Reservoir and Distribution System | Types, Merits and Demerits

What is Reservoirs?

A tank is used to store water is called reservoir. The storage may be public storage (clear water reservoirs, service reservoirs, balancing reservoir) and private storage (roof tank, underground tanks).

Distribution or service reservoirs are used in a distribution system to provide storage to meet fluctuation in demand of water, to provide storage for firefighting and emergencies such as breakdowns, repairs etc., and to stabilize pressures in the distribution system.

These reservoirs may be constructed of masonry (Brick or stone), concrete (plain or reinforcement or prestressed) or steel. These reservoirs are always closed to prevent contamination and growth of algae.

Purposes of Reservoir:

The reservoirs are necessary for the following purposes:

a. To provide a reserve against failure of main or in intermittent systems of supply.

b. To balance the fluctuations in the demand.

c. To reduce the pressures on the various appliances and instillations.

d. To maintain uniform pressure in the distribution system.

e. To meet the emergency demands such as firefighting.

Types of storage reservoirs

Reservoirs can be classified in the following ways:

A) According to use

1) Clear water reservoir:

Are used for storage of the filtered water until it is pumped or conveyed into the service reservoirs for distribution. Clear water reservoir must have minimum capacity to store filtered water corresponding to 14 16 hours average daily flow for storage when pumps are idle.

The reservoir should have divided into two or more compartment to enable for repair and cleaning. Such reservoirs are generally built underground or half below ground level and half above ground level depending upon the site condition and constructed with masonry and RCC.

2) Service reservoir or distribution reservoir:

It is used to store the filtered water to meet the widely fluctuating demands from clear water reservoir, to provide storage for firefighting and emergencies and to equalize operating pressures and is constructed before distribution system.

It is constructed with masonry and RCC. Elevated types are also popular. These service reservoirs should be designed for balancing, breakdown storage and fire storage. It serves the following purposes

a) Balance the fluctuation in demand.

b) Maintain uniform pressure in distribution system. 

c) Enable convenient pumping hours.

d) Meet emergency demand like fire.

e) Improve quality due to storage.

f) Distribution system becomes economical. 

g) Operation of distribution system becomes very easy.

If topography does not permit gravity distribution, an elevated tank is constructed for the distribution of water. They are either surface or elevated.

B) According to location or position:

 a) Surface/ground/non-elevated reservoir.

 b) Elevated reservoir.

a) Surface reservoir:

 They are either circular or rectangular generally constructed at high points in the city. These reservoirs are constructed at ground level or below ground level and hence these are called ground reservoirs or non-elevated reservoirs. Ventilators, overflow, inlet, outlet pipes, sluice values, washout value etc. is provided in it.

b) Elevated Reservoirs:

They are circular, rectangular or elliptical overhead tanks erected at a certain suitable elevation above the ground level and supported to towers. Water is pumped to the elevated reservoirs from surface reservoirs and then supplied to the consumers.

The various accessories provided for elevated reservoirs are Inlet pipe, outlet pipe, overflow pipe, ladders, Manhole, washout pipe, ventilators, water level indicators and lighting conductors for the protection against lighting.

Reservoir and Distribution System | Types, Merits and Demerits
Reservoir and Distribution System | Types, Merits and Demerits

C) According to materials used:

a. Earthen reservoir

b. RCC reservoir

c. Masonry reservoir

d. Steel reservoir etc.

D) According to the shape:

a. Circular reservoir

b. Rectangular reservoir

c. Egg shape reservoir

d. Spherical reservoir

Capacity Calculation of Balancing Reservoir

Breakdown storage/reserve:

It is the storage for emergencies such as the failure of pumps, electricity, or any other equipment and time for repairs.

It is difficult to assess because such breakdown may be minor or it may be major which may take days or weeks to repair hence generally 25% of the total capacity of the reservoir or about 1.5 to 2 times the average hourly supply may be considered as breakdown storage.

Higher value may be adopted, if the expectation of failure is very high. Normally in Nepal this storage is not considered.

Fire storage:

The water stored in the distribution reservoir for firefighting purposes is known as fire reserve. Fire storage is calculated by the following formula R= [F-P] T,

Where, R = fire reserve (liters), F= Fire demand from various formulas (liter/min), P=reserve fire pumping capacity in liter/minute and T = duration of fire in minutes. 

Balancing or equalizing storage:

Demand of water always keeps on varying from hour to hour, but the treated water always comes out of treatment plant at a constant rate.

Balancing reserve is that quantity of water to be stored in the reservoir for equalizing or balancing the fluctuations in the demand against the rate of the supply from treatment plant and the service reservoir designed for this purpose is called balancing reservoir.

It is conventionally calculated by means of mass curve or hydrograph indicating hourly rate of consumption. There are two commonly used methods to calculate Balancing storage.

a. Mass curve Method

b. Analytical Method

Mass curve Method:

The mass curve is the plot of cumulative supply (inflow) and cumulative demand (outflow) versus time.

Procedures for mass curve method:

  1. From previous given or collected data, take hourly demand (outflow) for 24 hrs. in the typical days (maximum, minimum & average)
  2. Calculate cumulative demand (outflow) and cumulative supply (inflow) and plot mass curve of demand (outflow) against time and mass curve of supply (inflow) against time to obtain two curves. [Two cases may arise; one is constant rated supply for 24 hrs. and the other is constant rated supply during limited time.]
  3. Now find the two max ordinates (one deficit and other surplus) between demand and supply curve as A and B respectively and calculate balancing storage as (A+B). [summation of A and B is the total ordinate between the two tangents drawn parallel to the supply line]
  4.  Repeat the procedure for all typical (minimum, maximum & average) days and finalize the max storage capacity for the worst day.

Analytical method:

  1. Calculate the cumulative hourly demand (outflow) and cumulative hourly supply (inflow) for 24 hrs. in tabular form.
  2. Find the hourly excess of outflow (deficit), excess of inflow (surplus), total outflow (TO) and total inflow (TI).
  3. Then note the maximum calculative surplus (MCS) and maximum calculative deficit (MCD).
  4. Then the capacity of balancing reservoir (CBR) is given by:
  5. If TI> TO, CBR-MCD-TI+TO and
  6.  If TISTO, CBR=MCS+MCD

The capacity of balancing reservoir depends on the inflow from source to the reservoir and the water demand of the area. The inflow from source to the reservoir may be either continuous supplying water at the constant rate for all the 24 hours or intermittent supplying water for certain fixed hours in day as in the case of pumping system.

The water supplied from the reservoir may also be either continuous or intermittent. Depending upon the system of inflow to the reservoir and the system of inflow from the reservoir, the design of reservoir capacity is divided into four cases as shown in table below:

CaseInflow to the reservoir Outflow from the reservoir
AContinuous Continuous 
BContinuous Intermittent
CIntermittentContinuous 
DIntermittentIntermittent

System of water supply

Water may be supplied to the consumers by the following two systems:

a. Continuous system

b. Intermittent system

a) 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: 

Merit

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

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

c. Adequate quantity of water is available at any time for firefighting.

d. Lesser pipe sizes are required.

Demerit

a. More wastage of water if the people do not possess the civic sense and do         not understand the importance of water.

b. If there is leakage in the system, large volume of water is wasted because of       long duration of flow.

c. On repairing supply may be interrupted during supply hours.

b) Intermittent system:

This method is adopted when either sufficient quantity of water is not available or sufficient pressure is not available. In this system the supply of water is divided into various zones and supplied for fixed hours or by turn. It is the most common system adopted in Nepal.

The timing is 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.

Merits:

i. Useful when either sufficient pressure or quantity of water id not available          at the source to meet the demand.

ii. At various distribution zones of the city, water can be supplied by turn.

iii. Repairing work can be done in non-supply hours.

iv. Leakage on the system causes less waster of water because of small                     duration of flow.

Demirits:

1. As a whole this system is inconvenience to the consumers because they               have to remain alert to collect the water supply.

2. People store water during non- supply hours. Consumers may not have               sufficient storage, which may cause in-sanitary condition.

3. No water is available for firefighting in non-supply hours and before the               system is on, may cause huge demand. fire

4. During the non-supply time, taps left open unknowingly or due to                         negligence, which led more waste of water.

5. It requires bigger sized diameter pipes.

6. During non-supply time pressure in the line may fall below atmospheric              pressure, which may induce suction of external matter and soil through leak     joint. The pipeline laid nearer sewers may leads severe contamination and         pollution.

7. Many valves will be required for maintaining the system to open or close             and to operate such values extra staff may be required.

8. It increases the wastage of water because most of people keep their taps           open unknowingly.

I hope this article on “Reservoir and Distribution System” remains helpful for you.

Happy Learning – Civil Concept

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