Design Criteria

Design Period Per Capita water Supply Continuous or Intermittent Supply Quality Standards Source of Water Supply Transmission of Water

Distribution System Water Treatment plant Disinfection Flow Measurement Pumping Service Reservoir

Design Period

Clause 2.2.6 of Manual stipulate design period, For some components it may be modified depending on its useful life, facility for carrying out extensions when required and interest rate so that expenditure far ahead of utility is avoided. Land for future extension should be acquired in beginning itself. Project components may be designed to meet the requirements of the following design period.

Sl.No.	Data Source	Design period in years
1	Storage by dams	50
2	Infiltration Works	30
3	Pumping
	i. Pump house (civil works)	30
	ii. Electric motors and pumps	15
4	Water treatment units	15
5	Pipe connection to several treatment units and other small appurtenances	30
6	Raw water and clear water conveting mains	30
7	Clear water reservoirs at the head works, balancing tanks and service reservoirs (overhead or ground level)	15
8	Distribution system	30

 

Per Capita water Supply

Factors affecting consumption:  
Larger the city, more rate of consumption. Individual  bungalows  consume more than flats. Slums consume less water. In hot weather consumption is more. The consumption rate is less in metered areas than that area where charges are levied on flat rate basis.

Domestic : 
Clause 2.2.8.3 of manual stipulate maximum water supply of 70 LPCD for towns without sewerage, 135 LPCD for towns with sewerage system existing/contemplated and 150 LPCD for metropolitan and Mega cities  with sewerage system existing/contemplated. Where water is provided through PSPs 40 LPCD should be considered. The un accounted for water is not included in above per capita supply. The LPCD figures include water for commercial, institutional and minor industries. However the bulk  supply to such establishments should be assessed separately.

Un accounted for Water :
The unaccounted for water in Indian cities is very high-some 40-50%. However the target should be to reduce it to 15%. Accordingly while designing a scheme provision of 15% for UFW/NRW should be taken and old schemes should be rehabilited to bring UFW/NRW to 15% level.

Fire Fighting :
Clause 2.2.8.3b of manual provide for fire fighting demand as a coincident draft on the distribution system along with a normal supply. Provision in kl/day of 100*√P (where, P=population in thousands) may be adopted for communities larger than 50000.

Institutional :
The water requirement for institutes / industries/ hotels/ hostels and hospital be taken separately as per Clause 2.2.8.3c of Manual

industrial:
Clause 2.2.8.3d of Manual gives water requirement of different industries.

 

 

Continuous or Intermittent Supply

Piped water supplies should be designed on continuous 24 hour basis. Intermittent supplies are neither desirable from public health point nor economical.

Quality Standards

Physical & Chemical :  
Clause 2.2.9 of Manual Table 2.2 gives recommended guidelines for physical and chemical parameters on the principal that safe water is an obligatory standard and physical and chemical qualities are optional within a range.

Bacteriological Quality :
Clause 2.2.9 of Manual, Table 2.3 gives recommended guidelines for Bacteriological quality

Virological Quality : 
Drinking water must be free of human enteroviruses to ensure negligible risk of transmitting viral infection. Clause 2.2.9 of Manual, Table 2.4 gives recommended treatment for different water sources to produce water with negligible virus risk

Frequency of Sampling : 
The minimum number of samples to be collected from distribution system should be as per Table 15.1 of Manual.

Source of Water Supply

Surface Water: 
Water collected from precipitation, Lakes, ponds, Dams, Rivers, Irrigation canals, sea water, waste water reclamation etc are surface sources. Study the availability and relative costs of supplying water and then decide source. The raw water from lakes, ponds, dams and rivers is extracted by constructing floating or fixed Intake. Intake can be with pumping sets on it or only draw water and through conduit carry to a suction well from where it is pumped. Clause 5.2.7.1 (c) stipulate design considerations. Rate of Silting in dams, Clause 5.2.7.1 (g( i)), stipulate 0.1 to 0.2 hectare meters per year per sq.kilometre. Evaporation in dams, Clause (5.2.7.1 (g(ii)), stipulate 2-2.5 m/year. NOIGEN-101 mixture of Cetyl and Stearyl alcohols indigenously available may be used for suppressing evaporation from lakes and reservoirs by spraying on water surface. A dose of 1.2 kg/hectare/day is adequate for wind velocities below 8 km/hour.

Ground Water: 
Hydrogeological Map of the area published by CGWB should be referred to know hydrogeological conditions, ground water potential and quality. Depleting water table over the years indicate more withdrawal of water than the recharge and ground water extraction from such areas should be restricted. Open well(Shallow well/dug well/sunk well), bored well,infiltration gallaries and radial wells are used to abstract ground water.

Pipe flow and Transmission of water

Laying & jointing :  
Pipes are laid underground with minimum cover of 1 m. Care should be taken to locate other utilities in ground and avoid damage to them. All specials and valves should be available and installed with pipe without leaving gap for subsequent installation. Width of trench at bottom shall provide 200 mm clearance on both sides of pipe. Pipe line shall be laid as straight as possible with minimum horizontal and vertical bends. The bend should not exeed 2 degree or as recommended by manufacturer. Provide proper bends and thrust blocks and anchors at bends and dead ends. Transportation, handling and storage should be proper and follow manufacturers recommendations. Pipes over 300 mm dia shall be handled and lowered into the trenches with the help of Crane or chain pully block. The socket end should face upstream when laying on flat ground and should face the up gradient when pipe runs uphill. All lumps, blisters and excess coating material shall be removed from socket and spigot end of each pipe and outside of the spigot and inside of the socket shall be wire-brushed and wiped clean and dry from oil & grease before the pipe is laid.

Testing of laid pipe line :
The field pressure to be imposed should not be less than the maximum of

1. 1.5 times the maximum sustained operating pressure,
2. Sum of the maximum sustained operating pressure and the maximum surge pressure.
Losses during test shall be less than 0.1 litre per mm of pipe dia per KM of pipeline per day for each 30 metre head of pressure applied.

Economic size of rising Main : 
Appendix 6.5 of Manual gives method for finding economical size. Based on it a design template on axle sheet can be used to find economic size by giving input values specific to the requirement. Excel templeate for design of rising main is given in the chapter 'General'

Appurtenances : 
On line valves are provided in larger mains at 1-5 km interval to facilitate repairs. Non rising spindle Sluice valves are used for isolating or scouring and not intended for continuous throttling as erosion of the seats and body cavitation may occur. Butterfly valves are used to reguate and stop the flow. In large size butterfly valves are cheaper and occupy less space. These may involve higher friction loss than sluice valve. Also not suitble for continuous throttling. Sluice valve and butterfly valve for higher sizes require geared hand wheel or power driven actuators. Scour valves are provided in valleys/depressions to dewater pipe line. Air valve size is one twelth of pipe diameter when it serves purpose of only release of air and one eighth when it serves purpose of admission as well as release of air. Kinetic air valves are used to release air entrapped during running of pipe line. Pressure relief valves, Check valves, Pressure Reducing Valves, Ball Float Valves, Shut off valves are used for specific purposes.

Peak factor: 
Economic size is when it runs 24 hours. As such power availability determines pumping hours. Peak factor for Rising main is "24/Pumping hours".

Minimum and maximum Velocity of flow: 
Low velocities reduce head loss and so energy cost but pipe dia required becomes larger. High velocities increase head loss and so energy but require smaller pipe dia. As such economic pipe dia should be selected. Maximum velocity may be 3 m/sec to prevent erosion.

Water Hammer: 
Water hammer pressures are computed as per clause 6.17.1. If the operating pressure plus surge pressure exeeds 1.1 times internal design pressure then protective device required. In no case maximum operating pressure plus surge should exeed field hydrostatic pressure.

 

Free Flow: 
Open channels and gravity aqueducts and tunnels provide free flow. Open channels have restricted use in water supply in view of losses due to evaporation and seepage and possibility of pollution and misuse (theft) of water. Aqueducts and tunnels flow three quarter full. mean velocities which will not erode channels after ageing range from 0.3 to 0.6 mps for unlined canals and 1 to 2 mps for lined canals.

Flow under Pressure:
Pressure aqueducts and Tunnels are ordinarily circular in section

Head Loss in Pipes:
Hazen-Williams formula (clause 6.2.1 a) for pressure conduits and Mannings formula (clause 6.2.1 b) for free flow conduits are generally used. Table 6.1 of Manual give value of Hazen William coefficient of roughness and Table 6.2 give value of Mannings coefficient of roughness. Modified Hazen william Formula, clause 6.2.4 obviates the limitations of Hazen William formula and can be used for more accuracy. Clause 6.2.8 give design recommendations for use of Modified Hazen Williams Formula and Table 6.4 provides value of roughness coefficient in MHW formulae.

Head loss due to Specials and Appurtenances :
Pipe line transitions and appurtenances add head loss which is expressed as KV^2/2g where v is velocity in m/sec and g is acceleration due to gravity in m/sec square. Value of K for different fittings are given in Table 6.5.

Minimum Velocity :
Minimum velocity be 0.6 mps to avoid deposition & corrosion. However where it is inevitable due to minimum pipe diameter criteria, lower velocities may be adopted with adequate provision for scouring.

Pipe Material Options :
Pipelines involve major investments in water supply as such judicious selection of pipe material is necessary. Selection basis should consider durability, life and cost which includes installation and maintenance cost. Pipe material options are CI,DI,MS,GI, RCC, PSCC, BWSC,AC,PVC, Polyethylene, GRP etc. Technical factors affecting decision are availability in market, internal pressure, roughness coefficient, hydraulic and operating conditions, internal and external corrosion problems, laying and jointing, type of soil, ease of trasportation, special conditions etc. The Manual has stipulated check list, Table 6.7, to facilitate decision for selection of pipe material

 

‘Characteristics of metallic, PSC and GRP pipes’ and ‘Characteristics of plastic, DI and GI pipes’ can be referred to adjudge suitability of pipe material option for particular use.’

Single contract for supply, laying &Jointing :
It may be desirable that all pipeline contracts are awarded on a single contract responsibility so that quality of assurance at various stages of manufacture, transportation, supply, laying, jointing, testing and commissioning is taken care by a single agency.

Distribution System

Pressure Requirement :
Clause 2.2.8.3 e stipulates that for towns where one-storeyed buildings are common and for supply to the ground level storage tanks in multi-storeyed buildings, the minimum residual pressure at ferrule point should be 7 m for direct supply. Where two-storeyed buildings are common, it may be 12 m and where three-storeyed buildings are prevalent 17 m or as stipulated by local byelaws.

Peak factor : 
Peak factor for 24*7 water supply is 3 for population of zone less than 50000, it is 2.5 for population in range  50000 to 200000 and it is 2 for population of zone above 200000.

Minimum Pipe Size:  
Minimum pipe size may be taken 100 mm.

Two pipes on wide roads:  
For roads wider than 25 m distribution pipe may be provided on both side of road.

Sluice Valves:  Sluice valves should be provided such as to isolate each District Metering Area (DMA).

Design of pipe network:  Design is done through customized software.

Water Treatment plant

Aeration :
Purpose: Aeration is to add oxygen in waters deficient in oxygen or for expulsion of carbon di oxide, hydrogen sulphide and other volatile substances causing taste and odour or to precipitate impurities like iron and mangenese. Limitations: Aeration requires significant head of water. Water is rendered more corrosive after aeration when dissolved oxygen content is increased though in certain circumstances it may be otherwise due to removal of carbon di oxide. For taste and odour removal, aeration is not highly effective but can be used in combination with chlorine or activated carbon to reduce their doses.

Types: Spray aeraters in which water is sprayed through nozzles in atmosphere.
Water fall/multiple tray aeraters/cascade aerators in which water falls along steps/trays in small height and pass through media. In diffused aeration air passes through water.

 

Chemical Handling & Feeding :
Feeding can be dry or in solution. Solution is fed through controlled feeders which are gravity or pressure type. There should be atleast 2 tanks for each chemical feed & capacity of each to hold 8 hour requirement.Freeboard should be atleast 0.3 m. Coating with bituminous tank for alum tank necessary but for corrosive chemicals lining of rubber/PVC/Epoxy resin required. Lifting tackle to lift chemicals to solution tank required for gravity feed. Each tank should be provided with atleast 0.75 m wide platform, railing of 0.75 m height be provided on platform. Platform should have 2 m clear head room & top of solution tank should not be higher than 1 m from floor of platform. Manual mixing for plants upto 2.5 MLD and for higher capacity mechanical mixers/compresses air/recirculation required. To regulate dose solution feed device is used by means of orifice rotameter/positive displacement pump/weir. Constant head orifice is the most common device.

Chemical Storage :
A storage of 3 months is advisable. In cases where major storage is provided at a place away from the feed equipment, a weeks storage space should be provided near the plant. Storage should be  damp proof & properly drained. For chemicals in bag, stack height should not exceed 2 m.

Coagulation & Flocculation:
Coagulation is produced by the addition of a chemical and rapid mixing (flash mixing) for obtaining uniform dispersion. Flocculation formation of settleable particles (floc) is achieved by gentle and prolonged mixing. Good flocculation with minimum coagulant dose and in least time occurs within optimum pH zone. Flocculation Time: usually require 15-30 minutes in summer and 30-60 minutes in the colder months. Hydrated lime or soda ash may be used when increase in hard ness is to be avoided. When ferrous sulphate is used as a coagulant pH should be above 9.5 to ensure complete precipitation of the iron. Coagulant aids:  a chemical which when used along with main coagulant, improves or accelerates the process of coagulation and flocculation by producing quick-forming, dense and rapid settling flocs. Finely divided clay, fullers earth, bentonites and activated carbon, polyelectrolytes are commonly used coagulant aids.

Rapid Mixing:

Sedimentation :
Plain Sedimentation: It is usually employed as a preliminary process to reduce heavy sediment loads from highly turbid waters prior to subsequent treatment such as coagulation/filtration. Settling Velocity of Discrete Particles is as per clause 7.5.2. Clariflocculators are videly used across country. The coagulation and sedimentation processes are effectively incorporated in a single unit in the clariflocculator. 2 or 4 flocculating paddles are placed equidistantly.

Tank Dimensions: Rectangular tanks length is commonly upto 30 m but larger lengths upto 100 m have been also adopted. length to width rato is 3:1 to 5:1. Circular tanks upto 60 m in diameter are in use but are generally upto 30 m to reduce wind effect. Square tanks are genarally smaller uually sides upto 20 m. square tanks with hopper bottoms having vertical flow  have sides generally less than 10 m to avoid large depths. Depths commonly used are 2.5 to 5 m with 3 m being a preferred value. Bottom slopes may range from 1% in rectangular tanks to about 8% in circular tanks. The slope of sludge hopper range from 1.2 V:1H to 2 V:1 H. Surface loading and detention pariods for various types of sedimentation tanks are given in clause 7.5.6. Inlets & Outlets: Normal weir loading s are upto 300 cum/day/meter.

Sludge Removal: In circular tanks where mechanical scrappers are provided, the floor slopes should not be flatter than 1 in 12, to ensure continuous and proper collection of sludge. For manual cleaning slope should be above 1 in 10. Power requirements are about 0.75 watt/sqm of tank area. The scrapping mechanism is rotated slowly at 30-40 minutes in one revolution or tip velocity of scrapper should be around 0.3 m/min or below. For sludge blanket type vertical flow settling tanks the slope of hopper should not be less than 55 degree to horizontal.

Tube Settlers: Tube settlers allow high loading rates and used for improving the performance of existing basins and also as a sole settling unit.

Filtration :
Slow SandFilter: Requires large land, sand and labour and as such may suit only for small capacities. It may be cost effective for rural and small communities. The design guide lines are given in table 7.3 of manual.

Rapid Gravity Filters (RGF): The distinctive features of RGF compared to slow sand filtration is careful pretreatment, higher filtration rate, coarser but more uniform filter media, backwashing by reversing flow. Standard filtration rate is 4.8-6 m/hour. Practice is tending towards higher rate (upto 10 m/hour) with better pretreatment and use of coarser sand (effectve size 1mm). Maximum area of one filter bed 100 sqm consisting of two halves of 50 sqm is recommended for plants greater than 100 mld. Also for flexibility of operation a minimum of 4 beds should be provided which can be reduced to 2 for smaller plants. Where filters are located on both sides of a pipe gallery, length to width ratio of filter bed is found to be 1.11 to 1.66 averaging about 1.25 to 1.33. A minimum overall depth of 2.6 m including free board of 0.5 m is adopted. It is not necessary to provide roof over the filters. The operating gallery should be roofed. Effectve size of sand shall be 0.45 to 0.7 mm, uniformity coefficient 1.7 to 1.3, ignition loss should not exceed 0.7% by weight, soluble fraction in hydrochloric acid shall not exceed 5% by weight, silica content not less than 90%, specific gravity 2.55 to 2.65 and wearing loss shall not exceed 3%. Usually depth of sand should be 0.6 to 0.75 m but for higher rate filtration when coarse medium is used deeper sand beds are suggested. Standing depth of water over filter varies from 1 to 2 m and free board of 0.5 m.

Under Drainage Systems: In case of central manifold and laterals (lateral pipes can be of CI, plastic, AC, concrete or other material) . A non ferous drain system is preferable where water has a low pH and is corrosive and when the correction for pH has to follow the filtration process. However AC pipes have a tendency to dissolve away in presence of low pH alum treated waters.), perforations vary from 5 to 12 mm in diameter and should be staggered at a slight angle from vertical axis of pipe, spacing of perforations in laterals may be 80 mm for 5 mm perforation to 200 mm for 12 mm perforation. Ratio of total area of perforations to total cross sectional area of lateral should not exceed 0.5 for perforations of 12 mm and should decrease to 0.25 for perforations of 5 mm. Ratio of total area of perforations to the entire filter area may be about 0.3%. The ratio of length to diameter of the lateral should not exceed 60. The spacing of laterals closely approximates the spacing of orifices and shall be 300 mm. The cross sectional area of manifold should be preferably1.5 to 2 times the total area of the laterals.

Filter Gravel: Size of gravel varies from 50 mm at bottom to 2 to 5 mm at the top with a depth of 0.45 m. In case of porous plate floor supported on concrete pillars, bottom gravel not required.

High RateBackwash: Back wash pressure is about 5 m in underdrains so as to expand sand 130-150% of its undisturbed volume. Normally wash water rate where no other agitation is provided is 600 lpm/sqm for a perid of 10 minutes. For high rate wash pressure may be 6-8 m and wash water rate of 666-750 lpm/sqm for 6-10 minutes. Capacity of back wash storage tank must be sufficient to supply wash water to two filter units at a time where the units are 4 or more.

Air Wash system: Free air at 600-900 lpm/sqm at 0.35 kg/sqcm is forced through underdrain for a period of 5 minutes following which wash water is introduced at a rate of 400-600 lpm/sqm.

In the practice of backwashing employing air and water wash together air is applied at a rate of 45-50 m/hour and water at 12-15 m/hour.

Dual Media Filters: Two media of different density and sizes are used. Top layer consists of lower density material like coal having larger particle size. Lower layer is higher density material like silica sand and have smaller diameter particles. In India anthracite coal is not easily available, the coarse material may consist of high grade bituminous coal or crushed coconut shell can be used. The effective size of coal (specific gravity 1.4) is usually 1 mm (0.85-1.6 mm range) with uniformity coefficient of 1.3 to 1.5 and depth of 0.3 to 0.4 m. The finer media-layer usually consists of 0.3-0.4 m thick silica sand (specific gravity 2.65) with effective size of around 0.5 mm (0.45 to 0.60) and uniformity coefficient of 1.3 to 1.5. In case of crushed coconut shell used as coarse media, the size ranges from 1 to 2 mm with depth of 0.3 -0.4 m, uniformity coefficient below 1.5, specific gravity 1.4, The sand used in conjunction with crushed coconut shell has effective size varying between 0.44 to 0.55 mm, uniformity coefficient below 1.5, sand depth 0.3 to 0.4 m. Filtration rate range recommended is 7.5 to 12 m/hour.

The back wash rates of 700-900 lpm/sqm are used.

Multi Media filter: Normally contain three media such as anthracite coal, silica sand and garnet sand with specific gravities 1.4, 2.65 and 4.2.

Performance of Rapid Gravity Filter: Filtrate turbidity should be less than 1 NTU,should be free from colour (3 or less on cobalt scale), filter run be not less than 24 hours with a head loss not exceeding 2 m, wash water consumption less than 2% of filtered quantity.

Disinfection

Satisfactory disinfection  is obtained by prechlorination to maintain 0.3 to 0.4 mg/l free available  residual throughout treatment or 0.2 to 0.3 mg/l free available residual in the plant  effluent at normal pH values. At higher pH of 8 to 9 at least 0.4 mg/l is required for complete bacterial kill with 10 minutes contact time. For 30 minute contact time dosage reduces to 0.2 to 0.3 mg/l. Normal concentration of chlorine destroys organisms associated with typhoid fever, dysenteries and various gastrointenestinal disorders. Cysts of E. histolytica causing amoebic dysentary are inactivated at higher dose of 0.5 mg/l of the free residual chlorine. To inactivate virus 0.5 mg/l of free chlorine for one hour contact time is required.Where water supply is infested with nematodes  0.4 to 0.5 mg/l of free available residual chlorine for six hour contact time is required. Application: Bleaching powder solution used for disinfecting small quantities and addition of gaseous chlorine through vacuum chlorinaters for bigger quantities ( as Bleaching powder is costlier than chlorine gas)are commomnly used.

Flow Measurement

Where to measure flow :  

• River flow gauging-upstream of intake,
• yield of open wells and tube wells,
• incoming water in intake structure,
• Flow at entry of WTP,
• Filtrate from each filter,
• Back wash water in WTP,
• Out going water from WTP,
• Water pumped from CWPS, RWPS,
• Flow to Service Reservoir or flow from service reservoir,
• Flow to distribution zone, sub zones,
• Measurement of flow to individual consumer

Measurement in open channals:  
Triangular notch, Rectangular notch, trapezoidal notch, Weirs,standing wave flumes,Ventury Flumes, Drops, velocity area methods, electro magnetic probe method

Measurement in Pipes:
Diffrential Pressure devices such as Ventury meters, Orifice Plates and Nozzles, Pitot tubes

Domestic water Meters:    
As per the amended IS 779 (ISO 4064) magnetically driven and hermetically sealed meters should be used. To improve quality of meters type test in CPRI recommended.

Bulk water Meters:    
Vane wheel type or helical type in sizes 50 to 300 mm conforming to IS 2373 are available in the market. In higher sizes full bore magnetic flow meters and ultrasound meters are also available.

Pumping

NPSH available (suction pressure-friction loss in foot valve, suction pippe and fittings-velocity head at suction face-suction head-vapour pressure) should be higher than the NPSH required. Piping: Suction pipe should be short and straight, bends of long radius, size one or two sizes larger than nominal suction of pump, use ecentric reducer so that no point on suction pipe is higher than suction part of pump, velocity in suction pipe 2 m/sec, velocity in bell mouth 1.5 m/sec, suction strainer should have area at least 3 times area of suction pipe. Discharge pipe may be one size higher than pump delivery, velocity may be 2.5 m/sec, discharge piping connection to a common header by a radial tee or 30-45 degree bend. Dismantling joint be provided between pump and valve. Valves: In case of suction lift provide foot valve. Provide vacuum pump priming when suction pipe larger than 300 mm. In case of positive suction head sluice valve or butterfly valve is provided in suction pipe for isolation. In delivery pipe a non return valve (NRV) and delivery valve ie sluice/butterfly valve should be provided. NRV should be between pump and delivery valve. Size of valve should match size of pipe.

Pump House Size: Minimum space between two adjoining pumps or motor should be 0.6 m for small and medium units and 1 m for large units. A clear space of 915 mm in width shall be provided in front of switch board. If there are any attachments or connections at the back of switch board space behind shall be either less than 230 mm or more than 750 mm in width measured from farthest part of attachment/conductor. Service bay for overhauling/repairs should be provided to accommodate largest equipment. A ramp or a loading unloading bay should be provided. Head room and material handling tackle for vertical pump should be able to lift, dismantle and carry motor and largest colomn assembly to maintenance bay.

Service Reservoir

Capacity can be worked out by mass curve between inflow pattern of water and outflow pattern of water. Capacity is about 25-35%of quantity of water to be supplied in case of continuous supply.