Environmental Engg.

Lesson-11 Quantity of water

The problem of estimating the quantity of water may be tackled by studying in detail the following two factors:

Rate of demand

The requirements of water for various uses are properly analysed and ultimately, the rate of consumption per head is worked out.

Population

The persons to be served by the scheme are calculated and estimate of future population if worked out with the help of suitable method.

Rate of demand

A small quantity of water is required by a man under normal conditions for his personal use.  But his demand of water for other purposes will naturally depend upon the standard of living and degree of culture.  In order to arrive at a reasonable value of rate of demand for any particular town, the demand of water for various purposes is divided under the following five categories:

1. Domestic purposes

2. Civic or public purposes

3. Industrial purposes

4. Business or trade purposes

5. Loss and waste

We will briefly analyse each category and will discuss how the quantity of water under each category is worked out for the purpose of estimating rate of demand of water.

Domestic purposes

The quantity of water required for domestic purposes can be sub divided as follows:

Drinking

A human body contains about 70 per cent of water.  The consumption of water by a man is required for various physiological processes such as blood formation, food assimilation, etc.  The quantity of water which a man would require for drinking depends on various factors.  But on the average and under normal conditions, it is about 2 litres per day.  This amount, as will be seen, is very small as compared to various other uses of water.  But it is most essential to supply water for drinking purposes with a high degree of purity.  If water for drinking contains undesirable elements, it may lead to epidemic.  In fact, the drinking water should be protected, potable and palatable.

Cooking

Some quantity of water will also be required for cooking.  The quantity of water required for this purpose will depend upon the stage of advancement of the family in particular and society in general.  However, for the purpose of estimation, amount of water required for cooking may be assumed as about 5 litres per head per day.

Bathing

The quantity of water required for bathing purpose will mainly depend on the habits of people and type of climate.  For an Indian bath, this quantity may be assumed as about 30 to 40 litres per head per day and for tub-bath, it may be taken as about 50 to 80 litres per head per day.

Washing hands, face etc.

The quantity of water required for this purpose will depend on the habits of people and may roughly be taken as 5 to 10 head per day

Household sanitary purposes

Under this division, the water is required for washing clothes, floors, utensils, etc. and it may be assumed to be about 50 to 60 litres per head per day

Private gardening and irrigation

In case of developed cities, there will be practically no demand of water for this purpose.  In case of undeveloped cities, private wells are generally used to provide water for private gardening and irrigation.  It is therefore not essential to include the quantity of water required for this purpose in case of public water supply project

Domestic animals and private vehicles

The amount of water required for the use of domestic animals and private vehicles is not of much concern to a water supply engineer.  With the growth and development of town, the cattle disappear and commercial stables come into existence. 

The requirement of water for domestic purposes roughly works about to 40% to 5% of the total water requirements per capita per day.

Civic or public purposes

The quantity of water required for civic or public purposes can be sub divided as follows:

Road washing

The roads with heavy amount of dust are to be sprinkled with water to avoid inconvenience to the users. On the average, the quantity of water required for this purpose may be taken as about 5 litres per head per day.

Sanitation purposes

In this division, water is required for cleaning public sanitary blocks, large markets, etc. and for carrying liquid wastes from houses.  The quantity of water required for this purpose will depend on the growth of civilization and may be assumed to be about 2 to 3 litres per head per day.

Ornamental purposes

In order to adorn the town with decorative features, fountains or lakes or ponds are sometimes provided.  These objects require huge quantity of water for their performance.  As far as Indian towns are concerned, the quantity of water required for this purpose may be treated as quite negligible since in most of the towns, the quantity of water available is not enough even with the most urgent needs of the society.

Fire demand

Usually, a fire occurs in factories and stores.  The quantity of wter required for fire fighting purposes should be easily available and always kept stored in the storage reservoir. 

In case of public water supply, fire demand is treated as a function of population and some of the empirical formulae, commonly used for calculating the fire demand are as follows

Buston’s formula

Q = quantity of water required in litres per day

P = population in thousands

This formula is used in England for moderate provision

John R. Freeman’s formula

             Q = quantity of water required in litres per minute

            P = population in thousands

Kuichling’s formula

                

            Q = quantity of water required in litres per minute

            P = population in thousands

National Board of Fire Underwriters’ formula

           Q = quantity of water required in litres per minute

            P = population in thousands

As for Indian conditions are concerned, a moderate allowance of one litre per head per day for fire demand will be sufficient.

Industrial purposes

The quantity of water required for industrial or commercial purposes can be sub divided as follows:

Factories

The quantity of water required for the processes involved in factories will naturally depend on the nature of products, size of factory, etc. and it has no relation with the density of population.  It is quite likely that the demand of water for factories may equal or even exceed the demand of water for domestic purposes.  The possibility of recycling of water in the plant will also have appreciable effect on the demand of water for a particular product

Power stations

A huge quantity of water will be required for working of power stations.  But generally, the power stations are situated away from the cities and they do not represent a serious problem to public water supply.

Railways

In most of the cases, the railways make their own arrangements regarding their water requirements and hence, the quantity of water to be consumed by railways is not ordinarily included in any public water supply system.

It is thus not possible to connect the requirement of water for industrial purposes to the population of the city.  It is therefore advisable to study each case independently in this regard and decide the quantity of water required for industrial purposes accordingly.  For a city with moderate factories, it is estimated that about 20 to 25 per cent of per capita consumption will be required for industrial purposes.

Business or trade purposes

Some trades such as dairies, hotels, laundries, motor garages, restaurants, stables, etc. require a large quantity of water.  Such trades are to be maintained in hygienic conditions and sanitation of such places should be strictly insisted.  The number of such business centres will depend upon the population and for a moderate city, an average value of about 15 to 25 litres per head per day may be taken as water requirements for this purpose.

Loss and waste

The quantity of water required under this category is sometimes termed as unaccounted requirement.  It includes careless use of water, leakage in mains, valves, other fittings, etc. unauthorized water connections and waste due to other miscellaneous reasons.  The quantity of water lost due to all these reasons is uncertain and cannot be effectively predicted. However, for the purpose of calculating the average rate of demand it may be estimated to be about 30 to 40 per cent of per capita consumption. 

Factors affecting rate of demand

There are various factors which influence the rate of demand of water.  These factors are to be analysed carefully and properly before arriving at the rate of demand for a particular locality.  Following are the factors affecting rate of demand

Climatic conditions

The requirement of water in summer is more than that in winter. So also is the case with hotter and cooler places.  In extreme cold, people may keep water taps open to avoid freezing of pipes.  This may result in increased rate of consumption. 

 Cost of water

The rate at which water is supplied to the consumers may also affect the rate of demand.  The higher the cost, the lower will be the rate of demand and vice-versa.

Distribution pressure

The consumption of water increases with the increase in the distribution pressure.  This is due to increase in loss and waste of water at high pressure.  For instance, an increase of pressure from 2 to 3 kg/cm² may lead to an increase in consumption to the extent of about 25 to 30 per cent.  The designer therefore should only provide for distribution pressure which is necessary for rendering satisfactory service.

Habits of population

For high value premises, the consumption rate of water will be more due to better standard of living of persons.  For middle-class premises, the consumption rate will be average while in case of slum areas, it will be much lower.  A single water tap may be serving several families in low value areas

Industries

The presence or absence of industries in a city may also affect its rate demand.  As there is no direct relation between the water requirement for industries and population, it is necessary to calculate carefully present and future requirements of industries.

Policy of metering

The quantity of water supplied to a building is recorded by a water meter and the consumer is then charged accordingly.  The installation of meters reduces the rate of consumption.  The engineers dealing with water supply schemes, recommend installation of water meters, when the following two conditions are existing:

1. Quantity of water that is available from the source is limited

2. Total cost of water supply scheme is an important consideration.

Quality of water

The improvement in quality of water may result in the increase of rate of consumption.   The public using the improved water will consider it safe and may make various uses of the available water.  On the other hand, if water has unpleasant taste or odour, the rate of consumption will come down.

Sewerage

The existence of sewerage system in a locality will lead to an increase in use of water for civic or public purposes.  The people will also use more quantity of water for flushing sanitary units such as urinals and water closets.

Size of city

Generally, the smaller the city, the lower is the rate of demand.  But the presence of a water-consuming industry in a small town may result in a higher rate of demand, even if the town is small. 

System of supply

The supply of water may be continuous or intermittent.  In the former case, water is supplied for 24 hours and in the latter case, it is supplied for certain duration of the day only.  It is claimed that the intermittent supply system will reduce rate of demand.  But sometimes, the results are proved to be disappointing, mainly for the following two reasons

• During non-supply period, the water taps are kept open and hence, when the supply starts, water flowing through open taps is unattended and this results in waste of water.

• There is tendency of many people to throw away water stored previously during non-supply hours and to collect fresh water.  This also results in waste of water.