Water Demand of a City: 5 Types | Water Quantity | Water Engineering

Following are the various types of water demand of a city or town: 1. Domestic Water Demand 2. Commercial and Industrial Demand 3. Fire-Demand 4. Demand for Public Use 5. Compensate Losses Demand.

Type # 1. Domestic Water Demand:

It includes the quantity of water required in the houses for drinking, bathing, cooking, washing etc. The quantity of water required for domestic use mainly depends on the habits, social status, climatic conditions and customs of the people. In India on an average, the domestic consumption of water under normal condition is about 135 litres/day/capita as per IS: 1172-1171.

In developed countries this figure may be as high as 350 litres/day/ capita. The increase in water consumption in developed countries is mainly due to use of air coolers, air conditioners, maintenance of lawns, automatic household appliances such as home laundries, disc washers etc.

Domestic Water Demand:

The total consumption in this demand, generally amounts to 55 to 60% of the total water consumption the breakup of 135 litres/day/person may be taken as shown in Table 5.1.

Type # 2. Commercial and Industrial Demand:

Commercial building and commercial centres include office building, warehouse, stores, hotels, shopping centres, health centres, schools, temples, cinema houses, railway and bus stations etc. The water requirements of commercial and public places may be up to 45 litres/day/capita. Table 5.1 (a) gives the water demand for buildings other than residences as per IS: 1172-1963.

Table 5.1(a)

Water supply Requirements for Public Building other than Residence (As per IS; 1172-1963 Indian Standard Code of Basic Requirements for Water Supply, Drainage and Sanitation).

The water required in the industries mainly depends on the type and size of industries which are existing in the city. The quantity of water required by industries is also expressed in terms of per capita demand. The water required by factories, paper mills, clothe mills, cotton mills, breweries, sugar refineries etc. comes under industrial use. The quantity of water demand for industrial purposes is around 20 to 25% of the total demand of the city.

Most of the big industries, universities and institutions generally have their own water supply arrangements from the private tube-wells.

Type # 3. Fire Demand:

Fires generally break in thickly populated localities and the industrial area, and cause serious damages of properties and sometimes lives of the people arc lost. Fire may take place due to faulty electric wires by short circuiting, fire catching materials, explosions, bad intention of criminal people or any other unforeseen mishappenings. If fires are not properly controlled and extinguished in minimum possible time, they lead to serious damages and may burn the cities.

All the big cities have full fire-fighting squads. As during fire-breakdowns large quantity of water is required for throwing it over the fire to extinguish it, therefore provision is made in the water works to supply sufficient quantity of water or keep as reserve in the water mains for this purpose, in the cities fire-hydrants are provided on the water mains at 100 to 150 m apart. Fire brigade men immediately connect these fire-hydrants with their engines and start throwing water at very high rate on the fire.

The minimum water pressure available at fire hydrants should be of the order of 1.0 to 1.5 kg/cm² and should be maintained even after 4 to 5 hours of constant use of fire hydrant.

Fire Demand:

Generally, in a moderate fire breakout, three jet streams are simultaneously thrown from each hydrant; one on the burning property and one each on adjacent property on either side of the burning property. The discharge of each stream should be about 1100 litres/nun. Hence in a big city having a population of say 45 lakhs, if six fires break out in a day and each fire stands for 3 hrs., the total amount of water required is given by

Q = 6 x 1100 x 3 x 60 x 3 (i.e. no of fires x discharge x time of each fire)

= 3564 x 10³ litres/day.

Then, the amount of water required per person, will be

=3564 x 10³/45 x 10⁵ < 1 lit/person

From the above example, it is clear that the total amount of water requires hardly to 1 lit/person/day for this purpose.

Quantity of Water:

The quantity of water required for fire-fighting is generally calculated by using the following empirical formulae:

(a) National Board of Fire Underwriters Formula:

Where Q = Quantity of water required in litres/min.

P = Population of the town in thousands.

Formula (a) is used when population of the town is upto 2 lakhs. When the population of the town is more than 2 lakhs, the provision of 54,600 litres/min is made with extra provision of 9,100 to 36,400 litres/min. for a second fire.

(b) Freeman formula:

Where F = Period of occurrence of fire in years

Both formulae (a) and (b) give high fire demand which is not suitable for Indian conditions. For Indian conditions Kuichling’s formula gives satisfactory results.

(c) Kuichling’s Formula:

For residential cities generally the following fire demand should be adopted as far as possible:

(i) For town having low building 2200 litres/minute

(ii) For town having higher buildings 4500 litres/minute

(iii) For costly colonies or valuable market and public places 7650 to 13,500 litres/minute.

(iv) For three-storeyed density populated colonies up to 27,000 litres/minute.

(d) Buston’s Formula:

All the above formulae do not take into account the type of locality, building, public places or Industrial area. They give same results for industrial as well as residential area. Chances of fire break up are Industrial area is more than in residential areas.

Type # 4. Demand for Public Use:

Quantity of water required for public utility purposes such as for washing and sprinkling of roads cleaning of sewers, watering of public parks, gardens, public fountains etc., comes under public demand. To meet the water demand for public use, provision of 5% of the total consumption is made while designing the water works for a city.

The requirements of water for public purposes shall be taken as given in Table 5.2.

Consumption of water for livestock shall be taken as given in Table 5.3.

Consumption of water for irrigation purposes in towns shall be taken as given in Table 5.4

Where possible unfiltered water can be directly used for the above public purposes. For supplying water for this purpose separate pipe lines are to be laid. For Fire-fighting purposes, this water (unfiltered) may also used.

Type # 5. Compensate Losses Demand:

All the water which goes in the distribution pipe does not reach the consumers. Some portion of this is wasted in the pipe lines due to defective pipe-joints, cracked and broken pipes, faulty valves and fittings. Sometimes consumers keep open their taps or public taps even when they are not using the water and allow continuous wastage of water.

In some way, some quantity of water is lost due to unauthorised and illegal connections. While estimating the total requirement of water of a town, allowance for these losses and wastages should also be done. Generally allowance of 15% of the total quantity of water is made to compensate for losses, thefts and wastage of water.

Per Capita Demand:

In community, water is used for various purposes as described above. For the purpose of estimation of total requirements of water, the demand is calculated on an average basic, which is expressed as so many litres/capita/day.

If Q is the total quantity of water required by a town per year in litres, and the population of the town is P. the per capita demand will be.

The per capita demand of the town depends on various factors and will be according to the living standard of the public and the number and type of the commercial places in the town etc.

For an average Indian town, the requirement of water in various uses is as under:

Total quantity of water required by the town per day shall be 270 multiplied with the total population of litres/day.