The process of distributing treated water to the consumers is called water distribution system.
The distribution system includes pumps, reservoirs, valves, water meter, pipe fittings, etc. The cost of the distribution system is about 40-70 % of the total of the entire scheme.
A good water distribution system aims to supply water to all the consumers whenever required in sufficient quantity with required pressure without any leakage.
1.1. Requirements of the good water distribution system
1. The system should be economical to maintain and operate.
2. Sufficient water should reach to all consumers.
3. It should be able to draw enough water during an emergency like fire fighting.
4. The system should be watertight and leakage should be least as possible.
5. Any types of contamination of water should not occur during the distribution.
6. It should not be laid below the sewer lines as far as possible.
7. The system should have adequate pressure but the pressure should not be high causing the bursting of pipes and fittings.
8. The distribution layout should be such that it should supply water even during repair and maintenance time.
1.2. Methods of the water supply system
There are two types of water supplying system.
i. Continuous system
ii. Intermittent system
1.2.1. Continuous system
This is the best method in which the water is supplied to the community during all 24 hours of a day.
In this system, a sufficient amount of water is always available for consumers to use and also for an emergency like a fire fighting. Due to the continuous circulation; water remains fresh but the losses will be more if there are leakages in the system.
1.2.2. Intermittent System
If adequate water is not available, the whole community is divided into several zones and water is supplied in each zone for a fixed time in a day or alternate days. The water flows in the consumer’s tap at certain intervals. So, it is called intermittent system.
1.3. Methods of Water Distribution System
For efficient distribution, it is required that water should reach to every consumer with the required rate of flow. Therefore, the same pressure in the pipelines is necessary which should force the water to reach every place. The distribution systems can be classified as follows:
1.3.1. Gravity System
When the distribution reservoir is located at a higher elevation than the target community; then water can be supplied with the gravity flow. This method is much suitable when the source is the river or impounded reservoir at sufficient height than the target community. Usually pumping water is not required at any stage of this type of distribution.
Advantages of the gravity method
1. No energy is required to operate the system as water is conveyed by gravity.
2. No pump is required.
Disadvantages of the gravity method
1. Not applicable in plain or flat terrain where an elevation source of water supply is not available.
2. Water loss by leakage is comparatively higher as the system remains under constant pressure.
3. Requirement of break pressure tanks to reduce the pressure in the pipelines.
1.3.2. Pumping System
In this system, water is supplied to the consumers with the help of pumps. Some extra pumps are installed for emergency causes like fire hazards, peak water demand, etc. This method is suitable if the source is at a lower elevation than the target community. However, this system of distribution becomes very expensive for long term use.
In this system, the water may be insufficient when the power fails. So diesel pumps are also managed for an alternative. This method has no problem with the pressure and maintaining the head at the consumer’s tap.
1. Water can be pumped only when required.
2. Low water loss due to leakage.
1. Break down of system occurs if power fails.
2. Maintenance and operation cost is high.
3. During pumping hours, the inflow of water through leaks may cause water contamination.
1.3.3. Dual System
It is a combination of a gravity system and pumping system. So it is called the Dual System.
a. One Way Dual System
In this system, water is pumped to the elevated reservoir and then supplied to the consumers by the action of gravity. The distribution network is connected with elevated reservoir but not with direct distribution pumps as shown in the figure above.
b. Two Way Dual System
In this system, separate pumps are provided for direct distribution and elevated reservoir where the elevated reservoir is used for backup in an emergency, power failure, etc. The Distribution network is connected with both elevated reservoir and direct distribution system.
1.4. Layouts of Distribution Network
In general, there are 4 types of a distribution network system. They are:
a. Dead End or Tree system
b. Gridiron System
c. Circular or Ring System
d. Radial System
1.4.1. Dead End or Tree System
This system is also referred to as a tree system. It consists of one main pipe from which a number of sub-mains bifurcate and from each sub-main several branch pipes separate out which are called laterals.
From laterals, connections are given to different houses. This type of distribution system is suitable for old cities. The system is easy to design and is cheap and simple. Water conveyance is only unidirectional in this system, so water can reach at a specific point solely through one route, thus if any fault creep in water system gets disturbed in that area.
This system has many dead ends which prevent the free circulation of water, thereby increases the possibility of contamination of water. Discharge can’t be increased in case of a fire breakout. Dead-end mains longer than 1,000ft should be at least 6 inches in diameter.
Advantages of the dead-end system
a. It is relatively cheaper.
b. Design and calculation of the dead-end system is easy.
c. Requires less number of valves; this makes it easier to determine discharges and pressures.
Disadvantages of the dead-end system
a. Single pipeline serves the region. One problem in the pipeline may lead to a cut of the water supply of a large area.
b. Presence of many dead ends makes stagnation of water in pipes.
c. Discharge of water is quite low.
1.4.2. Gridiron System
This system is also referred to as a reticular or interlaced system. The entire system consists of one main pipe which runs through the centre. And also consists of branches and laterals which runs in a grid pattern. All of these three pipes are interconnected.
Since the mains, branches and laterals are interconnected; dead ends are laminated and water reaches at different locations through more than one route. By closing cut-off valves of other areas pipes, water can be diverted to the affected area at the time of the fire. There is a very fewer chance of recontamination because there are no dead ends.
The design of this system is complicated because pipelines get water from different directions. Size of pipes is larger and more number of sluice valves are required. It is most applicable for a planned city where roads and street are provided in well planned rectangular and squares grid patterns.
Advantages of the gridiron system
a. Absence of dead-end reduces the chances of pollution due to stagnation.
b. During repair and maintenance work; the small region is only affected.
c. Availability of enough water at street fire hydrants.
Disadvantages of the gridiron system
a. Requirement of a huge number of cut-off valves.
b. Requirement of longer pipes with a larger diameter.
c. Difficult to determine discharge, pressure and velocities in the pipelines.
d. Less economical.
1.4.3. Circular or Ring System
In this system, the supply main forms a ring around the distribution area. The branches are connected cross-wise to the mains and also to each other. This system is most reliable for a town with well-planned streets and roads.
1.4.4. Radial System
In this system; the whole city is divided into parts and each part contains a centrally located distribution reservoir (elevated).
The distribution pipes are laid radially ending towards the periphery and are connected to the central distribution reservoir.
Advantages of the radial system
a. Easy to determine pipe size.
b. This system provides quick service.
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