DISTRIBUTION OF WATER

After treatment, water is to be stored temporarily and supplied to the consumers through the network of pipelines called distribution system. The distribution system also includes pumps, reservoirs, pipe fittings, instruments for measurement of pressures, flow leak detectors etc. The cost of distribution is about 40 to 70% of the total cost of the entire scheme. The efficiency of the system depends upon proper planning, execution and maintenance. Ultimate aim is to supply potable water to all the consumers whenever required in sufficient quantity with required pressure with least lost and without any leakage.

The requirements for a distribution System include;

1.      They should convey the treated water up to consumers with the same degree of purity

2.      The system should be economical and easy to maintain and operate

3.      The diameter of pipes should be designed to meet the fire demand

4.      It should safe against any future pollution. As per as possible should not be laid below sewer lines.

5.      Water should be supplied without interruption even when repairs are undertaken

2.11.1 System of Distribution

For efficient distribution it is required that the water should reach to every consumer with required rate of flow. Therefore, some pressure in pipeline is necessary, which should force the water to reach at every place. Depending upon the methods of distribution, the distribution system is classified as gravity, pumping, and dual or combined gravity and pumping systems.

Gravity System

When some ground sufficiently high above the city area is available, this can be best utilized for distribution system in maintaining pressure in water mains. This method is also much suitable when the source of supply such as lake, river or impounding reservoir is at sufficiently higher than city. The water flows in the mains due to gravitational forces. As no pumping is required therefore it is the most reliable system for the distribution of water.

Pumping System

Constant pressure can be maintained in the system by direct pumping into mains. Rate of flow cannot be varied easily according to demand unless a number of pumps are operated in addition to stand by ones. Supply can be affected during power failure and breakdown of pumps. Hence diesel pumps also in addition to electrical pumps as stand by to be maintained. During fires, the water can be pumped in required quantity by the stand by units.

          Combined Pumping and Gravity System

          This is also known as dual system. The pump is connected to the mains as well as elevated reservoir. In the beginning when demand is small the water is stored in the elevated reservoir, but when demand increases the rate of pumping, the flow in the distribution system comes from the both the pumping station as well as elevated reservoir. As in this system water comes from two sources one from reservoir and second from pumping station, it is called dual system. This system is more reliable and economical, because it requires uniform rate of pumping but meets low as well as maximum demand. The water stored in the elevated reservoir meets the requirements of demand during breakdown of pumps and for fire fighting.

2.11.2 Pumps

Pumping facilities are required wherever gravity can’t be used to supply water to the distribution system under sufficient pressure to meet all service demands. Pumps are driven by electricity, diesel or steam power.

All pumps may be classified as kinetic energy pumps or positive displacement pumps:

Table 2.9 Pump Types and Major Applications in Water and Wastewater

Major Classification	Pump Type	Major Pumping Applications
Kinetic	Centrifugal	Raw water and wastewater, secondary sludge return and wasting, settled primary and thickened sludge, effluent
	Peripheral	Scum, grit, sludge and raw water and wastewater
	Rotary	Lubricating oils, gas engines, chemical solutions, small flows of water and wastewater
Positive Displacement	Screw	Grit, settled primary and secondary sludges, thickened sludge, raw wastewater
	Diaphragm	Chemical solution
	Plunger	Scum and primary, secondary, and settled sludges; chemical solutions
	Airlift	Secondary sludge circulation and wasting, grit
	Pneumatic ejector	Raw wastewater at small installation (100 to 600 L/min)

The centrifugal pump and its modifications are the most widely used type of pumping equipment in the water treatment.

Centrifugal Pumps

Pumps of this type are capable of moving high volumes of water in a relatively efficient manner. The centrifugal pump is very dependable, has relatively low maintenance requirements, and can be constructed out of a wide variety of construction materials. The centrifugal pump is available in a wide range of sizes, with capacities ranging from a few gallons per minute up to several thousand pounds per cubic inch.    

The centrifugal pump consists of a rotating element (impeller) sealed in a casing (volute). The rotating element is connected to a drive unit or prime mover (motor or engine) that supplies the energy to spin the rotating element. As the impeller spins inside the volute casing, an area of low pressure is created in the center of the impeller. This pressure allows the atmospheric pressure on the water in the supply tank to force the water up to the impeller.  Because the pump will not operate if there is no low-pressure zone created at the center of the impeller, it is important that the casing be sealed to prevent air from entering the casing. To ensure the casing is airtight, the pump includes some type of seal (mechanical or conventional packing) assembly at the point where the shaft enters the casing. This seal also includes some type of lubrication (water, grease, or oil) to prevent excessive wear.

When the water enters the casing, the spinning action of the impeller transfers energy to the water. This energy is transferred to the water in the form of increased speed or velocity. The water is thrown outward by the impeller into the volute casing where the design of the casing allows the velocity of the water to be reduced, which, in turn, converts the velocity energy (velocity head) to pressure energy (pressure head). The water then travels out of the pump through the pump discharge. The major components of the centrifugal pump are shown in 2.10 below.

                                    

            

Figure 2.10 Centrifugal pump – major component (Spellman, 2003)

The general characteristics of the centrifugal pump are shown in the table below.

Table 2.10 Characteristics of Centrifugal Pumps (Spellman, 2003)

Characteristic	Description
Flow rate	High
Pressure rise per stage	Low
Constant variable over operating range	Pressure rise
Self-priming	No
Outlet stream	Steady
Works with high-viscosity fluids	No

Horse Power of Pump

The horse-power (H.P.) of a pump can be determined by calculating the work done by a pump in raising the water up to H height.

Let the pump raise ‘ρ’ kg of water to height ‘H’ m

Then workdone by pump = ρ X Q X H

     = WQH kgm/sec

Where W= density of water in kg/m3.

Q = water discharge by pump in m3/sec

 The water horse power = Discharge x Total head

                                                           75

                         

                       W.H.P.   =      ρ × Q × H

                                                     75

    

      Break Horse Power =      W. H. P     _

                                                Efficiency

                                 

                                       =      W × Q × H _                                      ………………….…..2.3        

                                                  75 × η

Selection of Pump

Basic data regarding the water availability like diameter, depth of the well, depth of the water table, seasonal variations of water table, drawdown duration of pumping and safe yield are to be collected accurately before selecting a pump.

There are many varieties of specifications and choices available in the market and it is a tricky problem facing an engineer to select the best suited for his requirement

          The following are to be noted when selecting a pump;

1.      Capacity and efficiency : The pump should have the capacity required and optimum

efficiency.

2.      Lift :  Suction head from the water level to the pump level.

3.      Head:  It is also called delivery head. Generally the total head (suction and delivery

             head) should meet all possible situations with respect to the head.

4.      Reliability: A reputed manufacture or similar make pump already in use may give the

failure rate and types of troubles.

5.      Initial cost: The cost of the pump and its installation cost should be minimal.

6.      Power : Power requirements should be less for operation.

7.      Maintenance: Maintenance cost should be minimal. Availability of spares and cost of

             spares are to be ascertained.

2.11.3 Pipes and Pipes Requirements

Pipes convey raw water from the source to the treatment plants in the distribution system. Water is under pressure always and hence the pipe material and the fixture should withstand stresses due to the internal pressure, vacuum pressure, when the pipes are empty, water hammer when the values are closed and temperature stresses.

A good piping material should possess the following characteristics:

1.      It should be capable of withstanding internal and external pressures.

2.      It should have facility of easy joints.

3.      It should be available in all sizes, transport and erecting should be easy.

4.      It should be durable.

5.      It should not react with water to alter its quality.

6.      Cost of pipes should be less.

7.      Frictional head loss should be minimal.

8.     The damaged units should be replaced easily.

There are different types of pipes which include, cast iron, steel, prestressed concrete, reinforced  concrete cylinder  (R.C.C.), asbestos concrete (A.C.), galvanized iron, and poly vinyl chloride (PVC) and plastic pipes (Venkateswara, 2005).

Table 2.11 Advantages and disadvantages of the different types of pipes (Venkateswara, 2005).

Type of Pipe	Advantages	Disadvantages
Cast iron pipes	1. Cost is moderate 2. The pipes are easy to join 3. The pipes are not subjected to corrosion 4. The pipes are strong and durable 5. Service connections can be easily made 6. Usual life span  is about 100 years	1. Breakage of pipes are large 2. The carrying capacity of these pipes decreases with the increase in life of pipes 3. The pipes are not used for pressure greater than 0.7 N/mm2 4. The pipes are heavier and uneconomical beyond 1200 mm diameter
Steel pipes	1. No. of joining are less because these are available in long lengths 2. The pipes are cheap in first cost 3. The pipes are durable and strong enough to resist high internal water pressure 4. The pipes are flexible to some extent and they can therefore  be laid on curves 5. Transportation is easy because of its light weight	1. Maintenance cost is high 2. The pipes are likely to be rusted by acidic or alkaline water 3. The pipes require more time for repairs during breakdown and hence not suitable for distribution pipes 4. The pipes may deform in shape under combined action of external forces
Prestressed concrete pipes	1. The inside surface of pipes can be made smooth 2. Maintenance cost is low 3. The pipes are durable with life period 75 years 4. No danger of rusting 5. These pipes do not collapse or fail under normal traffic loads	1. The pipes are heavy and difficult to transport 2. Repairs of these pipes are difficult 3. The pipes are likely to crack during transport and handling operations 4. There pipes are affected by acids, alkalis and salty waters
R.C.C. pipes	1. There are pipes are most durable with usual life of about 75 years 2. The pipes can cast at site work and thus there is reduction in transport charges 3. Maintenance cost is less 4. Inside surface of pipe can made smooth 5. No danger of rusting.	1. Transportation is difficult. 2. Repair work is difficult 3. Initial cost is high 4. These pipes are affected by acids, alkalis and salty waters
Asbestos concrete (A.C.) pipes	1. The inside surface of pipes are very smooth 2. The joining of pipe is very good and flexible 3. The pipes are anticorrosive and cheap in cost 4. Light in weight and transport is easy 5. The pipes are suitable for distribution pipes of small	1. The pipes are brittle and therefore handling is difficult 2. The pipes are not durable 3. The pipes cannot be laid in exposed places 4. The pipes can be used only for very low pressures
Galvanized iron pipes	1. The pipes are cheap 2. Light in weight and easy to handle 3. The pipes are easy to join	1. The pipes are affected by acidic or alkaline waters 2. The useful life of pipes is short about 7 to 10 years
P.V.C. and plastic pipes	1. The pipes are cheap 2. The pipes are durable 3. The pipes are flexible 4. The pipes are free from corrosion 5. The pipes are good electric insulators 6. The pipes are light in weight and it can easy to mould any shape	1. The co-efficient of expansion for plastic is high 2. It is difficult to obtain the plastic pipes of uniform composition 3. The pipes are less resistance to heat 4. Some types of plastic impart taste to the water.