Design of Public Water Supply Systems

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Design of Public Water Supply

Systems

Albert Kenyani Inima [7 Oct 2014


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Design of Water Supply System

1.Catchment [Out of scope]

2. Surface or subsurface water source [Out of scope]

3. Water treatment plant [See water treatment]

4. Water supply mains [trunks]

5. Water distribution main

6. Water delivery submains and branches

7. Water usage

8. Wastewater collection system

9. Wastewater treatment plants

10. Water source/water catchment


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Types of WDN Designs

Brand new WDN [rural areas]

Upgrading/replacement of existing de

[Capacity, technology, quality][Many

parameters stay the same]

Extending the existing system


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Stages in design of a WDN [Hardy Cro

Procedure]

0. WDN design objectives1. Agree on the WDN design period

2. Decide on the area that will be served

the design period.

3. Forecast the spatial and temporal popof this area during the design period.

4. Estimate water demand & check if the

sources are adequate to meet this dema


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Stages in design of a WDN [Hardy Cro

Procedure]

5. Decide on the type of WDN layout and WDN construction phases

6. Generate possible alternative dimensions of WDN components [

7. Use a suitable mathematical optimization procedure [Eg Hardy C

Method] to find out the pipeline OPTIONS that best meets requirem

8. Improve the pipeline by incorporating reservoirs, pumps and valv

9. If none of the options meets the requirements, go back to Step 5

going!


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Step 0: WDN design objectives

Design objective 1[Need/Market]: The number of people to be served and amount o

supplied [SMART]

Design objective 2[Need/Market]: The type of technology to be provided [eg piped w

system]

Design objective 3[Need/Market]: The way the technology WDN will be managed [H

water be supplied]

Design objective 4[Need/Market]: Is the project financially sustainable and profitable


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PART 1: DEMAND FORECASTING

[establishment of need/market for the W


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Hardy Cross Step 1: The project desig

period

Definition: The number of years when water supplybe more than water demand [The WDN should be

adequate].

Why do we need a Design Period [P]?

Many components of WDN are installed once dperiod P and are very difficult to change latter[L

reservoirs, trunk mains]


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Factors affecting the Design Period [L

The lifetime of the WDN components[Obsolescence or economic life] [ P m

less than N].

Population growth rate [If population i

increasing fast then design based on value of P].

Difficult of making future extensions a

modifications [As difficult increases P


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Factors affecting the Design Period [T

The project construction time[C] and projectcommissioning time [M] [P must be much grea

C + M].

Economy of scale [If the WDN cost decreases

considerably with the size of WDN, then a big P

be prefered] Design period must less than the RETURN per

[Recurrence period] of discharge on which the

based.


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WDN design life formula

R--Risk of failure [Uncertainty of populatchanges, WDN modification]

T-- Period design value exceeded once o

average

As R increases T will decrease.R=1/T


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Return Period


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Return Period


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HCP St 2 D id th th t il


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HCP Step 2. Decide on the area that wil

served during the design period.

How big and small will the area to serve be?

a) Strategic alignment with development needs This must fit into national development plan[eg Kenya Vision 2

Sustainable Development Goals 2030].

Fit into County Integrated Development Plan.

Fit into Catchment Development Plan.

[GOOGLE MAP]

HCP St 2 D id th th t il


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HCP Step 2. Decide on the area that wil

served during the design period P

b) The budget availabilityc) The return period

d) The unserved populations

e) The technology used [eg Gravity syste

HCP St 3 F t th ti l d t


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HCP Step 3. Forecast the spatial and tem

population during the design period.

Requires service area disaggregation into sma

homogeneous areas [with regard to population

and population growth] [Note maximum popula

allowed in areas [dwelling units per hectare]].

Get the latest population statistics showing pop

per sub-location and population growth. Forecast the populations in the sub-location at

of the design period


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Population Forecasting

There are 3 possible population growtscenarios [Increase, stay constant or

decrease].

New population [N+1] = Old populatio

+Population Decrease [Mortality] +Population Increase [Natality]- Emigra

Immigration

Population growth rate = Average cha


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Population Growth Models

Linear Population Growth [Low population dens

areas, controlled development limits maximum

of dwelling units per acre, mixed use not allowe

design periods[less than 15 years]].

Exponential Population Growth [Medium-high

population density areas, controlled developmemaximum number of dwelling units per acre, lim

mixed use allowed, moderate design periods[1

years]

Logistic Population Growth [General model tha


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Concluding Remarks on Population Grow


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Concluding Remarks on Population Grow

Modelling


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HCP Step 4 Estimate water demand & check if the


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HCP Step 4. Estimate water demand & check if the

sources are adequate to meet this demand.

Estimate the water demand for each disaggreg

area. Water demand= Design population forecast x a

per capita water demand

Find total water demand [Less than safe [minim

water yield] or yield at low probability of failure Design period]

Source locations [Single or Multiple Sources/ G

pumped WDN/ Continous or Intermittent

Pumping][Factors --Cost, topography, populatio


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Per Capita Water Consumption

Water used for:

Water use [Domestic & Industrial water Include

Fireflows

Unaccounted for water [Backwash, trunk main

leakage and stolen water] [45%of all water abs

from source for Nairobi]


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Per Capita Water Use Computatio

Take a representative sample of annual water usage [Water Abfrom source, water supplied from treatment plant, metered wa

for similar/same city when water use was normal [Without freq

shortages or with many non-repeat events such as the World C

Take the population census [estimated population].

Per capita water use = [Average annual water use]/Population

Note per capita water does not account for diurnal, weekly, mo

use variation patterns[See latter]


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Fireflows

Fire can be fought through installed fire sprinkler system or thr

fighter engine [hydrant VALVE] Highrise and commercial usually have fire sprinkler systems w

domestic buildings fire fighting is usually through fire engines.

Water flow allowance for fire fighting must be provided, which c

diverted to other uses such as domestic water but which must

available when needed [when there is a fire outbreak]

During a fire, fire engines recharge from the nearest firehydrants[Hydrants are not on every house]. Fire hydrants are c

fire flow at an available storage tank. Same also applies for fire

system.

Design fires last maximum of 2 hours. The design fireflows dep

size of supply pipe.


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PART B: WDN [PRODUCT] DESIGN[ENGINEERING]


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[ENGINEERING]

HC P Step 5. Decide on the type of WDN layout an


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phases

How will water be transported to the p

Do we layout the complete route[WDN

one go or do we roll out the route as t

population grows and water demandincreases?

a er ranspor a on rou e ayo[Geometry]


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[Geometry]

Generally follows existing or planned

street/road patterns for accessibility d

construction, O & M.

Two basic types of streets [Loop[ring]

dead end[tree branch/cul-de-sacstreet]][Open and closed streets]


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Comparision of loop and dead end str


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Comparision of loop and dead end str

Parameter Cul -de- sac Ring

Area used to build streets

[length, number and area]

Ease of adding new buildings

and facilities [Modifying street

traffic flow patterns]

Population density

Traffic congestion

Problem of getting lost

Average resident travel

distances

Street pollution [Noise &

WDN Layouts [water pipelines follow streets]


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WDN Layouts [water pipelines follow streets]

Dead End [Cul-de-sac/Tree Branch] WDN

Loop WDN [Gridiron WDN, Ring WDN, Radial and Circular WD

Components of a WDN [Delivery mains, distribution main, subm

branch pipelines]


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Components of a WDN


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Components of a WDN

The delivery main/trunk[Has no submains, usua

constant diameter, usually constant flow-rate throupipe, usually the longest and biggest pipe in the W

big population advisable to have more than ONE in

accident or maintenance, highest velocity flows an

pressures, capacity reached at end of design peridesigned to carry design flow]

Distribution mains [trunk][Has supply submains

decreases as you go downstream, may be designe

have tapering diameters, may handles high flows d


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Mains, Sub-mains and Branches


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Mains, Sub mains and Branches

Differentiate based on diameters

Valves are required for many flow regulation anmeasurement reasons[ Flow direction control, n

return[avoid biofouling and arrest water hamme

waves], pressure relief, fireflow boosting, isolat

non-paying clients, leak detection, flow measur


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Terminology


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gy

Static pressure ==No water flow in entire system

Residual pressure==Water flows everywhere exce

measured point


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Other LOOPED WDNs


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Radial WDNs

Circular WDNs


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Comparison of WDN


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Variable Gravity Fed WDN Pumped WDN

Topography Downhill Sloping Flat and Uphill Topogr

Cost

Pressure management Expensive [Dam

construction and long

routes]


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Comparison of WDN


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Variable Single Input Source WDN Multiple Input Source

WDN

Water quantity Higher

Water quality Steady Fluactuates

Emergency water shortage

management

Difficult

Comparison of WDN


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Variable Dead End WDN Loop WDN

Dominant mode of pipeflow Series fllows Parallel flows

Pipe sizes Bigger

Total Pipe lengths Much longer

Water availability during repairs

upstream

None Water may still flow normally

Water stagnation and biofouling A problem No water stagnation

Connecting new clients Difficult Easy

Firefighting Difficult Easier

Water leakage problem More serious

WDN Development in Phases


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If cost savings are substantial the Water Delivery a

distribution mains should be installed in one go.

Multiple water delivery mains may be installed at d

times [Increases capacity while providing an altern

during repairs]

Multiple water source iput lines can be installed at

times during the design period [Split the design pe

smaller periods].

Hardy Cross Procedure


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1. Agree on the WDN design period

2. Decide on the area/s that will be serveduring the design period.

3. Forecast the spatial and temporal pop

during the design period for these areas4. Estimate water demand & check if the

sources are adequate to meet the dema

Hardy Cross Procedure


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5. Decide on the type of WDN layout and phases

6. Generate possible alternative dimensions of WDcomponents [Pumps, pipes, valves, reservoirs]

7. Use a suitable mathematical optimization proced

Hardy Cross Method] find out the option that best

requirements.

8. If none of the options meets the requirements, g

to Step 5 and keep going!

HCP step 6. Generate possible alternative dimensions of the comp

WDN layout you have chosen[Pumps, pipes, valves, reservoirs]


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Step A: Skeletonize[simplify] the WDN layout and water demand notes [ for mat

expediency]

Step B: Select type of pipe and create judicious guesstimates of the pipe sizes[

commercially available pipes for delivery mains, distribution mains, sub mains an

required [You could do this by comparing with similar water projects undertaken

Step C: Generate many other probable pipe sizes as in Step B above [In Simulat

of these options can be done in EPANET].

Step D: Perform Hardy Cross Network mathematical calculations[or any other su

HCP step 6. Generate possible alternative dimensions of the comp

WDN layout you have chosen[Pumps, pipes, valves, reservoirs]


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]

Step E; Select the WDN design options that meet the required conditions of min

maximum pressures and velocities. [Hydraulic and Energy Grade Lines in fluid m

Step F; If none of the options meet the min-max conditions, select options that ca

by addition of reservoirs or/and by installing pumps at appropriate nodes [[increa

velocity].

Step G: Reanalyse as in step D and keep iterating until feasible solutions emerge

enforce design value limits.

Step H; Cost the feasible solutions and select the one with minimum cost [Const

costs included]

WDN Skeletonization


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Skeletonization


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Consider only trunk mains and submains [and bra

and leave out the detail.


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Step B: Select type of pipe and createjudicious guesstimates of the pipe sizes


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Hardy Cross Optimization


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Each Hardy Cross Procedure produces the flows and flow directions in the WDN

From the HCP flow values, we can compute the pressures and velocities at ever

We can check that pressure and velocity values at all nodes are within the requi

maximum range of values. Every option that produces pressure and velocity values that are within range is t

feasible but not economically feasible.

To check for economic feasibility we must work out the construction and operatio

the same period] for all selected technically feasible options. Only one of these o

the BEST option if all possible pipe diameter permutations were considered.

In practice we may not need to go for the BEST [all permutations considered] bu

of the few options we have Selected.


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ecap: ar y ross pt m zat oProcedure


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Estimate the water demand including fire flows from census and design period.

Decide on the general WDN layout from existing or planned street/road network.

Simplify the WDN to have a minimum number of demand nodes.

From a Google map measure the internode distances and node altitudes. Obtain the range of various pipes available and their unit costs.

Generate many possible options of various pipe diameters that can be used to de

system.

Perform Hardy Cross calculations and determine which options are technically fe

minimum and maximum node pressure and velocities].

If none meets these conditions consider incorporating pumps and reservoirs.

Repeat until you obtain a sample of technically feasible options. Select the long term least cost technically feasible solution as your optimal desig

Performing Hardy Cross ProcedureCalculations


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Reservoir Sizing


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Usually comes after pipe sizing.

Tanks are usually required for management of

PERIOD water demands.

During these peak periods supply from source

be able to meet demand unless the supply is ebig[OVERDESIGN].

PEAK WATER DEMAND

A d il it t d d A D D d [ADD] A


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Average daily per capita water demand =Average Day Demand [ADD]= Average annu

demand/365 days =C

Average annual water demand =sum of total annual water demand for N years/N year

a) Diurnal water use patterns [Water use is not uniform across 24 hours in a day]

Average Hour Demand[AHD]= C/24=R

=C x Hourly water use coefficient[multiplier]

Maximum hour water demand = peak hour demand [PHD]

= ADD x Peaking Factors [Multipliers]

Check to ensure total water use in a day = Total water available THAT day [In th


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PEAK WATER DEMAND

A erage eekl per capita ater demand 7C


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Average weekly per capita water demand = 7C

b) Weekly water use patterns [Water use is not uniform across 7 daweek]

Average daily water demand = C

Water demand for any given day =C x daily water use coefficie

Maximum daily water use = peak daily water demand

Check to ensure total water use in a week = Total amount of w

THAT week [In this case=7C]

PEAK WATER DEMAND



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b) Monthly water use patterns [Seasonality] [Water use is not uniformonths of the year]

Average monthly water demand = 30C

Water demand for any given month =30C x monthly water use

coefficient[multiplier]

Maximum monthly water use = peak monthly water demand

Check to ensure total water use in a month = Total amount of w

needed THAT month [In this case=30C]

Water Use Pattern Analysis

Need to start the year then month then week and lastly day


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Need to start the year, then month, then week and lastly day.

Cycles begin to repeat themselves once you come to the end.

only diurnal variation but not weekly or monthly variations, the patterns will be nearly a wave of constant wave length[ it can b

decomposed into sinusoidal curve usinf Fourier Analysis].

This repeating patterns are used to test WDN software. If they

simulate these repeating cycles then they are not CONVERGIN

[EPANET used to suffer this problem for 10 years since it was

the early 1980s]. If seasonality is present in water data then convergence can ta

number of years before it can be observed in the data plotted b


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example of Canada hourly peaking factobelow]


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Reservoir Sizing Methods

Reservoirs are structures that store water in the


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Reservoirs are structures that store water in the

We have 2 types of reservoirs [Water source

reservoirs[water storage reservoirs] and WDN

tanks[water service/distribution] reservoirs].

2 types of reservoirs [Single source input reser

multiple source input reservoirs]

Simulate the required reservoir size, the mannewhich it will be operated and the reliability of th

supplies due to presence of water reservior


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Reservoir components

Reservoir yield [Effective capacity] =Water sup


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Reservoir yield [Effective capacity] =Water sup

from reservoir to meet needs [Total volume -de

storage-fire suppression storage=OS+ES][Con

already available and new water inflows]

Critical period=period during which consumers

meet all their water requirements from water alstored in the reservoir[1-2 days].

Examples of Water Reservoir in W


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Arrangement of Reservoirs in a WDN [Bto smallest]

The water treatment tanks [open or closed top]


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[ p p]

The water distribution main tanks [Delivery tan

Clear well tank] The water distribution main tanks [Distribution t

The water sub-mains tanks [Sub-main water ta

The water branch line[service] tanks [Service/c

tanks]


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Reservoir sizing methods [Analytic or

simulation]

Ripple Diagram[Mass Curve Method]


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Secant Method


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Tank Water Height

Water flow must pass through tank [Can bypas


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Height in tank good indicator of unaccounted fo[leakages, stolen water, malfunctioning system

Used as a management tool [trouble- shooting

Valves

A device that measures, controls or regulates water flow in a WDN


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Valves includes many appurtenances in the WDN:

Water meters

Water Taps

Toilet tanks

Reservoir overflow regulators

Pumps

Uses of Valves

Without Valves WDN could never be able to function as designed [

enforcers} [WDN takes in water at the correct pressures and gives


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enforcers} [WDN takes in water at the correct pressures and gives

correct pressures]. Generally added at the tail end of the design:

Valves are used in WDN for various reasons:

Turning WDN system on and off

Prevention of system malfunction

Pollution control

System automation System O & M safety

Pressure control

Flow directioning

Bringing in water and taking out water[including leakage control]


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Tutorials

[Q &A Session]


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Conclusions[Next steps]{what should the learn

next in this quest for knowledge and jobs][insp

Attempting to meet market need.


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Engineering things that can now be done

Managerial things that can now be done

REFERENCES


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END


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Lecture Appendices


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Appendix A: Revision Question


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Appendix B: Revision checklist


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Appendix C: Local organizations/expe

dealing with this issue


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Appendix E: Relevant social media fo

and people to link with


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employed at XYZ as ABC next lecture

opener

Why should we not fire you?

Give the student list of problems? [Sta


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p [

Interview questions]--Why Should we

employ?

One student gives 5 minute answers.

Questions Comments by lecturer {Suitability]

The End


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Design of Public Water Supply Systems

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