i

ACKNOWLEDGEMENTS

The experience gained through this industrial training program will be of a great value in

future career. I am fortunate to have my training in Project Engineering Institutional Consultant

Service in the Jaffna-Kilinochchi Water Supply and Sanitation Project in Jaffna. Several people have

provided considerable encouragement and assistance to complete my training successfully. First, a

special acknowledgement is going to the project leader for assigning me as a trainee in their

organization and for giving me extremely valuable advices to do my training. I have a great pleasure to

tell my sincere thanks to the all other staff members of the organization.

I would like to express my sincere gratitude to the Industrial Training Career Guidance Unit of the

Faculty of Engineering and the Head of the Civil Engineering Department, for organizing and

coordinating my industrial program and for having made it a success experience.

Thanks to my training mates who have given me encouragement and support in various ways during

the training period.

ABERATHNE A.S.T

E/11/003

Faculty of Engineering

University of Peradeniya

ii

CONTENTS

Acknowledgements i

Contents ii

List Of Figures iii

List Of Tables iv

List Of Attribute v

Chapter 1 INTRODUCTION

1.1 Project Background 1

1.2 Project Out Line 1

1.3 Project Objective 4

Chapter 2 SEWER SYSTEM DESIGN

2.1 Basic Design Procedure 5

2.2 General Topography 5

2.3 Key Strategy 5

2.4 Design Population 5

2.4 Design The Waste Water Load And Other Factors 7

Chapter 3 SEWER PIPE NETWORK DESIGN

3.1 Software Used 10

3.2 Network Design 10

3.3 Hydraulic Analysis Of The Proposed Sewerage System 10

3.4 Model Set-Up 10

3.5 Sewer Pipe Network 11

Chapter 4 PUMPING STATION AND SUB CATCHMENT

4.1Pump Station Selection Criteria 18

4.2 Submersible Pumping Station 18

4.3 Sub-Catchment Division Of Jaffna 19

4.4 Major PS Sub-Catchments 20

4.5 Medium PS Sub-Catchment 20

4.6 Small PS Sub-Catchment 21

Chapter 5 CHAMBER SURVEY

5.1 Introduction 23

iii

5.2 Inspection Chamber(IC) Marking 23

5.3 IC Survey 24

Chapter 6 ISSUES & RISKS

6.1Shallow Bed-Rock 25

6.2 Ground Water 25

Chapter 7 CONCLUSION 26

iv

LIST OF FIGURES

Figure 1.1 Project Area 2

Figure 1.2 Project Management Frame 3

Figure 2.1 Design Flow Chart 5

Figure 3.1 Proposed Man Hole 15

Figure 3.2 Proposed Inspection Chamber 16

Figure 4.1 Division Of Sub Catchment 20

Figure 4.2 Main Pumping Station 20

Figure 4.3 Medium Pumping Station 21

Figure 4.4 Small Pumping Station 21

Figure 4.5 Main Network 22

Figure 5.1 Location Of The IC Point 24

Figure 5.2 IC Survey Sample Sheet 24

v

LIST OF TABLE

Table 2.1 Catchment No. 2 Sewerage Flow Estimates

Based on Projected GND Populations 8

Table 3.1 Depth of flow in the sewer system 12

Table 3.2 Pipe Roughness Coefficients for Gravity Sewers 13

Table 3.3 Pipe Roughness Coefficients for Pumping Mains 13

Table 3.4 Approximate Self Cleansing Velocities for Conventional Sewers 14

Table 3.5 Maximum Manhole Spacing for conventional sewers 14

Table 4.1 Wet Well Operation Requirement 17

Table 9.1 Recommended Design Parameters for Pump Stations 18

vi

LIST OF ABBREVIATIONS

ABBREVATION DISCRIPTION

JKWSSP Jaffna-Kilinochchi Water Supply and Sanitation Project

WWTP Waste water treatment plant

MC Municipal council

ADB Asian development bank

AFD Agence Fancies development

GOSL Government of Sri Lanka

NWSDB National water supply and drainage board

GND Grama Niladari division

CW Colebrook White

GRP Glass-reinforced-plastic

IC Inspection chamber

HDPE High Density Poly Ethylene

GIS Geographic Information System

PWWF Peak Wet Weather Flow

DSS Desired Standards of Services

GRP Glass Reinforced Pipe

1

Chapter 1 INTRODUCTION

I was directed by National Apprentice and Industrial Training Authority (NAITA) to gain a practical

knowledge in Engineering during my training period 20.10.2014-06.01.2015. I am assigned as a

Trainee Engineer in Project Engineering Institutional Consultant Service and I have been working in

the Jaffna-Kilinochchi Water Supply and Sanitation Project in Jaffna.

1.1 Project Background

There was no sewer system in Jaffna. Onsite wastewater systems was being used almost

exclusively in Jaffna. Sanitation facilities in Jaffna still include consists primarily of flush toilets with

septic tanks and vertical soakage ways. The only exception was piped sanitation systems at the Jaffna

Teaching Hospital in Jaffna. Otherwise, septic tank soil absorption system was the most common type

of wastewater system. Properties within the project area were served by septic tanks, the locations of

which were determined by a house connection survey. These septic tanks did not function adequately

due to the high ground water table prevalent in this area.

So the Jaffna-Kilinochchi Water Supply and Sanitation Project (JKWSSP) included for the design and

construction of a sewerage network, for a portion of the area within the Jaffna MC City center, with a

corresponding sewage treatment plant to be sited approximately 3km west of Jaffna MC. The sewage

treatment plant has been designed for an estimated capacity of 13,300 m3/day average flow. The

projected estimate for the year 2037 population for the project area was 80,000 persons. The estimated

waste water treatment plant (WWTP) capacity corresponds to 120,000 equivalent population, allowing

for all residential, commercial and non-residential population flows being accounted for, plus

infiltration. The sewerage works are proposed for phased implementation between Catchment No 1

and 2, mainly I involved with catchment two in figure 1.1 for IC marking and IC survey and when I

was there project was in design stage.

1.2 Project Out Line

Project Jaffna Kilinochchi water supply and sanitation project

Funded by Asian development bank (ADB)

Agence Fancise development (AFD)

Government of Sri Lanka (GOSL)

Project output improving water supply sewerage and sanitation infrastructure

Strengthening Jaffna water resource management

Building capacity of the national water supply and drainage board (NWSDB)

and improve the operational and maintenance capacity of the northern regional

support center

Project cost 267.93$ million

2

Project implement by National water supply and drainage board

(NWSDB)

Consultancy by Grontmij A/S Denmark in association with HIFAB international(Sweden),

Greentech consultance(Pvt), Lanka Hydraulic institute (LTD)and total

Management solutions company(PVT)

Fig 1.1: Project Area

3

microbiologist

Institution development specialist

Social/ community mobility officer

Team leader

Deputy team leader (Water)

Legal officer

Deputy team leader

(Sewerage)

Ground water modeler

Water supply Engineer

Resident Engineer

Water network design modeling Engineer

Resettlement specialist

Architect QS IT officer Electrical-

mechanical Engineer

Water resource Engineer

Structural Engineer

Water supply Engineer

Water treatment Engineer

QS IT

office

Accounting finance specialist

Resident engineer

Environment specialist

Sewerage treatment specialist

Sewerage network engineer

Elect-Mechanical Engineer

QS Architect Training

Engineers IT officer

Fig 1.2 Project Management Frame

4

1.3 Project Objective

The purpose of this project was to produce efficient design for the implementation of the preferred

engineering solution, which is technically sound, cost effective, safe, constructible and maintainable.

The principal objectives of the design was as follows:

Design the new sewerage serving the Jaffna study area to discharge into the Kalundai WWTP.

Allow in the design for connection of future developments, if any.

Design the sewers to have some spare capacity to carry the maximum amount of 13,000 m3 per

day.

Design the sewers in such manner that a self-cleansing velocity would be achieved even if the

maximum amount of sewerage per day will not be achieved.

Design a pumping station in such manner that it could be used in future without any major

modifications.

5

Chapter 2 SEWER SYSTEM DESIGN

2.1 Basic Design Procedure

Fig 2.1 Design Flow Chart

Basic map of the catchment

area

Digitizing households, roads

other important feature of the

catchment area in GIS

Existing topography data

from survey department

Improved GIS map with

feature attributes

Population survey (estimate

to 2037)

Identification of sub

catchment , pumping

station ect.

Evaluation of the pumping

station locations, alternatives

and sociology and EIA evaluation

Conceptual sewer gems

model

Identification of IC

(inspection chamber)

for household

Topography survey for

household features and

IC

Improving model adjusting

sewer network

Optimizing network

(depth of cut satisfying

various constrains) for

individual sub catchment

Finalizing

pumping station

location

Improving GIS maps

Identification of missing

by roads (walk through

survey)

Topography survey

for the missing roads

and by lands

6

2.2 General Topography

The Jaffna Project Area was generally flat, that was, there was no obvious hills and valleys but rather

mounds and depressions which were not immediately obvious. For this reason, the natural policy of

gravitating flows to low points and then pumping to treatment was identified. In the absence of an

obvious solution such as would be the case in critical areas.

2.3 Key Strategy

The Strategy for the Project Area was” To provide effective (i.e. trouble free) sewerage scheme with

the minimum number of Pumping Stations thereby keeping operation and maintenance costs to a

minimum”.

The following key factors will be followed in preparing the sewerage design:

Evaluating the topography

Reviewing all existing and proposed services

Population and associated flow generation

Establishment of design criteria for sewer and pumping stations

Verifying catchment areas and establishing effects of any adjacent catchments and,

Preparing and evaluating appropriate sewerage options for approval and design

development

2.4 Design Population

A household capacity of 7.8 persons per current household was applied. This was larger than typical

person per household rates, in order to reflect the projected increase in population from 2011 and up to

the 2037 target year, due to population growth through e.g. subdivision of lots, increase in apartment

buildings, etc., in the future. The projected increase from 2011 to 2037 was estimated at 48%, and

thus the value of 7.8 reflected a current household capacity of 5.27 persons. After all residential

information is input to the model, plus buildings with large residential populations, the household

average capacity of 7.8 was finally adjusted. This adjustment was made such that the total populations

compare favourably against the projected 2037 domestic population estimated from the Supplementary

Sewerage Concept Design Report (which were based on projected actual 2011 GND populations).

7

2.4 Design The Waste Water Load And Other Factors

2.4.1 Sewerage Flow Estimates Based On GIS Survey Information

Sewer-GEMS hydraulic model incorporating all residential lots and non-residential inputs from GIS

information, with detailed populations and calculated sewage flows.

There was expected to be approximately 5,000 residential and non-residential lots. This data needed to

be input to the GIS-based model. Information required includes for details of houses, multi-story

residential apartment occupancy and commercial buildings’ employees, schools and students, hospitals

and beds, and other commercial and government institutions.

The hydraulic model summation results for domestic populations was compared against the projected

population estimates from GND’s , which were based on actual 2011 GND populations projected to

2037. This was carried out by adjusting the estimated criteria for average number of persons per

household within the model, as described below.

The sewerage model flows was based on the following criteria:

All domestic flows to be based on actual house-lots itemised from GIS information

Per capita water-use of 120 l/c/d (average day) with;

Sewerage capture rate of 80%, giving a generated sewage flow of 96 l/c/d;

Non-domestic flows input to the model via actual institutions, will be assigned water-use

criteria as follows:

The student University and Campus to be assigned a peak flow of approx. 51.5 l/sec;

Schools at 30 l/student/Ave day, including infiltration and allowance;

Commercial employees at 96 l/employee/day average day sewerage capture;

Hospitals, 250 l/hospital bed/day sewerage capture including allowance.

Buildings need to be identified through the GIS and be assigned population information to be

input to the model, such as commercial and residential buildings, churches, schools, hospitals,

and any other.

Peak flows are calculated using a Peak factor of 4 times the average flow;

Infiltration is applied at 15% of peak dry weather flow.

The Catchment No.2 main pump station capacity was expected to be approximately 245 l/sec.

8

Pumping main lengths and sizes were revised, and were confirmed by running the sewerage model.

Similarly, trunk sewer and gravity sewer sizes and lengths were revised, and confirmed following the

detailed inputs of the redirected sewers and pump station locations into the hydraulic model.

2.4.2 Sewerage Flow Estimates Based on Projected GND Populations

The sewerage flow estimates based on the populations projected to 2037 from each GND population

are shown in Table 2.1. An estimate of the University sewage flows was also included by using an

approximate 8,064 equivalent campus population. Non-domestic flows were not identified to date from

the GIS and not allocated to each individual GND. Flow estimates will increase after this information

is received, but for the calculation purpose, an estimate of 25% of domestic flow has been applied at

this time.

Table 2.1: Catchment No. 2 Sewerage Flow Estimates Based on Projected GND Populations

Ward (GND) Population Peak

Domestic

Flow (as

m3/day)

Non-

Domestic

No.

Peak

Non-

Domestic

Flow (as

m3/day)

Total

Peak

Sewerage

Flow (as

m3/day)

Total Peak

Sewerage

Flow (l/sec)

Navanthurai South 2,759 1,218 25% 305 1,523 17.63

Navanthurai North 699 309 25% 77 386 4.47

Moor Street North 1,877 829 25% 207 1,036 11.99

Moor Street South 2,274 1,004 25% 251 1,255 14.53

Iyanar Kovilady 2,553 1,127 25% 282 1,409 16.31

Vannarpannai

North 2,964 1,309

25% 327 1,636 18.94

Vannarpannai NW 3,908 1,726 25% 431 2,157 24.97

Vannarpannai NE 3,301 1,458 25% 364 1,822 21.09

Neeraviyady 2,207 975 25% 244 1,218 14.10

Kantharmadam

NW 2,650 1,170

25% 293 1,463 16.93

Kantharmadam NE 437 193 25% 48 241 2.79

Kantharmadam SW 764 337 25% 84 422 4.88

University 8,064 3,561 25% 890 4,451 51.5

9

2.4.3 Infiltration and Inflow

Based on the Feasibility Report produced by SMEC in March 2006 and Inception Report by Grontmij

in March 2013, 15% of peak flow is considered as an infiltration.

2.4.4 Trade/Industrial Flows

Industrial/Trade flows were not included and were assumed to be the responsibility of the waste

generators themselves. The proposed sewerage system was purely of municipal nature.

2.4.5 Peaking Factors

Peaking factors accounted for variations in the flow throughout the day. The flow estimated represents

average daily flow and at different times of the day the actual flow in the sewer may be significantly

higher or lower than the average flow. The designer was concerned with peak flows for two reasons:

• To ensure that the system can convey flows without a significant backup in the sewerage system,

• To ensure that self-cleansing velocities (conventional sewers) are achieved at least once per day to

prevent deposition of solids.

Peak Factor (PF) = 4.7 x p-0.11 as provided by NWSDB

Where P was the population in 1000’s subject to PF > 3.The peaking factor was applied to the average

daily flow (DWF), to determine the peak sewage flow.

Hospitals* 0 0 * * * *

Schools* 0 0 * * * *

Commercial* 0 0 * * * *

TOTAL 34,457 15,216 * 3,804 * 19,020 * 220

10

Chapter 3 SEWER PIPE NETWORK DESIGN

3.1 Software Used

SewerGEMS software package was used to build the network model of the Jaffna sewerage

system. SewerGEMS is a hydraulic modelling and data handling software package. This software

is considered suitable for modelling and design of sewerage networks and complex ancillaries. It

allows the modelling of sewerage systems, analysis of the results permitting identification of

potential problem areas in the sewerage system and the design of sewerage system.

ArcGIS version 10.1 and MS Excel 2000 were used for data manipulation, analysis and

presentation. Wastewater Planning Users Group, Code of Practice (WPUG CoP) for the Hydraulic

Modelling of Sewer Systems (2nd Edition) standards have been adopted in construction of the

network model.

3.2 Network Design

Network design was carried out utilizing the collected population data from the planned surveys. Network

design was a joint exercise between the team .The network model run with the design constraint parameters

such as depth, velocity and slope constraints.

The model designed system carefully examined and based on the depth restriction, the pumping station

locations were determined. In our training time the model was run with steady state parameters. A detailed

Extended Period Analysis will be performed at later stage.

3.3 Hydraulic Analysis Of The Proposed Sewerage System

The network model was build and tested for population projections for year 2038. The wastewater

collection system always started at house with the kitchen, bathroom, laundry and toilet plumbing and

include the house connection to the branch and main trunk sewer through manhole system.

A complete gravity system in Jaffna was not possible so combination of gravity and pumping station was

considered. Pumping stations was proposed to convey the sewage flows to a higher point from there it can

gravitate to lower point where the next pumping station was proposed.

It was proposed to connect the grey-water and sewage from toilets through the house connection sewer,

branch sewer to the main trunk sewer. Since, there was no sufficient information available on the house

connection levels, only branch and trunk sewer connections were considered and after finishing the IC

marking, IC survey will be started to take the levels.

3.4 Model Set-Up

A detailed SewerGEMS model was developed incorporating all the proposed sewerage system

assets and topographical features.

General assumptions used in the interpretation of model build data include the following:

11

The outgoing pipe from a manhole is always lower than or equal to the level of the incoming

pipe

the pipe gradient is assumed to be constant i.e. an average gradient is used between upstream

and downstream nodes

Pipe diameters and shapes do not change between manholes

Pipe diameters increase down the system

Where possible, the interpolation of missing data was carried out by manual calculation. It was

necessary to emphasize that any hydraulic computer model output accuracy dependent on the

data used in its construction.

o The base model was built to cater for flows that arise from projected population for year

2028.

The model was run at PWWF with proposed pump stations modelled using fixed pumps.

Gravity mains were sized in accordance with the DSS and taken into account the topography of

the proposed routes as much as possible.

Rising mains were sized based on the minimum and maximum velocities specified in the DSS.

Consideration will also be given to the total head that could be achieved by a typical pump,

which is assumed to be 60 m.

The optimal staging of pump capacities was generally considered to be at 20 year intervals,

based on the typical pump and switchboard asset life. Gravity mains were sized to cater for

ultimate development and was not be staged due to the relatively high construction cost of

gravity mains compared to rising mains (even of the same diameter), which reduces the

financial benefit of staging.

In order to perform a decent design using the sewer model, the following data is required:

Population projections for the design horizon;

Ground levels or spot-heights along the proposed sewer route;

Proposed house-connection coverage;

Soil and ground water table data;

And to describe pumping stations in the model, the following data is required:

Proposed pumping station catchment

Location of pumping station and land availability

Groundwater condition;

12

3.5 Sewer Pipe Network

3.5.1 Depth of Flow

All sewers was designed to flow at a maximum depth ratio to the pipe inside diameter at peak

discharge (QP) per the following table.

Table 3.1: Depth of flow in the sewer system

Pipe Size (mm) Ratio d/D

200 0.50

300 0.50

400 0.60

500 0.60

600 0.60

800 0.70

1000 0.75

1500 0.75

3.5.2 Minimum Sewer Size

The minimum pipe diameter for public gravity sewer was 150 mm (as agreed with NWSDB) to

comply with the current local practice. Lateral pipes or house connections minimum diameter was of

100 mm.

3.5.3 Pipe Gradients

Pipe gradients for head lengths of conventional sewers were not less than 1 in 80, although this was

sometimes relaxed to 1 in 100 where connection of properties would not otherwise be possible.

Gradients of downstream lengths were then flattened to 1 in 100 as additional properties were

connected.

Once sufficient properties have been connected to allow meaningful hydraulic calculations to be

undertaken, pipe gradients were calculated to ensure adequate capacity and velocity in the pipe.

Gradients were steepened in some cases to minimise the number of ramp/backdrop connections, or

where the difference in level was insufficient to construct a ramp.

The minimum gradient for property connections was generally be 1 in 60.

13

3.5.4 Sewer Layout

The following was adopted in the design and layout of the sewers.

Sewers were designed to serve all existing property plots for the proposed land-uses. As per

international standard requirements, the trunk sewer diameters were not be less than 150mm

and for small bore sewers the minimum diameter is 100mm.

To avoid future conflict during any future maintenance (repair operations), minimum clearance

between all proposed pipelines, inspection chambers/manholes, rising mains and existing

utilities were 0.5 metres or as specified by the respective utility authority.

The Colebrook-White formula was used for the flow analysis. A global Colebrook White (CW)

pipe roughness coefficient of 0.6mm was established. The following design roughness

coefficients were used for gravity sewer and pumping mains as shown in below Tables.

Table 3.2: Pipe Roughness Coefficients for Gravity Sewers

Pipe Material Roughness

Coefficient

Normal 0.6

VC 1.5

DI 0.3

Table 3.3: Pipe Roughness Coefficients for Pumping Mains

Depth of the sewer was determined by hydraulic consideration. A minimum depth of

cover of 1.2m for conventional sewers.

Sewer pipes were PVC up to 300mm pipe and pipes between 375 - 600mm were

HDPE. Any pipes above 600mm were concrete pipe. The pipes were laid and protected

by a granular bed and surround. Where pipes were laid in more onerous conditions,

such as at shallow depth with a heavy traffic loading, a concrete bed and surround or

Velocities Roughness

Coefficient

Up to 1 m/s 0.3

Between 1.1 m/s and 1.5 m/s 0.15

Over 1.5 m/s 0.06

14

other means of support were used. When pipes were laid below the ground water level,

the granular surround were protected by a geo-textile membrane.

There were no special provision for sewer ventilation incorporated in the network

design. It was expected that sewers will be vented through the vent pipes at the

dwelling.

The hydraulic design of major and small-bore sewers were based upon charts for the Hydraulic Design

of Channels and Pipes using the appropriate roughness coefficient and a minimum design actual

velocity of 0.6 m/s in conventional sewers and 0.3 m/s for small-bore sewers at peak flow. Below

tables shows the minimum velocities and gradients for pipelines to give a self-cleansing velocity at

least once per day:

Table 3.4: Approximate Self Cleansing Velocities for Conventional Sewers

Pipe Diameter (mm) Velocity (m/s)

200-300 0.60

400 0.77

500 0.82

600 0.86

800 0.88

Maximum flow depth (d/D) 0.50

Maximum flow velocity (m/s) 2.5

3.5.6 Manholes

A manhole were proposed for each of the following cases:

Change of alignment

Change of slope

At each house connection to the main line

Change of pipe diameter

Change of pipe material

At the beginning of each sewer

Manhole spacing as in table 3.5 did not generally exceed 100m except on large diameter sewers (over 600mm),

and particularly those to be installed by micro-tunnelling, where manhole spacing up to 120m had been

considered. House connections were made to the line via laterals and in some cases to the manholes.

15

Table 3.5: Maximum Manhole Spacing for conventional sewers

Manholes on sewers were located at all junctions, changes in direction and gradient and at spacing to

suit the house connections. Manhole covers were provided with glass-reinforced-plastic (GRP) sealing

plates. Access to manholes was to be by GRP ladders.

Manholes were constructed from in-situ concrete. To resist corrosion the manholes were lined with

GRP or other approved liner, and was have tanking protection fixed to the outside.

The soffit of pipes entering a manhole was not be lower than the soffit of the outlet pipe except in

special cases to minimise turbulences.

Manhole internal surfaces, including the benching, were to incorporate a suitable corrosion resistant

lining such as Sulphur resisting cement.

Flushing and inspection points for small-bore sewers were installed at all changes in direction. These

chambers were provided at the junction of two sewers where the sewer depth was less than 2.5 m.

Chambers were provided at every 120 m, at each household connection point and at the upstream

terminal of the sewers or just upstream of the treatment facility.

3.5.7 Inspection Chamber

An Inspection Chamber was a clean-out generally installed at the property line of a building. It allowed

the municipality or city to access the sanitary or storm sewers without disturbing the building owner.

The municipality or city could service the laterals to the building with cleaning equipment for

blockages or they could camera the lateral for inspection purposes. An Inspection Chamber installed at

the property line could indicate whether the blockage was on the building owner’s side or the

city/municipality’s side and whose responsibility it was for cleaning. Inspection Chambers could also

be used for ‘sampling’ what was going through the lateral should the need arise to take samples.

Depth to base of inspection chamber

Maximum 1.5 m

Minimum 0.6m

So design average of 1.2m

Inspection chambers should extend to at least 1.0m meter into the property

Pipe diameter (mm) Max Spacing (m)

Above 150 mm and below 300 mm 100

Between 400 and 800 mm 120

16

3.5.8 Pipe Materials

There are a number of pipe materials which can be used for sewers and pumping mains. Sewers can

be constructed using asbestos cement, vitrified clay, concrete, PVC and GRP. Pumping Mains can be

constructed using PVC, GRP, and pre-stressed concrete and ductile iron.

Most types of pipe have been used in the Asia and it has generally been accepted that for sewers,

asbestos cement pipes were not suitable as they were not resistant to chemical attack arising from

septic sewage. This also applies to concrete pipes.

GRP pipes have been used successfully in some locations for both sewers and pumping mains. These

pipes have the advantage that they are resistant to both internal aggression (i.e. sulphuric acid formed

from hydrogen sulphide) and external aggression due to chlorides. However, they have the

disadvantage that their structural integrity is dependent on the backfill.

PVC pipes are not subject to degradation due to internal and external aggressive materials and have

been used extensively for sewers in the past with no history of problems. PVC pipes were used for all

sewers up to 300 mm diameter since they were readily available, cost-effective, durable and resistant

to chemical attack from hydrogen sulphide and chlorides. Pipe diameters above 300mm, HDPE pipes

were used and the same characteristics as PVC apply.

Ductile Iron pipes were used extensively for pumping mains. They are strong and with suitable lining

provide long term service. They can subject to erosion by aggressive ground conditions but this can be

overcome by suitable protection (tape wrapping).

3.5.9 Construction Material

The following materials are recommended for the construction of this scheme. Their selection is based

on availability, durability and cost.

Sewer Pipes (in trench) : PVC up to 300mm and HDPE above 300mm pipe diameter

Manholes : In situ concrete with GRP or other suitable linings

Manhole Covers : Ductile iron with a GRP sealing plate

Concrete : Sulphur Resistant Cement

Concrete Protection : Faces in contact with sewage - GRP lined

Faces in contact with ground – waterproof membrane

Reinforcement : High yield steel.

17

Chapter 4 PUMPING STATION AND SUB CATCHMENT

4.1 Pump Station Selection Criteria

Submersible pumping stations are generally selected for flows up to 100 l/s and wet well stations for

larger flows. However, each station should be treated on its own merits and the following

considerations assessed:

Space available for pumping station (submersible stations usually require less space);

Initial and final design flow;

Total head on the pumps;

Proximity of housing or public areas (opening submersible pump wells may create odor

nuisance).

For current scheme, submersible/wet-well pumping station arrangements were considered due to space

restrictions within the catchment.

The following table gives an overview on the wet-well sizing

Table 4.1: Wet Well Operation Requirement

Sewerage pump stations

Wet-well operating

requirements

V (m3) = 0.9 x pump rate (L/s)

N

Where N is the acceptable number of starts per hour

Pump Rate (L/s) = capacity of the largest duty pump

N = 12 for motors <= 15 kW

N = 8 for motors 15kW - 200 kW

N = 5 for motors > 200 kW

The minimum depth between duty start and duty off is 100 mm and

ideally should be 300 mm or greater.

4.2 Submersible Pumping Station

Submersible pumping stations should incorporate the following features:

Minimum of one duty and one standby pump;

Non-return valve and gate valves for isolation of each pump;

18

Valves to be in a separate, easily accessible chamber adjacent to the pump sump;

Air reaction operation level controls as follows:

- High level alarm (also float);

- Pump start;

- Pump stop;

- Low level pumps protection (also float).

The following table gives an overview on the recommended design parameters for pumping stations

Table 9.1: Recommended Design Parameters for Pump Stations

Design Parameters

Description Unit 10,000 < PE ≤

20,000

10,000 < PE ≤

20,000

PE >20,000

Type of station Wet well Wet well Wet well

Number of pumps

(all identical and work

sequentially)

2

1 duty,

1 stand-by

100% standby

4 (2 sets)

1 duty, 1 assist,

Per set

(100% standby)

6 (3 sets)

1 duty, 1 assist,

Per set

(50 % standby)

Pump design flow each at Q peak each at 0.5 Q peak each at 0.25 Q

peak

Maximum retention

time at Qave

min 30 30 30

Min pass through

openings

mm 75 75 75

Minimum suction and

discharge

openings

mm 100 100 100

Pumping cycle

(average flow

conditions)

Start/hour 6 min and 15

max

6 min and 15

max

6-15

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4.3 Sub-Catchment Division Of Jaffna

In order to achieve the conveyance of flows from various sub-catchments to the proposed

wastewater treatment facility; combination of gravity and pressure system was considered. The whole

Jaffna catchment was divided into several pumping station catchments that received flows to a low

level pumping station from there the flows were pumped to the next highest point to allow them to

flow under gravity.

Figure below shows the division of the Jaffna sewerage catchment into 4 major sub-catchments which

were served with gravity and pumping system.

Figure 4.1: Division of sub Catchment

Estimation of population for each sub-catchment was calculated based on the survey performed by the

TA’s allocated for the project by NWSDB. The Jaffna catchment was divided into 5 large, 12 medium

and 38 small pumping station catchments that eventually discharge them into the proposed wastewater

treatment system at Kalundai.

4.4 Major PS in Sub-Catchments

The current Jaffna catchment (proposed sewerage catchment) was divided into 5 major PS’s

that were operating at a flow capacity above 100L/s. These major PSs received flows from medium

and small PSs and convey them to the PSs downstream or in two cases to the proposed WWTP. Below

figure shows the distribution of major pumping stations within the project area.

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Fig 4.2: Main Pumping Station

4.5 Medium Ps in Sub-Catchment

Further, the major PSs were fed by 13 medium sized PSs that were operating at a flow capacity

between 20 and 100L/s. These medium PSs received flows from the small PSs and discharged them

into the major PS for further disposal. Below figure shows the distribution of major pumping stations

within the project area.

Fig 4.3 Medium pumping station

4.6 Small Ps in Sub-Catchment

Due to the topography, depth restriction of 4m and complexity of the project area, it was difficult to

run the gravity system for more than 1 km. So designed a gravity system that discharge into several

small PSs that was operate at a flow capacity up to 20L/s . These small PSs were spread all over the

catchment to receive flows for further disposal into the medium PSs. Below figure shows the

distribution of small pumping stations within the project area.

21

Fig 4.4 Small Pumping Station

Figure 4.5 indicate the whole main pumping network.

Fig 4.5: Main Network

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Chapter 5 INSPECTION CHAMBER SURVEY

5.1 Introduction

An Inspection Chamber is a clean-out generally installed at the property line of a building. It allows

the municipality or city to access the sanitary or storm sewers without disturbing the building owner.

The municipality or city can service the laterals to the building with cleaning equipment for blockages

or they can camera the lateral for inspection purposes. An Inspection Chamber installed at the property

line can indicate whether the blockage is on the building owner’s side or the city/municipality’s side

and whose responsibility it is for cleaning. Inspection Chambers can also be used for ‘sampling’ what

is going through the lateral should the need arise to take samples. The Mainline Adapt-a-Valve

Inspection Chamber is versatile in that it can be adapted to become an extendible backwater valve or

can be used to pressure test the lateral or isolate the lateral if need be. The body of the Mainline

Adapt-a-Valve Inspection Chamber has a special slot molded right into it that is designed to accept the

backwater valve gate or the test/isolation n gate. Inspection chambers may be constructed from

class B engineering bricks or precast concrete sections surrounded in concrete or in-situ concrete or

glass-fiber plastic.

5.2 Inspection Chamber(IC) Marking

Following regulation should be considered in IC marking

Upstream of inspection chambers, internal network should be allowed for at 1 in 60 grade

and

IC should be at least 5m away from the well.

In special causes, the above requirements could be considered to be relaxed to 1 in 100

grade for the internal pipework

House connection shall be provided through Inspection chambers at suitable

locations,

considering the location of the low end of the property , the location of the branch sewer,

and the location of the exiting septic tank

For battle-axe blocks, the IC should be placed within the main body of the property.

House connection shall be provided for all residential properties, non-residential

properties including properties with currently houses and empty plots

Marked IC point is shown in below

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Fig 5.1: Location Of The IC Point

5.3: IC Survey

Purpose of the IC survey is improve the SewerGems Model and add information. IC survey is done

with the help of surveys. Surveys use GPS technic to measure the points. At least 3 point measurement

is needed in a property. That’s outlet of the toilet, outlet of the bath and well, outlet of the kitchen and

location of the inspection chamber. From these reading with the help of Arc -GIC can calculate

the flow direction. Sheet used to marked IC show in below.

Fig 5.2 IC Survey Sample Sheet

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Chapter 6 ISSUES & RISKS

6.1 Shallow Bed-Rock

The limestone of Jaffna "is a grey yellow and white organogenic, porous limestone (reef limestone)"

and "is typically a compact, hard, partly crystalline rock". The depth of limestone existence

varies across the study area.

6.1.1 Geotechnical Data

It is understood that no site investigation works including geophysical survey or intrusive exploratory

holes are planned until the construction phase of this project. It is our opinion that if the site

Investigation is carried out after design completion, major changes in detailed design may be required

With considerable impact on cost and construction program.

In view of the above, consideration should be given to undertaking a site investigation

comprising intrusive exploratory holes and geophysical survey to support the geotechnical

detailed design. A geotechnical desk study, which will comprise collection and collation of

historical information from the general vicinities of the pipeline route, will be carried out to allow

determination of the scope of the site investigation. It is anticipated that the availability of significant

geotechnical information will greatly reduce the scope of the proposed investigation.

6.1.2 Impact Of Limestone Bedrock On Network Design

Since the modeler has no comprehensive information on the existing ground conditions within

the Jaffna study area, it has been assumed that the depth of weathered hard limestone is in the range of

4m.However, the Railway Contractors have provided bore-hole information along the railway line. As

per the survey information provided by them, the depth of limestone rock along South of Stanley Road

is in the range of 2.0m. This restricts both the sewer and pumping station depth to 2.0m.

At a depth of 2.0m, the pipe can only run for 300m to meet the minimum self-cleansing

velocity. This means, number of pumping stations within the study area might go over 90. This would

Increase the capital costs for PS and annual operation & maintenance costs. Since the modeler has no

evidence of depth of limestone in other areas within the catchment, it is difficult to arrive

with a meaningful and reasonable design.

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6.2 Ground Water

The groundwater table is located at 0m to 3m below ground level. The conductivity is very high. The

estimated shallow wells are over 80,000 in number constructed up to depths of 5m to 10m. Over 50%

of the wells in the Jaffna Peninsula have high salinity water. Most well-water falls into low to medium

sodium and phosphorus content. Nitrate or N levels in most agro-wells are higher than the permissible

level. Due to the disposal of sewage from pit-latrines, soakage ways and septic tanks, fecal

contaminated groundwater has been reported from several places in the Jaffna Peninsula

26

Chapter 7 CONCLUSION

Engineering is not an easy task as a carrier. It is essential to have good practical knowledge and also

experience to do the task effectively. As an engineer he is responsible to do several occupations. Once

he is an adviser, once he is a manager, once he is a supervisor likewise an engineer should have good

personality to handle everything properly. As an Engineer he should be able to associate every person

in the site well. He should be able to create good communication with each other to enhance the work

in the site.

As a trainee at Project Engineering Institutional Consultant Service and I have been working in the

Jaffna-Kilinochchi Water Supply and Sanitation Project in Jaffna. I experienced lot and was able to get

some basic knowledge regarding the field of civil engineering. As this is my first training I was able to

gain the knowledge not only about the engineering field but also who is an engineer, What are his

duties, How about his participation to the job well, In my training period I was able to understand how

the theoretical relations applied to the practical purposes. I got opportunity to associate people who are

working in various stages such as senior engineers, site engineers, managers, technical offices, labors

etc. As an undergraduate during my three month short training period I saw the reality of the

engineering world. I understood one thing we should always try to learning through observation to full

fill the necessity of the engineering knowledge to the world.