Sunday, May 12, 2019

Water demand

The quantity of water required by a community, society or industry is called water demand. It is normally expressed as litres per person per day or LPCD. While designing the water supply scheme for a region, it is necessary to determine the total quantity of water required for different purposes. The various types of water demands are:
  • Domestic water demand
  • Industrial or commercial water demand
  • Fire demand
  • Demand for public uses
  • Compensate losses demand
Domestic water demand refers to the water required by houses for drinking, cooking, washing, bathing etc. It depends upon habits, social status, climatic conditions and customs of people. In India, on an average, the domestic consumption of water under normal circumstances is 135 litres/day/capita according to IS1172:1171. This figure may be as high as 350 litres/day/capita in developed countries.

Industrial  or commercial water demand refers to requirements of water to commercial buildings or centres like office buildings, warehouses, stores, hotels shopping centres, health centres, schools, temples, cinema houses, railway and bus stations. The water demand in such cases  is 45 litres/day/capita. Universities, commercial buildings and centers including office buildings, warehouses, stores, hotels, shopping centers, health centers, places of worship, cinema houses, railways and bus stations fall under this category. Indian standard; IS1172:1963 sets water supply requirements for public buildings other than residences varying from 45 to 450 LPCD.

Quantity of water required for public utility purposes like washing and sprinkling on roads, watering of public parks, gardens, etc comes under public demand and 5% of the total consumption is set aside while designing water works for a city.

The quantity of water to be set aside for fire fighting is calculated by different formulae as listed below:
KUICHLING's FORMULA (This formula is best suited for Indian conditions)


BUSTON's FORMULA



FREEMAN's FORMULA


Where P is population in thousands

The following are the reasons for losses and wastage:
  • Defective pipe joints, cracked and broken pipes, faulty vaulves and fittings
  • Wastage by consumers
  • Unauthorised and illegal connections
While estimating total quantity of water required, an allowance of 15% of the total quantity of water is added to compensate for losses, thefts and wastage

Per capita demand = Total yearly requirement of city in litres / (365 * Design population)

Average per capita demand = 335 litres / hour / day
Domestic demand       = 200
Industrial demand       = 50
Commercial demand  = 20
Public use                   = 10
Losses and theft         = 55

TOTAL                       = 335

Factors affecting per capita demand

  • Size of a city
  • Climatic conditions
  • Demography and habits of people
  • Industrial and commercial activities
  • Quality of water supply
  • Pressure in distribution system
  • Sewerage facilities
  • System of supply
  • Cost of water
  • Policy of metering and method of charging
Factors affecting losses and wastes
  • Water tight joints
  • Pressure in distribution system
  • System of supply
  • Metering
  • Unauthorised connections
Variations in demand
  • Daily consumption
  • Seasonal variation
Maximum daily consumption = 1.8 * Average daily demand

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