour through Scotland,

where he several installations, mostly for industrial use. The basic design was that of a down

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flow filter and use of scraping to remove accumulated material, that is, schmutzdecke. The

hydraulic loading rate, sand size, sand bed depth, water depth and other design parameters

that were delineated became the basis for the practice that followed. The London filter, which

used the Thames River as its raw water source, was the first use of a treatment process for a

piped public water supply. The hydraulic loading rate, 0.15m/hr or 3.9mgd became a

common design criterion and is one of the criteria retained to current times. (Manual of

design for slow sand Filtration, 1991)

Slow sand filtration involves converting raw water into potable water. The structure (slow

sand filter) can preferably be rectangular or circular 1.2m in depth. However the length of the

structure is determined by the flow rate, which is usually 0.1 to 0.2m per hour (m3 per m2 per

hour). Slow sand filters differ from all other filters in that they work by using a complex

biological film (schmutzdecke or hypogeal) that grows naturally on the surface of the sand.

The sand itself does not perform any filtration function but simply acts as a substrate. The

Schmutzdecke is the layer that provides the effective purification in potable water treatment;

the underlying sand just provides support for this biological treatment layer during its

operational period of 10-20 days. Afterwards it is necessary to refurbish the filter, a few

millimetres of the sand is scraped off water and water re-circulated. The benefits of slow sand

filtration are:

• Proven reduction of protozoa and most bacteria

• High flow rate of up to 0.6 litres per minute

• Simplicity of use and acceptability

• Visual improvement of the water

• Production of sufficient quantities of water for all household uses

• Local production (if clean, appropriate sand is available)

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• One-time installation with low maintenance requirements

• Long life (estimated at more than 10 years) with no recurrent expenses

The drawbacks of slow sand filtration are:

• Not as effective against viruses

• No chlorine residual protection - can lead to recontamination

• Routine cleaning can harm the biolayer and decrease effectiveness

• Difficult to transport due to weight - high initial cost

Slow sand filtration (SSF) is most appropriate where there is funding to subsidize the initial

filter cost, available education for use and maintenance, locally-available sand, and a

transportation network able to move the filter. (Manual of Design for Slow Sand Filtration

chapter 1, 1991)

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2.3 RAPID SAND FILTRATION

Rapid sand filter or rapid or rapid gravity filter is one of the processes in water purification

and is used in large scale operations for example, municipal works. The filters use relatively

coarse sand as the filtering medium and wastewater passes through it either by gravity or

after being pressurized. Chemical additives, such as coagulants, are often used in conjunction

with the filtration system to ensure thorough purification.

Advantages

1. Compared to a slow sand filter it has a much higher flow rate of about 200million

gallons of water per acre per day.

2. Area of land required is less.

3. Utilizes less sand.

4. It is less affected by quality of raw water.

Disadvantages

1. It cannot remove bacteria

2. Requires high level of skilled supervision

3. High maintenance and operational costs

4. Produces large volumes of sludge

5. Does not improve taste nor odor

6. Without coagulants or flocculants it cannot get rid of pathogens

Comparing slow sand filtration and rapid sand filtration, the former is a better option for a

number of reasons but most importantly because:

• Rapid sand filtration requires more water for backwashing

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• Rapid sand filtration produces large volumes of sludge

• Slow sand system has a longer life with considerably low initial and maintenance cost

Table _ Summary of water quality characteristics measured over the course of study

testing two filtration systems. Where individual tank values were measured, values were

averaged for each tank within a treatment’s water recirculation system then averaged across

systems.

Table 1: (North American Journal of Aquaculture table 1, 66:261-270, 2004)

This test demonstrated that both rapid and slow sand filters are effective methods but slow

sand filters are better.

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2.4 NANOFILTRATION

This is a liquid-phase separation removing dissolved solids, carried out by means of

membranes, with a relatively high transmembrane pressure. It applies the use of nanometer

sized cylindrical through-pores that pass the membranes at 90 degrees. Nanofiltration

membranes have pores of between 1-10 Angstrom, smaller than that used in microfiltration

and ultrafiltration.

2.5 ULTRAFITRATION

It is a combination of membrane filtration where forces such as pressure and concentration

gradients force flow through a semi-permeable membrane. Suspended solids and high

molecular particles are retained in what is known as a retenant while water and low molecular

matter is allowed to pass through the permeate. This technology is mostly used in industries

especially when dealing with protein matter. It is different from nanofiltration and

microfiltration based on the size of the molecules it filters (103-106 Da). UF can be used for

the removal of particulates and macromolecules from raw water to produce potable water.

They have been used to either replace existing secondary (coagulation, flocculation,

sedimentation) and tertiary filtration (sand filtration and chlorination) systems employed in

water treatment plants or as standalone systems in isolated regions with growing populations.

UF processes are currently preferred over traditional treatment methods for the following

reasons:

• No chemicals required (aside from cleaning)

• Constant product quality regardless of feed quality

• Compact plant size

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• Capable of exceeding regulatory standards of water quality, achieving 90-

100% pathogen removal

UF processes are currently limited by the high cost incurred due to membrane fouling and

replacement. Additional pre-treatment of feed water is required to prevent excessive damage

to the membrane units. (Wastewater Treatment Plant Operator Certification Manual, 2011)

2.6 CARBON FILTERING

Charcoal purifies water through the process of carbon filtering. The activated carbon in the

charcoal will attract the water contaminants that have negative charges. Each particle/granule

of charcoal provides a large surface structure allowing contaminants the maximum possible

exposure to the active sites within the filter media. Activated carbon works via a process

called adsorption, whereby pollutant molecules in the fluid to be treated are trapped in the

pore structure of the carbon substrate. Mostly the substances that are removed from water

with the use of charcoal are chlorine, volatile organic compounds and sediments, also taste

and odour. One pound (450g) of activated carbon contains a surface area of about 100acres

(40ha). (Handbook for Sediment Quality Management, 2005)

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Figure 1: Filtering using different layers of course and fine sand, gravel and charcoal.

2.7 FLOCCULATION

In terms of Polymer science it is the reversible formation of aggregates in which the particles

are not in physical contact. It is also the process where colloids come out of suspension in

form of floc either spontaneously or by adding a clarifying factor. In the flocculated system,

there is no formation of a cake, since all the flocs are in the suspension. (Wastewater

Treatment Plant Operator Certification Manual, 2011)

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2.8 COAGULATION

It is a chemical reaction which occurs when a chemical or coagulant is added to the water

encouraging colloidal material to join together into small aggregates (flocs). Coagulation

removes dirt and other particles suspended in water. Alum and other chemicals are added to

water to form tiny sticky particles called "floc" which attract the dirt particles. The combined

weight of the dirt and the alum (floc) become heavy enough to sink to the bottom during

sedimentation. (Indian Institute of Technology Kanpur web courses, 2013)

2.9 SEDIMENTATION

In water treatment process, sedimentation is the process of removal of suspended particles

that are heavier than water by gravitational settling. Sedimentation basins also known as

settling tanks or clarifiers are large tanks in which water is made to flow very slowly in order

to promote settling of particles or flocs. Most raw water will contain mineral and organic

particles. The density of mineral particles is usually between 2000 to 3000 kg/m3 and can

easily settle out by gravity. Organic particles, on the other hand, have densities ranging from

1010 to 1100 kg/m3and take a long time to settle by gravity. In conventional water treatment,

coagulants are used to destabilize particle to form larger and settable solids. (Indian Institute

of Technology Kanpur web courses, 2013)

Several factors affect the separation of settleable particles from water. These include:

� Particle size

The size as well as type of particles to be separated affect the efficiency of a settling

tank. Sand and silt can be easily removed owing to their density. The velocity of the

water-flow channel can be slowed can be slowed down to less than one foot per

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second, and most of the gravel and grit will be removed by simple gravitational

forces. However, colloidal material (particles that remain suspended in the water and

make it appear cloudy) will not settle until the material is coagulated and flocculated

by the addition of a chemical, such as iron or aluminium sulphate. The shape of a

particle also affects its settling since for example, it is easier for a round particle to

settle compared to an irregularly shaped particle.

� Water temperature

Once temperature of the water being treated decreases so does the rate of settling. As

a result detention time increases as the water cools down. As the temperature

decreases the plant operator must alter the coagulant dosage so as to compensate for

the decreased settling rate. A wastewater treatment plant has the highest flow in the

summer when temperatures are considerably high and settling rates faster. During the

cold season when water temperatures have dropped, settling velocity is slow and

detention time is increased so as to allow flocs and particles settle.

� Currents

Some of the water currents that occur in a sedimentation basin are density currents

and eddy currents (produced water flowing into and out of tank). Currents are

advantageous in that they assist in flocculation. However currents tend to distribute

particles all over, increasing the rate of floc settling as a result. Such a problem can be

solved by introduction of baffles so as to avoid short circuiting in the tank.

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2.10 CHLORINATION

Water chlorination is the addition of chlorine (Cl2) to water to render it safe for human use.

Chlorine further protects human beings using chlorinated water from waterborne diseases.

Pathogens commonly found in wastewater effluents are E. coli, Streptococcus, Salmonella,

Shigella, mycobacterium, Pseudomonas aeroginosa, Giardia lamblia and enteroviruses.

Tacnia, Ascaris and hookworm ova may be present in raw sewage. All of these

microorganisms can make people sick. Disinfection to remove or inactivate microorganisms

is the most important step in wastewater treatment to prevent downstream users from

contracting waterborne infectious diseases caused by microbes traditionally present in

wastewater. Chlorination plays a key role in the wastewater treatment process by removing

pathogens and other physical and chemical impurities. (Wastewater Treatment Plant Operator

Certification Manual, 2011)

Chlorine's important benefits to wastewater treatment are:

• Disinfection

• Controlling odor and preventing septicity

• Aiding scum and grease removal

• Controlling activated sludge bulking

• Controlling foaming and filter flies

• Stabilizing waste activated sludge prior to disposal

• Foul air scrubbing

• Destroying cyanides and phenols

• Ammonia removal

• Chlorine is obtained from common salt (NaCl). It is a gas at atmospheric pressures

but liquefies under pressure and the liquefied gas is transported and used as such.

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2.11 ULTRAVIOLET TREATMENT

UV is Ultraviolet Radiation, an energy band within the electromagnetic energy spectrum. Its

wavelength is between that of visible light and x-rays and it has been found to be an effective

method for destroying germs in a water supply. Different germs can tolerate different

amounts of UV light and therefore require varying amounts of UV energy to be destroyed. By

definition, dosage is the intensity of UV light multiplied by time. The intensity is the amount

of UV energy that the UV lamp produces at a certain distance from its surface per square

centimeter of the lamp’s area. The time is the period it actually takes the water to travel inside

the UV chamber.

UV destroys germs by causing a molecular change in their DNA makeup that prevents them

from multiplying and destroys the ability to spread disease. When germs cannot multiply,

they are considered dead.

UV has many advantages over other disinfection processes:

• UV is effective and quick. No need for holding tanks and reaction times. No need for

storing chemicals.

• UV does not alter the taste of water, which makes it ideal for use in bottling plants

and food processing applications.

• UV is safe. NO need to add or handle hazardous chemicals or risk polluting the

environment.

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• UV is compatible with all other water treatment processes. No need for de-

chlorination if using RO systems. In fact, UV enhances the use of other water

treatment by keeping them free from germs.

• UV is economical. Almost always, the cost of UV disinfection units is much less than

the cost of chemical treatment systems. The cost of service and maintenance of UV

units is very low. The electrical running cost of an UV unit in a house is about that of

a regular light bulb.

• UV is more effective against viruses than chlorine.

• Easy installation. UV units are very easy to install and require very little space.

Limitations OF UV Water Treatment

Excessive bacteria counts in water may require additional UV dosages or chemical treatment.

Other factors, such as water temperature, should also be considered. UV units are normally

designed to operate best between two and forty degree Celsius. Freezing will cause damage

to the UV unit and water temperature higher than forty degree C will cause a reduction in UV

energy and therefore, a reduced germ kill ratio. (Wastewater Treatment Plant Operator

Certification Manual, 2011)

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2.12 PH ADJUSTMENT

pH is the measure of free hydrogen activity in water. In more practical terms pH is the

measure of how acidic or basic a substance is. Measured on a scale of 0-14, solutions with a

pH of less than 7 are considered acids while those with a pH of 7 or more are bases. In any

pH adjustment system, bases are used to neutralize acids while acids are used to neutralize

caustics; the term caustic and base, although not truly synonymous, are often exchangeable.

The byproducts are normally salts and water.

In the initial step of all kinds of wastewater treatment, pH adjustment is used to achieve the

desired set point. After any necessary pH adjustment, addition of flocculants and mixing of

flocculants is utilized to precipitate liquid/solid separation. These flocculants will envelop

most suspended solids reducing them to non-hazardous waste.

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3.0 THEORETICAL FRAMEWORK

To achieve the set objectives several theories had to be utilised according to the procedure to

had to be followed to come up with the design work for the recycling system.

3.1 SLOW SAND FILTER

A slow sand filter is simple in operation, design and construction. It is made up of a bed of

sand supported by a layer of gravel, confined within a box with appurtenances to deliver and

remove water. Within the gravel support, and on the floor of the box, are underdrains to

remove the filtered.

3.1.1 Flow Rate

Flow rate should be between 0.1 and 0.4 metres per hour ideally for best results. We use flow

rate per unit area per unit of time because we are interested in quantifying how fast the water

flows past a given level in a given area. For example; 20 litres per hour from a 25 cm

diameter container does not allow the same sand particle contact time as 20 litres per hour

from a 75 cm diameter container, and sand particle contact time is critical for allowing

maximum purification. This is how to figure the flow rate by just measuring how many litres

flow per hour:

Flow rate in meters per 1 hour = [(litres per 1hr) ÷1000]÷(area of sand bed surface in square

meters)

3.1.2 Detention Time – Hydraulic Retention Time, HRT

� = � �� hrs

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3.1.3 Head Loss

This is caused by flow through the schmutzdecke and the sand bed. As the filter is operated,

the schmutzdecke develops and its hydraulic resistance increases causing most of the

headloss. Clean-bed headloss is about 10cm, but the level depends upon the following

factors:

i. Hydraulic loading rate

ii. Temperature

iii. Sand bed media characteristics

Darcy’s Law integrates these variables into an equation and can be applied to flow in the

laminar range through any porous medium, including the schmutzdecke.

1) Darcy’s Law. Headloss within any porous medium is described by Darcy’s law

Figure 2

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� = −� ℎ� ��

Where V= superficial velocity, also called hydraulic loading rate (m/hr)

ℎ�= headloss available across the filter bed from headwater to tailwater (m)

z= flow distance through porous media (m)

k= hydraulic conductivity of porous media (m/hr)

dh/dz= hydraulic gradient, loss of head/unit length of flow (m/)

V is synonymous with Hydraulic Loading Rate, HLR:

�� = ��

HLR=hydraulic loading rate, defined as flow divided by plan area of sand bed (m3/m2/hr

or mgad)

Q= flow of water (m3/s or ft3/s)

A=plan area of filter bed (m2)

For a homogenous porous media, Darcy’s law is usually expressed in finite terms:

� = −� ℎ� ∆��

In which ℎ� = headloss across bed (m)

∆�=depth of filter bed (m)

The headloss ℎ� determines the superficial velocity, v, through the sand bed. Meaning for

given sand and a given headloss, v is determined by the equation above.

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2) Temperature Effect. The following equation expresses Darcy’s equation in terms of

intrinsic hydraulic conductivity, k’:

� = �′� × ��∆� In which k’=intrinsic hydraulic conductivity (N/m)

�=dynamic viscosity of water at a given temperature (N-s/m2)

This equation shows that the hydraulic conductivity, k, of a sand is a function of the

dynamic viscosity, µ, a fluid property and the intrinsic hydraulic conductivity, k’, a

porous media property.

3) Intrinsic Hydraulic Conductivity. The intrinsic hydraulic conductivity, k’, of a

clean sand bed is a function of the sand size, the sand distribution, and the aggregation

of the sand. The hydraulic conductivity cannot be predicted, it can only be measured

by tests. The table below gives hydraulic conductivity data from a laboratory test,

from several pilot filters (30.54cm in diameter), and from several full-scale slow sand.

When the intrinsic hydraulic conductivity, k’, is measured the viscosity factor is

removed from concern, permitting evaluation in terms of only a porous media

characteristic. The k’ values may be used to monitor whether bed clogging is taking

place and to determine whether a sand being considered for an installation has an

intrinsic hydraulic conductivity that falls within an expected range.

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Hydraulic Conductivities for Slow Sand Filters

Installation �� UC Testing

Method

k’ (N/m) K (25º C)

(m/hr)

Empire c 0.21 2.67 Full-scale 6.6× 10 ! 2.65

Empire “ “ Lab Column “ “

100 Mile

House

0.25 3.5 Full-scale 5.05× 10 !

2.04

CSU pilot

plants

Phase I

0.27 1.63 Pilot plants 4.6 × 10 ! 1.46

Phase II 0.29 1.53 Pilot plants 8.9 × 10 ! 3.56

Phase III 0.13 1.60 Pilot plants 2.5 × 10 ! 1.01

CU pilot

plants

0.22 2.50 Pilot plants 6.78× 10 !

2.74

Table 2: Hydraulic conductivities of different slow sand filters

The most important factor to note is that coarse sand does not filter as well as fine sand; and

fine sand offers more resistance to the flow of water than coarse sand. 0.15-0.35 is the ideal

effective size with a uniformity coefficient of less than 2. However all the sand in a layer

should be the same effective size. The smaller effective size should be in the top 30 or 40 cm

layer of sand. The gravel on the bottom should be large enough not to pass through the holes

in the drain pipes and small enough to prevent sand from seeping into them. The drain pipes

should be covered by at least 6 inches (15.24cm) of gravel. Generally, the smaller the

effective size of the sand used the better. If the effective size is too small however, water will

not flow through the filter fast enough. If the effective size is too large the biolayer will not

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form effectively and the filter will not purify water. (Manual of Design for Slow Sand

Filtration chapter 3, 1991)

3.2 CHLORINATION

The chlorine demand of a water sample is defined as the difference between the concentration

of chlorine added to the sample and the concentration of the chlorine residual remaining at

the end of a predetermined contact time.

As a strong oxidizing agent, chlorine kills via the oxidation of organic molecules. Chlorine

and its hydrolysis product, hypochlorous acid are neutrally charged and therefore easily

penetrate the negatively charged surface of pathogens. It is able to disintegrate the lipids that

compose the cell wall and react with intracellular enzymes and proteins, making them non-

functional. Microorganisms then either die or are no longer able to multiply.

When dissolved in water, chlorine converts to an equilibrium mixture of chlorine,

hypochlorous acid (HOCl), and hydrochloric acid (HCl):

Cl2 + H2O→HOCl + HCl

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Table 3 (Source: Wolfe et al. 1984)

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3.3 COAGULATION

Coagulation - the addition of a coagulant followed by rapid mixing resulting in the

destabilization of colloidal and fine suspended solids, initial aggregation of destabilized

solids.

Factors affecting coagulation

Raw water source - changes in water quality such as turbidity and suspended

solids

- need adjustment in chemical dosage rates and chemicals used

Temperature - low temperature affects the efficiency of coagulation process

Suspended solids - the nature of suspended solids affects the coagulation

efficiency

Alkalinity - changes the pH of the water and eventually affects coagulation and

precipitation

Colloidal Stability

Colloids - do not agglomerate naturally,

- are stable and are too small to settle out in a reasonable time, also called as stable

sols,

- have large surface area to volume ratio

- are usually negatively charged

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Examples of settling velocities

Particles dia. (mm) Size typical of Settling velocity (m/s)

1 coarse sand 0.23

0.01 silt 1 x 10-4

0.0001 large colloid 1 x 10-8

Table 4

Colloids are usually negatively charged and surrounded by ions of opposite charge

Design of Mix Tanks

Rapid mix requirements:

- need to rapidly disperse the coagulant

- Detention time usually less than 1 min but can be as high as 2 minutes

- High turbulence

- A measure of the extent of shearing and mixing within a tank is given by G – velocity

gradient

G can be visualized as being the measure of the relative velocity of 2 particles at a given

distance apart for instance, two particles moving 1 m/s relative to each other and at a distance

0.1 m apart will have a G of 1/0.1 = 10 s-1

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A general equation which relates the power input needed for mixing:

2/1

V

PG

µ=

Where G = velocity gradient (s-1)

P = power input (watts = Nm/s)

V = volume of mixing basin (m3)

µ = viscosity (Ns/m2)

Values for G and td

___________________________________________________________________

td (s) 20 30 40 >40

G (s-1) 1,000 900 790 700

____________________________________________________________________

Table 5

Use of mechanical mixers

-Impeller driven mixers are the most efficient devices

-Relationship relating power input and the diameter of impellers and speed is given by:

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P = φ ρ N3 D5

Valid for turbulence mixing with Re = ρ ND2/µ > 10,000

Where P = Power (Nm/s)

φ = power number or impeller constant

ρ = density of the liquid (kg/m3)

N = rotational speed (rev/s)

D = diameter of impeller

Points to note – impeller diameter should be within 30 to 50% of the width of the tank, if not

will move the water and will not get efficient mixing.

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3.4 SEDIMENTATION

Sedimentation is the process by which suspended particles settle by gravity. If the particle is

falling in the viscous fluid under its own weight due to gravity, then a terminal velocity, or

settling velocity, is reached when this frictional force combined with the buoyant force

exactly balances the gravitational force. The resulting terminal velocity (settling velocity) is

given by Stokes equation:

�* = + ,-./-0123+4�

Where:

vs=settling velocity (m/s)

(vertically downwards if ρp > ρf, upwards if ρp < ρf)

g = gravitational acceleration (m/s2)

ρp= mass density of the particles (kg/m3)

ρf =mass density of the fluid (kg/m3)

R=particle diameter

Retention time is given by:

5 = �� = �6 × 7�

Weir loading rate or weir overflow rate is the quantity of water flowing over a unit weir

length of the tank in a day. Given by:

8�� = 9:;<=�5>(@A )5;5�:<>C=:>DE5ℎ(@)

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Rectangular basins are more suited for large scale wastewater treatment compared to circular

basins. This is because rectangular basins tend to have:

∗ Low maintenance cost

∗ High tolerance to shock overload

∗ Predictable performance

∗ Cost effectiveness

∗ Reduced risk of short circuiting

3.4.1 Design Parameters

Rectangular Circular Upward flow

Settling velocity(mm/s) 0.1-0.5 ________ _______

Horizontal velocity(m/hr) 14-15

Surface loading(@A/@G�H)

10-50 10-45 28-43

Retention time(hrs) 1.5-4 1.5-4 2-3

Outflow weir loading

(@A/@ day)

100-450 100-450 100-450

Average depth(m) 1.75-3.0 1.5-2.5 4.25-7.75

Plan dimensions(m) Up to 100m long;

length: width 4:1 to

5:1

3.3-30 diameter 5-9 square

Base slope 1:25 to 1:100 1:6 to 1:8 1:1 to 1:2

Table 6

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4.0 PRELIMINARY DESIGN SKETCHES

SKETCH OF THE DESIGN PROCESS

Figure 3

COAGULATION

SEDIMENTATION

SLOW SAND FILTER

CHLORINATION

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5.0 METHODOLOGY

5.1 SITE SELECTION

Design of the wastewater recycling system begun by identifying a suitable location for

construction. Factors considered were topography of the area, elevation, its proximity from

the treatment plant and area of land available. Assessing the land makes it possible to identify

where specific units of the system will be located and how water could flow by means of

gravity without having to pump it and increase cost of production as a result. The highlighted

area pointed in figure 5 is the segment of the land that is suitable for handling the structures

of the treatment system.

5.3 LABORATORY ANALYSIS OF WASTEWATER

Before commencing the treatment process quality of the water is to be known, this was done

through laboratory examinations. Some of the parameters to be measured are:

- Temperature

- Conductivity

- Dissolved oxygen

- pH

- Total suspended solids (TSS)

- BOD

- COD

- Total dissolved solids (TDS)

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These tests were done at the Kipevu WTTP laboratory under the supervision of the laboratory

technicians. The results for the month of June 2013 were summarized in the table:

Effluent Report for 3rd

June to 30th

June 2013

PARAMETERS

pH TDS DO COD BOD TSS

No of

samples

targeted

4 8 4 4 4 4

No of

samples

tested

20 20 20 2 2 20

No of

samples

that

complied

20 10 16 NONE NONE NONE

%

Compliance

100 50 80 0 0 0

Average 8.1 987.3 3.17 162.5 110.5 131.8

Standards

for effluent

6.5-8.5 1000µs/cm 20 50 30 30

Table 7

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6.0 ANALYSIS AND DESIGN

The recycling system is comprised of the following segments to be synchronized into a single

working unit:

-sedimentation

-coagulation

-slow sand filtration

-chlorination

-storage and supply

6.1 COAGULATION DESIGN

The flow rate is known and a detention time (t) is selected, next is computing:

i. The basin dimensions

ii. Power required

iii. Impeller specifications

• Choosing a design: circular or square.

• Get the values of viscosity and specific weight of the water at the operational

temperature

• Having Q, V, G, t, g and µ

• Plugged into the velocity gradient equation, G and calculating the Power (P) that

needs to be imparted to water. It is the same equation regardless of the type of the

mixer chosen.

i) Basin dimensions

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Flow of 262,000m3/day distributed in 3 compartments with a detention

time of 40minutes. 3 compartments are used in series with decreasing

values of G (G1=50/s, G2=20/s and G3=10/s) for the third, second and

first compartments respectively. A minimum of three compartments are

used in order to minimize short circuiting and to facilitate tapered

flocculation which is also facilitated by decreasing values of G.

�I>=�E>J = K�LG�LA�A =26.7/s

In order for Detention time to be correct 50,000 ≤ J ∗ � ≤ 100,00

J� = 26.7 × 40 × 60P = 64,080

Detention time is there satisfactory.

Basin Volume, V=Flow*Detention time

26200024 × 3600 × 40@CD = 121.3@A

Depth of the entire basin is 15m, therefore top area is

= 121.3 15 = 8.1@G�

Square compartments with length and width of L each:

3� × � = 8.1@G � = RS.�A = 1.64@

L=W=1.64m

Total length of basin=3*L=3*1.64=4.92

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Total volume of basin, V=(4.92*1.64*15)=121.032 cubic metres

ii) Power required

J = T U� × I

Making P the subject of the formula above to get value of power:

U = JG��

µ=0.001(Ns/m2)

V=121.032 (m3)

For the first compartment (G=50), P= (502*121.032*0.001)=302.58W

For the second compartment (G=20), P= (202*121.032*0.001) = 48.41W

For the third compartment (G=10), P= (102*121.032*0.001) = 12.10W

iii) Impeller specifications

The tank has two baffles, 6 impeller blades and the turbine has an operating speed of 100rpm.

Impeller diameter is given by:

P = φ ρ N3 D5

5/1

3N

P D

φρ=

= R(A�G.KSLVS.V�L�G.��)�.WA×XXX.!×Y�� W�� Z[\ = 0.66@

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The impeller diameter is 40% of the width of the tank, which lies between the recommended

range of 30%-60% of the tank width.

6.2 SEDIMENTATION TANKS

Assuming the construction of four tanks, the flow will be equally distributed among them:

GWG,��V = 65,5000@A

Using a length to width ratio of 4:1 and a length of 20m, width will be 5m. The recommended

length to width ratio should between 4:1 and 5:1 for sedimentation tanks.

Surface Loading Rate

]�� = � �=>�� = 65,500(20 × 5) = 655@/�H

Retention Time

Depth of each tank is 3.5m,

� = �� = (20 × 5 × 3.5)65500 = 0.00534�H ≈ 8@CD

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Weir Loading Rate

Figure 4

Applying a rectangular weir:

8�� = �:>DE5ℎ;9<>C= = 65,50020 = 3,275@G/�H

Dimensions of each tank are 20m long by 5m wide by 3.5m deep.

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6.3 FILTRATION TANKS

Design of slow sand filter

Design Parameter Metric

1 HLR 0.2 m/hr

2 Filter bed area A=Q/HLR

3 Depth of filter bed

Initial 0.92m

Final 0.70

4 Sand specification

Effective size, d10 0.40 mm

Uniformity Coefficient 2.67

5 Depth of gravel support 0.52 m

6 Drains

Laterals (diameter/area) 1.5m/9.75m

Main A=1/600*area

drained

7 Depth of supernatant watera

1m

8 Headloss permitted 0.5m

Table 8

-Cover is not necessary since Kipevu has a fairly hot climate.

-a supernatant water depth should be sufficient to prevent hydraulic scour.

Filtered water=262,000/24hr=10,917 m3/hr

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Amount of backwater utilized=2% of total volume

Design flow rate=5m/hr

Time lost during backwash=45min

Length to width ratio= 1:1.5 to 1:1.33

6.3.1 The required flow of water

= 10,917 × 102% × 2423.75ℎ= = 11,253@A/ℎ=

Area of filter bed=Q/HLR= 11253/5=2,250.6m2

4 filtering units of equal area: 2250.6/4=562.65m2

Taking filter bed width to be 21m, length=562.65/21=26.8m

The length width ratio is acceptable 1.28 because it is within the range of 1:1.5-1:1.33.

6.3.2 Sand Depth

Depth of sand bed is checked against breakthrough index of flocs. Hudson formula:

`× ab ×c d� = ef × +4b, ++b

Where:

Q=filtration rate (m/hr)

d=sand particle size

H=total head loss (m)

L=depth of sand bed (m)

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Bi=Breakthrough index (ranges between 0.0004 and 0.006)

Taking Bi to be 0.0004, Q=4×5m/hr=20m/hr and d=0.40mm

d = ef × +4b++b` × ab = g. gggh × +4b++b+g × (g. h)b = 4i. jbkl

Sand bed of 92cm depth.

6.3.3 Gravel distribution

The depth, L, of gravel layer of particle size d can be computed using the following equation:

L=2.54×K×log d

(10≤K≤12) taking K to be 11.

Size 2 5 10 20 30 50

Depth (cm) 9.17 21.30 30.48 39.66 45.02 51.78

Table 8

For gravel of size d=50mm depth that corresponds is 51.78 approximately 52cm.

6.3.4 Under drainage system

Plan area for each filter=21×26.8=562.8m2

Total area for perforations=0.003×area of filter

=0.003×562.8=1.688 m2

total area of laterals=number of units × total area of perforations

=4×1.688=6.752 m2

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Area of central manifold=(4-1)× area of lateral

=3×6.752=20.256

Diameter of central manifold=R+g. +mj hn� = i. +ol

A pipe of 1.5m diameter is commercial.

Assuming a spacing of 3m between subsequent laterals

Number of laterals required=26.8×1.5/3=13

Length of each lateral=1/2 width of filter-1/2 diameter of manifold

=21/2-1.5/2=9.75m

6.3.5 Filter Air Wash

Rate at which air is supplied is estimated to be 5m/min

Duration of air wash is 10min

Quantity of air required per unit=(5×10×562.8)=2810m3

Backwash clear volume=backwash duration×filter rise rate×total SA of filters

=(40×10/60×21×562.8)=78,792m3

6.4 CHLORINATION TANK

A chlorine dose of 2.5mg/l conforms to the WHO standards for disinfection and residual

concentration.

Amount of chlorine require in a day=10197×24×2.5×10-3=611.82kg

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6.5 CONTACT TANK

Contact tanks are commonly used to disinfect drinking water prior to distribution. These

tanks are usually open chambers split by a series of baffles. Sub-dividing the chambers helps

to control the flow of water through the tanks and improves the chlorine disinfection process.

To be effective, chlorine disinfection requires a minimum residence time for water to remain

in the tanks. In addition to maximizing the residence time provided by a tank, care must also

be taken to eliminate "dead-spots" where water can remain for many days.

Minimum retention time=30min

Flow=10m3

minimum tank volume=(10×30×60)=18000m3

6.6 STORAGE TANK CAPACITY

The required storage capacity for a reservoir is a summation of balancing, breakdown and fire

reserve. The balancing reserve can be estimated from hourly consumption of water for the

town. This is 20-60litres per hour per capita. With a population of 939,370 the demand per

hour can be computed as:

=30/1000×939,370= 28,181 cubic metres

Assuming a circular tank of height 15.5m inclusive of a free board of 0.5m.

Cross-sectional area of the tank=28,181/15=1,880 square metres

Diameter= R�SS�×Vp = 49 ≅ 50@

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7.0 CONCLUSION AND RECOMMENDATION

This design meets most set standards and criteria for water treatment system designs. The set

objectives of the project are also met within the scope of the project. Designs of the

sedimentation and filter systems cater for the volumes of water being treated as set out to be

achieved.

The Kipevu treatment plant functions for about 9 hours in day due to lack of proper funding

for the high electricity bill. It is recommended that the plant functions for 24hours a day so as

to receive maximum volumes of wastewater and to keep this treatment system running all

through as well. Another recommendation would be to construct a concrete fence to surround

the area so as to intensify security.

Some recommendations that involve the design are use of filter mats and pre-ozonation:

Filter mats placed on top of the surface of the slow sand filters provides longer times and a

simpler cleaning technique than with conventional filters. Filter cleaning only requires the

removal and cleaning of the fabric.

Pre-ozonation may increase organic precursor removals in slow sand filters by increasing the

production of lower-molecular-weight and more biodegradable compounds from large

organic molecules that are more resistant to biodegradation.

Flow measurement considerations should be done at the start of every subprocess in the

system. Each water treatment plant is required to have a flow measuring device capable of

measuring the anticipated flow including variations within accuracy of ten percent (10%).

This can be accomplished by use of an ultrasonic flow measurement. The liquid level is

measured by determining the time required for an acoustic pulse to travel from a transmitter

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to the liquid surface where it is reflected and returned to a receiver. This ensures that all

volumes are accounted for and any leaks detected in time.

Future designs could be established from the storage tank so as to convey the treated water

through a separate pipe network to deliver water to the residents of Mombasa County for

non-potable usage.

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8.0 REFERENCE

Cesar Marron

Slow Sand Filtration Water Treatment Plants Design Operation and Maintenance, 1999

G. L. Karia and R. A. Christian

Wastewater Treatment Concepts and Design Approach 2006 by PHI Learning Private Ltd,

New Delhi

Operation of dual drinking and non-potable water networks in Paris

Water Supply Volume No 8, page 193-200 IWA Publishing 2008

Stover, E.L., Haas, C.N., Rakness, K.L. And Scheible, O.K. (1986).

Design Manual: Municipal Wastewater Disinfection. Cincinnati, OH, US Environmental

Protection Agency.

UN-HABITAT, 2008. Waste Water management Plan for Madhyapur Thimi Municipality

Volume 1 Main Report. UN-HABITAT Water for Asian Cities Programme Nepal,

Kathmandu.

Hand book for Sediment Quality Assessment, S. L. Simpson, GE Chariton, AA Stauber, JL

King, SK Chapman. CSIRO 2005 page 22

Wastewater Treatment Plant Operator Certification Manual, Kentucky Department of

Environmental Protection 2011, page 143-153.

WARMA Service Charter, 2011

WHO Library Cataloguing-in-Publication Data

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WHO/UNICEF Joint Monitoring Programme for Water Supply and Sanitation

World Health Organisation and UNICEF 2006

WPCF Task Force on Wastewater Disinfection (1986). Wastewater Disinfection Manual of

Practice No. FD-I 0. Alexandria, VA, Water Pollution Control Federation

www.dualwan.org/load-balancing.html

http://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_13.pdf

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9.0 APPENDICES

Appendix A (Photographs)

Photograph 1 ......................................................................................................8

Photograph 2.......................................................................................................57

Photograph 3........................................................................................................58

Photograph 4........................................................................................................59

Photograph 5........................................................................................................60

Appendix B (Sketches and figures)

Figure 1.........................................................................................................18

Figure 2.........................................................................................................26

Figure 3.........................................................................................................38

Figure 4.........................................................................................................46

Figure 5.........................................................................................................62

Figure 6.........................................................................................................63

Figure 7.........................................................................................................64

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Appendix C (tables)

Table 1.......................................................................................................15

Table 2.......................................................................................................30

Table 3.......................................................................................................32

Table 4.......................................................................................................34

Table 5......................................................................................................35

Table 6......................................................................................................38

Table 7......................................................................................................41

Table 8......................................................................................................47

Photograph 2: A satellite map of the wastewater treatment plant.

A satellite map of the wastewater treatment plant. Source: Google maps

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Google maps

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Photograph 3: A sedimentation tank full of wastewater. Source: Kipevu WWTP 2013

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Photograph 4: An empty sedimentation tank full. Source: Kipevu WWTP 2013

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Photograph 5: Under-drain system and supporting gravel layers in a slow sand filter

(Municipal Waterworks in Amsterdam)

university of nairobi school of engineering...

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