Post on 26-Aug-2014
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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