Unit:7 Reservoirs and distribution System of water supply note(Second year second parts)

Introduction

After the treatment of water , it is distributed to the target community for domestic , industrial and public uses. Water is to be stored temporarily and supplied to the consumers through the network of pipelines is called distribution system. A distribution system consists of main pipes, service pipes, different reservoirs, valves, pumps, hydrants, meters, stand post 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.

Requirements of the Good Water Distribution System

1.The system should be economical to maintain and operate.
2.Sufficient water should reach 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 type 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.

7.1 Water storage (Reservoir) : Clear water reservoir , service reservoir

Reservoirs are the tanks or basin which are used to store the water for various purposes . The larger the capacity of the reservoir , the better will be reliability of water supply system. But larger capacity of the reservoir will increase its cost. Depending on its purpose, the reservoir can be classified as follows.

a.Clear water reservoir

b.Service reservoir

a.Clear water reservoir: Clear water reservoir is used to store the water that has been treated and clear. It is provided at the end of the treatment process within the water treatment plant premise. It is generally located below the ground so that water can be conveyed by gravity from treatment unit to the reservoir. It is considered a part of the water treatment plant. The water from the clear water is conveyed to the service or distribution reservoir. The clear water should have capacity of 14 to 16 hours of daily water demand. The larger capacity will provide better reliability of the water supply system because the supply of water can be maintained without interruption for long time when water treatment units and other upstream components are under repair and maintenance.

b.Service Reservoirs; Service reservoirs or distributions reservoirs are used in a distribution system to provide storage to meet fluctuations in demand of water , to provide storage for fire fighting and emergencies such as breakdowns , repairs, etc. and to stabilize pressures in the distribution system The service reservoirs should be located near to the community as far as possible to supply the water to the consumers in shortest possible time with minimum loss of heads in pipes. Besides, it will save cost of the pipelines as smaller pipes are required in the transmission lines from the source to the service reservoirs.

Types of service reservoir:

• Surface Reservoir

• Elevated Reservoir

Construction of Service Reservoir:

(1) Surface Reservoir

They are also called as ground or non-elevated reservoir. They are rectangular or circular in shape. The supply may be by gravity flow or by pumping. Pumping is avoided as far as possible to minimize the cost. They are constructed with two compartments so that one can be used when other is being repaired. The two compartments are connected by control valves. It consists of an inlet pipe. The reservoir contains ventilation for aeration. Overflow pipes are provided at full supply level to maintain a constant level in the reservoir. The deposited sludge can be removed through washout pipes during occasional cleaning. The outlet pipes are placed at the higher level than the washout pipes.

* Elevated Reservoirs

They are also known as overhead tanks. They are rectangular ,circular or elliptical in shape. It consists of the inlet for entry, the outlet for the exit, ventilation for aeration and overflow pipe at full supply level to maintain a constant level. It consists of the ladder to reach the top of the reservoir and manhole for regular inspection.

7.2 Distribution system : gravity ,pumping and dual system

Methods of Water Distribution System

For efficient distribution, it is required that water should reach every consumer with the required rate of flow.

Therefore, the some pressure in the pipelines is necessary which should force the water to reach every place.

The distribution systems can be classified as follows:

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 of 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.

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 as an alternative.
This method has no problem with the pressure and maintaining the head at the consumer’s tap.

Advantages

1.Water can be pumped only when required.
2.Low water loss due to leakage.

Disadvantages

1.Break down of system may occur if power fails.
2.Maintenance and operation cost is high.

3.Dual system

• This is also known as combined pumping and gravity system.

•The pump is connected to the mains as well as to an elevated reservoir . In the beginning when the demand is small the water is stored in the elevated reservoir but when the demand increases the rate of pumping, the flow in the distribution system comes from both the pumping station as well as elevated reservoir.

•As in this system water comes from two sources named 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.
Following are the main advantages of this system.
→The system is economical, efficient and reliable and adopted practically everywhere.
→Pumping at constant rate increases efficiency.
→Water is available even during failure of pump and power.

7.3 Layout of Distribution System: Dead End, Grid Iron, Ring and Radial System (Introduction Only)

1. Dead end or tree system:

SM=SUB MAIN SC=SERVICE CONNECTION

B=BRANCH

• = CUTOFF VALVE

Dead End or tree System

In this system , one main pipeline through the center of the area to be served and from both sides of the main , the sub main takes off. The submains are further divided into several branches from which service connections are given to the consumers. So the network of pipelines covers the entire area as like as branch of a tree and no cross connection is done but various dead ends are available . Mostly adopted in towns or cities developed in haphazard way. Eg: Kathmandu valley. The system has following advantages and disadvantages:

Advantages:

• The design calculations are simple and easy. It is possible to determine the discharge and pressure in each pipe very accurately.
•The pipe diameters are to be designed for the population likely to be served by them. This leads to cheap and economical design.
•The lying of pipe is simple.
• Less numbers of cutoff valves is required.

Disadvantages:

•Accumulation of sediment and stagnation because of dead ends and quality of water is degraded.

• If the system fails by breaking of pipe or closed for repair , whole locality beyond that point can’t get water.

• Many scour valves are required and less successful in maintaining satisfactory pressure in the far areas.

•The water available for fire fighting is low because supply can neither be increased nor be diverted.

2. Grid Iron System

In this system , one main pipeline through the center of the area to be served and from both sides of the main , the sub –mains take off in perpendicular direction then branch lines inter connects all the sub mains so that water can be circulate through entire distribution system. This system is suitable for cities planned in the rectangular pattern or grid iron pattern.

• = CUTOFF VALVE SM= SUB MAIN B= BRANCH

Advantages:

•Circulation of water is available hence no stagnation or sediment deposit and no chance of pollution.

•Due to interconnection, water is delivered at every point of the system with minimum head loss.

• Repair or damage in any section affects only in very small area.

• Fire fighting water can be easly available by diverting the water from the other sections to the affected area using valves.

Disadvantages:

• More number of cutoff valves and longer length of pipes are required and costly in lying.

• Design of pipes and calculation of pressure is laborious, complicated and difficult.

• The analysis of discharge , pressure and velocities in the pipeline is difficult and

Cumbersome

• Overall cost is high.

2. Ring or Circular System

In this system ,entire distribution area is divided into small circular or rectangular blocks and main pipelines are laid on the periphery of those blocks. Those peripheries of the mains are then linked in the interior of the area as shown in figure. In this case water at any point is available from any two directions. This system is suitable for cities having well planned roads and roads in circular or rectangular pattern. The length of main pipe is larger and hence large quantity of water is available for firefighting and all other advantages and disadvantages are same as in grid iron system.

SM=SUBMAIN

• = CUTOFF VALVE

4.Radial System:

In this system , the layout is reverse of circular or ring system. Water flows towards the outer periphery. In this system the entire distribution area is divided into small circular or rectangular zones and each center of zone is provided with a distribution reservoir and water is flowed radially towards periphery from branches as shown in figure. In this system has high pressure of distribution and gives quick and efficient water distribution and hence more efficient in fire fighting . This system is suitable for cities having radial road network. The major disadvantages of this system is that it requires more resevoirs and all other advantages and disadvantages are same as in grid iron system.

DR=Distribution reservoir

SM=Sub main B=Branch

7.3 System of Supply: Continuous and Intermittent system

Water may supplied to the consumers by the following two system.

1.Continuos system

2.Intermittent system

1.Continuous System

If water is supplied to the consumers for all 24 hours of a day from a system of supply ,it is called the continuous system. It is the best system and has following advantages and disadvantages.

Advantages:

→Water is available every time hence no need of private storage tank.

→No stagnant in the pipe at any instant hence fresh water is always available.

→Adequate quantity of water is available at any time for fire fighting.

Disadvantages:

→More wastage of water if the people do not possess the civic sense and do not understand the importance of water.
→If there is leakage in the system ,large volume of water is wasted because of long duration of flow.
→On repairing, supply may be interrupted during supply hours.

2.Intermittent system

If water is supplied to the consumers only fixed hours of a day from a system of supply ,it is called the intermittent system. It is the most common system adopted in Nepal. The timing are fixed normally at the morning and evening. Timing may be changed to suit climatic and seasonal condition of the year. As soon as possible the basic steps should be taken to change it into continuous system.