Detailed Design Report

ATHI WATER SERVICES BOARD
CONSULTANCY SERVICES FOR A FEASIBILITY STUDY AND

DETAILED DESIGN FOR SEWERAGE SYSTEM IN JUJA AND
THIKA MUNICIPALITY
(PHASE II)
DRAFT DESIGN REPORT
FRAME Consultants Ltd Athi Water Services Board

Consulting Civil, Structural & Geotechnical Engineers Africa Re-Centre 3rd Floor, Hospital Road
Pension Towers, Loita Street
P.O Box 45283-00100 Nairobi, Kenya
P.O. Box 58624-00200, Nairobi - Kenya,
Tel: 254 020 2724292/3
Tel: 020 2213744/2251505
Fax: 254 020 2724295
Fax: 020 2213990
E-mail: info@frameconsultants.com Email:
Website:
Website:www.frameconsultants.com
MARCH 2013
AWSB in partnership with MLHUD Consultancy Services for a Feasibility Study
and Detailed Design for Sewerage System in
Juja &Thika (Phase II)
TABLE OF CONTENTS
LIST OF TABLES........................................................................................................... iv
LIST OF FIGURES..........................................................................................................v
ABBREVIATIONS.........................................................................................................vi
EXECUTIVE SUMMARY...............................................................................................i
1. INTRODUCTION.....................................................................................................1
1.1 GENERAL......................................................................................................................1
1.2 OBJECTIVE OF THE STUDY....................................................................................1
1.3 SCOPE OF THE STUDY..............................................................................................2
1.3.1 FEASIBILITY STUDY FOR JUJA AND THIKA MUNICIPALITIES....................2
1.3.2 DETAILED DESIGN PHASE.....................................................................................3
1.4 ORGANIZATION AND MANAGEMENT OF THE STUDY..................................4
1.5 SCOPE OF THIS REPORT..........................................................................................4
2. PROJECT AREA BACKGROUND INFORMATION..........................................5
2.1 THE PROJECT AREA.................................................................................................5
2.1.1 LOCATION.................................................................................................................5
2.1.2 TOPOGRAPHY...........................................................................................................5
2.1.3 CLIMATE....................................................................................................................6
2.1.4 GEOLOGICAL AND SUB-SOIL CONDITIONS......................................................8
2.2 SOCIO-ECONOMIC INFRASTRUCTURE..............................................................9
2.2.1 GENERAL...................................................................................................................9
2.2.2 DISEASES.................................................................................................................11
2.3 WATER SUPPLY INFRASTRUCTURE..................................................................11
2.3.1 JUJA TOWN..............................................................................................................11
2.3.2 THIKA TOWN..........................................................................................................11
2.4 SANITATION INFRASTRUCTURE........................................................................12
2.4.1 JUJA TOWN..............................................................................................................12
2.4.2 THIKA TOWN..........................................................................................................12
3. POPULATION AND DEVELOPMENT PLANNING.........................................15
3.3 GENERAL POPULATION........................................................................................15
3.4 DESIGN PERIOD........................................................................................................16
3.5 POPULATION PROJECTIONS................................................................................17
3.5.1 PREVIOUS STUDIES ADOPTED DEMOGRAPHIC TRENDS............................17
3.5.2 ASSUMPTIONS........................................................................................................18
3.5.3 RESIDENTIAL POPULATION................................................................................19
3.5.4 INSTITUTIONAL POPULATION...........................................................................19
3.5.5 COMMERCIAL POPULATION...............................................................................19
3.5.6 INDUSTRIAL POPULATION..................................................................................20
FRAME Consultants Ltd Draft Design Report

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3.6 SETTLEMENT PATTERNS......................................................................................20
3.7 CURRENT LAND USE...............................................................................................21
3.8 CURRENT DEVELOPMENT PLANS......................................................................21
3.8.1 THE THIKA DISTRICT STRATEGIC PLAN (2005-2010)....................................22
3.8.2 THE THIKA DISTRICT DEVELOPMENT PLAN (2008-2012).............................22
3.9 FUTURE LAND USE PLANNING............................................................................22
4. WATER DEMAND AND WASTEWATER GENERATION.............................24
4.1 GENERAL....................................................................................................................24
4.2 WATER DEMAND......................................................................................................24
4.3 WASTEWATER GENERATION..............................................................................25
4.3.1 DOMESTIC WASTEWATER FLOWS....................................................................25
4.3.2 PRE-TREATED INDUSTRIAL WASTEWATER FLOWS....................................25
4.3.3 INFILTRATION INFLOW (I/I)................................................................................26
4.3.4 TOTAL WASTEWATER FLOWS...........................................................................26
5. THE RECOMMENDED SEWERAGE WORKS.................................................27
5.1 GENERAL....................................................................................................................27
5.2 PHASE IIA – IMMEDIATE WORKS.......................................................................27
5.3 PHASE IIB – FUTURE WORKS...............................................................................27
6. FIELD INVESTIGATIONS...................................................................................29
6.1 GENERAL....................................................................................................................29
6.2 TOPOGRAPHICAL SURVEY...................................................................................29
6.3 HYDROLOGICAL INVESTIGATIONS..................................................................29
6.4 GEO-TECHNICAL INVESTIGATIONS.................................................................29
7. DESIGN CRITERIA...............................................................................................30
7.1 GENERAL....................................................................................................................30
7.2 DESIGN BASICS.........................................................................................................30
7.3 DESIGN CRITERIA...................................................................................................30
7.3.1 SEWERS....................................................................................................................30
7.3.2 SEWAGE TREATMENT..........................................................................................33
7.4 CONSTRUCTION METHODS..................................................................................37
8. DETAILED DESIGN OF THE PROJECT...........................................................38
8.1 GENERAL....................................................................................................................38
8.2 SEWAGE COLLECTION AND CONVEYANCE SYSTEM.................................38
8.3 SEWAGE TREATMENT WORKS...........................................................................39
8.4 EFFLUENT AND SLUDGE DISPOSAL..................................................................44
9. OPERATION AND MAINTAINANCE OF FACILITIES..................................45

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9.1 GENERAL....................................................................................................................45
9.2 SEWER SYSTEM........................................................................................................45
9.3 TREATMENT WORKS..............................................................................................45
9.3.1 ROUTINE OPERATION AND MAINTENANCE ACTIVITIES............................45
9.3.2 START-UP PROCEDURE FOR WASTE STABILIZATION PONDS...................46
9.3.3 DE-SLUDGING AND SLUDGE DISPOSAL..........................................................47
9.4 OPERATION AND MAINTENANCE HUMAN RESOURCES............................47
9.5 OPERATION AND MAINTENANCE MATERIAL RESOURCES......................48
10. RESETTLEMENT ACTION PLAN..................................................................50
11. ENVIRONMENTAL IMPACT ASSESSMENT...............................................51
12. PROJECT COST ESTIMATE AND FINANCIAL ANALYSIS.....................52
12.1 CAPITAL COST ESTIMATES..............................................................................52
12.2 OPERATIONAL AND MAINTENANCE COST ESTIMATES........................52
12.3 FINANCIAL ANALYSIS........................................................................................52
12.4 CONCLUSION AND RECOMMENDATIONS...................................................52
13. PROJECT IMPLEMENTATION PLAN..........................................................53
13.1 GENERAL................................................................................................................53
13.2 CONSTRUCTION CONTRACT PERIODS........................................................53
LIST OF ANNEXES

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LIST OF TABLES
Table 2.1: Mean Climatic Data for the period 2011-2013...............................................................8

Table 2.2: Demographic Details of Administrative Areas within the Project Area.......................10
Table 2.3: Demographic Indicators................................................................................................10
Table 3.1: Population by Administrative Areas.............................................................................15
Table 3.2: Demographics of area to be served by the Project........................................................15
Table 3.3: Growth rates adopted in the various studies within and around the project area..........17
Table 3.4: Summary of Design Period and Growth Rates.............................................................18
Table 3.5: Assumed Population Classification..............................................................................18
Table 3.6: Population estimates for the various design horizons...................................................19
Table 3.7: Institutional Establishments projections.......................................................................19
Table 3.8: Commercial population projections..............................................................................20
Table 3.9: Summary for Industrial population...............................................................................20
Table 4.1: Water Demand in the Various Design Horizons...........................................................24
Table 4.2: Estimated Domestic Wastewater Flows........................................................................25
Table 4.3: Wastewater Flows Estimates........................................................................................26
Table 7.1: Peak Factors..................................................................................................................32
Table 7.2: Maximum Spacing of Manholes...................................................................................33
Table 7.3: Minimum Sizes of Manholes........................................................................................33
Table 7.4: Permissible volumetric loading and Percentage removal of BOD...............................34
Table 8.1: Design summary of sewage collection and conveyance system...................................37
Table 8.2: Primary Design Parameters for the Sewage Treatment Works.....................................38
Table 8.3: Design details of the inlet works units for the various S.T.W......................................40
Table 8.4: Design Details of waste stabilization ponds.................................................................41
Table 13.1: Construction Stage Implementation Schedule............................................................50

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LIST OF FIGURES
Figure 2.1: Location of Project Area................................................................................................6

Figure 3.1: Population Density in and around the Project Area.....................................................16
Figure 5.1: Layout of Works to be implemented in the various phases.........................................28

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ABBREVIATIONS
AWSB - Athi Water Services Board

CBS - Central Bureau of Statistics
EIA - Environmental Impact Assessment
MEWNR - Ministry of Environment, Water & Natural Resources
MLHUD - Ministry of Lands, Housing & Urban Development.
WRMA - Water Resources Management Authority
MWI - Ministry of Water & Irrigation
NAMPSIP - Nairobi Metropolitan Services Improvement Project
NEMA - National Environmental Management Authority
RAP - Resettlement Action Plan
RUJWASCO - Ruiru-Juja Water and Sewerage Company
RFP - Request for proposal
RSP - Ruiru Sewerage Project
STW - Sewerage Treatment Works
THIWASCO - Thika Water and Sewerage Company
ToR - Terms of Reference
WASREB - Water Services Regulatory Board
WaSSIP - Water and Sanitation Services Improvement Project
WSP - Waste Stabilization Ponds

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EXECUTIVE SUMMARY

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1. INTRODUCTION
1.1 GENERAL
This report will investigate current service level, operations and adequacy of the existing
sewerage infrastructure in terms of quality and quantity with a view to determining their
feasibility, adequacy, operational, efficiency and augmentation of potential within the frame
work of providing sanitation services to Juja and Thika towns. This report identifies the most
viable and economic options for intervention thereby laying a basis for preliminary designs.
Athi Water is one of the eight Water Boards under the Ministry of Environment, Water and
Natural Resources created to bring about efficiency, economy and sustainability in the provision
of water and sewerage services in Kenya. Athi Water is created under Section 51 of the Water
Act 2002 serving a population of over 4.5 million.
The Board ensures the provision of quality and affordable water and sewerage services in its area
of jurisdiction through its twelve (12) appointed Water Services Providers (WSPs) namely:
 Nairobi Water and Sewerage Company

 Thika Water and Sanitation Company
 Limuru Water and Sewerage Company
 Ruiru-Juja Water and Sanitation Company
 Runda Water Ltd
 Kikuyu Water Company
 Karuri Water and Sanitation Company
 Gatundu Water and Sewerage Company
 Githunguri Water and Sewerage Company
 Gatanga Community Water Scheme
 Kiambu Water and Sanitation Company
 Karimenu Community Water and Sanitation Company
1.2 OBJECTIVE OF THE STUDY
The objective of the study was to come up with suitable infrastructure designs for managing
human waste through water borne sanitation system. The main objectives were therefore to:-
 Carry out Feasibility Study for Juja Urban and Thika Municipality Sewerage Project. For
Thika Municipality, the Consultant will pay special attention to the possible integration of
the Southern Parts of Thika Municipality (Athi River Basin).
 Formulate and design a sewerage project that takes into consideration the Environmental
Management and Co-ordination Act of 1999 and Water Act 2002 and will serve the area
for various planning horizons up to year 2030.

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 Prepare Detailed Design and develop Design Report, Drawings and Tender Documents.
Prepare integrated sewer designs for both Juja and Southern Parts of Thika Municipality.
 Undertake an Environmental Impact Assessment (EIA) for the Proposed Project and
prepare the necessary documentation.
 Prepare a Resettlement Action Plan (RAP)
1.3 SCOPE OF THE STUDY
The study involved not less than the following tasks:-

1.3.1 FEASIBILITY STUDY FOR JUJA AND THIKA MUNICIPALITIES
The Study was preceded with two (2) stages as follows:
Stage 1: Desk Study

This stage involved the following:
 Reviewing the Spatial Plan of Nairobi Metropolitan Region and identifying the
contribution of Juja and Thika to the said Spatial Plan and the Nairobi Metropolitan
Strategy.
 Collection of data and information relevant to the Project
 Reviewing and documentation of any previous and existing studies and plans.
 Establishment of criteria to be adopted for formulation of a Phased Sewerage Master
Plan.
 Environmental investigations in line with the current legislations.
Stage 2: Field Visit

This stage involved the following:-
 Carrying out a detailed assessment of available water sources (surface and ground water)
within the Project Area.
 Site assessment to include:
− Current water connections and coverage
− Potential sewer connections
− Existing population to be served
 Reviewing of existing By Laws Constraints
 Assessment of wastewater generated in Initial, Future and Ultimate stages
 Obtaining Socio-economic information relevant to Cost Benefit Analysis (CBA)

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 Undertaking a detailed financial and economic analysis of the proposed project.
 Overall project evaluation including environmental effects on the beneficiaries.
 Carrying out Preliminary Design of the major structures in the proposed project
 Carrying out Preliminary Geotechnical Survey on soil properties for the proposed sewage
treatment plants.
 Identification, on preliminary basis, suitable location of lateral, trunk sewers and sewage
treatment works.
 Holding Stakeholder Workshop and based on the inputs of the stakeholders come up with
the preferred option for Juja and Thika
 Reviewing existing and potential indirect discharge of effluent from commercial
customers into the sewerage system and propose improvements. This includes a cadaster
of commercial customers in Juja & southern parts of Thika Municipality, including the
quality and quantity of discharge, and the description of pre-treatment facilities these
customers have, or should have.
1.3.2 DETAILED DESIGN PHASE
The scope of services in the detailed study for the preferred scenario for Juja and Some Southern
Parts of Thika Municipality include:
- Finalization of estimation of wastewater generation.

- Zoning of wastewater generation to identify proper sewer pipe sizes.
- Detailed survey of lateral and trunk mains, at agreed lengths, and sewage treatment
works.
- Calculation of sewage treatment works biological, physical and chemical processes.
- Siting and designing of manholes in line with the Feasibility Study Report
recommendations.
- Detailed designs of primary, secondary and tertiary sewers and sewage treatment works,
including sludge handling and disposal/reuse facilities. The possibility of generating
electricity in the Wastewater Treatment Plant will be explored.
- Tackling of hydrogen sulphide generation.
- Make BOQs, drawings and tender documents.
- Develop an operational and financial sustainability model for the project.
- Any other work that may be agreed with and assigned by the Client

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1.4 ORGANIZATION AND MANAGEMENT OF THE STUDY
The feasibility design for Juja and integrated detailed design for Juja and Thika towns is
organised in line with our Technical Proposal and the Contract Agreement. The Consultant has
used all the proposed staff for the assignment The Consultant has put in place the logistical
means and equipment necessary for the assignment and the overall execution of the works. The
Consultant has made use of its office in Nairobi during execution of the feasibility study and will
do the same during the detailed design works. The office is fully furnished and equipped with the
necessary engineering, computing, and communication equipment. Computerized plotters to be
used during the designs have been installed with the latest software for analysis and production
of design drawings, details and documents.
Vehicles and data collection equipment are also available for use during field investigations, data
collection and review.
The project is being managed by the Project Director/Team Leader, Eng F.M Kimani, assisted by
the Deputy Team Leader, Eng Bernard Wanjohi. The various Consultants’ senior specialist staffs
are responsible for the execution of the various specialist activities and are providing the
technical backstopping services to the study team. The Consultant has started engaging specialist
staff not directly involved in the project to provide quality assurance checks.
1.5 SCOPE OF THIS REPORT
This report investigates current service level, operations and adequacy of the existing sewerage
infrastructure in terms of quality and quantity with a view to determining the Rehabilitation and
Augmentation potential within the frame work of providing sanitation services to Juja & Thika
Towns. The report identifies the most viable and economic options for intervention thereby
laying a basis for preliminary designs. [UPDATE FOR DETAILED DESIGN REPORT]

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2. PROJECT AREA BACKGROUND INFORMATION
2.1 THE PROJECT AREA
1.3.1 LOCATION
1.3.1.1 Juja
Juja town is located in Thika West District in Kiambu County, 10 km South-East of Thika Town
and 12km North-West of Ruiru, latitudes -1.1833 and Longitude 37.1167 about 30km North of
Nairobi. It is the administrative centre for Jomo Kenyatta University of Agriculture and
Technology (JKUAT). The project area can be accessed through the A2 (Nairobi – Thika) dual
highway or the Nairobi – Nanyuki railway line.
The northern boundary of Juja is the Komu River, whilst the southern boundary is the Theta-
Thiririka Rivers. The eastern boundary is the Nairobi-Athi Rivers.
1.3.1.2 Thika
The Thika Municipality is situated in Thika West District, Kiambu County, approximately 45 km
North-East of Nairobi. It stretches approximately 24km in an east-west direction and tapers from
a width of about 2.5km on western side to 5.0km on the Eastern edge. The total area of the
municipality is 93km2. The main Nairobi/Nyeri road, which runs in a north-south direction,
dissects the Municipality just to the west of the commercial and administrative center of the
town.
The northern and southern boundaries of Thika Municipality are well demarcated by rivers which
flow in an easterly direction. The Karimenu-Chania-Thika Rivers form the northern boundary
while the Komu Rivers forms the Southern boundary.
1.3.2 TOPOGRAPHY
The topography of the project area is dominated by the Aberdare Mountains to the West. To the
East of the town rises the only mountain in the immediate vicinity, Ol Doinyo-Sabuk.
The Eastern slopes of the Aberdares have been dissected by swift-flowing streams. The project
area is situated strategically where most of these rivers merge and flow together to the East of the
project area.
Generally, the area covered by the Municipality slopes gently from an altitude of 1560m at the
western boundary down to 1440m A.S.L towards the eastern edge.
To the West of the A2 Nairobi-Thika road, there are undulating coffee plantations with many
large scattered trees. The major part of the undeveloped area to the east of the A2 Nairobi-Thika
road is uncultivated plains with scattered trees and shrubs. The land falls steeply to the river
valleys with heavy vegetation.

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SOUTH
SUDAN ETHIOPIA
Mandera
Lodwar Moyale
Marsabit
UGANDA
Wajir
SOMALIA
Eldoret Isiolo
Meru
Kisumu
Mt. Kenya Garissa
Nakuru
Thika
NAIROBI
RUIRU/JUJA/THIKA
PROJECT AREA
Lamu
INDIAN
Malindi OCEAN
TANZANIA Mombasa
Figure 2.1: Location of Project Area

1.3.3 CLIMATE
1.3.3.1 General
The major parameters of the Climate of the study area are controlled by the large-scale pressure
systems of the western Indian Ocean and the adjoining continents, and its day-to-day weather is
related to the very considerable day-to-day variation in the detail of these pressure systems. In
addition, differences in topography lead to a diversity of climates due to the tendency for greater
cloudiness and precipitation at higher altitudes and on windward slopes, and lower temperatures
with increasing altitude.

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In the study area, the year can be sub-divided on the basis of predominant wind direction and
general weather characteristics, into four well-marked seasons.
- North-East Monsoon: This season runs from December to March and is warm, clear and
sunny with persistent north-easterly winds and occasional showers.
- Rainy Season; “Long Rains”: The “Long Rains” usually run from Late March to May
bringing warm rainy weather with light winds varying from North-Easterly to South-
Easterly by the end of the period.
- South-East Monsoon: From June to September, weather is usually cool and overcast
with persistent south easterly winds, little rain drizzle which becomes heavier towards the
end of the season.
- Rainy Season; “Short Rains”: The “Short Rains” usually occupy the month of
November when the weather is warm, sunny and showery, with a light wind changing
from south-Easterly to north-Easterly during the month.
There are two meteorological stations at Thika that monitor climatic conditions. They are located
at the Thika Horticultural Research Station (0º 50’S and 37° 04’E; altitude, 1549m) and Thika
Agromet Research Station (1º 01’S and 37° 06’E; altitude, 1477m).
1.3.3.2 Temperatures
The annual mean maximum temperature is 25.58°C, while the annual mean minimum
temperature is 15.43ºC. The annual average temperature is 20.51°C. The mean minimum and
maximum temperatures for the period 2011-2013 are given in Table 2 .1.
1.3.3.3 Rainfall
Precipitation in the project area is mainly bi-modal. Rainfall data for the period 2011-2013 is
given in Table 2 .1. The mean annual rainfall measured during these periods was 987.43mm.
The mean monthly rainfall in the wettest month (April) was 262.5mm. There is usually no
rainfall during the months of July, August and September.
1.3.3.4 Evaporation
The rate of evaporation recorded during the periods 2011-2013 using an evaporation pan Type-A
are shown in Table 2 .1. The annual mean evaporation is 1,452mm, with a mean monthly
maximum of 182mm in March. The daily net evaporation rate is 3.98mm.
1.3.3.5 Relative Humidity
The relative humidity in percentages, measured at 0600 and 1200, for the period 2011-2013 is
given in Table 2 .1. The average annual values are 78.3% in the morning and 49.6% in the
afternoon. There is not a great monthly variation in humidity; with the highest reading been
around 84.3%.

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1.3.3.6 Sunshine
The number of hours of daily sunshine recorded for the period 2011-2013 is given in Table 2 .1.
The annual average daily sunshine is 6.7hours, with monthly high and low means of 9.0 and 2.9
hours observed in the months of January and August respectively.
The Table 2 .1 below gives the mean data of the various climatic elements obtained from the
Thika Agromet Research Station for the period January 2011 to October 2013.
Table 2.1: Mean Climatic Data for the period 2011-2013

Mean Mean Relative Mean
Temperature (0C)
Month Rainfall Evaporation Humidity (%) Sunshine
Max. Min. (mm) (mm) 0600H 1200H (hours)
January 27.8 12.90 28.04 151 72.7 37.5 9.0
February 24.3 13.92 22.94 170 61.2 34.5 8.8
March 24.7 14.28 123.18 182 61.0 34.0 8.0
April 26.9 16.57 262.50 125 83.3 55.7 8.3
May 26.1 15.43 91.47 100 82.3 56.3 6.7
June 24.5 14.10 32.51 85 83.7 55.5 6.0
July 23.7 12.97 3.37 81 82.7 52.5 5.0
August 24.1 12.60 19.57 79 84.3 56.0 2.9
September 26.4 13.44 23.07 112 83.7 47.3 3.7
October 26.5 18.65 93.00 131 82.3 52.3 6.4
November 25.9 18.96 172.00 110 83.5 61.0 7.6
December 26.1 21.39 115.80 126 78.5 53.0 7.7
25.58 15.43 987.43 1,452 78.3 49.6 6.7
1.3.4 GEOLOGICAL AND SUB-SOIL CONDITIONS
1.3.4.1 Geology
The eastern half of the project area is formed by rocks of the basement system, while to the west,
lavas, pyroclastic and sediments of the tertiary age are exposed. In general, the rocks in the area
can be divided into three main age groups:
i. Archean: Basement System

ii. Tertiary: Volcanic and Sediments
iii. Recent: Soil and Alluvial Deposits

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The Basement System is believed to represent an original sedimentary series of limestones.
Shales and sandstones, into which basic magma has been intruded. Intense compression with
rising temperatures has resulted in these rocks being transformed into a highly folded
metamorphic series.
The volcanics and sediments occur in the western half of the area, forming the plains south of the
project area. The volcanic and sediments are probably all of the tertiary age but, as no fossils are
available and no age determination have been made, it is not known whether they range into
Pliocene.
The term “building stone” is used to describe the series of massively bedded, fine-grained tuffs or
claystones occurring in the area.
Recent deposits in the area include soils and alluvial sand deposits. The soils are normally
residual weathering deposits, whose composition is controlled more by the physical conditions of
formation than by the type of rock from which they were derived. In general, the well drained
soils are sandy and the badly drained soils black cotton soils. On the basement system, red latiric
soils are the most common, but in places they give way to more sandy soils and even black
cotton soils. The variation of soils on the volcanic rocks is not so great, the principal soils being
black and grey clays.
Another important feature of the soils on the plains south of Thika is the development of mound
topography. The mounds are usually about 20 to 50m in diameter, and on top of the mounds, the
underlying rock, which is usually composed of cemented brecciated material, is exposed or
covered by only a few centimetres of soil. In between the mounds, the soil cover is much deeper.
1.3.4.2 Groundwater
In the volcanic areas, where the volcanic rocks rest on an old beveled land surface, an aquifer is
probably provided by the weathered gneiss of that surface. Numerous boreholes have been sunk
in the area, particularly through the volcanic rocks. In general, it may be said that boreholes sited
on volcanic areas are more successful than those sited in Basement System rocks.
1.3.4.3 Soil Conditions
The depth to rock or non-penetrative hard soil level varies considerably throughout the area. The
total variation in depth is 0-3.9m with an average of 1.2m. The soil layers are deeper west of the
Nairobi-Thika road, with a variation of 0.35-3.9m and an average of 1.75m, and on the plain
south-east the total variation is 0-3.8m with an average of 0.95m. In most of the area, there is a
topsoil layer of 0.1m containing some humus. Underneath the topsoil there is reddish-brown clay
or grayey-black clay. In the deeper layers the soil is mixed with murram or weathered rock
material.
1.4 SOCIO-ECONOMIC INFRASTRUCTURE

2.2.1 GENERAL
The project area has a diverse population distribution varying from one sub-location to the other.
The western parts of the project area are the most densely populated while the lower regions with
low population density. In addition to Thika being a rich agricultural district, it is one of the

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leading industrial districts of the country. However, there are isolated pockets of poverty in the
project area, mainly found in urban slums in Thika Municipality, and Juja Town.
The project area is proposed to cover:-

 Parts of Theta & Mugutha sub-locations in Ruiru District,
 Kiaora, Milimani, Kalimoni, Komo, Weteithie in Juja Division Thika West District,
 Karimenu, Kamenu, Kianjau, Komo in Thika West division Thika West District,
 Githima & Munyu sublocations in Thika East District.
Table 2 .21 below shows proposed project areas various administrative divisions, their
population and population densities.
Table 2.2: Demographic Details of Administrative Areas within the Project Area
Area
Sub- Population Households Density
District (km2)
Location
Male Female Total
Theta 11,318 11,816 23,134 6,184 31.1 743
Ruiru
Mugutha 7,975 7,912 15,887 4,789 59.5 267
Kiaora 11,008 10,888 21,896 7,137 15.5 1,4100
Milimani 6,290 5,948 12,238 4,096 30.4 403
Kalimoni 10,165 9,696 19,861 6,039 121.3 164
Komo 2,168 1,993 4,161 1,514 39.2 106
Thika
Weteithie 11,190 10,426 21,616 7,409 29.6 731
West
Karimenu 4,453 4,961 9,414 2,706 26.4 356
Kamenu 18,750 18,727 37,477 12,164 33.3 1,126
Kianjau 9,887 8,100 17,987 6,627 7.8 2,299
Komu 18,499 18,173 36,672 12,579 6.7 5,460
Thika Githima 1,626 1,694 3,320 843 27.3 122

East Munyu 1,308 1,259 2,567 665 5.0 509
Total 114,637 111,593 226,230
Table 2 .3 below summarizes some of the demographic indicators.
Table 2.3: Demographic Indicators2

Population 226,230
Number of males 114,637
1
Source: 2009 Kenya Population and Housing Census,
2
Source: 2009 Kenya Population and Housing Census& Thika District Strategic plan 2005

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Number of females 111,593
Dependency Ratio 100:73
Population growth rate 2.8%
2.2.2 DISEASES3
The most prevalent diseases in Juja and Thika are Malaria, HIV/AIDs and Broncho-pneumonia
while the childhood diseases include anaemia, marasmus, eye infection, pneumonia, malaria,
Kwashiorkor etc.
HIV/AIDs are a major health problem with the prevalence averaging 34%. With regard to bed
occupancy, about 60% of the hospital beds are occupied by patients with HIV/AIDs related
diseases. The age group 20-49 years is the most affected, majority of who are females. This has
resulted in high increase in number of HIV/AIDs orphans in the district and loss of families’
incomes which is directed towards addressing the pandemic in the household. The main causes of
the spread of HIV/AIDs in the district include unsafe sexual behaviour, drug abuse especially
drinking of illicit brews, high levels of peer pressure and ignorance of facts, family breakdowns
etc.
The socio-economic impact of HIV/AIDs in the district include the drop-out rates in high
schools, female and children headed families, loss of manpower and high mortality and
morbidity rates, orphans and street children etc.
1.5 WATER SUPPLY INFRASTRUCTURE

2.2.3 JUJA TOWN
Juja Town is served by water from the nearby Ndarugu River. Water from the source undergoes
full treatment before supply. The Government of Kenya, through the Athi Water Services Board,
operates and maintains the water supply with current production of 4,000m 3/day. This is in
addition to the old system which has production of 1000m3/day.
2.2.4 THIKA TOWN
Under the Thika Water Supply Augmentation Phase II Project (TWS Project), whose
implementation was completed in late 1991, the municipality has an adequate water supply
system. The water treatment works has a design capacity of 36,660m3/day.
The reticulation network is in three separate pressure zones, each served by gravity pipe-work
from elevated storage tanks. Pressure zones A and C are both located on the western side of the
Nairobi/Nyeri Road. The main pressure zone is zone B which covers the whole area of the town
3
Implementation of the National Population Policy for Sustainable Development

11
east of the Nairobi/Nyeri road. This zone incorporates most of the developed area including the
industrial areas.
1.6 SANITATION INFRASTRUCTURE
2.2.5 JUJA TOWN
Juja town has no existing water borne sewerage system. The rapid population growth and
migration to the area has caused tremendous expansion of the town requiring the provision of
adequate sewerage services. At present the modes of wastewater/sewage disposal in the area is
through pit latrines, septic tanks and compost pits among others.
Jomo Kenyatta University of Agriculture and Technology has a sewage treatment plant that
serves the University and parts of Juja town.
2.2.6 THIKA TOWN
2.2.6.1 Description of the Main System
The main sewerage system serves most of the developed areas of the municipality that lie in the
northern and central parts of the town. The area covered by the system is about 15km 2. This
includes the town centre, the industrial area, the main residential developments, and the site and
service scheme of Makongeni.
The first phase of the sewerage system was constructed in 1965. This covered an area of about
3km2 serving the town centre, with a trunk sewer along the Thika river valley leading to a
conventional sewage treatment works.
The second phase was implemented in 1973. This involved the expansion of the sewer network,
with new areas being connected by new branch sewers, the provision of new trunk sewers, and
the construction of new treatment facilities to the south of the municipality. A pumping station
was constructed at the inlet to the old treatment works that pumped flows to a new trunk sewer
carrying the sewage to the new treatment site by gravity. The new works comprise an inlet works
for preliminary treatment followed by a set of waste stabilization ponds. With the commissioning
of the new scheme, the old conventional system was abandoned.
Rehabilitation works were carried out in 1988-89, during which new lines were laid parallel to
old lines, some lines were diverted, and others were replaced.
As the township continued to grow, new lines have been built and others extended to cover new
development. Among these, the sewer serving the area of Kiboko was built in 1990-91. Another
sewer lines serves the staff houses of UTI industries and the adjoining USAID area. Additional
sewer lines have also been laid in mortgage housing scheme.
The pumping station that was built as part of the 1972 scheme is the only one in the whole
sewerage system. It is located in the adjacent to the original conventional treatment works and
serves the catchment area of the old works. Rehabilitation of the station was undertaken at the
end of 1991. This involved stripping down of the pumps and replacement of bushes, bearings and
other parts. Civil aspects of the station were also rehabilitated. However, the pumps were
recently out of operation for more than 12 months due to the theft of electrical switchgear. When

12
the pumps are out of action, untreated sewage either overflows directly to the river or is diverted
through the old treatment works. None of the treatment units of the works is operational, so the
flow passes through the works and across fields, where it is used for irrigation, and from there to
the river.
To the north of the town centre, the estate of Bendor has an independent sewerage system that
was constructed in 1972-73. The eastern half of the area discharges into two small stabilization
ponds near the Thika river, while the western half discharges into a large septic tank. In the upper
part of the estate individual septic tanks are used by the residents.
The army barracks which are located in the eastern part of the Municipality, are partly served by
an independent network of sewers that connects to the public sewerage system, and a septic tank
that serves the northern area of the site. Another small scheme of sewers connected to a septic
tank is operated by the United Nations at a refugee centre, which is located in the north east of
the municipality.
It is estimated that 56% of the present population of the town is connected to the sewerage
system, while 6% are served by septic tanks. Of the remaining 38% of the population, most
households have built their own pit latrines although open defacation is used in some areas with
scattered dwellings. The septic tanks are located mainly in the low density developments around
Bendor and Maborokomo estates and other areas of scattered dwellings to the west of the Nairobi
road. The two principal areas where pit latrines have been built are in the informal housing areas
of Kiandutu and Kiangombe which lie to the South of the Garissa Road.
2.2.6.2 Sewerage of the Army Zone
The land and buildings occupied by the army are mainly served by a sewerage system which
discharges into the town system just upstream of the town’s waste stabilization ponds. A section
of the area is served by a septic tank. It is operated and maintained by the army.
The area occupied by 12th Army Battalion covers about 61/2km2 , of which the central and south-
western parts have buildings that are sewered.
The area is sewered by a 400mm diameter c0oncrete trunk main which runs centrally through the
quarters from North to South. At the southern end the sewer turns westwards to connect to the
trunk sewer leading to the waste stabilization ponds. The sewer is very deep over much of its
length, with a maximum depth of 12.8 meters. The sewer is frequently blocked and the depth
makes cleaning difficult.
2.2.6.3 Existing Sewage Treatment Works
The only sewage treatment works that serves Thika was commissioned in 1973. The works
consist of a set of waste stabilization ponds preceded by an inlet works where preliminary course
screening, grit removal, communitors, and flow measurement facilities are provided. The
stabilization ponds comprise primary, secondary, and maturation ponds in series. Staff housing is
provided, together with a small office and laboratory. There are 8 one bed roomed units at the
new sewage treatment works site.

13
The pond system was designed for a capacity of 6100m 3/day. The design criteria that were used
are:-
Sewage influent strength - 370 mg BOD5/1.

Surface loading rate - 225 kg BOD/ha/d
Minimum retention time in Primary Ponds - 14 days
Minimum retention time in Secondary Ponds - 10 days
A 675mm dia concrete trunk main discharges to the inlet channel. The course screen is broken
and rusty and is of little use. The “Pista” grit trap has been out of order for many years due to
stolen motors and other electrical parts. The communitors are not functioning and the flow is
diverted through a hand-raked screen in the by-pass channel. This screen is in good condition.
The flow recorder has been out of operation for many years, but flow measurements can be taken
at the flume by measuring the depth of flow on a scale fixed to the channel wall just upstream of
the flume. Provision has been made for flow splitting for any future expansion of the ponds.
The distribution of flow to the primary ponds is not proportionate due to blockages in some lines.
The inlet pipe for pond PI is rusted and it allows sewage to discharge onto the ground surface.
2.2.6.4 Other Sanitation Facilities
Sanitation facilities other than waterborne sanitation that are used in Thika include septic tanks
and pit latrines.
9.1

14
3. POPULATION AND DEVELOPMENT PLANNING
3.3 GENERAL POPULATION
The population estimates were obtained from the census data (2009) based on location
administrative boundaries. The project area encompasses the following locations:
 Theta location consisting of Theta sub-location,

 Mugutha location (parts of it) consisting of Mugutha sub-location
 Juja location consisting of Kiaora and Milimani sub-locations
 Kalimoni location consisting of Kalimoni sub-location
 Komo location consisting of Komo and Weteithie sub-locations
 Biashara location (parts of it) consisting of Karimenu sub-location
 Makongeni location (parts of it) consisting of Kamenu, Kianjau and Komu sub-locations.
Table 3 .4 below gives a summary of the project area and the respective population from the
2009 national census data:
Table 3.4: Population by Administrative Areas

Sub- Population Area %
District Households Density
Location Male Female Total (km2) Covered
Ruiru Theta 11,318 11,816 23,134 6,184 31.1 743 100%
Mugutha 7,975 7,912 15,887 4,789 59.5 267 53%
Thika Kiaora 11,008 10,888 21,896 7,137 15.5 1,410 100%
West Milimani 6,290 5,948 12,238 4,096 30.4 403 100%
Kalimoni 10,165 9,696 19,861 6,039 121.3 164 100%
Komo 2,168 1,993 4,161 1,514 39.2 106 100%
Weteithie 11,190 10,426 21,616 7,409 29.6 731 100%
Karimenu 4,453 4,961 9,414 2,706 26.4 356 60%
Kamenu 18,750 18,727 37,477 12,164 33.3 1,126 20%
Kianjau 9,887 8,100 17,987 6,627 7.8 2,299 20%
Komu 18,499 18,173 36,672 12,579 6.7 5,460 20%
Thika Githima 1,626 1,694 3,320 843 27.3 122 100%
East Munyu 1,308 1,259 2,567 665 5.0 509 100%
TOTAL 114,637 111,593 226,230 72,752 433.1
The population to be served by the proposed Juja-Thika Sewerage Project (Phase II) is
summarised in Table 3 .5 that follows.
Table 3.5: Demographics of area to be served by the Project

% of Total
Sub- Area Population
District Density Population sub-location
Location (km2) served
served
Ruiru Theta 31.1 743 23,134 100% 23,134
Mugutha 59.5 267 15,887 53% 8,420
Thika Kiaora 15.5 1,410 21,896 100% 21,896

15
% of Total
Sub- Area Population
District Density Population sub-location
Location (km2) served
served
West
Milimani 30.4 403 12,238 100% 12,238
Kalimoni 121.3 164 19,861 100% 19,861
Komo 39.2 106 4,161 100% 4,161
Weteithie 29.6 731 21,616 100% 21,616
Karimenu 26.4 356 9,414 60% 5,648
Kamenu 33.3 1,126 37,477 20% 7,495
Kianjau 7.8 2,299 17,987 20% 3,597
Komu 6.7 5,460 36,672 20% 7,334
TOTAL 401 550 220,343 135,400
Figure 3 .2 below shows the population densities of the project area location in terms of
administrative locations.
Figure 3.2: Population Density in and around the Project Area
3.4 DESIGN PERIOD
The design period is taken to be 20 years. The population and water demands are projected to the
year 2035. By using a population growth rate of r% per annum, the actual present, projected
future and ultimate human and commercial entities populations are calculated assuming
geometrical increase and the growth rates reducing towards the ultimate design year since the

16
town is expected to have limited growth rates. The compound formula is applied ona10-year and
20-year basis for future and ultimate water demand requirements.
r n
P n=P o 1+ (100 )
Where; Po - Present population
Pn - Population
r - Annual growth rate
n - Number of years between Po and Pn
3.5 POPULATION PROJECTIONS
3.3.1 PREVIOUS STUDIES ADOPTED DEMOGRAPHIC TRENDS
The Howard Humphreys ‘’Long Term Development Plan’’ Regional Studies (1986) was based
on Inter-censal Data from 1969 to 1979 and projections were presented up to the year 2010.
Subsequent Studies by other Consultants – Sogrea / Cape Consult (2005), Uniconsult (March
2007) and Seureca/Cas (August 2007) have based their analysis for Water Demand on Inter-
censal Data 1969 – 1979, 1979 – 1989 and 1989 – 1999 respectively.
This design has the benefit of recently published population data by Central Bureau of Statistics
for the Year 2009. A comparison of growth rates adopted under the previous studies with actual
inter-censal growth rates is given in Table 3 .6 below.
Table 3.6: Growth rates adopted in the various studies within and around the project area4
Population
Source Period
growth rate
1 Central Bureau of Statistics (Inter-censal Results) 1969 - 1979 4.9%
1979 – 1989 4.7%
1989 - 1999 4.8%
1999 – 2009 3.9%
2 Howard Humphreys – Long Term Development 1985 - 1995 5.3%
Plan (Regional Studies - 1986) 1995 – 2000 5.0%
2000 - 2010 4.5%
3 Nairobi Water and Sewerage Emergency Physical 1999 – 2006 3.4%
Investments Project, Diagnostic of the Current 2006 – 2008 3.3%
Situation – Sogreah / Cape Consult (January 2005) 2008 – 2010 3.2%
2010 – 2012 3.1%
2012 – 2014 3.0%
2014 – 2016 2.9%
2016 – 2018 2.8%
2018 – 2020 2.7%
4 Water Supply and Sewerage Services Demand 1999 – 2005 4.3%
Forecast for Nairobi City Uniconsult Kenya (March 2005 – 2010 4.3%
2007) & Nairobi Water and Sewerage Institutional 2010 – 2020 3.7%
4
Feasibility Study and Master Plan for Developing New Water
Sources for Nairobi and Satellite Towns

17
Population
Source Period
growth rate
Restructuring Project, Hydraulic Network Modeling 2020 - 2030 3.0%
–Seureca / Cas Consulting Engineers (August 2007)
3.3.2 ASSUMPTIONS
The following assumptions have been made for the calculation of the population projections:
i. The base year of the population has been taken as 2009, initial year as 2015, future year
as 2025 and the ultimate year as 2035, with the growth rates adopted as shown in Table 3
.7 that follows.
Table 3.7: Summary of Design Period and Growth Rates

Item Year Growth Rate
Base year 2009 -
Initial year 2015 4.3%
Future year 2025 3.7%
Ultimate year 2035 3.0%
ii. The number of institutions and commercial establishments has been assumed to be even
spread within the project area.
iii. The income group classification applied in the population projections is as shown on
Table 3 .8 below. This has been adopted from the Uniconsult report on Water and
Sewage Demand Forecast for Nairobi City and Satellite towns.
Table 3.8: Assumed Population Classification

Class Rate
High class 18.40%
Medium class 61.60%
Low class 20.00%
The population estimates was done in to consideration of Kenya’s Vision 2030

Development Plan which aims to make Kenya a middle income country by the year 2030,
it is expected that the people in the various income brackets shall increase with the nation
having fewer low income families and a higher proportion of high income earners.
iv. The population within a sub-location is uniformly distributed. Average population density
within sub-locations has thus been used to compute the population within the project area.
v. The primary schools within the project area are assumed to be day schools with WC
while all secondary schools are assumed to be boarding schools.

18
3.3.3 RESIDENTIAL POPULATION
Table 3 .9 is a summary of population projection to be served by the Project in the initial, future
and ultimate stages.
Table 3.9: Population estimates for the various design horizons.

Sub-location 2009 2015 2025 2035
Annual Growth Rate (%) - 4.3% 3.7% 3%
Theta 23,134 29,782 42,829 57,559
Mugutha 8,420 10,840 15,589 20,950
Kiaora 21,896 28,188 40,537 54,478
Milimani 12,238 15,755 22,657 30,449
Kalimoni 19,861 25,569 36,770 49,415
Komo 4,161 5,357 7,704 10,354
Weteithie 21,616 27,828 40,019 53,782
Karimenu 5,648 7,271 10,456 14,052
Kamenu 7,495 9,651 13,880 18,654
Kianjau 3,597 4,633 6,663 8,955
Komu 7,334 9,443 13,580 18,251
Githima 3,320 4,274 6,146 8,260
Munyu 2,567 3,305 4,753 6,388
TOTAL 141,289 181,907 261,605 351,582
3.3.4 INSTITUTIONAL POPULATION
The institutions in the project area include educational, health and administrative establishments.
The Table 3 .10 below gives a summary of the projected population of the institutions in the
various design horizons.
Table 3.10: Institutional Establishments projections

Base Present Initial Future Ultimate
Component Units
(2008) (2013) (2015) (2025) (2035)
Day Schools Persons 27,366 33,779 36,747 52,846 71,021
Boarding Schools Persons 6,054 7,473 8,130 11,692 15,714
Hospitals Beds 86 107 117 169 228
Health Centres Numbe 91
r 113 123 177 238
Administrative offices Persons 63 78 85 123 166
3.3.5 COMMERCIAL POPULATION
The project area has a number of commercial entities which provide a range of services to the
residents. These include but not limited to bars, hotels, shops, workshops, petrol stations among
others. The retail market and private sector businesses are very active in Thika. They create
employment and income generating opportunities for the local community and the environs.
Table 3 .11 that follows below gives the details.

19
Table 3.11: Commercial population projections5

Component Units
(2008) (2013) (2015) (2025) (2035)
Hotels Beds 44 54 59 85 114
Bars and restaurants Number 33 41 44 64 86
Shops Number 696 859 935 1,344 1,806
3.3.6 INDUSTRIAL POPULATION
Industrial establishments in the project area vary from very big employing many people to small
industries employing a few people. The industries are of different forms that can be grouped into
agro-based factories, Chemical processing industries, engineering and manufacturing factories.
Details of the industries are in Table 3 .12 below.
Table 3.12: Summary for Industrial population6

Component
(2008) (2013) (2015) (2025) (2035)
Agro-based 7 8 9 14 18
Chemical 4 4 5 8 10
Engineering 4 4 5 8 10
Manufacturing 7 8 9 14 18
Total 22 24 28 44 56
3.6 SETTLEMENT PATTERNS
The district is densely populated with distribution determined by climatic conditions;

infrastructure, food availability and proximity to urban areas among other factors. The settlement
patterns in the areas of the project area highly correspond with natural resources endowment. In
the high and medium potential areas there are high population densities. In the arid and semi-arid
areas populations densities is much lower and high absolute levels of poverty is prevalent. Urban
areas in Thika and Juja are growing fast due to rural-urban migration, availability of education
institutions and opportunities for employment in the industries. Population also varies from the
towns and rural areas. In the rural areas population density is low compared to the urban centers.
Urban settlement is mostly concentrated within 3km of the major roads, i.e. Nairobi-Thika Road
and Thika-Garissa Road.
5
Data obtained from the Thika District Development plan 2008-2012
6
Data obtained from the Thika District Development plan 2008-2012

20
3.7 CURRENT LAND USE
The project area is generally divided into four land use categories: industrial, institutional
(schools, hospitals, hostels), commercial and residential (high, medium and low-income
residences together with informal settlements). Industrial development was until recently the
main employer and income source for most of the population. At present there are over 20 major
industries and more than 100 minor industries.
The land prices vary depending on their location and proximity to the highway with the most cost
reducing as you move away from the major roads. The price of land in the project area has
multiplied five times in the last three years ago.
3.8 CURRENT DEVELOPMENT PLANS
The current development plan in the country is the Vision 2030. This is a long term development
blueprint. The Kenya Vision 2030 aims at transforming Kenya into a newly industrializing
middle income country providing a high quality of life to all its citizens in a clean and secure
environment. The vision is anchored on three key pillars; Economic, Social and Political
Governance.
The rapid population growth has had serious effect on social and economic development. This is
manifested in increased unemployment, high dependency ratio, increased demand for health
services, increased demand for agricultural land, more need for fuel and forest products, over-
crowding in educational facilities, more demand for better housing, high levels in poverty
indices. Poor farming practices coupled with lack of concern about forest conservation in the
district has resulted in soil erosion and environmental degradation. All these developments have
taken place in an environment of insufficient integration of population and environmental
concerns in the development process.
The major development challenges and cross cutting issues concerning the entire project area are:
 Poor infrastructure
 Poor marketing
 Low producer price
 High staff turn-over
 Small land sizes
 Inadequate health facilities
 School enrolment
 Poverty
 HIV/AIDS
 Gender inequality
 Disaster management
 Unemployment
 Environmental conservation and management
 Global warming and climate change
 People with disabilities

21
 ICT
 Security
The current development plans for the district are outlined in the following documents:
3.3.7 THE THIKA DISTRICT STRATEGIC PLAN (2005-2010)
This is a localization of the National Plan of Action (NPA) for the implementation of the
National Population Policy for Sustainable Development (NPPSD) contained in Sessional Paper
No. 1 of 2000. The NPPSD outlines a wide range of strategies and actions that need to be
undertaken to meet the set goals, objectives and targets through a multi sectoral and multi-
dimensional integrated approach involving all stakeholders in the population and health sector.
The Plan was developed in to address specific population and development needs of the Thika
district.
3.3.8 THE THIKA DISTRICT DEVELOPMENT PLAN (2008-2012)
This is the 8th series of the District Development plans (DDP) for the 148 districts as of October
2008. It provides linkages with long-term plan policies as enshrined in the Vision 2030.It is a
planning document detailing the measures and strategies the district will undertake to achieve
national aspirations as contained in the national development.The DDP articulates medium term
policies and objectives which are further translated into short term Strategies, programmes and
projects to be implemented under the Medium Term Expenditure Framework (MTEF). The latter
ispart of the financial reforms to strengthen financial discipline, accountability and efficient and
effective delivery of services to the people. The MTEF sectors identified include: Agriculture
and Rural Development Trade, Tourism and Industry; Physical infrastructure, Environment,
Water and Sanitation; Human Resource Development, Research, Innovation and Technology;
Special programmes; Public Administration and Governance, Justice, Law and Order.
3.9 FUTURE LAND USE PLANNING
There are many instances of upcoming mega residential developments as a result of the
infrastructural improvements in the area. These include:-
 Buffalo Hills-this is a controlled development that sits on 335 acres and features an 18-
hole golf course,
 Chania Gardens,
 Thika Greens -this project occupies 1,706 acres of land. The projects feature amenities
like schools, shopping malls, a health facility, and restaurant among others,
 Safaricom SACCO plots,
 Oak Valley Development,
 Garden City,
 Migaa,
 Golden Mile Park,

22
 Castle Park- Windsor homes have secured 100 acres of land under an agreement with the
land owner and are planning to set up 10,000 units next to Castle Breweries,
 Leisure Village,
 Juja South Estate- this project acquired approximately 30 hectares of land at Juja. Water
requirements for the proposed South Estate have been estimated at 500 cum per day,
 Bahati Ridge among others.

23
4. WATER DEMAND AND WASTEWATER GENERATION
4.1 GENERAL
The water demand and wastewater generated from population classes were calculated as
proposed in the Ministry of Water and Irrigation Practice Manual for Water Services in Kenya
(2005). Further reference has also been made to WHO Reports No. 4 (Design and Selection
Criteria for Community Water Supplies) and 9 (Selection and Design Criteria for Sewerage
Projects).The calculated water demand and wastewater generated is as shown in Annex II and
summarized in Table 4 .13 and Table 4 .15 respectively.
4.3 WATER DEMAND
The daily per capita water consumption has been based on the Ministry of Water and Irrigation
Practice Manual for Water Supply Services in Kenya (2005).Table 4 .13 gives a summary of the
estimated water demand within the Project Area at the different design horizons.
Table 4.13: Water Demand in the Various Design Horizons

Average Daily Water Demand (m3/d)
Component Unit Rate
2013 2015 2025 2035
Domestic
High Class (18.4%) l/h 250 7,692 8,368 12,034 16,173
Medium Class (61.6%) l/h 150 15,450 16,808 24,172 32,486
Low Class (20%) l/h 75 2,508 2,729 3,924 5,274
Institutional
Day Schools with WC l/h 25 844 919 1,321 1,776
Boarding Schools l/h 50 374 407 585 786
Hospitals l/bed 200 21 23 34 46
Health Centers l 5,000 565 615 885 1,190
Administrative Offices l/h 25 2 2 3 4
Commercial
Hotels l/d 300 16 18 26 35
Bars and Restaurants l/d 500 21 23 33 44
Shops l/d 100 86 94 135 181
Industrial l/d 20,000 20 22 35 55
Daily Water Demand (m3/d) 27,599 30,026 43,185 58,048
Unaccounted for Water (m3/d) 20% 5,520 6,005 8,637 11,610
Total Water Demand (m3/d) 33,119 36,031 51,822 69,657

24
4.4 WASTEWATER GENERATION
In this project area daily average water demand has been adopted to enable in estimating the
wastewater generation and future and ultimate projections. The following components of
wastewater flow have been considered in this study;
i. Domestic Wastewater Flows

ii. Pre-treated Industrial Wastewater Flows
iii. Infiltration Inflow (I/I)
4.3.1 DOMESTIC WASTEWATER FLOWS
This is waste originating from residential areas, institutional establishments and commercial
centres.
So as to generate these flows, domestic, institutional and commercial average daily water
demands have been multiplied with a factor which is a ratio of sewage quantity to water
consumption. In this study, various factor of the average daily water demand has been applied so.
The domestic waste water flows for the project area are as shown in Table 4 .14;
Table 4.14: Estimated Domestic Wastewater Flows

Wastewater Flows (m3/day)
Factor
Component Present Initial Future Ultimate
(%)
(2013) (2015) (2025) (2035)
High Class Housing 75% 5,769 6,276 9,025 12,130
Average Class Housing 80% 12,360 13,447 19,338 25,989
Low Cost Housing 85% 2,132 2,319 3,335 4,483
Day Schools with WC 85% 718 781 1,123 1,509
Boarding Schools 85% 318 346 497 668
District Hospitals 80% 17 19 27 36
Dispensaries 80% 452 492 708 952
Administrative Offices 85% 2 2 3 4
Hotels 85% 14 15 22 29
Bars and Restaurants 80% 16 18 26 35
Shops 85% 73 79 114 154
4.3.2 PRE-TREATED INDUSTRIAL WASTEWATER FLOWS
Due to stringent environmental laws, industrial wastewater is expected to be pre-treated to

acceptable standards prior to draining into the sanitary sewer system. The quantities of the pre-
treated industrial wastewater flows have been estimated as a factor of the average daily industrial
water demand as indicated in table 6.2.
4.3.3 INFILTRATION INFLOW (I/I)

25
This the total quantity of water entering the sewer system through such means as defective pipes,
pipe joints, connections, manhole walls, from sources such as roof leaders, yards drains, swamps,
surface runoff etc.
An allowance of 15% of the sum of Domestic and Pre-treated Industrial wastewater flows has
been used in this project.
The fact that some allowance for unauthorised and unavoidable stormwater has been made
should not be used as an excuse to permit the indiscriminate entry of storm water into the
sewerage system. Such practise must be considered illegal, if sewers are not to be overwhelmed
during periods of heavy rain.
4.3.4 TOTAL WASTEWATER FLOWS
The total estimate of average daily wastewater flow in this study in the various design horizons is
as tabulated below;
Table 4.15: Wastewater Flows Estimates

Present Initial Future Ultimate
Component
(2013) (2015) (2025) (2035)
Domestic Wastewater 21,870 23,793 34,218 45,988
Pre-treated Industrial Sewage 16 18 28 44
Infiltration Inflow 3,283 3,572 5,137 6,905
Total 25,169 27,382 39,383 52,937

26
5. THE RECOMMENDED SEWERAGE WORKS
5.1 GENERAL
From the Integrated Feasibility Study Report, the recommended sewerage system has been
proposed to be implemented in two Phases. These phases are Phase IIA (To be implemented
before the initial design horizon-2015) and Phase IIB (To be implemented before the Future
design horizon - 2025)
Long-term growth of the project area has been considered to ensure that the sewers have
sufficient capacity. Provisions have also been made for future extensions of the sewerage system
and due allowance has been made to deal with these additional sewage flows.
Summary of the recommended scope of work in the various project implementation phases is as
discussed in the following sub-sections. Figure 5.1 shows the layout of the works to be
implemented in the various phases.
5.2 PHASE IIA – IMMEDIATE WORKS
The recommended sewerage works to be implemented in this Phase will serve the densely
populated areas of the project area. The scope of work to be undertaken at this stage includes;
a) Construction of reticulation sewer systems in already settled and densely populated areas.
The sewer system is recommended to be constructed for the ultimate sewage flows.
b) Construction of the trunk sewers along the Mugutha, Theta, Thiririka, Ndarugu,
Kamuguti and Komu rivers. The trunk sewers are also to be constructed for the ultimate
sewage flows but will only cover the lengths from the treatment works inlet works to the
areas already settled.
c) Construction of a new treatment works at Kalimoni Site, Ndarugu Site and Extension of
the Existing Thika S.T.W. It’s recommended that the waste stabilization ponds system be
designed to handle initial sewage flow in their respective catchment areas.
Detailed design details of the recommended system to be implemented under this phase are
discussed in Chapter 8.
5.3 PHASE IIB – FUTURE WORKS
The works to be carried out in this future stage comprise of the following;
a) Extension of sewer reticulation and trunk sewers to cover settlements not covered by
Phase IIA where additional settlements will have come up.
b) Construction of the trunk sewer along Kurakuta River.
c) Construction of a new treatment works at Juja Farm and Munyu sites. This WSP system
should be designed to handle the remaining design flows from their respective drainage
basins.

27
AWSB in partnership with MLHUD Consultancy Services for a Feasibility Study and Detailed Design for Sewerage System in Juja &Thika (Phase II)
Figure 5.3: Layout of Works to be implemented in the various phases

28
6. FIELD INVESTIGATIONS
7.1 GENERAL
7.2 TOPOGRAPHICAL SURVEY
7.3 HYDROLOGICAL INVESTIGATIONS
7.4 GEO-TECHNICAL INVESTIGATIONS

29
7. DESIGN CRITERIA
7.1 GENERAL
Appropriate design criteria and technical parameters have been used in this sewerage project
feasibility study project. The Consultant has reviewed several documents and has also used his
wide experience in the Water Supply and Sanitation Sector to ensure integration of local and
international conditions, economics and engineering practices. This Chapter describes the design
criteria, assumptions and methodology adopted in the detailed design of the recommended
sewerage system as discussed in Chapter 5.2.
7.2 DESIGN BASICS
The sewerage collection system has been designed to carry sewage mainly composed of:
- Sewer from domestic (residential) establishments,

- Sewer from commercial establishments,
- Sewer from institutional establishments,
- Industrial wastewater flows,
- Infiltration Inflow (I/I) to sewer lines and manholes.
A separate sewerage system has been designed that is not expected to take any direct storm
water.
A daily per capita contribution of 55g of BOD5 has been provided for domestic, institutional and
commercial establishments where laboratory results are not present. Industrial establishments
draining into the sewer systems will be assumed that have pre-treated their waste to similar
parameters.
7.3 DESIGN CRITERIA

5.3.1 SEWERS
5.3.1.1 Construction Material
The pipe materials have been selected taking into consideration availability, durability and cost.
Circular pipes have been adopted in this design. PVC pipes are recommended for sewers with

30
bore less or equal to 300mm. Precast Concrete pipes have been adopted for the sewers greater
than 300mm.lines. Steel pipes are recommended for all river aerial crossings, Class A and B road
crossings, rail crossing and deep sewers. Different pipe bedding types have been used depending
on the nature of the ground. These have been classified as bedding type A, B, C, and D.
i. Concrete Pipes - Spun Concrete Pipes relevant Kenyan and British Standards are
manufactured locally by a number of Companies. They form the most common choice for
Sewer Pipes. Spigot and socket jointed pipes are manufactured in sizes ranging from
100mm to 915mm diameter, and ogee jointed pipes are available in sizes from 100mm to
1525 mm diameter.
Rubber ring, flexibly jointed pipes are manufactured in sizes ranging from 150mm to
975mm diameter and strength classes M and H, or reinforced. Pipes are vertically cast in
vibrated moulds and supplied moulds and supplied with rubber rings. Rigid jointed pipes
require jointing with tarred hessian and cement mortar.
Pipes will be laid on a concrete bed and provided with a haunch and surround or
reinforcement to meet the particular loading requirements.
ii. Upvc Pipes - Un-plasticised PVC pipes are manufactured in Kenya in metric sizes up to
450mm diameter. A number of firms are involved in production, including Polypipes Ltd
and Eslon.

31
iii. Glass Fıber Reınforced Polyester (GRP)Pıpes & Fıttıngs - Glass fibre reinforced
polyester pipes are imported and sizes range between nominal diameters 250mm to
4000mm. They are lighter, corrosion resistant and are much easier to install when
compared to Steel and Concrete pipes. The considerably higher costs associated with
importation however prohibit its use.
iv. Steel Pipes - Steel pipes are manufactured locally. They are generally used in for
locations where sewers are exposed or in situations such as river crossings. Protection
against corrosion is required internally and externally. Bitumen sheathing is normally
used, the external sheathing generally being reinforced with glass fibre windings.
5.3.1.2 Hydraulic Design
Hydraulic design of the sewers for the Project has been based on the Manning Equation with n
value of 0.013. The following expressions have been used;
2 1
1 A
Q= A ( )
n P
3
s2
Where;

32
A = Cross-sectional area of flow, m2
n = Manning’s coefficient
P = Wetted Perimeter (m)
s = Sewer Slope (mm/m)
In order to minimize the deposition of solid matter in sewers and the corrosion of concrete sewers
and manholes resulting from the evolution of hydrogen sulphide, a minimum velocity of flow in
sewers of 0.75m/s should be achieved at least once every. It has also been ensured that the
velocity is at least 0.6m/s when the sewage is flowing at its Dry Water Flow (DWF). To avoid
excessive abrasion from grit and sediments, peak velocities have been limited to a maximum of
3.0m/s at peak flow.
5.3.1.3 Peak Factors (FR) and Peak Flows (QR)
The following Peak Factors were proposed in the Nairobi Masterplan for sewers, sanitation and
drainage (1997) report has been adopted;

33
Table 7.16: Peak Factors
Average Flows Peak Factor
Less than 500m3/d 7.5
1,000m3/d 6.6
5,000m3/d 5.5
10,000m3/d 5.0
50,000m3/d 3.8
More than 100,000m3/d 3.1
The peak flow rate (QR) has been calculated using the following expression;
QR = FR (DWF – INF) + INF

34
Where:-
FR is the peak flow factor
DWF is the Dry Weather Flow (l/s)
INF is the Total Infiltration Rate
5.3.1.4 Infiltration Inflow (I/I)
This study has adopted a 15% allowance for unauthorized but unavoidable entry of groundwater
and storm water.
5.3.1.5 Minimum Size of Sewers
In order to reduce the risk of blockages and to simplify maintenance, the minimum size of all
primary trunk sewers is 300mm diameter.

35
5.3.1.6 Depth of Sewers
The depths of sewers have been provided to be sufficient to take the gravity flows of sewage
from premises. Maximum economic sewer depths have been used depending on the ground
conditions, depth of the water table, local topography, size of the sewer and its proximity to
buildings. However, sewers should rarely be deeper than 8m.
The normal minimum depth to the crown of sewers has been maintained at 1m.
5.3.1.7 Pipe Gradients
Minimum Slope (Reference: Dar-es-Salaam Master Plan – Sewerage and Sanitation Study)
- Minimum slope of sewer = 0.2% (1:500)

- Absolute minimum slope of sewer = 0.1% (1: 1,000)
- Minimum slope of lateral connection = 3% (1:33)
5.3.1.8 Manhole Spacing and Manhole Diameter

36
Manholes are provided on sewers at all changes of direction, sewer level, at every junction and
throughout the systems at intervals sufficiently close to simplify sewer cleaning.
The maximum distances at which manholes should be spaced and the minimum sizes of
manholes are given in Table 7 .17 and Table 7 .18 respectively.
Table 7.17: Maximum Spacing of Manholes

Sewer Pipe Diameter Maximum
(mm) Spacing (m)
450 or less 60
525 – 825 90
900 and above 120
Table 7.18: Minimum Sizes of Manholes

Diameter of largest Diameter of manhole rings
sewer (mm) (mm)
225to 300 1050
375 to 600 1200
675 to 900 1500
5.3.2 SEWAGE TREATMENT7
5.3.2.1 General
Treatment of wastewater by stabilization ponds is an appropriate method of wastewater treatment

in developing countries with tropical warm climates; provided adequate inexpensive land is
available. This treatment method makes use of sunlight, wind, temperature, bacteria and algae to
produce a high quality of re-usable final effluent.
Construction of the ponds is simple and straightforward which mainly comprises of earth moving
and minor concrete and pipe works, which can be executed easily.
Wastewater stabilization ponds (WSP) are highly efficient in removal of BOD (up to 70%) and
pathogens (up to 99.99%) in wastewater.
7
D Mara, Sewage Treatment in Hot Climate, 1986

37
5.3.2.2 Inlet Works
The proposed inlet works comprise the following main components:
i. Screening: The screening mechanism adopted is by use of manually raked coarse and
fine screens. The screens have been provided at an inclination of 60 o to the horizontal at
a flow velocity of 0.6m/s. The width of the channel needed for screens is found by;
Q peak
W=
s
νD ( )
b+ s
Where ν - flow velocity, m/s
D - effective depth of flow, m
s - bar spacing, mm
b - bar thickness, mm
ii. Grit removal: The proposed mechanism adopted is the use of a trapezoidal cross-
section, constant velocity grit channel controlled by a Parshall Flume. The top width,
W, of the trapezoidal channel is given by:-
5 Q Peak
W=
D
Where:- D - Depth of flow
The length of the grit channel, L, has been taken to be 20 times the depth of flow, D, to
allow for inlet and outlet turbulence and variations in settling velocity.
iii. Flow Measurement: It has been adopted that a Parshall flume been used for this
purpose, as well for flow controlling in the grit removal mechanism discussed above.
The Parshall flume has been selected based on ISO9826:1992 Standard – Flow
Measurement in open channel. Use of a digital flow recorder is incorporated.
5.3.2.3 Influent BOD Loading
For the estimation of the BOD, the following equation has been used;
1000 B
Li =
q
Where:- Li - waste water BOD,

B - BOD contribution (g/capita/day) of 40, and

38
q - Wastewater flow rate (L/capita-equivalent/day).
5.3.2.4 Waste Stabilization Ponds
i. Anaerobic Ponds: The anaerobic ponds have been designed based on volumetric BOD
loading.
For permissible volumetric loading, the following table shows the design criteria
adopted;
Table 7.19: Permissible volumetric loading and Percentage removal of BOD
Volumetric BOD5 loading

(g/m3/d )
Temperature Percentage BOD5 Removal
(0C) (%)
<10 100 40
10-20 20T – 100 2T + 20
20-25 10T + 100 2T + 20
>25 350 70
The effective volume of anaerobic pond, VA, is calculated using the following
expression;
Li Q i
V A=
λv
Where: - Li - Influent BOD5 loading, mg/l

39
Qi - Influent sewage flow, m3/day
λv - Volumetric BOD5 loading rate, g/m3/day
The mean hydraulic retention time for the anaerobic pond, θA, is calculated using;
VA
θA=
Qi
The geometry of the pond is calculated from the following expression;
D
V A= [ ( LW ) + ( L−2 sD ) (W −2 sD )+ 4 (L−sD) ]
6
Where: - D - Effective depth of pond, m

L - Length of pond at Top Water Level, m
W - Width of pond at Top Water Level, m
s - Horizontal slope factor (i.e. 1 in s)
Side slope ratio of 1:3, Length-Breadth ratio of 3:1 and a meter freeboard has been
adopted.
ii. Facultative Ponds: Facultative ponds have been designed on the surface loading basis.
The following formula has been adopted for sizing the mid-depth area;
10 Q Li
AF=
λs
Where:- λs - Design Permissible BOD5 Loading Rate, kg/ha/day

Q - Average Flow in the pond, m3/day
Li - Influent BOD5 loading from anaerobic pond, mg/lit
The permissible surface loading rate value increases with temperature, ToC. The global
permissible value of the surface-loading rate to be used in design will be estimated from
the following equation:

40
λ s=350 ( 1.107−0.002T )T−20
An effective depth of 1.75m has been used as recommended by Mara, Sewage

Treatment in Hot Climate. (1986). Side slope ratio of 1:3, Length-Breadth ratio of 3:1
and a meter freeboard has been adopted.
The effective retention time in the facultative ponds is calculated using the following
equation;
2 AF D
θF=
(2Q+ 0.001 A F e)
Where:- θF - retention time in the facultative pond, days
AF - Surface area of the facultative pond, m2
D - Effective depth of the pond, m
e - Net evaporation rate per day, mm
The effects of seepage have been assumed to be negligible and hence not taken into
account.
iii. Maturation Ponds: Maturation ponds are designed on basis of faecal coliform removal
by use of first order kinetics as recommended by D. Mara (1986).

41
The equation for the design is as follows:
Ni
N e=
1+k T θ M
Where Ni = influent number of coliform per 100 ml

Ne = effluent number of coliform per 100 ml
kT = first order rate constant for FC removal, d-1
θM = Retention time, d
The retention time for maturation pond, θM, is given by:-
1
Ni
θM=
1
[(
k T N e ( 1+k T θ A ) ( 1+k T θ F ) )]
n
−1
Where A, F and M refer to anaerobic, facultative and maturation ponds, and n is the
number of maturation ponds assuming equal sized ponds.

42
The value of the first order rate constant, kT, is given as follows depending on the
design temperature of the area;
k T =2.6 ( 1.19 )T −20
A check will also be made on the BOD5 of the first maturation pond, which must be
lower than that of the last facultative pond. The maximum loading is taken as 75% of
that on the last facultative pond.
The area of the maturation pond is then calculated from the equation;
2 Qθ M
A M=
(2 D+0.001 e θ M )
Where:- e - net evaporation rate per day, mm
They are wholly aerobic thus the effective depth adopted should be 1.2m.

43
5.3.2.5 Sludge Handling
i. Grit Removal: The grit will be removed manually in to trash rack for disposal. A truck
will be used in transport of the grit in to solid waste disposal site or to a landfill.
ii. Sludge Removal: The quantities of sludge expected to be produced in the anaerobic
ponds has been estimated using the following criteria;
- Amount of solids is 55grams/capita-day,

- Water content in the sludge is 90%
- Density of sludge has been estimated at 1,050kg/m3.
De-sludging has been designed to be done by use of pumping.
iii. Sludge Drying Beds: The sludge drying bed has been designed on the basis of loadings
of beds as dry solids per year. The criteria used has assumed that the loading of the beds
as dry solids per year as 175Kg/m2/year.
5.4 CONSTRUCTION METHODS
The majority of the construction provides no special technical difficulties, apart from those allied
to rock excavation. Standard construction techniques currently employed in Kenya will be
satisfactory.

44
8. DETAILED DESIGN OF THE PROJECT
8.1 GENERAL
The sewerage collection and conveyance system has been designed to carry municipal sewage
mainly composed of:
a) Domestic wastewater (includes residential, commercial and institutional wastewater),

b) Industrial wastewater,
c) Unauthorised but unavoidable groundwater infiltration and storm water inflows.
The treatment works has been designed to deal with medium strength sewage with industrial
wastewater expected to be pre-treated before draining to the proposed system.
This Chapter considers the components of Phase IIA-Immediate works, which have been
introduced in Chapter 5.2.
8.2 SEWAGE COLLECTION AND CONVEYANCE SYSTEM
The following Table 8 .20 shows a summary of the design details for reticulation and trunk
sewers. The sewer pipes have been designed for ultimate sewage design flow.
Table 8.20: Design summary of sewage collection and conveyance system

Sewer Pipe
Sewer Pipe Sewer Pipe No. of
Sewer Diameter
Length (m) Material Manholes
(mm)
Mugutha River Trunk Sewer
Theta River Trunk Sewer
Thiririka River Trunk Sewer
Kamuguti River Trunk Sewer

45
Sewer Pipe
Sewer Pipe Sewer Pipe No. of
Sewer Diameter
Length (m) Material Manholes
(mm)
Komu River Trunk Sewer
Juja Reticulation Sewers
Thika South Reticulation

Sewers
8.3 SEWAGE TREATMENT WORKS

8.3.1 GENERAL
The proposed sewage treatment works for Juja and Thika South are provided at different
locations depending on the natural drainage patterns. The designed STW are:-
i. Kalimoni STW, serving Ruiru and Juja area, is located near Thiririka River,
ii. Ndarugu STW, located along Ndarugu River will serve mostly Juja.
iii. Thika STW is an existing works located along Komu River. Under the scope been
designed, rehabilitation and extension of Thika STW will serve Southern parts of Thika
Municipality.
Apart from Thika STW, land for the other two treatment works sites is under individual
ownership, hence the client will need to compensate the owners and transfer ownership before
construction can commence.
The sewage treatment works recommended will comprise of the following units:-
a) Inlet works with coarse and fine bar screens,

b) Horizontal flow grit removal channels,
c) Flow measuring Parshall flumes,
d) Waste Stabilization Ponds,
e) Utility Building consisting of offices, workshop and Generator Room for Kalimoni and
Ndarugu sites,
f) One double grade 9 staff house for Kalimoni and Ndarugu sites.
The basic design parameters for each of the sites are as shown in Table 8 .21 that follows;

46
Table 8.21: Primary Design Parameters for the Sewage Treatment Works
Sewage Treatment Works
Parameter
Juja (at Ruiru) Thika
1. Population served, persons
- Present (2013) 63,576 45,023
- Initial Horizon (2015) 69,161 48,978
- Future Horizon (2025) 99,460 70,435
- Ultimate Horizon (2035) 133,667 94,659
2. Drainage Area Served, km2 153.7 61.8
3
3. Daily Water Demand Estimates, m /d;
- Present (2013) 12,715 8,707
- Initial Water Demand (2015) 13,835 8,551
- Future Water Demand (2025) 19,897 13,627
- Ultimate Water Demand (2035) 26,745 18,318
4. Daily Wastewater Flows, m3/d;
- Present (2013) 9,651 7,278
- Initial Wastewater Flows (2015) 10,500 6,522
- Future Wastewater Flows (2025) 15,099 11,391
- Ultimate Wastewater Flows (2035) 20,293 15,312
5. Mean ambient Temperature of the Coldest Month, 18 18
0
C
6. Daily Per Capita BOD5 loading rate, mg/caput/day 40 40
8.3.2 INLET WORKS
The Inlet Works has been designed to consist of coarse and fine screens, grit removal channel,
flow measuring Parshall flumes, an over flow chamber and flow splitting chambers. It’s at the
inlet works that preliminary treatment of the sewage is achieved.
The coarse and fine bar screens, grit removal channel and Parshall flumes have been provided in
two (2No.) sets so as to accommodate ease of operation of the system if a unit is taken out of
service either for servicing or during breakdown.
The units provided in the inlet works are described below;

47
i. Manually Raked Bar Screens: This is the first step in the removal of the coarse solids.
Here, the influent wastewater will be passed through coarse bar screens and then fine bar
screens. The designed screens have the following dimensions;
a. Coarse Bar Screens: This consist of bars 10mm thick, 50mm deep and placed
such that they slope at 600 to the vertical so as to have maximum area and ease of
cleaning using a hand held rake. This will have a clear spacing between the bars of
50mm.
b. Fine Bar Screens: This consist of bars 10mm thick, 50mm deep and placed such
that they slope at 600 to the vertical so as to have maximum area and ease of
cleaning using a hand held rake. This will have a clear spacing between the bars of
25mm.
ii. Grit Removal Channel: This has been designed to effect removal of grits, consisting of
sand, gravel, or other heavy solid materials that have subsiding velocities. A horizontal
flow, velocity-controlled grit chamber type has been designed.
iii. Flow Measurement Parshall Flumes: This has been incorporated at the main inlet
channel.
A measuring scale is installed on the upstream side of the flume which will be read every
day. However, the actual measurement is to be carried out by use of an electroflo recorder
that measures the depth of water on the upstream side of a flume, and records on a
pedestal. These instrument records the rate of flow and also the total amount that flows.
iv. Overflow Chamber: An overflow chamber has been incorporated to assist in regulating
the quantities of sewage that flows in to the waste stabilization ponds. The overflow
chamber incorporates the use of a transverse weir and a stilling chamber. Excess sewage
flow will be drained into the overflow/outfall open channel that drains into the nearby
rivers.

48
v. Flow Splitting Chambers: This has been provided to split the flow into the various
parallel streams of waste stabilization ponds. The flows have been provided to be
controlled by use of sluice gates operated using rotary wheels. An emergency open
channel has been provided to bypass the waste stabilization ponds in to the river via the
overflow/outfall channel.
The design details of the units of the inlet works for the sewage treatment works sites are
presented in the table below;
Table 8.22: Design details of the inlet works units for the various S.T.W
Unit
1. Coarse Screens,
Cross-sectional Area, m2 0.5
No. of units, Nr 2 2
No. of bars per unit, Nr 16
Thickness of bars, mm 10
Spacing of the bars, mm 50

49
Unit
2. Fine Screens,
Cross-sectional Area, m2 0.5
No. of units, Nr 2 2
No. of bars per unit, Nr 27
Thickness of bars, mm 10
Spacing of the bars, mm 25
3. Grit Removal Channel,
No. of Units, Nr 2 2
Length of each channel, m

50
Unit
Width of each channel, m
Depth of each channel, m
4. Flow Measuring Parshall Flumes, ISO 9826:1992
Number of units, Nr 2 2
Upstream step slope
Upstream stilling channel length, m
Throat breadth, m
Downstream opening, 0

51
Unit
8.3.3 WASTE STABILIZATION PONDS
A Waste Stabilization Ponds system has been provided for the treatment of the sewage. The
ponds are designed such that the works are functional even during maintenance period when one
system is closed. A series comprising of Anaerobic, Facultative and three (3No) Maturation
ponds have been found to be adequate to fully treat the sewage to acceptable levels in each site.
The Table 8 .23 below summarizes the design characteristics of each of the ponds in each of the
sewage treatment works sites;
Table 8.23: Design Details of waste stabilization ponds

Retentio Effluen Effluent
Paralle Pond Dimensions (m) n Time t BOD5 FC
Pond l Ponds (days) (mg/l) (/100ml)
(Nr)
D FL.L T.W.L EM.L
JUJA (AT RUIRU) SEWAGE TREATMENT WORKS – 10,500m3/d
4.0
Anaerobic 2
0
1.7
Facultative 2
5
1.5
1st Maturation 2
0

52
Retentio Effluen Effluent
Paralle Pond Dimensions (m) n Time t BOD5 FC
Pond l Ponds (days) (mg/l) (/100ml)
(Nr)
D FL.L T.W.L EM.L
1.5
2nd Maturation 2
0
1.5
3rd Maturation 2
0
THIKA SEWAGE TREATMENT WORKS EXTENSION WORKS – 6,522m3/d

4.0
Anaerobic 3
0
1.7
Facultative 3
5
1.5
1st Maturation 3
0
1.5
2nd Maturation 3
0
1.5
3rd Maturation 3
0
D – Effective depth of the pond

FL.L – Ponds Floor Level
T.W.L – Ponds Top Water Level
EM.L – Ponds Embankment Level
8.3.4 OUTLET WORKS
In each of the sites, an outfall sewerline has been provided that drains the effluent treated waste
water in to the nearby rivers.
8.3.5 ANCILLARY WORKS
For effective operation and maintenance of the sewage treatment works, the following ancillary
works have been incorporated in the designed system;
i. Utility Building: This building has been provided to house the day to day administrative
duties within the site. The building provided incorporates offices, a laboratory, workshop,
storage room and back-up generator room. This has been provided at the Kalimoni and
Ndarugu sites only.

53
ii. Double Grade 9 Staff House: This has been provided for accommodation of staff who
works during night hours. This has been provided at the Kalimoni and Ndarugu sites only.
iii. Landscaping, Fencing and Gate House: It has been provided that surplus excavated
material is used in general site levelling. Also planting of grass within the site has also
been provided. A chain link fence has been provided around the site constructed with
concrete fence posts. A standard gate with a pedestrian gate has been provided at the
entrance and an additional pedestrian gate provided close to the outfall. A gate house has
also been provided at the main entrance gate. This works has been provided for all the
sites.
iv. Access Road, onsite roads and storm water drainage works: An access road has been
provided from the nearest road to the S.T.W. Onsite road work will consist of a parking.
This has been provided at the Kalimoni and Ndarugu sites only.
v. Electrical works: KPLC mains power connection has been provided to site for site works
and lighting. A small standby generator has also been provided for site lighting only. This
works has been provided for the Kalimoni and Ndarugu Sites only.
vi. Onsite Water and Sanitation Works: All site buildings have been provided with water
and sanitation services. Water will be provided by RUJWASCO. XXm 3 capacity elevated
water tanks has been provided in each of the site for water storage. Sewage generated in
the site buildings have been provided to be drained directly into the nearest Anaerobic
Pond. This works will be undertaken in the Kalimoni and Ndarugu sites only.
8.3.6 REHABILITATION WORKS FOR THIKA S.T.W
The existing sewage treatment works for Thika are currently working beyond the design
capacity. The following works have been proposed to be undertaken as part of the rehabilitation
of the existing system at Thika STW;
i. Construction of a new inlet works complete with coarse and fine bar screens, grit
channels, flow measuring Parshall flumes and overflow chamber to replace the current
broken inlet works;
ii. De-sludging all 6No. primary facultative ponds;
iii. Minor works like bush clearance, reinstatement of drainage system and repair of fence.
8.4 EFFLUENT AND SLUDGE DISPOSAL
It is recommended that the effluents from the sewerage treatment process be disposed of in the
following two ways;
i. The effluent wastewater to be disposed of in to the nearby rivers downstream of any

water abstraction points as follows;

54
a. For Kalimoni S.T.W, the effluent of the last maturation ponds can be disposed of
to the nearby Thiririka River,
b. For Ndarugu S.T.W, the effluent of the last maturation ponds can be disposed of
to the nearby Ndarugu River,
c. For Thika S.T.W, the effluent of the maturation ponds can be disposed of to the
nearby Komu River.
ii. The final sludge remnant after open air drying to be sold to farmers for use as fertilizer
for the nearby plantations.

55
9. OPERATION AND MAINTAINANCE OF FACILITIES
9.1 GENERAL
So as to ensure that the designed sewerage system serves its purpose, it is imperative that the
different component that makes up the system are operated and maintained in such a way to
obtain optimum results. Operation and maintenance should cover the sewer lines and the sewage
treatment works facilities.
9.2 SEWER SYSTEM
The sewer systems form the backbone of the sewage conveyance system and hence adequate care
must be taken to ensure that their function is not hindered. Though the sewers have been
designed to be self-cleansing, during the early stages of operation of the system, the flow in the
sewer lines will be very low and a certain amount of settlement of solids may take place thereby
requiring flushing. Also some sand may enter the system from other sources causing settlement
to take place at low flows. Periodic inspections must be carried out to detect such siltation and
remove it by flushing.
The operation and maintenance strategy for sewers should be guided by the following
principles:-
- Maintenance of free flow of sewers within the lines by removal of all obstructing
materials and regular de-silting of the sewer lines,
- Planned and systematic ground and subterranean inspections on a regular basis using
various techniques including sewer cameras and physical visual observation. These
inspections are aimed at checking the condition of the sewer flow within the conduit and
unusual subsidence or corrosion of sewers.
Additionally, a sewer inspection team can be constituted from time to time to fit in the inspection
of the entire sewer network. Inspection can be incorporated as part of the normal sewer operation
and maintenance activity.
9.3 TREATMENT WORKS
9.3.1 ROUTINE OPERATION AND MAINTENANCE ACTIVITIES
Maintenance requirements of the designed system are very simple, but must be carried out
regularly to avoid serious odour, fly and mosquito nuisance. Ponds must be properly operated
and maintained so as to obtain the designed efficiency and life. Although a minimum of attention
from the operator is required, failure to follow the simple operating procedures can seriously
affect the efficiency of the ponds. The daily and routine operations performed as follows:-
- Removal of screenings and grit from the inlet works and disposing at the designated place;

56
- Cutting the grass on the embankments and removing it so that it does not fall into the pond
so as to prevent the formation of mosquito-breeding habitats; the use of slow-growing
grasses minimizes this task;
- Removal of floating scum and floating aquatic plants from the surface of facultative and
maturation ponds so as to maximize photosynthesis and surface re-aeration and obviate fly
breeding;
- Spraying the scum on anaerobic ponds with clean water or pond effluent to prevent fly
breeding. This scum should however not be removed as it aids the treatment process;
- Removal of any accumulated solids in the inlets and outlets chambers of the ponds;
- Repair of any damage to the embankments caused by burrowing animals;
- Desludging of Anaerobic ponds when they are a third full of sludge as discussed in Section
9.3.3; and,
- Repair of any damage to external fences and gates.
The operators must be given precise instructions on the frequency at which these tasks should be
done, and their work must be constantly supervised. The supervisor/ foreman should be required
to complete at weekly intervals a pond maintenance record sheet. The operators may also be
required to take samples and some routine measurements.
9.3.2 START-UP PROCEDURE FOR WASTE STABILIZATION PONDS
The proposed waste stabilization pond systems comprising of anaerobic, facultative and
maturation ponds system should preferably be commissioned at the beginning of the hot season
so as to establish as quickly as possible the necessary microbial populations to effect waste
stabilization. Prior to commissioning, all ponds must be free from vegetation. Facultative ponds
should be commissioned before anaerobic ponds: this avoids odour release when anaerobic pond
effluent discharges into an empty facultative pond. It is best to fill facultative and maturation
ponds with freshwater so as to permit the gradual development of the algal and heterotrophic
bacterial populations. If freshwater is unavailable, facultative ponds should be filled with raw
sewage and left for three to four weeks to allow the microbial population to develop; a small
amount of odour release is inevitable during this period.
Anaerobic ponds should be filled with raw sewage and seeded, where possible, with digesting
sludge from, for example, with sludge from local septic tanks. The ponds should then be
gradually loaded up to the design loading rate over the following week (or month if the ponds are
not seeded).
Care should be taken to maintain the pond pH above 7 to permit the development of
methanogenic bacteria, and it may be necessary during the first month or so to dose the pond
with lime; or soda ash. If due to an initially low rate of sewer connections in newly sewered
towns the sewage is weak or its flow low, it is best to by-pass the anaerobic ponds until the
sewage strength and flow is such that a loading of at least 100 g/m1/d can be applied to them. (It

57
is also necessary to by-pass an anaerobic pond whilst it is being de-sludged, so the by-pass
should be a permanent facility).
9.3.3 DE-SLUDGING AND SLUDGE DISPOSAL
Anaerobic ponds require de-sludging when they are one third full of sludge (by volume). This
occurs every nth year, where n is given by the following expression:-
V
n=
3 Ps
Where:- V - Volume of anaerobic pond, m3

P - Population served
S - Sludge accumulation rate, m3/person-year
The usual design value of s is 0.04m 3/person-year. Sludge removal can be readily achieved by
using a raft-mounted sludge pump that is commercially available or they can be assembled
locally. The sludge has been designed to be discharged into a sludge drying bed. Although waste
stabilization ponds sludge has a comparatively better microbiological quality, its disposal must
be carried out in accordance with the relevant Kenya national and Kiambu county regulations
governing sludge disposal.
9.4 OPERATION AND MAINTENANCE HUMAN RESOURCES
In order that preventive, routine and corrective O&M tasks are properly done, the WSPs must be
adequately staffed. The level of staffing depends on the type of works to be carried out. It is
recommended that a Sanitation/Sewerage Section be established within the Water and Sanitation
Services Providers set-up for RUJWASCO and the existing section at THIWASCO be
strengthened to take charge of the maintenance of the sewerage system. This sewerage sections
should have adequate staff and funds allocated for operation and maintenance of the system.
Comprehensive training should be given to the staff in order to ensure that best practices are
introduced early.
The number of staff for every position will vary from one scheme to the other. Table 9 .24
shows the proposed number of human resources to be mobilised in every scheme.
Table 9.24: Proposed mobilization of key personnel

ITEM Units Juja (at Ruiru) Thika
Sewerage System Operator - RUJWASCO THIWASCO
Sewerage Superintendents nr 1 1
S.T.W Supervisors nr 1 1
Sewers Supervisors nr 1 1
S.T.W Laboratory Technicians nr 2 2
Sewage Plant Technicians nr 3 3
Electro-Mechanical Technicians nr 1 1
Sewers Operators nr 2 2

58
Pump Operators nr 1 1
Other Support Staff nr 10 10
Figure 9 .4 shows the proposed staffing for the Sewerage/Sanitation Department necessary for
proper operation and maintenance of the sewerage investments.

59
BOARD OF
DIRECTORS
MANAGING
DIRECTOR
TECHNICAL
MANAGER
SEWERAGE
SUPERINTENDENT
Sewage Treatment Sewers Management

Works Unit Unit
SEWERS
S.T.W SUPERVISOR
SUPERVISOR
S.T.W ARTISANS SEWERS

- Laboratory Technicians OPERATORS
-Sewage Plant Technicians
- Electro-Mech Technician
OTHER SUPPORT STAFF

- Pumps Operators
- Drivers
- Watchmen
- Messengers
- Cleaners
- Labourers
Figure 9.4: Proposed Staffing Structure of the Sewerage Department
9.5 OPERATION AND MAINTENANCE MATERIAL RESOURCES
For the proposed staff to effectively carry out their assigned duties effectively, relevant material
resources need to be mobilised. These material resources include;

60
- Inspection and Transport vehicles,
- Offices and Laboratory furniture, and
- Tools and equipment for the sewage treatment works and sewers operation.
The number and type of material resources to be provided varies from one scheme to the other.
Table 9 .25 gives the proposed number and type of the resources needed to be mobilised once
the designed sewerage systems are operational.
Table 9.25: Proposed mobilization of material resources

Sewerage System Operator - RUJWASCO THIWASCO
4WD Double Cabin Pick-up truck nr 1 1
Motorcycles nr 1 1
Heavy Duty Lawn Mowers nr 1 1
Jetting/Vacuum/Exhauster truck nr 1 1
Mobile workshop converted pick-up nr 1 1
Laptop Computers complete nr 3 3
Deskjet Printers nr 1 1
Workshop tools and equipment Item 1 1
Laboratory Furniture and equipment Item 1 1
Staff Houses Furniture Item 1 0
Administrative offices Furniture Item 1 0
9.2

61
10. RESETTLEMENT ACTION PLAN

62
11. ENVIRONMENTAL IMPACT ASSESSMENT

63
12.PROJECT COST ESTIMATE AND FINANCIAL ANALYSIS
12.1 CAPITAL COST ESTIMATES
12.2 OPERATIONAL AND MAINTENANCE COST ESTIMATES
12.3 FINANCIAL ANALYSIS
12.4 CONCLUSION AND RECOMMENDATIONS

64
13.PROJECT IMPLEMENTATION PLAN
13.1 GENERAL
The construction works sites are envisaged to be vast and widespread, with heavy earthworks
required for the ponds, requiring experienced Contractor with sound technical and financial
capabilities.
The complexity and diverse nature of the works will require him to have a strong team of
qualified and experienced technical personnel capable of managing, executing and delivering the
project. Due to this, it is expected that the bidders will be pre-qualified before being allowed to
proceed to the bidding stage. As a result, only competent bidders shall be allowed to bid for the
works.
Subject to the source of funding for the construction works, international tenders could be called
for to allow interested firms fulfilling the procurement guidelines set by the funding agency to
bid. The successful bidder shall then execute a contract with the Athi Water Services Board,
which shall act as the Employer. The FIDIC Conditions of Contract shall be used as these are
well understood by most of the Contractors worldwide. Bills of quantities shall be measured in
the Civil Engineering Standard Methods of Measurements (CESMM), which is well understood
both locally and internationally.
As required under the FIDIC Conditions of Contract, the following shall form the Construction
Team:-
Employer: Chief Executive Officer, Athi Water Services Board;

Project Manager: Chief Manager, Infrastructure, Athi Water Services Board;
Project Engineer: Technical Manager, Athi Water Services Board;
Engineer’s Representative: Consulting firm(s) to be appointed;
Main Contractor: Successful bidding firm accepted by the Employer;
Sub-contractor(s) Firms nominated by the Employer and/or employed by the
main Contractor.
13.2 CONSTRUCTION CONTRACT PERIODS
It’s estimated that the project can successfully be implemented in 24 months once a suitable
contractor has been appointed and handed the site.
Table 13 .26 that follows presents the project implementation proposal for the project. A period
of 4 months is envisaged for tendering and award. A 12-month Defects Liability Period has also
been provided, whereby the chosen Contractor shall attend to any faults arising from execution of
the works.
Table 13.26: Construction Stage Implementation Schedule

Activity Months Execution of Activities

65
Tendering and Award 4
Construction 24
Defects Liability Period 12

66

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ATHI WATER SERVICES BOARD

CONSULTANCY SERVICES FOR A FEASIBILITY STUDY AND

DRAFT DESIGN REPORT

FRAME Consultants Ltd Athi Water Services Board

FRAME Consultants Ltd Draft Design Report

FRAME Consultants Ltd Draft Design Report

FRAME Consultants Ltd Draft Design Report

Table 2.1: Mean Climatic Data for the period 2011-2013...............................................................8

FRAME Consultants Ltd Draft Design Report

Figure 2.1: Location of Project Area................................................................................................6

FRAME Consultants Ltd Draft Design Report

AWSB - Athi Water Services Board

FRAME Consultants Ltd Draft Design Report

FRAME Consultants Ltd Draft Design Report

 Nairobi Water and Sewerage Company

1.2 OBJECTIVE OF THE STUDY

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1.3 SCOPE OF THE STUDY

The study involved not less than the following tasks:-

The Study was preceded with two (2) stages as follows:

Stage 1: Desk Study

Stage 2: Field Visit

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1.3.2 DETAILED DESIGN PHASE

- Finalization of estimation of wastewater generation.

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1.5 SCOPE OF THIS REPORT

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2. PROJECT AREA BACKGROUND INFORMATION

2.1 THE PROJECT AREA

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Figure 2.1: Location of Project Area

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1.3.3.5 Relative Humidity

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Table 2.1: Mean Climatic Data for the period 2011-2013

1.3.4 GEOLOGICAL AND SUB-SOIL CONDITIONS

i. Archean: Basement System

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1.3.4.3 Soil Conditions

1.4 SOCIO-ECONOMIC INFRASTRUCTURE

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The project area is proposed to cover:-

Thika Githima 1,626 1,694 3,320 843 27.3 122

Table 2 .3 below summarizes some of the demographic indicators.

Table 2.3: Demographic Indicators2

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1.5 WATER SUPPLY INFRASTRUCTURE

2.2.4 THIKA TOWN

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2.2.6.2 Sewerage of the Army Zone

2.2.6.3 Existing Sewage Treatment Works

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Sewage influent strength - 370 mg BOD5/1.

2.2.6.4 Other Sanitation Facilities

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3.3 GENERAL POPULATION