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(Para 10.3 OF CPHEEO)

1.Peak Factor:   

The per capita rate of water supply indicates only the average consumption of water per day per person over a period of one year.  In the design of water supply distribution system, it is to be recognized that consumption varies with the season, month, day and hour.  As far as the design of distribution system is concerned, it is the hourly variation in consumption that matters.  The fluctuation in consumptions accounted for, by considering the peak rate of consumption ( which is equal to average rate multiplied by a peak factor) as rate of flow in the design of distribution system.  The following peak factors are recommended for various population figures: 

1.For population less than 50,000    3.0     

2.For a population range of 50,000 to 2,00,000   2.25 

3.For population above 2,00,000    2.0 

4.For Small Water Supply Schemes  (Where supply is effected through standposts for only 6 hours)    3.0

2. Residual Pressure:   

Distribution system should be designed for the following minimum residual pressures at ferule points:  

Single storey building = 7m 

Two storey building = 12m 

Three storey building = 17m 

Distribution system should not ordinarily be designed for residual pressures exceeding 22 meters. Multistoried buildings needing higher pressure should be provided with boosters

3.Minimum Pipe Sizes   

Minimum pipe sizes of 100mm for towns having population upto 50,000 and

150mm for those above 50,000 are recommended.

For dead ends, less than 100mm can be considered.

If it is a grid, less than 100mm can be used in situations where no further expansions contemplated.

4. Elevation of Reservoir   

The elevation of the service reservoir should be such as to maintain the minimum residual pressure in the distribution system consistent with its cost effectiveness. The hydraulic gradient in the pipe should normally be between 1 and 4 per thousand at peak flow.  A suitable combination of pipe sizes and staging height has to be determined optimization of the system. The staging height of service reservoirs is normally kept as 15-20m.

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

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

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Aim : To improve the raw water quality to the drinking water standards and stop water borne transmission of epidemics .

Methods of treatment : 

Depends on the nature of source and its water quality .

Subsurface source Generally Chlorination will be sufficient except where iron is present .

Surface source : 1.Aeration ( if required) 2.Pre chlorination ( optional ) 3.Sedimentation – either plain or with coagulation and flocculation , 4.post chlorination ●

1. Aeration

Aim : i.to remove objectionable tastes and odours . ii.for expulsion of carbon dioxide, hydrogen sulphide . iii.to precipitate impurities iron and manganese present. iv.For increasing the dissolved oxygen content to water for imparting freshness.

Types of aeration i.Spray – Type ii.Multiple tray or water fall iii.Cascade type iv.Diffused air aerators v.Mechanical aerators

2. Pre – Chlorination

Aim : i.to prevent biological growth in raw water ii.for reduction of colour . iii.for destruction of some taste & odour producing compounds . iv. for oxidation of iron, manganese and hydrogen sulphide . v.to aid coagulation. vi.for minimizing post-chlorination dosage .

Dosage : 1 to 5 ppm depending on the degree of pollution .

3. Plain Sedimentation

Aim : To separate suspended impurities from water by gravitation .

Detention period : One to several days for sedimentation without subsequent filtration to 4 hours for sedimentation in conjunction with filters .( much longer settling time for basins preceding slow sand filters than for rapid sand filters ) .

Loading rate: 2.4 to 24m³ / day / m²

4 Chemical dosing

Aim : i.For coagulation, flocculation . ii.disinfection and softening . iii.algal and corrosion control . iv.for fluoridisation .

Types :

(i) Dry feed .

(ii) Solution feed .

Strength of solution :To be not more than 5% for manual feed and not more than 10% for mechanical feed.

Alum is the most common coagulant used and economical.

Lime is also added when PH and alkalinity are low

Dosage for alum : 20-100 mg / 1 (1-5 grain / gallon )

Dosage for lime: About one third that of alum

Density of lime = 670 kg / m3 , Density of alum = 980 kg / m3

5. Flash mixing

Aim : To disperse the coagulant evenly in the water.

Generally used when flow exceeds 300 Cu.m / hour.

Head loss : 0.20 – 0.60 m of water

Ratio of tank dia. To height : 1.1 to 3.0

6. Coagulation and Flocculation

Aim :The addition of a coagulant like alum promotes the formation of micro floes which are the nuclei for the absorption of turbidity and colour causing particles.

During flocculation, the micro floc particles formed during rapid mixing are brought together to aggregate into larger rapidly settle able floes by controlled agitation of water .

Detention Period :15-30 minutes in flocculation zone .2-3 hours in settling or clarifier zone . Dosage :To be decided by Jar Test .

Coagulation:  colloidal destabilization

•Typically, add alum (aluminum sulfate) or ferric chloride or sulfate to the water with rapid mixing and controlled pH conditions

•Insoluble aluminum or ferric hydroxide and aluminum or iron hydroxo complexes form

•These complexes entrap and adsorb suspended particulate and colloidal material. 

Flocculation: Slow mixing (flocculation) that provides for for a period of time to promote the aggregation and growth of the insoluble particles (flocs).

The particles collide, stick together abd grow larger

The resulting large floc particles are subsequently removed by gravity sedimentation (or direct filtration) Smaller floc particles are too small to settle and are removed by filtration

7. Sedimentation

Aim: To remove readily settling sediments such as sand, silt, coagulated impurities such as colour and turbidity and precipated

The range of surface loadings and detention periods for average design flow for different types of sedimentation tanks are as follows:

8. Filtration

Aim i. to separate the suspended and colloidal impurities in the water .

ii. to produce sparkling and aesthetically attractive water free from disease

producing organism .

Types :

a. Slow Sand Filters : Slow sand filters can provide a single step treated for 46 polluted surface waters of low turbiding (< 20 Ntu)

b. Rapid Sand Filters : The rapid sand filter comprises of a bed of sand serving as a single medium granular matrix supported on gravel overlaying an under drainage system.

Turbidity is measured in NTU: Nephelometric Turbidity Units. The instrument used for measuring it is called nephelometer or turbidimeter, which measures the intensity of light scattered at 90 degrees as a beam of light passes through a water sample.

Rapid sand filtration is a purely physical drinking water purification method. Rapid sand filters (RSF) provide rapid and efficient removal of relatively large suspended particles.

Two types of RSF are typically used: rapid gravity and rapid pressure sand filters. For the provision of safe drinking water, RSFs require adequate pre- treatment (usually coagulation-flocculation) and post- treatment (usually disinfection with chlorine). Both construction and operation is cost-intensive. It is a relatively sophisticated process usually requiring power-operated pumps, regular backwashing or cleaning, and flow control of the filter outlet.

Rapid sand filtration is common in developed countries for the treatment of large quantities of water where land is a strongly limiting factor, and where material, skilled labour, and continuous energy supply are available. The major parts of a gravity rapid sand filter are:

1.Chamber: filter tank or filter box

2.Filter media (sand)

3.Gravel support

4.Under drain system

5.Wash water troughs.

Slow sand filters 

Slow sand filters may be used where there is sufficient land and space, as the water must be passed very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration

Membrane Filtration

Membrane types & example full-scale configurations:

•Microfiltration  ~ 0.1 to 100 μm

•Ultrafiltration  ~ 0.005 to 10 μm

•Nanofiltration  ~ 0.5 nm to 1 μm

Highly effective particle removal

•Reverse osmosis ~ 0.01 nm to 0.1 μm

 Dissolved contaminant removal

9. Wash water Gutter :

Horizontal travel of dirty water over the surface of filter shall not be more than 0.6 to 1.0 m before reaching the gutter .

Bottom of gutter should clear the top of Expanded sand by 50 mm or more .

Upper edge of gutter should be placed as far above the surface of the undisturbed sand surface as the wash water rises in 1 minute .

10. Back wash .

Back wash should be arranged at such a pressure that the sand expands to about 130 to 150 of its undisturbed volume or 5 m head of water as measured in under

drain. Normally the wash water is applied at 36 m (600 lpm/m2) for a period of 10 minutes

11.Pressure Filters

Same principle as gravity type rapid sand filters; but water is passed through the filters under pressure. Tank axis may be vertical or horizontal.

12.Post Chlorination :

Aim : For disinfection of potable water by the use of gaseous chlorine or chlorine compounds to destroy bacteria through the germicidal effects of effects of chlorine; may be done at head works / treatment works and supplemented by additional chlorination in loose pockets of distribution system.

Dosage: When prechlorination is adopted relatively small doses will be required generally 1 to 2mg/l.

Often the most critical step in protection of consumer against pathogenic   microorganisms :  organisms are killed (or “inactivated”) by reaction with various chemical oxidants.

Commonly-used disinfectants: •“Free” chlorine – Applied as Cl₂(g) or NaOCl (HOCl is the active disinfectant in either case) •Chloramines, or “Combined” chlorine – Applied either as pre-formed NH₂Cl, or by mixing NH₃ and HOCl •Chlorine dioxide – Applied as ClO₂(g) •Ozone – Applied as O₃(g) (no long-term residual) •Ultraviolet light – Applied via submerged UV lamps (no residual)

Ozone treatment

Though the principle is relatively simple, this method needs special equipment, supply of pure oxygen and trained operators. Ozone is generated by passing pure oxygen through an ozone generator. It is then bubbled through a gas diffuser at the bottom of an absorption column, in a direction opposite to the flow of raw water. Retention or contact time is critical and the size of the absorption column depends on the water flow.

Ultraviolet irradiation treatment

 This method is often used to treat drinking water. Successful commercial installations have been made to purify sea water in large fish processing plants.

· U-V rays in the range of 2500-2600 Angstrom units are lethal to all types of bacteria.
· There is no organoleptic, chemical or physical change to the water quality.
· Overexposure does not have any ill effects.

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A pump is a device which converts mechanical energy into hydraulic energy. It lifts water from a lower to a higher level and delivers it at high pressure. Pumps are employed in water supply projects at various stages for following purposes: •To lift raw water from wells. •To deliver treated water to the consumer at desired pressure. •To supply pressured water for fire hydrants. •To boost up pressure in water mains. •To fill elevated overhead water tanks. •To back-wash filters. •To pump chemical solutions, needed for water treatment.

Classification of pumps

(i) Classification based on principles of operation

• Displacement pump

• Centrifugal pumps

• Air –lift pumps

• Impulse pumps

(ii) Classification based on type of power required

• Electrical driven pumps

• Gasoline engine pumps

• Steam engine pumps

• Diesel engine pumps

(iii) Classification based on the type of services

• Low lift pumps

• High lift pumps

• Deep-well pumps

• Booster pumps

• Standby pumps

The selection of a particular type of pumps depend upon the following factors

• Capacity of pumps

• Number of pump units required

• Suction conditions

• Lift (total head)

• Discharge condition, and variation in the load

• Floor space requirement

• Flexibility of operation

• Starting & priming characteristics

• Initial cost and running costs

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Various types of Joints

• Spigot and Socket Joints or Bell & Spigot Joints

This types of joints is mostly used for cast iron pipes For the construction of this joint the spigot or normal end of one pipe is slipped in socket or bell mouth end of the other pipe until contact is made at the base of the base of the bell.

• Expansion Joints This joint is used at such places where pipes expand or contract due to change in atmospheric temperature and thus checks the setting of thermal stresses in the pipes

• Flanged Joints

This joint is mostly used for temporary pipe lines, because the pipe line can be dismantled and again assembled at other places.

Flexible Joints

• Sometimes this joint is also called Bell & Socket or Universal Joint. This joint is used at such places where settlement is likely to occur after the lying of the pipes. This joint can also be used for laying of pipes on curves, because at the joint the pipes can be laid at angle. This is a special type of joint. The socket end is cast in a spherical shape.

After this the retaining ring is slipped which is stretched over the bead. Then a rubber gasket is moved which touches the retainer high. after it split cast iron gland ring is placed, the outer surface of which has the same shape as inner surface of socket end.

Mechanical Joints

• This type of joints are used for jointing cast Iron, Steel or wrought Iron pipes, when both the ends of the pipes are plain or spigot. There are two types of mechanical joints.

Dressers- Couplings

• It essentially consists of one middle ring, two follower rings and two rubber gaskets. The two follower rings are connected to-gather by bolts and when they are tightened,

Victaulic Joint

• In this type of joints a gasket or leak-proof ring is slipped over both the ends of the pipes. This gasket is pressed from both the sides by mean of half iron couplings by bolts. The ends of pipes are kept sufficient apart to allow for free expansion, contraction and deflection.

Screwed Joints

The joint is mostly used for connecting small diameter cast iron,  wrought iron and galvanized pipe. The end of the pipes have threads outside, while socket or couplings has threads on the inner side

Collar Joint

This type of joint is mostly used for joining big diameter concrete and asbestos cement pipes. The ends of the two pipes are brought in one

level before each other. Then rubber gasket between steel rings or jute rope socked in cement is kept in the grove and the collar is placed at the joint so that it should have same lap on both the pipes.

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Concrete Pipes

These pipes may be precast or Cast-in-situ plain concrete pipe may be used at such places where water does not flow under pressure, these pipes are jointed with Bel &Spigot Joints. Plain Concrete pipes are up to 60 cm dia only, above it these are reinforced.

Advantages of R.C.C Pipes

• Their life is more about 75 years

• They can be easily constructed in the factories or at site

• They have least coefficient of thermal expansion than other types of pipes . Hence they do not require expansion joints

• Due to their heavy weight, when laid under water, they are not affected by buoyancy, even when they are empty.

• They are not affected by atmospheric action or ordinary soil under normal condition.

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Steel pipes

The Construction of these pipes is similar to wrought iron pipes, it is occasionally used from main lines and at such places where pressure are high and pipe dia is more. Steel pipes are more stronger, have very light weight and can withstand high pressure than cast iron pipes. They are also cheap, easy to construct and can be easily transported.

The disadvantages of these pipes is that they cannot withstand external load, if partial vacuum is created by emptying pipe rapidly, the pipe may be collapsed or distorted. These pipes are much affected by corrosion and are costly to maintain The life of these pipes is 25 to 50 years, which is much shorter as compared to cast Iron Pipes Steel pipes are not used in distribution system, owing to the difficulty in making connections.

• The joints in steel pipes may be made of welding or riveting, longitudinal lap joints are made In riveted steel pipes up to 120 cm dia.

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Advantages of CI Pipes

• Ease in jointing the pipes

• Can withstand high Internal pressure

• Have a very long design life. (100 years)

• They are less prone to corrosion.

Dis-advantages of CI Pipes

• They are heavy and difficult to transport

• Length of pipe available as less (2.5 to 5.5m) so more joints are required for laying the pipes so chances of leakage also Increases.

• They are brittle so they break or crack easily.

Wrought Iron Pipes

• Wrought Iron Pipes are manufactured by rolling the flat plates of the metal to the proper diameter and welding the edges. If compared with cast Iron, these are more lighter, can be easily cut, threaded and worked, give neat appearance if used in the interior works. But it is more costly and less durable than cast iron pipes. These pipes should be used only inside the buildings, where they can be protected from corrosion. Wrought Iron pipes are joined together by couplings or screwed and socketed joints. To Increase the life of these pipes sometimes these are galvanized with zinc.

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Conveyance

There are two stages in the transportation of water:

•Conveyance of water from the source to the treatment plant.

•Conveyance of treated water from treatment plant to the distribution system.

In the first stage water is transported by gravity or by pumping or by the combined action of both, depending upon the relative elevations of the treatment plant and the source of supply.

In the second stage water transmission may be either by pumping into an overhead tank and then supplying by gravity or by pumping directly into the water-main for distribution.

Free Flow System

In this system, the surface of water in the conveying section flows freely due to gravity. In such a conduit the hydraulic gradient line coincide with the water surface and is parallel to the bed of the conduit. It is often necessary to construct very long conveying sections, to suit the slope of the existing ground. The sections used for free-flow are: Canals, flumes, grade aqueducts and grade tunnels.

Pressure System

In pressure conduits, which are closed conduits, the water flows under pressure above the atmospheric pressure. The bed or invert of the conduit in pressure flows is thus independant of the grade of the hydraulic gradient line and can, therefore, follow the natural available ground surface thus requiring lesser length of conduit. The pressure aqueducts may be in the form of closed pipes or closed aqueducts and tunnels called pressure aqueducts or pressure tunnels designed for the pressure likely to come on them. Due to their circular shapes, every pressure conduit is generally termed as a pressure pipe.

When a pressure pipe drops beneath a valley, stream, or some other depression, it is called a depressed pipe or an inverted siphon.

Depending upon the construction material, the pressure pipes are of following types: Cast iron, steel, R.C.C, hume steel, vitrified clay, asbestos cement, wrought iron, copper, brass and lead, plastic, and glass reinforced plastic pipes.

Cast Iron Pipes

• Cast – Iron Pipes are mostly used in water supply schemes. They have higher resistant to corrosion, therefore have long life about 100 years.

Cast Iron pipes are manufactured in lengths of 2.5 m to 5.5 m. The fittings of these pipes are also manufactured in sand molds having core boxes. These fittings are also weighed, coated with coal tar and finally tested. Cast-Iron pipes are joined together by means of Bell and Spigot, Threaded or flanged Joints.

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Factors Governing Location of Intake •The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.

Design Considerations

sufficient factor of safety against external forces such as heavy currents, floating materials, submerged bodies, ice pressure, etc. should have sufficient self weight so that it does not float by upthrust of water.

Types of Intake

Depending on the source of water, the intake works are classified as follows:

According to type of source

• River Intake • Canal Intake

• Reservoir Intake • Lake Intake

According to position of Intake

• Submerged Intake • Exposed Intake

According to presence of water in the tower

• Wet Intake • Dry Intake

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