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Chapter: Environmental Engineering - Demand For Water

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Water Demand: Per Capita Demand (Rate Of Demand) (Q)

FACTORS AFFECTING PER CAPITAL DEMAND: 1. Size and type of city 2. Climatic conditions 3. Class of consumers 4. Quality of water 5. Pressure in the distribution system 6. Sewerage Facilities 7. System of supply 8. Policy of metering system 9. Cost of water

WATER REQUIRED TO COMPENSATE IN THEFTS, WASTES, etc..

 

It 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, etc…. These losses should be taken into account while estimating the total requirement. These losses can be reduced by careful maintenance and universal metering. Even in the best managed water works this amount is usually taken as 15% of the total consumption.

 

 

DESIGN PERIOD

 

A Water supply scheme includes huge and costly structures like dams, reservoirs, treatment works, penstocks etc…., which cannot be replaced or increased in their capacities, easily and conveniently for example. The water mains including distribution pipes are laid underground and cannot be replaces or added easily without digging the road or disrupting the traffic. In order to avoid these future complications of expansions, various components of w.s.s are purposely made larger, so as to satisfy the community meets for a reasonable years to come. The future period or the number of years for which a provision is made in designing the capacities of the various component of the w.s.s. is known as DESIGN PERIOD. It should be neither too long nor should it be too short. Normally 20-30 years is considered for distribution system.

 

PER CAPITA DEMAND (RATE OF DEMAND) (Q)

 

It is the annual average amount of daily water required by one person and includes the domestic use, industrial and commercial use, public use, wastes, thefts, etc…

 

= Per capita Demand in litres/day/head =

Total yearly water requirement of the citty in Litres / 365xDesign population

q=V/365P

For an Average.Indian town. As per I.S recommendations the per capita demand may be taken as given in table below.

 

USE  CONSUMPTION (LPCD)

         

Domestic use        135

         

Industrial use       50

         

Commercial use    20

         

Civic or public use         10

         

Waste, theft. Etc…        55

         

Total 270 lpcd

 

The above figure or 270 lpcd when multiplied by the population at the end of the design period shall give the total annual average water requirement of the city/day. When multiplied by 365 will give the volume of the yearly water requirement in litres.

 

Generally the per capita demand valuesranges between 10-300 lpcd. These variations in total water consumption of different cities or towns depend upon various factors.

 

FACTORS AFFECTING PER CAPITAL DEMAND

 

1.     Size and type of city

2.     Climatic conditions

3.     Class of consumers

4.     Quality of water

5.     Pressure in the distribution system

6.     Sewerage Facilities

7.     System of supply

8.     Policy of metering system

9.     Cost of water

 

PROBLEM ON RATE OF DEMAND

 

Work out the rate of demand of water for an average Indian city. Make your own assumptions wherever necessary.

 

Soln: The total requirement of water for various purposes is worked out separately as under.

PURPOSE  CONSUMPTION (lpcd)

 

1.DOMESTIC      5

Drinking     5

Cooking      55

Bathing       30

Washing Clothes and utensils 10

Washing and Cleaning of houses Flushing of latrines    30

Total : 135

 

2. CIVIl or PUBLIC      

a. Road washing   5

b. Sanitation purpose    3

c. ornamental purpose   1

d. fire demand      1

Total:          10

 

3. Industrial purposes    50

 

4. commercial purposes 20

 

5. waste, theft, etc…      55

 

Total 270  Lpcd

 

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