Water demand is the amount of water required to fulfill the demand of the consumers.
While designing a water supply scheme, it is necessary to evaluate the amount of water demanded by the public and to find out the 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 following are various types of water demand:
Domestic water demand.
Industrial water demand.
Institution and commercial water demand.
Demand for public uses.
Water required to compensate losses in wastes thefts.
3.1 Domestic Water Demand
This includes the water required in residential buildings for drinking , cooking , bathing etc.
Domestic water demand accounts for 55 to 60% of the total water consumption.
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 (litre per head per day) or lpcd (litre per capita per day).
Although it can be reduced to 135 lpcd for economically weaker sections and LIG colonies (Low Income Group) depending upon prevailing conditions.
In a developed country like the U.S.A , this figure usually goes as high as 340 lpcd.
The total domestic water demand shall be equal to the total design population multiplied by per capita domestic consumption
3.2 Industrial Water Demand
The industrial water demand represents the water demand of industries which are either existing or likely to be started in future, in the city for which water supply is being planned.
This quantity varies with the number and types of industries present in the city.
This consumption, under ordinary conditions (by considering small scattered industries only ) is 50 lpcd.
In industrial cities , the per capita water requirement may vary as high as 450 lpcd.
3.3 Institution And Commercial Water Demand
It includes the water requirements of institutions such as hospitals, hotels, restaurants, schools and colleges, railway stations etc.
On an average, a per capita demand of 20 lpcd is usually considered to be enough to meet such commercial and institutional water requirements. Although this demand may be as high as 50 lpcd for highly commercialized cities
3.4 Demand for Public Uses.
This includes water requirements for watering of public parks, gardening, washing and sprinkling on roads etc.
On this account a nominal amount not exceeding 5% of the total consumption may be provided.
A figure of 10 lpcd is usually added on this account, while computing total water requirement
3.5 Fire Demand
During the fire breakdown large quantities of water is required for throwing it over fire to extinguish it, therefore provision is made in the water work to supply sufficient quantities of water or keep as a reserve in the water mains for this purpose.
Rate of fire demand is worked out by following formulas:
𝑸 = 𝟑𝟏𝟖𝟐 √𝑷
Q = quantity of water in litres per minute
P = population in thousands
For a central congested high valued city
when population less than or equal to 200000
𝑸=𝟒𝟔𝟑𝟕 √𝑷 (1 -0.01√𝑷)
when population > 200000
A provision for 54,600 litres/minute may be made with an extra additional provision of 9100 to 36,400 litres/minute for a second fire
For a residential city.
Small or low building,
Larger or higher buildings,
𝑸 = 𝟓𝟔𝟔𝟑 √𝑷
3.6 Water Required to Compensate For Losses in Wastes And Thefts
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.
Allowance of 15% of total quantity of water is made to compensate for losses, theft, and wastage of water.