Predicting Demand for Water

PREDICTING DEMAND FOR WATER

The various types of water demand which a city may have is classified as:

1. Domestic water demand (Residential)

2. Industrial water demand

3. Institutional and Commercial water demand

4. Demand for public uses

5. Fire demand

6. Water required to compensate losses in wastes and thefts

 

1. Domestic water demand

• The water required in residential buildings for drinking, cooking, bathing, gardening, flushing etc.

• The domestic water demand is 50 to 60% of the total water consumption.

• The demand may vary according to the living conditions of consumers (LIG/ MIG/HIG).

• LIG-Low income group

Table 1.1: Minimum domestic water consumption for Indian cities

 

2. Industrial water demand

• Represents the water demand of industries (existing and anticipated in future)

• The demand varies according to the number and type of industries in the city.

• The average per capita consumption for industrial needs is 50 lpcd.

• In Industrial cities, the per capita water requirement is 450 lpcd.

• Water demand of certain industries is given below:

a. Automobile - 40 kilo litre per vehicle produced

b. Leather - 40 kilo litre per tonne produced

c. Textile - 80 to 140 litre per tonne produced

 

3. Institutional and Commercial water demand

Water requirement of Institutions such as Hotels, Hospitals, Schools, Colleges, Offices, Railway Stations, Factories etc.

The water demand depends on the nature of city and number of commercial establishments.

On an average, the per capita demand is 20 lpcd.

 

4. Demand for public or civic use

The quantity of water required for public utility purposes such as watering of public parks, gardening, washing and sprinkling on roads, public fountains etc.

It accounts to 5% of the total water consumption

On an average, it is 10 lpcd

 

5. Fire demand

In thickly populated and industrial areas, fire outbreaks may cause serious damages. The quantity of water required for fire fighting is called fire demand and it is stored in Storage reservoirs. The minimum water pressure available in fire hydrants should be 100 to 150 kN/m2 (10 to 15 m of water head).

For cities having population > 50,000, water required for fire fighting in kilo litre.

= 100 √P, where P = population in thousands.

Rate of Fire Demand

Empirical formulae are used to calculate fire demand.

(i) Kuichling's Formula

Q=3182 √P mano no

Q = Water required in litre/minute

P = Population in thousands

(ii) Freeman formula

Q-1136[P/10+10]

(iii) National Board of Fire Under Writers formula

(American Insurance Association)

(a) For a central congested high valued city

(i) Population ≤ 2,00,000

(ii) Population > 2,00,000

(b) For a residential city, the water required for fire fighting is as follows:

Small buildings - 2200 litre/minute

Larger buildings - 4500 litre/minute

High rise Apartments - 7650 to 13,500 litre/minute

(iv) Buston's formula

This formula takes into account probability of occurrence of fire.

t - duration of fire (minimum 30 min)

R- period of occurrence of fire (minimum 1 year)

 

6. Water required to compensate losses (thefts/wastes)

(i) Leakage/overflow from service reservoirs

(ii) Leakage from main/ service pipe connections

(iii) Leakage/losses on consumer's premises (unmetered)

(iv) Leakage from public taps

(v) Defective pipe joints

(vi) Cracked pipes

(vii) Loose valves/ fittings

(viii) Unauthorised water connections

(ix) Damaged meter

 

PER CAPITA DEMAND

The per capita demand (q) in litres per day per head (annual average daily consumption per person)

= Total yearly water requirement of city in litres / 365×Design population

 

Factors affecting per capita demand

1. Size of City / Type of Community - The fluctuations in demand depends upon th size of city.

•  Large city-fluctuations are less and demand is more

• Small city - demand is less

• Residential Community - more fluctuations in demand

•  Industrial Community - Fluctuation is less

2. Standard of living/habits of people

•  Higher the standard of living - demand for water is more

3. Climatic conditions

• Hot climate - Usage of water will increase (bathing, lawn sprinkling etc.)

• Cold climate - Water is wasted to prevent freezing of pipes

4. Quality of water

• Good quality water - usage

• Poor quality water - usage is less.

5. Pressure in the supply

• High pressure - increased usage.

• Low pressure - decreased usage.

6. System of supply - water supply may be continuous (24 hrs) or intermitter Intermittent supply reduces the demand.

7. Sewerage - Flushing system increases water demand.

8. Policy of metering - Use of water decreases when the supplies are metered.

9. Water rates - Increase in water rates reduces the consumption

10. Age of community - Older communities use less water. Developing new communiti require large quantity of water for construction works.

11. Lawn sprinkling - Enforcement of lawn sprinkling regulations can reduce pe demands.

 

VARIATIONS IN DEMAND

The per capita consumption of water varies seasonally, monthly, daily and hourly.

 

(i) Seasonal variations

Summer season - large use of water.

Winter season - lesser use of water.

Rainy season - much lesser use of water.

(ii) Daily variations

Sundays / Holidays - more usage of water.

Weekdays - less usage of water.

(iii) Hourly variations

Early morning (0 to 6 hours) - less consumption of water.

Between 8 to 11 am - Peak consumption of water.

Between 1 to 4 pm - Water consumption is less.

Between 7 to 9 pm - high or peak consumption of water

Late night hours - Minimum consumption of water

The hourly variations in water demand is shown in figure 1.8

 

Assessment of Variations in Demand

(i) Maximum daily consumption 180% of average daily demand 100 ylish mumix=1.8

Where q = average daily demand

(ii) Maximum hourly consumption (peak demand) = 150% of its average hourly cosumption

= 1.5 x Average hourly consumption of maximum day

(iii) Maximum hourly consumption of maximum day

1.5 x [Max. daily demand /24]

Effect of variations in demand on the design capacities of different components of a water supply scheme.

1. Source of supply - Designed for maximum daily consumption

2. Pipe Mains (Type I & II) - Designed for maximum daily consumption

3. Filter and other units of water treatment plant - Designed for maximum daily consumption or twice the average daily consumption (including reserve for repairs)

4. Pumps - Same as filter and WTP units

5. Distribution system (Type III) - Designed for maximum hourly demand of maximum day plus reserve for fire

6. Service reservoir -  Designed for hourly fluctuations, fire demand, emergency reserve etc.