Sizing of Under Ground and Over Head water tanks.

The availability of continuous running tap water is a luxury in our country. This article deals with the sizing of domestic and flushing water tanks only. It does not deal with rain water harvesting tanks or solar water tanks.

Domestic water consumption in our country is increasing as the population moves more and more to the cities. Buildings and housing complexes have a high density of population in a small area and hence the systems for the water supply in these complexes need to be properly designed. The sizing of the Under Ground Tanks (UGT) and the Over Head Tanks (OHT) has to be done properly. The sizing is also based on the balance between the civil costs of the construction and the amount of time the pumps have to run to fill these tanks.

The National building code of India provides guidelines on the calculation of the water demand and the sizing of water tanks. The quantity of water to be stored shall be calculated taking into account the following factors:

• ·         Hours of supply at sufficiently high pressure to fill up the overhead storage tanks;
• ·         Frequency of replenishment of overhead tanks, during the 24 h;
• ·         Rate and regularity of supply; and
• ·         Consequences of exhausting storage particularly in case of public buildings like hospitals.

 The first step to the design of the tanks is calculating the water demand of the building/housing complex.

The water demand shall be calculated as per the guidelines of NBC. It is recommended to take the lower limit of 150 litres/person/day.The split of domestic and flushing water is as follows.

Domestic water – 105 lpd and flushing water – 45 lpd

The number of people (population) in a building complex has to be provided by the architect on the basis of the utilisation of the building. The total water requirement can hence be calculated.

Total domestic water = population x 105   ---- (a)

Total flushing water = population x 45    ---- (b)

This gives the daily demand of water required (a) + (b).

This is divided on the following basis; 1/3^rd  day storage in the OHT and 2/3^rd storage in the UGT. The storage in the UGT may be increased upto 1.5 times if the above factors are relevant and need to be accounted for.

All other water requirements have to added to this to arrive at the total storage capacity

1.       Fire fighting storage

2.       Irrigation water requirement

3.       Make up water for water bodies.

4.       Make up water for cooling towers if any.

The sum totals of these requirements are to be calculated separately for the UGT and OHT.

Considering a water depth around 2 to 2.5 m the size of the tank can be calculated .

References : NBC PART 9 PLUMBING SERVICES — SECTION 1 WATER SUPPLY, DRAINAGE AND SANITATION

Any comments and suggestions are welcome.