Whenever an engineer is given the duty to design a water supply scheme for a particular section of the community it becomes imperative upon him/her, to first consider the demand, and then the second requirement is to find sources to fulfill that demand.
It is very difficult to precisely assess the quantity of water demanded by the public since there are many variable factors affecting water consumption.
The various type of water demands, which a city may have, may be broken down into the following classes:-
(1) Domestic water demand
(2) Industrial water demand
(3) Institution and commercial water demand
(4) Demand for Public use
(5) Fire demand
(6) Water required to compensate losses in wastes and theft.
In order to estimate, as correctly as possible, the total water demand, all these demands must be considered and suitable provision made, depending upon the needs of those people for whom the water supply scheme is to be designed.
Domestic Water Demand
This includes the water required in residential buildings for drinking, cooking, bathing lawn sprinkling gardening sanitary purpose etc. The amount of domestic water consumption per person shall vary according to the living conditions of the consumers.
As per IS:1172-1993, the minimum domestic consumption for a town or a city with full flushing system should be taken at 200 l/h/d ; although it can be reduced to 135 l/h/d for economically weaker sections.
Industrial Water Demand
The industrial water demand represents the water demand of industries, which are either existing or are likely to be started in future, in the city for which water supply is being planned. This quantity will thus vary with the number and type of industries present in the city. The ordinary per capita consumption on account of industrial needs of a city is generally taken as 50 liters/person/day.
Institutional and Commercial Water Demand
The water requirements of institutions such as hospitals hotels schools, colleges, railway stations, offices etc. should also be assessed. The quantity will certainly vary with the nature of the city and with the number of commercial establishments. On an average, a per capita demand of 20 l/h/d is usually considered.
Demand for Public Use
This includes the quantity of water required for public utility purposes such as watering of public parks, washing, and sprinkling on roads, use in public fountains etc. For most of the water supply schemes in India, these needs are not believed as essentials, and a figure of 10 l/h/d is usually added on this account.
In thickly populated and industrial areas, fires generally break out and lead to serious damages, if not controlled effectively. Therefore, a provision should be made in water supply scheme for fighting fires.
Water Required to Compensate Losses in Thefts and Wastes
This includes the water lost in leakage due to bad plumbing or damaged meters, stolen water due to unauthorized water connections and other losses and wastes. These losses should be taken into account while estimating the total requirements.