What is the Per Capita Demand
The Per Capita Demand is defined as 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. It may, therefore, be expressed as
Total yearly water requirement of the city can, therefore, be worked out by using the above equation, provided the per capita demand is known or assumed. The per capita demand depends upon various factors (discussed below) and will vary according to the living conditions of the consumers and also with the extent and type of industries developed or likely to be developed in that region. For an average Indian city, as per recommendations of I.S. code, the per capita demand (q) may be taken as given in table below-
|Sl No||Use||Demand in l/h/d|
|4||Civik or Public Use||10|
|5||Wastes & Thefts, etc||55|
|Total =||335 |
= Per Capita Demand(q)
This figure of 335 litres/head/day when multiplied by the prospective population at the end of the design period, shall give the annual average water requirement of the city per day. When multiplied by 365, it will give the volume of the yearly water requirement in litres.
Factors Affecting Per Capita Demand
The annual average demand for water (i.e. per capita demand) considerably varies for different towns or cities. This figure generally ranges between 100 to 360 litre/capita/day for Indian conditions. These variations in total water consumption of different cities or towns depend upon various factors, which must be thoroughly studied and analysed before fixing the per capita demand for design purposes, these factors are discussed below:
- Size of the City
- Climatic Conditions
- Types of Gentry and Habits of People
- Industrial and Commercial Activities
- Quality of Water Supplies
- Pressure in the Distribution System
- Development of Sewerage Facilities
- System of Supply
- Cost of Water
- Policy of Metering and Method of Charging
Let’s discuss them one by one.
(1) Size of the City
The per capita demand for big cities is generally large as compared to that for smaller towns. This is because of the fact that in big cities, huge quantities of water are required for maintaining clean and healthy environments. For example, big cities are generally sewered, and as such require large quantities of water (a sewered house requires four to five times the water required by an unsewered home). Similarly, in a big city, commercial and industrial activities are generally more, thus requiring more water. Affluent rich living in air-cooled homes may also increase the water consumption in cities.
In fact, the effect of population on the size of the city is an indirect one, because even a smaller town may have high water consumption, if it is fully industrialised or is having some industry requiring tremendous quantities of water, or if rich affluent people are living in it. On average, the per capita demand for Indian towns may vary with the population, as shown in the following table.
|Sl No||Population||Per Capita Demand|
|1||Less than 20,000||110|
|2||20,000 – 50,000||110 – 150|
|3||50,000 – 2,00,000||150 – 240|
|4||2,00,000 – 5,00,000||240 – 275|
|5||5,00,000 – 10,00,000||275 – 335|
|6||Over 10,00,000||335 – 350|
*The figures shown here are liable to variation up to about 25%.
(2) Climatic Conditions
At hotter and dry places, the consumption of water is generally more, because more of bathing, cleaning, air cooling, sprinkling in lawns, gardens, roofs, etc. are involved. Similarly, in extremely cold countries, more water may be consumed, because the people may keep their taps open to avoid freezing of pipes, and there may be more leakage from pipe joints since metals contract with cold.
(3) Types of Gentry and Habits of People
Rich and upper-class communities generally consume more water due to their affluent living standards. Middle-class communities consume average amounts, while the poor slum dwellers consume very low amounts. The amount of water consumption is thus directly dependent upon the economic status of the consumers.
(4) Industrial and Commercial Activities
The pressure of industrial and commercial activities at a particular place increases the water consumption by large amounts. Many industries require really huge amounts of water (much more than the domestic demand), and as such, increase the water demand considerably. As pointed out earlier, the industrial water demand is having no direct connection with the population or the size of the city, but more industries are generally situated in big cities, thereby increasing the per capita demand for big cities. However, for a properly planned zoned city, the water requirement can be more accurately predicted by estimating the industrial and commercial demands separately.
(5) Quality of Water Supplies
If the quality and taste of the supplied water is good, it will be consumed more, because in that case, people will not use other sources such as private wells, hand pumps, etc. Similarly, certain industries such as boiler feeds, etc., which require standard quality waters will not develop their own supplies and will use public supplies, provided the supplied water is up to their required standards.
(6) Pressure in the Distribution System
If the pressure in the distribution pipes is high and sufficient to make the water reach at 3rd or even 4th storey, water consumption shall definitely be more. This water consumption increases because of two reasons:
- People living in upper storeys will use water freely as compared to the case when water is available scarcely to them.
- The losses and wastes due to leakage are considerably increased if this pressure is high. For example, if the pressure increases from 20 m head of water, (i.e. 200 kN/m2) to 30 m head of water (i.e. 300 kN/m2), the losses may go up by 20% to 30%
(7) Development of Sewerage Facilities
As pointed out earlier, the water consumption will be more, if the city is provided with a ‘flush system’ and shall be less if the old ‘conservation system’ of latrines is adopted.
(8) System of Supply
The water may be supplied either continuously for all the 24 hours of the day or may be supplied only for peak periods during the morning and evening. The second system, i.e. the intermittent supplies, may lead to some saving in water consumption due to losses occurring for lesser time and a more vigilant use of water by the consumers. But at many places, the intermittent supplies may not give much saving over the continuous supplies, because of the following reasons:
- In intermittent supply system, water is generally stored by consumers in tanks, drums, utensils, etc., for non-supply periods. This water is thrown away by them even if unutilised as soon as the fresh supply is restored. This increases the wastage and losses considerably.
- People have a general tendency to keep the taps open during non-supply hours, so that they may come to know of it as soon as the supply is restored. Many a times, water goes on flowing unattended even after the supply is restored, thus resulting in wastage of water.
(9) Cost of Water
If the water rates are high, lesser quantity may be consumed by the people. This may not lead to large savings as the affluent and rich people are little affected by such policies.
(10) Policy of Metering and Method of Charging
Water tax is generally charged in two different ways-
- On the basis of meter reading (meters fitted at the head of the individual house connections and recording the volume of water consumed).
- On the basis of certain fixed monthly flat rate.
In the 2nd case, i.e. when the supplies are unmetered and the charges are fixed, people generally do not practice economy in the use of water, because they think that they have to pay only a fixed amount irrespective of the quantity of water used by them. They are, therefore, generally liberal in consuming water, and many a times, leave their taps flowing unused. All this leads to high wastage and high consumption of water.
However, when the supplies are metered, people use only that much of water as much is required by them. Although metered supplies are preferred because of lesser wastage. They generally lead to lesser water consumption by poor and low income groups, leading to unhygienic conditions. Moreover, meters put unnecessary hinderance to the flow, resulting in loss of pressure and increased cost of pumping. Meters are also liable to be stolen and the cost of installing, repairing and reading the meters is generally high. In a good water works management, it is, therefore, generally desired to work out the economics of metering and to balance the cost, of meterage against the value of water conserved by reduction of wastages. At certain places, the saving in water due to meterage has really been large enough, as to permit postponing the otherwise needed extensions of supplies.
I hope this article will help you to the factors affecting per capita demand in water supply system. You may also want to see my other post from my Blog. If I have missed anything here, please let me know about that in the comment below this post.
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