The various types of water demands for a town or city may be divided into the following five categories: 1. Domestic Water Demand 2. Commercial and Industrial Water Demand 3. Demand for Civic or Public Use 4. Fire Demand 5. Loss and Waste of Water.
In order to estimate the total water demand of a town or city it is necessary to consider the quantity of water required to fulfill each of these demands. As such it is also indicated how the quantity of water required for each of these categories of water demand may be determined.
Type # 1. Domestic Water Demand:
This includes the water which is required for use in private residences, apartment houses, etc., for drinking, cooking, bathing, washing of clothes, washing of utensils, washing and cleaning of houses and residences, lawn watering, gardening and sanitary purposes (such as flushing of water closets, etc.). The amount of domestic water demand depends on the living conditions of the consumers.
As per the Indian Standard, IS: 172-1993 in the design of a water supply scheme for a town or city with full flushing system minimum water supply of 200 litres per head per day should be provided for domestic water demand. Out of 200 litres per head per day of water, 45 litres per head per day of water may be taken for flushing requirements and the remaining quantity of water is for other domestic purposes. The breakup of 200 litres per head per day may be approximately taken as shown in Table 2.2.
It is also mentioned in the Indian Standard, IS; 1172-1993 that the minimum value of water supply given as 200 litres per head per day may be reduced to 135 litres per head per day for houses for Low Income Groups (LIG) and Economically Weaker Section (EWS) of society, depending upon prevailing conditions. As such for small towns or cities minimum water supply of 135 litres per head per day may be provided for domestic water demand. The breakup of 135 litres per head per day may be approximately taken as shown in Table 2.3.
The total quantity of water required to meet the daily domestic water demand for the town or city may be obtained by multiplying the total domestic water demand in litres per head per day by the total design population. The total domestic water demand generally amounts to about 40 to 60% of the total water demand.
In a developed country like USA the domestic water consumption is as high as about 340 litres per head per day as against a low figure of 200 litres per head per day for our country. This is so because due to high standard of living in the developed countries more water is consumed in air cooling, air conditioning, automatic household appliances (such as dish washing machines, home laundries, garbage grinders, etc.), flushing system, car washing, etc.
Type # 2. Commercial and Industrial Water Demand:
ADVERTISEMENTS:
This includes the water demand of commercial and other establishments such as offices, hospitals, hotels, restaurants, cinemas, schools, etc., and factories or industries. On an average for a town or city with moderate factories or industries a provision of about 20 to 25% of the total water consumption is generally made in the design of water supply project for these uses.
However, the quantity of water required for this purpose will vary considerably with the nature of the town or city and with the number and types of commercial and other establishments and factories or industries. In small residential towns or cities the commercial and industrial use may be as low as 40 litres per head per day but in industrial towns or cities it may be as high as 400 litres per head per day. As per the Indian Standard, IS: 1172-1993 the minimum water requirements for buildings other than residences are as shown in Table 2.4.
Further as per the Indian Standard, IS: 1172-1993 the water requirements for railway and bus stations and terminals and airports are as shown in Table 2.5.
Notes:
1. The number of persons shall be determined by the average number of passengers handled by the station daily; due consideration may be given to the staff and vendors likely to use facilities.
2. Consideration should be given for seasonal average peak requirements. The requirements of water indicated in Table 2.4 for factories or industries are meant only for the general use of their employees and for drainage and sanitation purposes in the factory or industry. However, in addition to the quantity of water indicated in Table 2.4, the factories or industries would require a considerable quantity of water for the production processes involved in the factories or industries for their products.
The quantity of water required for the production processes involved will depend on the nature of products, size of factory or industry, etc., and it has no relation with the density of population. Table 2.6 gives the typical water requirements for some of the factories or industries.
The demand of water for the production process in a factory or industry will also be affected by the possibility of recycling of water in the plant. However, it is quite likely that even with recycling of water the demand of water for the production process in the factories or industries may be much higher. As such for some of the factories or industries they develop their own water supply system, in which case they have little or no demand on the public water supply system.
With the increase in the number of factories or industries in a town or city, the industrial demand of water would increase. It is therefore necessary to know the present requirement of water for the industrial use and also to estimate the future industrial demands. In a well-planned town or city the factories or industries are usually located in a separate zone. Such zoning of a town or city may considerably help in estimating future industrial demands.
Type # 3. Demand for Civic or Public Use:
This includes the quantity of water required for civic or public utility purposes such as watering of public parks or gardens, road washing, sprinkling of water on dusty roads, cleaning public sanitary blocks, large markets, etc., use in decorative features such as public fountains, etc. For most of the water supply projects in India, depending upon the availability of water, a provision of usually 5 to 10% of the total consumption of water is made to meet these demands.
Type # 4. Fire Demand:
It is the quantity of water required for fire-fighting purposes. In thickly populated and industrial areas, fires may break out and may result in severe damages if not controlled effectively. As such for almost all the big and medium size towns or cities provision should be made in the water supply schemes for meeting the demand of water for fire-fighting purposes.
ADVERTISEMENTS:
The necessary quantity of water required for fire-fighting purposes should be easily available and always kept stored in the storage reservoir. The fire hydrants are usually fitted in the water mains at a distance of not more than about 150 metres. When a fire breaks out, pumps installed on fire-fighting trucks are rushed to the site of fire occurrence and these pumps, when connected to fire hydrants, are capable of throwing water with high pressure. The fire is thus brought under control.
Generally for a moderate fire break out three jet streams are simultaneously thrown from each hydrant—one on the burning property and one each on the adjacent property on either side of the burning property. The discharge of each stream should be about 1100 litres per minute. Thus in a town or city having a population of say 30 lakhs, if it is assumed that four fires may break out in a day and each fire may last for say 3 hours, then the total quantity of water required to meet the fire demand
The above example indicates that to meet the fire demand the requirement of water per head per day is usually quite low as compared to the requirement of water for other purposes. However, during a fire large quantity of water would be required to be supplied in a short period which would result in a high rate, of water consumption. The high rate of water consumption during a fire would therefore be the deciding factor in fixing the capacities of the pumps, reservoirs and service pipes of the distribution system in a water supply project.
In the design of a water supply project the quantity of water required to meet the fire demand is usually determined by using various empirical formulae. In these empirical formulae the quantity of water required to meet the fire demand has been expressed as a function of population. Some of these empirical formulae which are commonly used for determining the quantity of water required to meet the fire demand are given below,
(a) Buston’s Formula:
This formula is used in U.K. for moderate provisions.
(b) Kuichling’s Formula:
(c) John R. Freeman’s Formula:
in which Q is quantity of water required to meet the fire demand in U.S. gallons per minute; and P is population in thousands.
Since 1 U.S. gallon = 3.785 litres, the above noted converted formula is obtained].
(d) National Board of Fire Underwriters’ Formula and other Recommendations:
The formula and other recommendations of the National Board of Fire Underwriters (now known as American Insurance Association) for determining the quantity of water required to meet the fire demand are as given below:
(A) For Business Towns or Cities:
(i) When population is less than or equal to 2 lakhs
in which Q is quantity of water required to meet the fire demand in U.S. gallons per minute; and P is population in thousands.
Since 1 U.S. gallon = 3.785 litres, the above noted converted formula is obtained.]
On the basis of the above formula the fire-flow capacities required for the towns or cities with different populations are as given in Table 2.7.
(ii) When population is more than 2 lakhs, then in addition to a provision of 45420 litres per minute as required for a population of 2 lakhs, a provision of 7570 to 30280 litres per minute may be made for a second fire.
(B) For Residential Towns or Cities:
For residential towns or cities the required fire-flow capacities depend upon the character and congestion of the buildings as indicated below:
(i) Sections where buildings are small and of low height
Not less than 1893 litres per minute (or 500 U.S. gallons per minute).
(ii) Sections with larger or higher buildings upto 3785 litres per minute (or 1000 U.S gallons per minute).
(iii) Sections with closely built buildings, or buildings approaching the dimensions of hotels, or high-value residences 5678 to 11355 litres per minute (or 1500 to 3000 U.S. gallons per minute).
(iv) Densely built sections of three-storey buildings.
Upto 22710 litres per minute (or 6000 U.S. gallons per minute).
For the sake of comparison the fire demand of water for a city having a population of one lakh is computed by using each of the four empirical formulae as indicated below:
It may be observed that the formulae at (b), (c) and (d) give higher rates of fire-flow and therefore a storage for only 2 hours duration with these rates may be considered as the sufficient allowance for the total stand-by storage capacity or the volume of water required to meet the fire demand. However, as per the recommendations of the National Board of Fire Underwriters, in U.S.A a storage for 5 to 10 hours duration at the rates indicated in Table 2.7 is considered as the minimum requirement of water to meet the fire demand.
All the above noted empirical formulae, however, suffer from a drawback that these are not related to the type of the area served. Thus, although the probability of occurrence of fire with a given duration may be greater for an industrial area than for a residential area, these formulae will give same values of the fire demand for both industrial as well as residential areas each having the same population.
In the water supply schemes of our country for making provision of water to meet the fire demand different recommendations have been made in- (i) the Manual on Water Supply and Treatment brought out by the Government of India, and (ii) the Indian Standard, IS: 9668-1990, each of which is as indicated below.
(i) The Manual on Water Supply and Treatment recommends that for towns/cities with population more than 50 000 to meet the fire demand provision may be made based on the following formula-
(ii) The Indian Standard, IS: 9668-1990 recommends that for towns/cities the water for fire fighting shall be provided at the scale of 1800 litres per minute for every 50 000 population or part thereof for towns up to 3 lakhs population and an additional 1800 litres per minute for every 1 lakh population of more than 3 lakhs. A storage for 4 to 24 hours duration at this rate may be provided to meet the fire demand.
Type # 5. Loss and Waste of Water:
The water in this category is sometimes termed as unaccounted-for water. This includes the water lost due to leakage in mains, valves and other fittings, worn or damaged meters, meter slippage, theft of water through unauthorised water connections, and loss and waste of water due to other miscellaneous reasons. The loss and waste of water due to all these reasons should be taken into account while estimating the total requirements of water.
However, the quantity of water lost and wasted due to all these reasons being uncertain it cannot be precisely predicted. As such in the design of a water supply project in order to account for the loss and waste of water a provision of about 30 to 40% of the total water consumption is usually made. However, if the distribution of water is entirely through meters and if the distribution system is well maintained, it is possible to bring down the loss and waste of water to about 10 to 15% of the total water consumption.
Factors affecting loss and waste of water and measures to reduce the same.
In a distribution system the losses and wastage of water should be reduced to a minimum possible value, for which it is necessary to study the factors affecting the losses and wastage of water.
The various factors which affect the losses and wastage of water and the measures to reduce them are as indicated below:
(i) Water Tightness of Joints:
If due to bad plumbing the joints in the supply mains are not watertight, a lot of water may be lost and wasted through leakage from such joints. The leakage of water can be reduced by careful and better plumbing and regular maintenance, thereby ensuring that the joints are watertight.
(ii) Pressure in the Distribution System:
High pressure in distribution pipes may lead to higher leakage losses of water, and also wastage of large amount of water through open taps. As such water should be supplied at the lowest possible pressure that will provide satisfactory service with minimum loss and wastage of water.
(iii) System of Supply of Water:
The water may be supplied intermittently or continuously for 24 hours. When water is supplied intermittently the leakage losses and consequent wastage of water would be less than those when water is supplied continuously for 24 hours. However, when water is supplied intermittently there may be more wastage of water because the consumers may throw away the earlier stored water to waste in order to fill again the fresh water when water supply is resumed.
Moreover, where an intermittent supply is not metered, the consumers may leave water taps open during periods of non-supply, which may remain open and unattended when the supply is turned on. This would result in a lot of wastage of water.
(iv) Metered or Unmetered Water Supply:
When the water supplies are metered the wastage of water is considerably reduced, because the consumers use water more carefully as they have to pay for the quantity of water actually consumed by them. On the other hand in the case of unmetered water supply the consumers are charged at a fixed monthly flat rate irrespective of the quantity of water consumed by them. Thus with unmetered water supply the consumers use water carelessly resulting in more wastage of water.
(v) Unauthorised Water Connections:
Water may be stolen through unauthorised connections, resulting in a considerable loss of water. Such unauthorised water connections should be detected and checked by severely punishing the defaulters, so as to reduce such losses of water.