design of an improved grey water recycling...

UNIVERSITY OF NAIROBI

SCHOOL OF ENGINEERING

DEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS

ENGINEERING

ENGINEERING DESIGN PROJECT

DESIGN OF AN IMPROVED GREY WATER RECYCLING

SYSTEM

NAME : MOGAKA ALPHAS OMBESE

REG NO : F21/0031/2008

SUPERVISOR: ENG. S.C ONDIEKI

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

of Science in Environmental and Biosystems Engineering.

APRIL 2013

ii

DECLARATION

This project is my work and has not been presented for any degree in any other

university.

SIGNATURE DATE

MOGAKA ALPHAS OMBESE

STUDENT

This project has been submitted for the examination with my approval.

SIGNATURE DATE

ENG. S C ONDIEKI

SUPERVISOR

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ABSTRACT

Students residing in Mugabe Hostel of the College of Agriculture and Veterinary

Services of the University of Nairobi have been plagued with inconsistent supply of

water to the halls of residence. Sanitation has been greatly hampered due to this

especially in the toiletry section.

This project proposes a grey water recycling system that will provide recycled

water specifically for flushing the lavatories. The grey water recycling system

components are designed and they consist of: Diversion system, piping system,

filtration system, storage system and the pumping system. They various systems

were tested on the EPANET software and it proved successful.

The study begins with the introduction and objectives and the literature review

together with the theoretical frame work highlighted. The methodology, data

collected, results and analysis are discussed within. Finally, the conclusions

recommendation and design drawings are provided.

The findings of the project are compiled in this report.

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ACKNOWLEDGEMENTS

This project completion has been made possible with the assistance of a

number of persons to whom I would like to express my sincere gratitude.

I am indebted to my supervisors Eng. S. C Ondieki for his continued guidance,

suggestions, comments and encouragement throughout the period and

completion of this study.

I would also like to express my gratitude to the Maintenance manager and his

staff for the assistance they afforded me during the study.

Finally, I owe special thanks to my family for their encouragement and moral

support. Thank you all for your prayers.

MAY GOD BLESS YOU ALL

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Table of Contents DECLARATION ............................................................................................................................................... ii

ABSTRACT ..................................................................................................................................................... iii

ACKNOWLEDGEMENTS ................................................................................................................................ iv

LIST OF FIGURES ......................................................................................................................................... viii

LIST OF TABLES ............................................................................................................................................. ix

1. INTRODUCTION ..................................................................................................................................... 1

1.1. Background Information ............................................................................................................... 1

1.2. Problem Statement ....................................................................................................................... 1

1.2.1. Project Justification ................................................................................................................... 2

1.2.2. Problem Analysis ....................................................................................................................... 2

1.3. Site Analysis and Inventory ........................................................................................................... 2

1.4. Objectives...................................................................................................................................... 3

1.4.1. Specific Objectives .................................................................................................................... 3

1.5. Statement of Scope ....................................................................................................................... 3

2. LITERATURE REVIEW ............................................................................................................................. 4

2.1. Grey Water .................................................................................................................................... 4

2.2. Potential of Greywater Reuse ....................................................................................................... 5

2.3. Benefits of Grey Water recycling .................................................................................................. 6

2.4. Quantification of grey water ......................................................................................................... 7

2.5. Direct Method ............................................................................................................................... 8

2.6. Indirect Method ............................................................................................................................ 9

2.7. Components of Greywater Treatment Systems ........................................................................... 9

2.8. Components of Greywater Treatment Systems ......................................................................... 10

2.8.1. Diversion System ..................................................................................................................... 10

2.8.2. Piping system .......................................................................................................................... 10

2.8.3. Filtration unit .......................................................................................................................... 10

2.8.3.1. Upflow-downflow filter ................................................................................................... 11

2.8.3.2. Multi-media filter ............................................................................................................ 11

2.8.3.3. Slow sand filter ................................................................................................................ 11

2.8.3.4. Horizontal Roughing Filter .............................................................................................. 12

2.8.4. Delivery and Storage System .................................................................................................. 13

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2.8.5. Pumping Unit .......................................................................................................................... 13

3. THEORETICAL FRAMEWORK ............................................................................................................... 14

3.1. Grey Water .................................................................................................................................. 14

3.2. Biochemical Oxygen Demand (BOD) ........................................................................................... 14

3.3. Site Selection ............................................................................................................................... 15

3.4. Head loss ..................................................................................................................................... 15

3.5. The Pump characteristics ............................................................................................................ 16

3.6. Sizing of conveyance pipes ......................................................................................................... 17

3.7. Design of storage tank ................................................................................................................ 18

3.8. Tank Loadings .............................................................................................................................. 20

4. GENERATION OF CONCEPT DESIGN .................................................................................................... 21

5. METHODOLOGY .................................................................................................................................. 22

5.1. Study Area ................................................................................................................................... 22

5.1.1. Location ................................................................................................................................... 22

5.1.2. Land Cover .............................................................................................................................. 23

5.1.3. Topography ............................................................................................................................. 23

5.1.4. Water Supply System .............................................................................................................. 23

5.1.5. Waste Management................................................................................................................ 23

5.2. Data Collection ............................................................................................................................ 23

5.2.1. Water Consumption Data ....................................................................................................... 23

5.3. Design .......................................................................................................................................... 24

5.3.1. Design of the diversion system ............................................................................................... 24

5.3.2. Filtration and Treatment System ............................................................................................ 24

5.3.3. Delivery and Storage System .................................................................................................. 24

5.3.4. Overall Design Layout ............................................................................................................. 24

6. RESULTS AND ANALYSIS ...................................................................................................................... 25

6.1. Water Consumption Data ........................................................................................................... 25

6.2. Diversion System ......................................................................................................................... 27

6.2.1. Piping ....................................................................................................................................... 27

6.2.2. Junction Chamber ................................................................................................................... 27

6.3. Filtration and Treatment System ................................................................................................ 27

6.3.1. Screen ...................................................................................................................................... 27

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6.3.2. Equalization or Settling Tank .................................................................................................. 28

6.3.2.1. Design Criteria for an Equalization Chamber .................................................................. 28

6.3.3. Filtration Unit .......................................................................................................................... 30

Disinfection ............................................................................................................................................. 31

6.4. Collection / Storage Tank ............................................................................................................ 31

6.4.1. Material ................................................................................................................................... 32

6.4.2. Shape and Position .................................................................................................................. 32

6.4.3. Dimensions of the storage tank .............................................................................................. 33

6.5. Pumping and Piping System ........................................................................................................ 35

6.5.1. Pipes ........................................................................................................................................ 35

6.5.2. Pumps...................................................................................................................................... 35

6.6. COST BENEFIT ANALYSIS ............................................................................................................. 37

6.7. SCHEMATIC DIAGRAM OF A GREY WATER RECYCLING SYSTEM ..................................................... 38

6.8. Water Quality Monitoring ........................................................................................................... 39

6.8.1. Odour Control ......................................................................................................................... 39

6.9. Maintenance of Greywater Treatment System .......................................................................... 40

7. CONCLUSION ....................................................................................................................................... 42

8. RECOMMENDATION ........................................................................................................................... 43

9. REFERENCES ........................................................................................................................................ 44

10. APPENDICES .................................................................................................................................... 46

OBSERVATION / DATA SHEET .................................................................................................................. 46

TANK MATERIAL PROPERTIES ................................................................................................................. 48

3D VIEWS OF THE SYSTEM PARTS ........................................................................................................... 49

EQUILIZATION CHAMBER ........................................................................................................................ 51

FILTRATION UNIT .................................................................................................................................... 52

STORAGE TANK ....................................................................................................................................... 53

PUMPING SYSTEM .................................................................................................................................. 54

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LIST OF FIGURES

Figure 1: Water Treatment Cycle

Figure 2: Google Maps Location of Mugabe Hostel.

Figure 3: Equalization Chamber

Figure 4: Filtration Unit

Figure 5: Storage Tank

Figure 6: Pump

Figure 7: Schematic Diagram of a Grey Water Recycling System

Figure 8: Tank Material Properties

Figure 9:3D Views of the System Parts

Figure 10: Different Views of the Recycling Unit

Figure 11: Equalization Chamber

Figure 12: Filtration Unit


Figure 14: Pumping System

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LIST OF TABLES

Table 1: Average Volume of Water Consumed In a Day

Table 2: Design Criteria for an Equalization Chamber

Table 3: Filtration Unit Parameter

Table 4: Pipe Sizes

Table 5: Pump Characteristics

Table 6: Observation / Data Sheet

Table 7: Summary of the System Maintenance

1. INTRODUCTION

1.1. Background Information

Water is becoming a rare resource in the world. It is therefore essential to reduce

surface and ground water use in all sectors of consumption, to substitute fresh

water with alternative water resources and to optimize water use efficiency through

reuse options.

These alternative resources include rainwater and greywater. This project will focus

on greywater treatment and its use as an alternative water resource in a students’

hostel.

Greywater is commonly defined as wastewater generated from bathroom, laundry

and kitchen. Due to rapid industrialization and development, there is an increased

opportunity for greywater reuse in developing countries such as Kenya.

Consequent to rapid growth in population and increasing water demand, stress on

water resources in Kenya is increasing and per capita water availability is reducing

day by day.

Recycling grey water for purposes of flushing toilets is therefore a viable option.

However the volume of grey water produced should be sufficient enough to warrant

such a project.

1.2. Problem Statement

Mugabe Hostel has been plagued with inconsistent water supply. This has had

severe consequences on the sanitation of the hostel, especially on the toiletry

section. Students are therefore forced to fetch water from the groundwater tanks,

ferry it to the wash rooms for their use while the toilets had no source of water. This

forced critical thinking on ways of solving this problem, hence developing a system

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where grey water from laundry and showers can be reused to flash the toilets will

ensure a constant supply of water and freeing fresh water for better consumption.

1.2.1. Project Justification

The need to uphold sanitation standards in Mugabe Hostels has prompted critical

thinking to come up with solutions. Various ways of supplementing the existing

water supply to the hostel have been exhausted. In the recent past two boreholes

had been sunk to supplement the water deficit being experienced at the Hostel.

However, they are now trying to cover up the boreholes that had been sunk in the

area and after cleaning and testing, the water discovered has been ruled out as

unsuitable for human consumption.

This project can therefore act as a plan B to the water shortage problem if

implemented.

1.2.2. Problem Analysis

Given the large quantity of grey water being discharged from the hostel, recycling it

and using it to flush the toilet will not only ensure constant supply of water for

flushing the toilets but also effectively reduce the strain on fresh water supply i.e.

fresh water needed for flushing will be zero.

1.3. Site Analysis and Inventory

The case study for this project is based on Mugabe Hostel in the College of

Agriculture and Veterinary Sciences of the University of Nairobi. The Hostel is

located in Upper Kabete, off Waiyaki way. This is where proper sanitation is

hindered due to the inadequate water supply to the hostel. Most of the water used is

in the showers, for laundry and toilet flushing. This therefore means that once the

tanks are empty the state of the lavatories becomes dire. The system is well suited

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since the topography of the region will allow for flow of the waste water downhill

hence the recycling process will be by gravity.

1.4. Objectives

The Overall Objective is to design a grey water recycling system for Mugabe Hostel.

1.4.1. Specific Objectives

� Determine the quantity and quality of the grey water

� Design the various system components for the grey water collection

1.5. Statement of Scope

• This project involves the design of a grey water recycling system for Mugabe

Hostel.

• Water consumption and quality evaluation will also be determined

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2. LITERATURE REVIEW

2.1. Grey Water

Grey water is wastewater generated from domestic activities such as laundry,

bathing and dishwashing. It comprises of about two-thirds of domestic water use. It

gets its name from the cloudy appearance and from its status as neither being fresh

nor polluted (Wikipedia 2009).

Water from the toilets is sewage or black water which indicates it has fecal matter

and urine hence cannot be recycled.

Grey water is easier to treat and recycle because of the low levels of contamination.

If the grey water is harvested on a separate plumbing, the grey water can be

recycled, stored and re-used. Filtration units and microbial digestion can be used to

provide clean water for flushing toilets or even watering garden plant (Myca, 2002)

Most grey water is easier to treat and recycle than blackwater, because of lower

levels of contaminants. If collected using a separate plumbing system from

blackwater, domestic grey water can be recycled directly within the home, garden

or company and used either immediately or processed and stored. If stored, it must

be used within a very short time or it will begin to putrefy due to the organic solids

in the water. Recycled grey water of this kind is never safe to drink, but a number of

stages of filtration and microbial digestion can be used to provide water for washing

or flushing toilets. Some grey water may be applied directly from the sink to the

garden or container field, receiving further treatment from soil life and plant roots.

Given that grey water may contain nutrients, pathogens, and is often discharged

warm, it is very important to store it before use in irrigation purposes, unless it is

properly treated first. (Asano, 2004)

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Figure 1: Water treatment cycle

2.2. Potential of Greywater Reuse

Reuse of greywater serves two purposes:

• Reduces fresh water requirement

• Reduces sewage generation

The amount and quality of greywater will in part determine how it can be reused.

Irrigation and toilet flushing are two common uses, but nearly any non-contact use

is a possibility. Toilet flushing can be done either by direct bucketing or by pumping

treated greywater to an overhead tank connected by suitable piping to the toilets.

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The grey water recycled can be used for the following purposes.

• Toilet flushing

• Floor cleaning

• Irrigation

• Gardening

• Car washing

• Construction

2.3. Benefits of Grey Water recycling

The concept of wastewater recycling has been continuously developed and various

benefits have arisen due to the recycled water. They include:

� Lower fresh water extraction from rivers and aquifers hence reduced strain

on resources

� Reduce strain on septic system or treatment plant - Greywater makes up the

majority of the household wastewater stream, so diverting it from the septic

system extends the life and capacity of the system. For municipal systems,

decreased input means more effective treatment coupled with cost savings.

� Indoor use e.g. toilet flushing

� Develop otherwise unsuitable real estate - A grey water recycling system,

along with the use of composting toilets, can enable the development of

property that is unsuitable for a septic system.

� Groundwater Recharge - Greywater recycling for irrigation replenishes

groundwater, helping the natural hydrologic cycle to keep functioning.\

� Irrigation and Plant growth - Greywater can support plant growth in areas

that might otherwise not have enough water.

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� Maintain soil fertility - The nutrients in the grey water are broken down by

bacteria in the soil and made available to plants. This helps to maintain soil

fertility.

� Enhance water quality - The quality of groundwater and surface waters are

much better preserved by the natural purification processes the grey water

undergoes in the top layers of the soil than by any engineered water treatment.

(Estray, 2009)

Most European countries have already adopted the concept of grey water recycling

but on a low scale level i.e. domestically. But in the United States, various by-laws

restrict the implementation and storage of the grey water systems thereby

inhibiting the spread of this system

In order to carry out such a project, various design parameters have to be taken into

consideration. They include:

• Water Supply

• Quantity of grey water produced

• Land topography for the site

• Soil characteristics

• Cost

• Use of the recycled water (Jeppesen, 1994)

2.4. Quantification of grey water

Determination of greywater generation and flow rate is the first requirement in the

design of greywater collection, treatment and reuse system. Reliable data on

existing and projected flow rate must be available for the cost-effective greywater

treatment system design.

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Following methods are proposed for quantification of greywater:

2.5. Direct Method

I. Water Meter

In the water meter method, a meter is provided at the outlet of the drain connecting

bathrooms, kitchen and cloth washing place (laundry). If not possible, the meter can

be placed at the inlet of the greywater collection tank which can be connected to

bathroom, kitchen and laundry.

Small plumbing modification in the piping system will allow collection of greywater

system which can be easily measured. This system can be fitted in residential

schools where variation in greywater quantity is not expected.

II. Bucket Method

This is the simplest form of greywater quantification wherein greywater is collected

in a bucket of known volume at the outlet of bathroom, laundry or kitchen. This

method is cheap and suitable where greywater quantity remains almost constant for

a substantial time period. The method is manual and precautions are required to

avoid any human contact with greywater. The method is described below:

• Identify outlet

• Keep a 20 liter bucket at outlet of bathroom and laundry

• Start stop watch and measure time for filling of 20 liter bucket

• Measure during 24 hour cycle

• Measure once per month

• Measure only during February, March and April

• Find out average value of greywater per day

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2.6. Indirect Method

Greywater quantity is about 50-60% of total water consumption. The quantity of

water consumed can also be used to quantify greywater. Indirect method also

includes correlation between a variable and greywater generation. A correlation is

developed between number of students in the hostel and greywater generation

based on the data collected.

2.7. Components of Greywater Treatment Systems

Greywater treatment process varies from simple devices that divert greywater for

direct application such as irrigation to complex systems involving sedimentation

tanks, filters, bioreactors, pumps and disinfection systems.

Advances in the effectiveness and reliability of wastewater technologies have

improved the capacity to produce reused water that can serve as alternative water

source in addition to meeting water quality protection and pollution abatement

requirements (Lazarova, 2000).

A number of technologies have been applied for greywater treatment worldwide

varying in both complexity and performance (Jefferson et al., 2001).

The following greywater systems considering non-contact application are

considered:

Primary treatment - pre-treatment to secondary treatment

• Screening

• Equalization

Secondary treatment

• Gravel filtration

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• Sand filtration

• Chlorination

2.8. Components of Greywater Treatment Systems

2.8.1. Diversion System

A diversion system is used to convey the grey water away from the sewers. A fully

engineered system will incorporate a sump tank and surge tanks.

A simple plumbing device diverts greywater in the wastewater drainage line to a

junction chamber for recycling.

2.8.2. Piping system

Combined wastewater is usually conveyed in 7.5 to 10 cm of pipes in residential

areas. Since grey water carries some solids, most of the solids tend to scrap the

bottom of the pipe in bigger pipe sizes, while small diameter pipes tend to get

clogged. Therefore, medium pipe sizes are preferred to either large or small

diameter sizes.

2.8.3. Filtration unit

The type of filter required for a greywater system depends largely upon the amount

of greywater to be filtered, the type of contaminants present and end use. A drain

filter is an easy and inexpensive way to filter lint and hair out of bath or laundry

water. A simple cloth bag tied over the end of a bathroom pipe may be sufficient for

irrigating outdoors or similar applications. Filtration is one of the most important

operations in the greywater purification process (Wright, 1986).

Though screening and sedimentation process remove a large proportion of

suspended matter, they do not effectively remove fine flock particles, colour,

dissolved minerals and microorganisms. In filtration, water is passed through a

filter medium in order to remove the particulate matter not previously removed by

sedimentation. During filtration, the turbidity and colloidal matter of non-settleable

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type protozoan cysts and helminth eggs are also removed. It is to be mentioned that

protozoa are stopped in the gravels, the bacteria by the medium gravel and the

viruses by the sand(Wright, 1986).

The filter types are as below:

2.8.3.1. Upflow-downflow filter

As the name suggests, raw greywater is put into the bottom of first column of filter

and collected at the top of second column. This water is again fed to the third

column of filter from the bottom and is collected at the top of fourth column. The

number of columns depends on quality of greywater and expected use of greywater

and optimally upflow-downflow filter contains four columns. The filter media varies

with the column and may contain gravel, coarse sand, fine sand and other material

such as wooden chips, charcoal etc (Wright, 1986).

2.8.3.2. Multi-media filter

Multi-media filters are filled with a variety of media in order of increasing size, for

example, fine sand, coarse sand, gravel, stone, and wood chips to a total depth of

0.75 m to 1 m. The inlet is provided at the top so that the filtered water is collected

through outlet in the bottom. A vent is provided at the top for letting out odorous

emissions, if generated in the filter. Media can be taken out for washing periodically

depending on the greywater characteristics and quantity. Replacement of local filter

media is also a feasible alternative (Wright, 1986)

2.8.3.3. Slow sand filter

Slow sand filters are shallow layers of stone, medium gravel, and pea gravel beneath

a deep layer of sand. A slow sand filter will have greywater load of 0.1 to 0.2��/

��/hr. These gravity filters may be constructed in a 200 liter drum or similar

12

container that is of suitable size. Features that should be part of a filter include a

perforated plate or some other device to distribute water evenly over the top, a

concrete funnel in the bottom to help water drain to the perforated drain pipe, and a

cover and vent to prevent odors. The bottom of the filter should be filled with stones

that are too large to enter the drain pipe.

Slow sand filters require regular cleaning and replacement of the top layer of media.

Multi-media filters require less frequent cleaning, but all layers must be cleaned or

replaced when maintenance is required. Routing greywater through a settling tank

before filtering reduces contaminant load and can lengthen the interval between

cleanings (Wright, 1986).

2.8.3.4. Horizontal Roughing Filter

The horizontal flow prefiltration technique using coarse gravel or crushed stone as a

filter media is also a sound alternative in handling turbid waters. The main

advantage of the horizontal flow prefilter is that when the raw water flows through

it, a combination of filtration and gravity settling takes place which invariably

reduces the concentration of suspended solids. The effluent from the pre-filter,

being less turbid, can be further easily treated with slow sand filter.

Horizontal flow prefiltration may be carried out in a rectangular box similar to a

basin used for plain sedimentation filled with various filter media. The raw water

inlet is situated at one side of the box and the outlet at the opposite side. In the main

direction of flow the water passes through various layers of graded coarse materials

in the sequence coarse-medium-fine (Wright, 1986).

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2.8.4. Delivery and Storage System

Grey water may be stored in tanks of different materials e.g. concrete tanks, steel

tanks, fiber glass tanks, plastic tanks or site built tanks. After storage, the water is

delivered to various units like cistern tanks and irrigation outlets.

The storage tank is should be a concrete tank given that treatment is done. Chlorine

is added to the recycled water to treat it and remove bad odour and hence the tank

needs to be strong enough to with stand constant movement and wear and tear.

2.8.5. Pumping Unit

Efficient centrifugal pumps are ideal where water requirements are substantial and

only single-phase power, and sufficient power available.

These are normally low cost balanced and with rigged construction. It has no

centrifugal switch, require less operational and maintenance cost with no air lock

problems

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3. THEORETICAL FRAMEWORK

3.1. Grey Water

The Various sources of grey water include: the kitchen, the laundry, the bathrooms,

sinks and showers. These sources do not carry water that is likely to contain disease

causing organisms. However the grey water is still needs to be recycled to remove

the solid matter, soap remains and also kill any bacteria in the water. To achieve

this, the BOD5 ft the water needs to be established to determine the content of

oxygen in the water and how it affects the grey water.

The following are some of the theoretical framework that need critical analysis for

the success of this project

3.2. Biochemical Oxygen Demand (BOD)

BOD curves are constructed to determine how oxygen in the water is affected by

oxidisable material. The process of decomposition is usually a monomolecular/ first

order reaction; hence the following differential equation is used:

�� = k’ (La-y)

Where; La=total biochemical oxygen demand at time t=0

y=consumption of oxygen

k ’= rate constant for the biochemical oxidation

The smaller k’ is the slower the decomposition.

K’ for black water is 0.1.

Some sources of of common contaminants in grey water include:

• Food material

• Detergents, soaps and cchemicals

• Salts

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3.3. Site Selection

In the process of assessing the suitability of sites for constructing greywater

treatment system, important considerations are as below:

Approximate size of 15 - 20 m land in the school campus for reuse system has been

considered

Topography and natural slope: the topography of the sites and contours can be

established using standard surveying procedures. The slope of the site is an

important factor in controlling surface ponding, runoff and erosion. A minimum of

2% slope of area is recommended. l Soil type: Soil type and properties are the key

factors in the design and operation of greywater reuse systems. The main

characteristics necessary for the evaluation of the soil for the purpose of greywater

reuse are soil texture, soil structure, corrosiveness, submergence, infiltration rate

through topsoil and percolation rate in the sub-strata. Percolation rates can be

determined using percolation tests and compared with textural classification charts.

Infiltration rates can be determined using a cylinder infiltrometer (Christov et al.,

1995). As sandy lighter soils can absorb more greywater, and heavier soils with a

high clay content absorb less (Greenhouse People's Environmental Centre, 2002)

therefore soil having structural stability i.e., stable clay/silt, hard strata soil is

recommended for greywater reuse system construction. Black cotton soil and sandy

soil should be strictly avoided

3.4. Head loss

It occurs due to passage of water through the filter media. The head loss equation is

derived from the fundamental equations of head loss. This can be calculated either

by using the Hazen Williams equation or the Darcy-Weisbach equation. Their

equations are as follows:

� Hazen Williams

� Darcy-Weisbach equation

3.5. The Pump characteristics

The three principle performance parameters relating to pump selection are as

follows:

o Flow / capacity

o Total Head

o Suction Lift;

a) CAPACITY: The required capacity was determined by relating

capacity to the total daily demand. Daily demand was estimated and the

hourly requirement computed by dividing the daily demand by the number of

hours the pump is required to work.

Hazen Williams equation

Weisbach equation:

The Pump characteristics


CAPACITY: The required capacity was determined by relating the storage



hours the pump is required to work.

16


the storage



17

b) TOTAL HEAD: This comprises of static head, dynamic head (friction loss) and

pressure head.

Total head = Static Head + Dynamic Head + Pressure head

c) Suction Lift: Centrifugal pumps have the capability of creating a vacuum in a

suction pipe which enbles them to suck water from below their setting level.

Max. suction lift is determined by the formula shown below:

�� = A- NPSH-��-��- �

Where: A – Atmospheric pressure

NPSH – suction x-stics of the pump

�� – Friction loss in the suction pipe

�� – Water vapour pressure

3.6. Sizing of conveyance pipes

The size of the required pipe can be calculated using the following formula:

� = �� × � ……………………………………5

Where:

Q= Flow (m3/s)

D= Pipe size (m)

V= Velocity (m/s), usually assumed to be 0.8m/s

While V is assumed and Q is known, it is easy to calculate the diameter/ size of the

required pipe. After getting the pipe size, it is then necessary to apply the same

formulae again to get

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3.7. Design of storage tank

In the design of the storage tank different elements are considered.

� The shear forces and moments caused by the horizontal earth pressure, on the

wall

� The net moment due to the earth pressure on the top and bottom faces of the

inner footings in the top.

� The moment due to earth pressure causes the tension in the bottom face of the

outer footing

The following cases will be considered for reinforcement:

1. When only water pressure acts

2. When the tank is empty.

Maximum and minimum pressure at the base will be given by:

�� = �� +

!� "………………………………..4

��#$ = �� −

!� "………………………………….5

Where: Q=pressure either maximum or minimum

W= total weight

B= breadth

e=eccentricity

Total horizontal pressure exerted by water will be given by:

& = '()

�

Where: p=total horizontal pressure

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W=total weight

H=effective height

Maximum available friction=*+

Where: * =frictional coefficient

W= total weight

Factor of safety== *+ �,-.⁄

Where: �,-.=maximum pressure at the base

Bending moments:

1. Short span: 01. = 21. × + × 3�.

Where: Msx=moment in the shorter span

245 = the coefficient in the shorter span

W is the ultimate design load

2. long span:016 = 216 × + × 3�.

Where: Msy=moment in the long span

216= coefficient in the long span

W=ultimate design load

Lx=length of shorter span

Areas of reinforcement, As

20

74=8

9.;<=>×?

Where: As= area of reinforcement

M=moment

Fy=the respective stress (steel)

3.8. Tank Loadings

Are of 2 types;

• Internal loading caused by Hydrostatic and active pressures

• External loading-Hydrostatic pressures, Active soil pressures and reaction of

the ground

In order to meet the regulations as provided in the British Standard Codes of

practices, BS 5337,

• Minimum thickness of the wall, t = 100 mm

• T = 2.5% depth of the tank +25 mm

• Check limits for crack width for the water retaining structure, Cr = 0.15

The allowable stress in the steel reinforcement was reduced such that

Force N = A21@A × f21A

Where B21A = 100N/mm2

21

4. GENERATION OF CONCEPT DESIGN

I. Determining the quantity and quality of the grey water produced

weekly. This is achieved through determination of the average water

consumed per week by inquiry, observation and estimation

II. Determining a suitable stable site for putting up the system

components. Achieved through carrying out geotechnical tests such as soil

sampling and testing

III. Designing of the various system components of the recycling unit. The

following systems for the grey water recycling unit will be designed:

� Diversion System

� Filtration and treatment system

� Delivery and Storage system

� The design layout from CAD drawings

IV. A working grey water recycling system will be modelled to show the

directions of the different flows of fresh water and grey water.

22

5. METHODOLOGY

5.1. Study Area

5.1.1. Location

Mugabe Hostel is located in The University of Nairobi Upper Kabete Campus at

longitude 1°15’16.37” S and latitude 36°43’39.21” E and at an elevation of 6165 ft.

The proposed site for the grey water recycling unit will be at an elevation of 6170 ft

to allow for flow of water by gravity during treatment.

Figure 2: Google maps location of Mugabe Hostel.

23

5.1.2. Land Cover

The hostel is surrounded by plant cover majorly grass, trees and live fence. There

are various footpaths crisscrossing around the hostel and roads nearby.

5.1.3. Topography

The area is a relatively steady slope from south west to north east. The area is hilly

with various flat stretches of land.

5.1.4. Water Supply System

The current water supply system to the hostel is from the Nairobi Water and

Sewerage Company and supplemented by various boreholes sunk within the

campus. The inconsistency of the supply of Water from the mains has been an

inconvenience in pumping the water to the overhead tanks prompting numerous

ground water tanks to be erected to store the water

5.1.5. Waste Management

Since the sewerage system in the hostel is not connected to the Nairobi Sewerage

system, the waste water produced is directed towards a septic tank that ha s a

capacity of about 40,000 litres. Other than occasional leakages, the system has been

able to hold up to the waste produced from the hostel.

5.2. Data Collection

5.2.1. Water Consumption Data

The water consumption data was collected using the observation/data sheet as seen

in the appendix. The average amount of water consumed in the hostel was compute

from approximation, inquiry and estimation of the daily average volume of water in

the roof tank.

Other sources of information regarding data was sourced from the internet and

books

24

5.3. Design

5.3.1. Design of the diversion system

For maximum efficiency, the different components of the diversion system will be

assessed and the suitable material for each part will be determined

5.3.2. Filtration and Treatment System

The most appropriate treatment method which will produce more pure water is

selected and incorporated within this system. A simple charcoal-sand filter is

designed with the sizes of the filters and arrangement of the sand and charcoal

being crucial to the efficiency of the filter.

The location and slope of this system is crucial in facilitating the removal of waste at

a relatively low cost.

5.3.3. Delivery and Storage System

The delivery system will majorly comprise of a pump which will assist in increasing

the head to the overhead tank to allow for water to flow by gravity and different

pipe sizes for distributing the recycled water to various locations.

A suitable tank will also be determined with the capacity and material being crucial

to its longevity. The size of the tank will be determined from the average volume of

grey water collected daily.

5.3.4. Overall Design Layout

A schematic layout of the complete design which will incorporate each of th above

components will be illustrated via CAD drawings

25

6. RESULTS AND ANALYSIS

6.1. Water Consumption Data

The table below shows data obtained from Mugabe hostel in a span of 7 days in

order to get the average volume of water consumed in a day

Table 1: Average volume of water consumed in a day

Area/Day 1(l/d) 2(l/d) 3(l/d) 4(l/d) 5(l/d) 6(l/d) 7(l/d) Average %age

Bathroom

and Sinks 3500 3500 3500 3500 3500 4000 4000 3600 60%

Toilet 1500 1500 1500 1500 1500 2000 2000 1800 30%

Leakages 100 100 100 100 100 100 100 100 1.67 %

Other

uses 500 500 500 500 500 500 500 500 8.33%

TOTAL 5800 5800 5800 5800 5800 6600 6600 6000 100%

The amount of water consumed is calculated per day using the equation below:

CDEDFGHI�(3/LDI) = �3GHNOP/+OOQ�LDIP/+OOQ

Average volume demanded per person = 80 l/day

Average volume demanded by hostel = 80 l/day × 100pple = 8000 l/day

Average Volume supplied per day (litres) =6000

Average Volume per day (��) = 5.55�R

26

Approximate number of people in the hostel per day = 100 pple

Average volume per person per day = S�!TU!VWXY�!Z!T��(X#�T! )

[W�X$Y�\!TW�Z!WZX!#$�]!]W �!X

=^^^_�__

= 55.5 l/day

NB: 55.5 l/day is the volume of water available per person per day due to the water

supply

80.0 l/day is the volume of water demanded per person per day in the hostel

This grey water system is supposed to provide the deficit i.e. 25.5 l/day

Total amount of water supplied = tank capacity * no. of times the

water is pumped to the per day overhead tank

= 3000 l * 2

= 6000 l/day

Volume of grey water produced per day

No. of students = 100

Approximate number of times flushing the toilets = 150

Capacity of the cistern = 15 litres

Amount of black water produced per day = 15 × 150

= 2250 l/day

27

Approximate volume of grey water produced = 5500 – 2250

= 3250 l/day

Percentage of grey water produced = R`^_ ___ × 100%

= 54.2 %

6.2. Diversion System

6.2.1. Piping

The existing piping system conveyed waste water from both the toilets and

bathrooms towards the septic tank. The piping conveying the greywater should be

diverted away from the pipe system carrying the black water.

6.2.2. Junction Chamber

Grey water which originally headed towards the septic tank is diverted to a junction

chamber which facilitates draining out greywater from different sources. The

dimension of junction chamber is determined based on providing sufficient storage

to handle peak hourly volume. The standard dimension of the junction chamber is

kept at about 0.3 m x 0.3 m x 0.5 m i.e. a capacity of for a hostel having greywater

generation of 2000-3000 l/day. Froth removal generated from bathroom and

washing may prove necessary.

6.3. Filtration and Treatment System

6.3.1. Screen

Screen can be a mesh with less than 10 mm size to remove coarse particles. The

screens can be placed at the inlet to the piping system of sources such as bathroom,

sinks etc. to remove large particles and prevent an overload of particles at the outlet.

The screens can be cleaned manually and solids disposed off along with solid waste.

28

6.3.2. Equalization or Settling Tank

Equalization or settling tank is required to collect grey water for continuous flow to

the filters for treatment and facilitates in settling of course particles. It also allows

for balance flow by taking into account maximum flow of greywater generated

during morning hours due to bathroom use. Adequate aeration and mixing must be

provided to prevent odors and solids deposition in equalization tank and this is

achieved b by providing baffles.

Removal efficiency of suspended solids in sedimentation tanks depends on surface

area and depth of tank. Surface loading rate is the basic guidance parameter for

determining size of tank. The design criteria for equalization or settling tank

presented in table below.

6.3.2.1. Design Criteria for an Equalization Chamber

Loading = ��a9b/c-6��de1×fc-6

= 135.5 l/day

Table 2: Design Criteria for an Equalization Chamber

PARAMETER RANGE

Detention time (hrs) 1-2

Surface Loading (l/h/��) 100 – 200

Depth of Tank (m) 0.8 – 1.0

Length to Width ratio 3:1 to 4:1

29

Figure 3: Equalization chamber

6.3.3. Filtration Unit

The most suitable filtration unit should consist of coarse sand

sand. This will

Figure 4: Filtration unit

Filtration Unit

The most suitable filtration unit should consist of coarse sand-charcoal and fine

30

charcoal and fine

31

Disinfection

The best method of disinfecting the water is through chlorination. Chlorination of

the recycled water becomes necessary since it has a number of benefits

• Kills pathogens in the water

• Removes odour of the recycled water

• It vaporizes in the water after disinfection

• Is readily available

• Cheaper compared to other forms of disinfection

Chlorine is available in liquid form or in tablet form and it can be added to the

collection tank and provided with adequate aeration for maximum efficiency and to

reduce toxicity.

6.4. Collection / Storage Tank

A collection or storage tank is required to have the appropriate capacity to handle

the average daily generation of treated greywater. In case the greywater generation

PARAMETER FILTER

Number of compartments 3

Media and size (mm)

Coarse sand

Coarse sand

Fine Sand

Hydraulic loading (m3/m2-hr) 0.1-0.2

Depth of media (m) 0.5

32

is large with a capacity of more than 4000 liter/day, collection tank may have

capacity to handle half of the quantity of greywater generated per day.

6.4.1. Material

The storage tank is constructed from concrete material because of the following

properties

� It durable and lasts longer than other materials

� It is a reliable material

� It has a high compressive strength

� It is cheaper than most materials

� It is isotropic and hence suitable for water retaining structures

� Is non-corrosive with chlorine

6.4.2. Shape and Position

A rectangular tank is suitable for use as a storage tank. The best position for this

tank should be underground and at a distance from the septic tank. It should also be

higher than the septic tank as a measure of control. The various pipes conveying the

water along the system should be buried a distance of 1m below the ground to avoid

damage or hinder movements of other users.

The major parts of the tank that will need to be designed include:

� Base of the tank or bottom slab

� Tank walls

� Top slab or cover of the tank

33

6.4.3. Dimensions of the storage tank

Length: width ratio = 4:1

Depth of tank =1 m

Volume of grey water produced = 3.25 ��/day

Volume = l × w × d

Volume =4+�d

3.25 = 4 × +� × 1

0.8125 = +�

W = √0.8125)

= 0.9 m ≈ 1m

L = 4 × 0.9

=3.61 m ≈ 3.6 m

Therefore the storage tank dimensions are 3.6 × 1 × 1

34


35

6.5. Pumping and Piping System

6.5.1. Pipes

The Various pipes that link the different components are installed in the system and

should comprise of the following:

6.5.2. Pumps

An electric pump is a preferred choice since the hostel has ample electric power.

Efficient centrifugal pumps are ideal where water requirements are substantial and

only single-phase power, and sufficient power available.

These are normally low cost balanced and with rigged construction. It has no

centrifugal switch, require less operational and maintenance cost with no air lock

problems. The pump should have a minimal yield (Q) of 1000 liter/hour and should

be a high head/low discharge pump

Table 5: Pump characteristics

Property Values

Lift 22 m

Flow rate 5.0m�/h

Voltage 240 v ac

36

Figure 6: Pump

37

6.6. COST BENEFIT ANALYSIS

The grey water recycling system will also save the overall cost of piped water by

reducing the intake. Calculating the total cost of water saved at current rates is:

Cost of water: 1 litre = Kshs 0.002

Approximate of 3000 litres of water recycled per day

= 3000 l/day × Kshs 0. 02

= 60 Kshs / day

In a Month = 60 × 30

= Kshs 1,800 per month

In a year = 1800 × 12

= Kshs 21,600 per year

6.7. SCHEMATIC DIAGRAM OF A GREY WATER RECYCLING SYSTEM

SCHEMATIC DIAGRAM OF A GREY WATER RECYCLING SYSTEM

38

SCHEMATIC DIAGRAM OF A GREY WATER RECYCLING SYSTEM

39

6.8. Water Quality Monitoring

Various precautionary measures should be taken to ensure that the fresh and grey

water do not mix. These include

1. No cross connection of the grey water piping system with the fresh water

piping system

2. Use of different colour pipe network

3. Prevention of mosquito breeding in the system

6.8.1. Odour Control

Good design and maintenance practices will reduce odour problems in greywater

treatment system without the use of chemical addition or air treatment. The

charcoal and chlorine also assists in removing the odor from the water. the

following measures are recommended to minimize odour problem:

I. A minimum slope of 2-3 % should be provided so as to ensure sufficient flow

through system when in operation

II. Baffles should be provided at the entrance of sedimentation tank and in

collection tank for aeration.

III. The closed conduit system should be avoided. If a closed conduit system is

unavoidable, length should be minimal with adequate velocity to scour the

pipe.

IV. Deposited solids should periodically be removed from equalization tank

V. Natural/ artificial coagulants should be added to sedimentation tank to aid in

the coagulation process.

VI. Addition of chemicals such as calcium nitrate, hydrogen peroxide, potassium

permanganate, hypochlorite and chlorine added to the system to oxidize the

sulphate bearing ingredients of greywater. This is only necessary if the

system cannot be designed in such a way to prevent formation of anaerobic

conditions

40

VII. Filters should be washed with clean water and filter media should be

periodically replaced.

VIII. Chlorination of final effluent also helps in minimizing odour

IX. Collection sump can be covered and vent pipe can be provided to let out the

odourous compound

6.9. Maintenance of Greywater Treatment System

The success of a greywater reuse system will depend on maintaining the system.

Any defect must be rectified as soon as it becomes apparent. Greywater systems

require regular maintenance e.g. weekly cleaning or replacing filters, periodic

desludging, and manually diverting greywater back to sewer and flushing of

drainage lines.

The following procedures may be required to be undertaken once the system starts-

up:

• Weekly maintenance of systems with filtering devices e.g. screens

• Sedimentation tanks require desludging every month

• Warning signs should be maintained in good order

• Protection from any contact with greywater to ensure that exposed body

areas that come into contact with greywater are immediately washed; not

make contact with the mouth or face either directly

• Use of greywater only for toilet flushing and to completely avoid use for anal

cleaning or hand washing

41

Summary of the system maintenance

Treatment Unit Activity Frequency Purpose

Equalization Tank De-sludging Every fort night Maintain the volume

of equalization tank

Filtration Unit Cleaning of filter

media

Every fort night

Maintain the

efficiency

Refill the upper

layer

Every fort night Overcome chocking

problem

Chlorination Maintain proper

dose

Daily Disinfection and

Odor control

Collection tank Reuse of water Every 2 days Maintain the quality

of greywater

42

7. CONCLUSION

The grey water produced within the hostel comprised of about 55% of the total

amount of water used and therefore the water recycling saves up to an equivalent

percentage off the mains supply which is a capacity of about 3000 litres per day or

3m�.

The recycled water is of good quality and can therefore be used primarily for

flushing toilets and cleaning of the pavements. Other uses will be watering the

flower bed surrounding the hostel and improve beautification.

This system will also be able to save on the cost of water supplied by the Nairobi

water and sewerage company. An estimate of about Kshs 21,600 will be saved on a

yearly basis.

Given that this system will have an economic life similar to that of the hostel, it

translates that with proper maintenance, the system will be able to serve the

students and improve the sanitation of the hostel.

43

8. RECOMMENDATION

This project should be implemented to improve the sanitation levels of the hostel

and reduce the long term costs in water supply.

The project can also be modified to handle the various volumes of grey water

produced by other hostels in the campus and ultimately be implemented in each and

every hostel.

In case of a system with anaerobic, iodine tablets can be used for disinfection

instead of chlorine.

44

9. REFERENCES

Jeppersen B. and Solley D. 1994: Domestic Grey water reuse: Overseas practice

and its applicability to Australia. Research Report No 73. Urban Research

Association of Australia, Brisbane city Council

Little, V. 2000: Residential grey water reuse: The Good, The Bad, The healthy.

Tucson, AZ: The Water Conservation Alliance of southern Arizona (Water CASA),

2009

Mullegger E, Langergrabber G. Jung H. Starkl M. and Laber J. 2003: Potential for

Grey water treatment and reuse in rural areas, 2nd International Symposium on

ecological sanitation

Myca T. 2002. The Effect of Portable vs. Grey Water on radish Growth

Peter L.M. Veneman and Bonnie Stewart 2002: Grey water characterization And

Treatment Efficiency; Final Report for The Massachusetts Department of

Environmental Protection, Bureau of Resource Protection, December 2002

Poff, J A. 2006 The Use of Grey Water as a Water Conservation Method: Civil

Engineering, Horticulture &Landscaping, urban Water Infrastructure, and Soil

&Crop Sciences, Colorado State University. Fort Collins, CO: Colorado State

University. Preview of a study taking place at the Colorado State University

Esrey S. 1998. Ecological Sanitation Edited by Uno Windblad. Published by the

department of Natural resources and the Environment, Sida, S-105 25 Stockholm,

Sweden

Roesner, L.2006. Long-term effects of Landscape Irrigation Using Household Grey

water -Literature Review and Synthesis. Water Environmental Research

45

Foundation. Fort Collins, Colorado: Water Environmental Research Foundation and

the Soap and Detergent Association

Schneider L. 2009. Grey Water reuse: Rule Development Committee Issue Research

Report Final, July 2009

Turner S. 2001. “Will there be Enough Water in The Next Century?” Microsoft

Encarta Encyclopedia

Seigrist, Robert L. 1978. Management of Residential Grey Water: Small Scale Waste

Management Project, University of Wisconsin. Madison, Wisconsin: University of

Wisconsin, 1978. Technical Memorandum

Wright M. 1986: Safe Use of Household grey water, Guide -106- New Mexico State

University

A-Boal, D. Christov, Lechte, P. and Shipton, R. 1995. Installation and Evaluation of

Domestic grey water reuse Systems: Executive Summary. Department of Civil and

Building Engineering, Victoria University of Technology, Victoria, Australia: Victoria

University of technology, 1995. Technical Memorandum

Al- Jayyousi, Odeh.2002. Focused Environmental Assessment of Grey water Reuse

in Jordan. Civil Engineering Department Applied Science University, College of

Engineering. Amman, Jordan: Environmental Engineering Policy (2002) 3,

2002.pp.67-73, study

Brown, C.2009. Recycled Water: Risks, benefits, economics and regulation by

system scale. New Zealand Land Treatment Collective conference proceedings

(Technical Session 30): Recycling of Water. Taupo 25-27 March 2009

46

10. APPENDICES

OBSERVATION / DATA SHEET

TO DETERMINE THE QUANTITY OF WATER CONSUMED IN THE HOSTEL

HOSTEL NAME…………………

NUMBER OF OCCUPANTS…………………..

Main Source of Water…………..

Other Sources

Are conditions of the drainage system good? (Y/N)

If no, what method of waste disposal is used?

………………………………………………………………………………………….

……………………………….……………………………………………………………………………………………

……………………………………

How many bathrooms are present

……………………………………………………………………………………………………………

How many toilets are present

…………………………………………………………………………………………………………………..

How many wash sinks are present

……………………………………………………………………………………………………………

Approximately how much water is used?

…………………………………………………………………………………………………

47

What is the capacity of the storage tank?

………………………………………………………………………………………..........

How long does it take to empty?

……………………………………………………………………………………………………………..

Are there any leakages?

……………………………………………………………………………………………………………………………

Approximate how many times the toilets are flushed per day

………………………………………………………………....

Any other uses of water in the

hostel………………………………………………………………………………………………………..

Approximate how much water is used for these

uses………………………………………………………………………………..

48

TANK MATERIAL PROPERTIES

TANK MATERIAL ADVANTAGES DISADVANTAGES

plastic

fiberglass commercially available,

alterable and moveable,

little maintenance light weight

Must be sited on smooth solid,

level footing pressure proof for

below ground installation

polyethylene Commercially available alterable,

moveable, affordable, available in

variety of sizes ,install above or

below ground

UV –degradable, must be

painted or tinted, pressure proof

for below ground installation

Trash cans (20-50 gallons) Commercially available,

inexpensive

Must use new cans, small

storage capacities

Metal

Galvanized steel tanks Commercially available, alterable

and movable, available in variety of

sizes, film develop inside to prevent

corrosion

Possibly corrosion and rust, must

be lined for potable use, only

above ground use

Steel drums (55-gallons) Commercially available, alterable

and moveable

Verify prior to use for toxics,

prone to corrosion and rust ,

small storage capabilities

Concrete

Ferroconcrete Durable and immoveable, install

below or above ground

Potential to crack and leak,

neutralizes acid rain

Monolithic/poured-in-place Durable, immoveable, versatile,

install below or above the ground,

decreases rainwater corrosiveness

Potential to crack and leak,

permanent, neutralizes acid rain,

in clay soil, do not place

underground

Stone, concrete block Durable and immoveable, keeps

water cool in hot climates

Difficult to maintain, expensive

to build

3D VIEWS OF THE SYSTEM PARTS

3D VIEWS OF THE SYSTEM PARTS

49

50

DIFFERENT VIEWS OF THE RECYCLING UNIT

51

EQUILIZATION CHAMBER

52

FILTRATION UNIT

53

STORAGE TANK

54

PUMPING SYSTEM

design of an improved grey water recycling...

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