Water Quantity Water Supply Design Till 12th April

Components of water supply scheme

i. Collection:Dams ,reservoirs, intake,pumping station, tubewells

ii. Transmission : Conduit, valves, pumping station

iii. Treatment :Sedimentation, coagulation, filtration, disinfection, storage

iv. Distribution :Pumping station, Overhead reservoir, feeders, main, pipes, valves, fire hydrants

Future water requirement

• Selection of per capita water consumption(WC)

• Future population forecast• Design period

Economical period of design Number of years in future for which proposed facility would

meet the demand of the community • Length of life of structure↑ (steel pipes-25yrs,concrete

pipes-75yrs)- dsn. Period ↑ • Ease of extension ↑ dsn. period↓• First cost ↑ dsn. period↓ • Rate of interest ↑ dsn. period↓• Economy of scale i.e size (per unit production cost

decreases with increase in scale of facility)• Lead time(preception of project to start) ↑ dsn. period ↑

200 mm diameter water supply pipe 1km long serves 3000 persons; Cost =Rs 300000 ;Cost/head=Rs100

400 mm diameter water supply pipe 1km long serves 12000 persons; Cost =Rs 4800000 ;Cost/head=Rs 40

Design flows and design periods for water supply components

1. Source of water supply:a. Large dams, impounding reservoir(max. daily

demand),transmission main(conduit)-costly difficult to enlarge(25-50 yrs)b. Tubewells (5yrs)design-peak hourly (w/o storage) -Max daily consumption(storage)

Design flows and design periods for water supply components

2. Water treatment plant-easy to enlarge(10-15yrs) Dsn flow -max. daily flow3. Pumping stations-easy(10 yrs)Dsn flow –peak hourly flow,max. daily

flow+fireflow, average and min. demand4. Distribution system- Difficult(25yrs)Dsn flow-(max daily+fire demand)/peak hourly

Problem 1

A community is expected to reach a population of 35000 in 20 yrs. It has present population of 28000 with average water consumption of 16x106 lit/day. The existing water treatment plant has a design cpapcity of 28350 m3/day. Assume an arithmetic rate of population growth. Determine in which year the existing plant will reach its design capacity. Assume the plant to be designed on max. daily consumption.

Solution Problem 1

Pf =35000 , PO =28000,K=(35000-28000)/20=350person/yr

Avg WC per capita per day=(16x106 lit/day)/28000 persons=571.4Lit/capita/day(Lpcd)Max daily WC=1.5x571.4=857.14Lit/capita/dayPf for water treatment plant=(28350x103)/857.1=33075 persons

Pf = PO  + K(tf-to)

33075=28000+350(tf-to) ; (tf-to) =14.5≈14 yrs

Problem 2

A small community had a population of 65000 and 85000 in the year of 1995 and 2005 respectively. Assuming a geometric growth rate and an average WC of 300lit/cap/day. Calculate the design flow for the treatment plant and the transmission main from current year. Select an appropriate value for design period.

• P1995 =65000 , P2005=85000,

• Pf = POe Kn =85000= 65000x(e10K)

K=0.0268person/yrFor transmission main design period=25 yrs(design yr=2037)Treatment plant =15 yrs(design yr=2027)Pf (transmission main)= 85000x(e0.0268x(2037-2005))=200387.15

Pf (treatment plant)= 85000x(e0.0268x(2027-2005))=153277.7

Max daily WC=1.5x300=450Lit/capita/dayCapacity for transmission mains= 200387.15 x450=90174.2m3/dayCapacity for treatment plant = 153277.7 x450=68974.96m3/day

Solution Problem 2

Problem 3The present population of a community is

160000 increasing at a geometric growth rate of 0.043 per yr. The present water requirement of the community are fully met by a number of tube wells installed in the city. The average WC is 350l/c/d using a design period of 15 yrs. Calculate the number of additional tube-wells of 3.4m3/min capacity to meet the demand of design period.

Solution problem 3Avg WC=350l/c/d; design period=15yrsPresent population=Po=160000;K=0.043Pf = POe Kn =160000X(e0.043x15)=304957.92

Additional poulation=304957.92-160000=144957.92Total WC= 350x144957.92=50735272l/d=50735.2m3/dayTubewell capacity=3.4x60x24=4896m3/dayWith storage/ overhead reservoir(OHR)max. Daily WC=1.5x50735.2=76102.9m3/dayTotal no. of tube wells=76102.9/4896=15.5≈16No overhead reservoir(OHR)Peak hourly flow=2.25x50735.2=114154.4m3/dayTotal no. of tubewells wells=114154.4/4896=23.3≈24

Sources of water Rainwaterdissolution of CO2-H2CO3 Surface water

suspended solids(silt clay) inorganic salts,

oils, organics, nutrients, pesticides,

pathogens from municipal, agricultural

runoff

Groundwater Depth>35msafe wrt micorbial contaminantsGenerally hard water

Collection works

Parts of intake structure1.Opening/strainer/grating 2.Conduit to convey water to sump

3.Sump/well from where water is pumped to treatment plant

Intake :Device placed in a surface water source to permit withdrawal of water from that sources

Types of Intake :Single port :constant/fixed depth (river, lake)Multiport :selective draft from various depths(reservoir)

Factors affecting intake structure

Water AvailabilitySediment TransportEnvironmental RegulationClimatic ConditionsInitial and Maintenance DredgingOperation and Maintenance

Reservoir intake

Lake intake(single port)

River intake (single port)

Technical & Environmental Considerations

Dams• Location narrow gorge• Solid foundation design• Adequate height for storage to meet draft

requirement at max daily flow• Upstream free from pollution(preferably uninhabited)• No discharge of industrial municipal agriculture

runoff (Rawal dam)• Watershed area heavily forested to avoid siltation in

reservoir

Technical & Environmental Considerations

Intakes• Location away from pollution sources(Hanna lake-

Quetta)• Adequate submergence of ports to avoid floating

debris and meet navigational requirement • Adequate elevation of conduit from stream/lake bed

to avoid bed debris• Entering velocity <0.15m/s to avoid small fish

intrusion

Distribution systemGravity distribution• Natural slope, spring at peak (Muree, D.G.Khan)• Economical• Site specific• Design problems

Distribution by pump w/o storage

• Not practicable• High electricity cost• Operator role important(constant attendance)• Power /tubewell or fire breakdown problem• Pressure variation• Pumps at peak hourly flow• Several pumps to conform varying demand

Pump with storage

Pump with storage

• Excess water pumped during periods of low consumption stored in OHR

• High consumption periods water drawn out to augment pumped water

• Constant pumping rate• Economical as pumping rate max. daily flow

instead of peak hourly flow• More reliable due to fire fighting reserve

Water supply pipes

• Durability• First cost• Maintenance cost• Type of water to be conveyed(corrosive)• Carrying capacity(size)

Cast iron pipes

• Life-100 yrs(C-100(new)-130(old))• Joints-flanged joints with rubber gasket• Use-distribution system, treatment plants,

pumping stations & where rigidity strength and joint tightness is required

• Disadvantages :Corrosion reduce capacity by 70%-must be lined with cement/bitumen

Steel pipes• Avg life-25-50 yrs (C=100)• Jointing by welding• Less carbon then CI• Use-trunk mains, seldom used in distribution

system due to difficulty in jointing • Advantages: Stronger and lighter then CI for

high pressure pipes ,Cheaper then CI• Disadvantages :Cannot withstand vacuum,

Corrosion susceptible

Galvanized iron pipe• Life:20 yrs (C=100)• Dipping CI pipe in molten Zn• Size :1/2”-24”• Resistant to corrosion• Use: distribution system , mains, esp where soil

is rocky & excavation can’t be done and pipe is to be laid in open eg. In hilly areas , plumbing

Concrete pipe• Avg life-75 yrs(C=138-152)Usual size of RCC pipe >400 mm

• Advantages :inexpensive relatively ,withstand high internal pressure /external load , corrosion resistance, long life, minimal bedding.

• Disadvantages: Manufactured on site/near site, heavy, exact pipe fittings to be laid in advance

Asbestos cement• Avg life-30-50 yr (C=140-130)• Size 100-600mm• Available length-4m• Use :distribution system , transmission mains• Advantages :immune to acid salt, corrosion, less

cost laying and jointing , Less pumping cost due to less friction

Polyvinyl chloride• Expected life-25 yrs 50 yrs(C=140)• Size :350 mm size 12mm-600mm• Mainly used for domestic plumbing easy to install

handle, distribution system• Advantages :light weight, easy to install ,Cheaper ,

corrosion resistance, ease of jointing, easy maintenance ,long service life, small pumping cost

• Disadvantages :Weak to sustain load, piling, PVC brittle in sunlight

Ductile pipe

• Similar to CI except increased ductility• Mg addition with low sulpher and

phosphorous content• More expensive then CI• Stronger tougher and elastic then CI