In this article we will discuss about:- 1. Estimating the Water Demand for a Town or City 2. Variations in the Rate of Demand of Water 3. Effect of Variations in Demand of Water.
Estimating the Water Demand for a Town or City:
For estimating the water demand or the quantity of water required for a town or city the following three factors are required to be known:
It represents the average consumption or demand of water for various purposes per person (or per head, or per capita) per day. It is usually expressed as the consumption or demand of water for various purposes in litres per person (or per head) per day (lphd), or litres per capita per day (lpcd).
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If ∀ is the total quantity of water in litres required per year for a town or city having a population P, then the average consumption or demand of water per capita per day over a period of a year, or the annual average rate of demand of water q in litres per capita per day (lpcd) is given by the following expression
A water supply project should be so planned that it has a sufficient capacity to meet the demand not only for the present times but also for a reasonable future period or number of years. The future period or the number of years for which a provision is made while planning and designing a water supply project is known as design period.
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The design period should neither be too long nor too short. A too long design period will result in a heavy financial burden on the present generation while a too short design period may render the project to be uneconomical.
The consideration of a design period is generally affected by the following factors:
(a) Useful Life of the Component Structures:
The design period should not exceed the useful life of the component structures.
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(b) Ease and Difficulty in Future Expansion:
If future expansion involves lot of difficulties then a longer design period should be chosen. On the other hand if future expansion of a water supply project can be undertaken with ease then a relatively shorter design period may be considered.
(c) Availability of Funds:
If only limited funds are available then a shorter design period will have to be considered and vice-versa.
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(d) Rate of Interest on the Borrowings:
If the money to be borrowed for a water supply project is available at a lower rate of interest, then a longer design period may be economically justified and the same may be adopted.
(e) Anticipated Rate of Population Growth, Including Possible Shifts in Communities, Industrial and Commercial Establishments:
If the rate of increase of population is less, a longer design period may be considered and vice versa. Similarly if there is a possibility of shifting of communities, industries and commercial establishments to a particular town or city then initially the water supply project for the town or city may be designed for a shorter design period with a provision of further expansion as and when needed.
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In practice a design period of 20 to 30 years is generally considered sufficient for the design of a water supply project.
It represents the total number of persons residing in a town or city at any particular time who are to be served by the water supply project. A water supply project is planned to meet not only the present requirements but also the future requirements. As such it is essential to know the present population of the town or city and also to estimate the future population. There are several methods for estimating the future population.
Variations in the Rate of Demand of Water:
The average consumption or demand of water per head per day. The rate of demand of water, however, does not remain constant but it varies with the seasons or month of the year, with the days of the week, and with the hours of the day.
These variations in the rate of demand of water are termed as:
(i) Seasonal or Monthly Variations:
The rate of demand of water varies considerably from season to season (or month to month). In summer season the average rate of demand of water is usually 30 to 40 per cent above the annual average rate of demand of water, because more water is required for drinking, bathing, washing of clothes, air coolers, etc.
On the other hand in winter season the average rate of demand is about 20 per cent lower than the annual average rate of demand of water because of less requirement of water for domestic uses. Similarly during rainy season the rate of demand of water will be much less.
The rate of demand of water also varies from day to day. This is due to change in the day to day climatic conditions, or due to the day being a holiday or some festival day. Thus on a rainy day the requirement of water will be much less that on the other day which may be dry and hot.
Similarly on a dust storm day more water will be required for washing and cleaning of entire house, bathing, washing of clothes, etc. Further the rate of demand of water is usually more on Sundays than on the other days of the week. Similarly on some festival day the rate of demand of water will be more than on a normal day.
The maximum daily demand of water per head or the maximum rate of demand of water on the day of maximum use of water (or the maximum day for the year) is generally taken as 180 per cent of the annual average daily demand of water per head or the annual average rate of demand of water.
The demand of water also varies from hour to hour of the day. A typical graph showing the hourly variation in the rate of demand of water (expressed in million litres per head per hour) is shown in Fig. 2.1. It may be seen from the graph that the peak or maximum demand of water usually occurs in the morning from about 7 a.m. to 9 a.m. and in the evening from about 7 p.m. to 9 p.m.
Further in the early morning hours the demand of water is at its minimum and also during noon from about 11 a.m. to 1 p.m. the demand of water is less. The maximum hourly demand of water per head is generally taken as 150 per cent of the average hourly demand of water per head on the day of maximum use of water (or the maximum day for the year).
Assessment of Maximum Demands of Water:
For designing the various components of a water supply scheme it is necessary to assess the maximum monthly, daily and hourly demands of water. The maximum monthly, daily and hourly demands of water are usually expressed as multiples of the corresponding average demands of water.
Each of these maximum demands of water may be assessed as indicated below:
(i) Maximum Daily Demand of Water:
The maximum daily demand of water per head or the maximum rate of demand of water on the day of maximum use of water (or the maximum day for the year) is generally taken as 180 per cent of the annual average daily demand of water per head or the annual average rate of demand of water.
Thus if q is the annual average daily demand of water per head or the annual average rate of demand of water, then the maximum daily demand of water per head or the maximum rate of demand of water on the day of maximum use of water (or the maximum day for the year)
(ii) Maximum Hourly Demand of Water:
The maximum hourly demand of water per head is generally taken as 150 per cent of the average hourly demand of water per head on the day of maximum use of water (or the maximum day for the year).
The average hourly demand of water per head on the day of maximum use of water (or the maximum day for the year),
The maximum monthly, weekly, daily and hourly demands of water may also be calculated by using the formula given by R.O. Goodrich which is as noted below:
Coincident Draft or Coincident Demand of Water during Fire Fighting:
During a fire there will be sudden large draft or demand of water for fire fighting in addition to the usual demand of water for the consumers. Therefore in the design of the distribution system of a water supply scheme a provision should be made to meet the fire demand in addition to the maximum demand of water for the consumers. It is, however, highly improbable that the demand of water for fire fighting will occur at the same time when there is maximum hourly draft or demand of water for the consumers.
As such the maximum requirement of water during a fire is determined by adding the fire demand to the maximum daily demand of water (i.e., 1.8 times the annual average daily demand of water) and not to the maximum hourly demand of water. The maximum daily demand of water is thus known as coincident draft or coincident demand of water during fire fighting.
Generally the sum of the fire demand and the maximum daily demand of water is more than the maximum hourly demand of water. However, if the sum of the fire demand and the maximum daily demand of water is less than the maximum hourly demand of water, then the distribution system of the water supply scheme will be designed for the maximum hourly demand of water.
Effect of Variations in Demand of Water:
Figure 2.2 shows four typical layouts of water supply schemes. In each of the layouts A and B an impounding reservoir created on the upstream of a dam is used as a source of water supply. Since the impounding reservoirs are usually at higher elevations, the water may flow under gravity and therefore pumps are not needed. In the layout A it is assumed that no treatment of water is required and hence from the impounding reservoir water is directly supplied to the service reservoir from which water is supplied to the distribution system.
In the layout B water from the impounding reservoir is first supplied to the filters and then to the service reservoir which supplies water to the distribution system. In the layout C water is obtained from a number of wells from which it is pumped to the treatment plant.
The treated water is then either directly supplied to the distribution system or it is first pumped to a service reservoir from which it is supplied to the distribution system. In the layout D water is obtained from a river from which it is pumped to the filters. The water is then pumped to a service reservoir from which it is supplied to the distribution system.
The various components of a water supply scheme should be designed not only to fulfil the average daily demand of water but to fulfil the maximum demand of water as and when it arises and also the variations in the demand of water. The following recommendations may generally be adopted for deciding the design capacities of the various components of a water supply scheme.
The source of water supply should have sufficient capacity to meet the maximum daily demand of water. Generally the impounding reservoirs have very large capacity, but in the case of rivers and wells used for water supply it should be ensured that these have sufficient capacity to meet the maximum daily demand of water.
Conduits or Pipe Mains Carrying Water from the Source to the Service Reservoir:
The conduits or pipe mains carrying water from the source to the service reservoir (conduits I and II in Fig. 2.2) should be designed for the maximum daily demand of water.
Conduits or Pipe Mains Carrying Water from the Service Reservoir to the Distribution System:
The conduits or pipe mains carrying water from the service reservoir to the distribution system (conduit III in Fig. 2.2) should be designed for the sum of the fire demand and the maximum daily demand of water (i.e., for the fire demand plus the coincident draft or coincident demand of water) or the maximum hourly demand of water, whichever is more.
Both the low lift and high lift pumps should be designed for the maximum daily demand of water plus some reserve for break-downs and repairs. Thus the pumps may be designed for 2 to 3 times the annual average daily demand of water instead of 1.8 times the annual average daily demand of water.
Further if the pumps are not working for all the 24 hours, then the above indicated design rates for the pumps should be multiplied by the ratio of 24 hours to the number of hours for which the pumps are working, to arrive at the actual design rates for the pumps.
Filters and other Units at Treatment Plant:
The filters and other units at treatment plant should be designed for the maximum daily demand of water plus some reserve for break-downs and repairs. Thus these may be designed for 2 times the annual average daily demand of water instead of 1.8 times the annual average daily demand of water.
The distribution system should be designed for the sum of the fire demand and the maximum daily demand of water (i.e., for the fire demand plus the coincident draft or coincident demand of water) or the maximum hourly, demand of water whichever is more.
The service reservoir is designed to take care of the hourly fluctuations in water consumption, fire demand, emergency reserve and the provision required when pumps have to pump the entire day’s requirement of water in limited hours of the day. Ordinarily the service reservoir is designed to hold or store the quantity of water which would be enough for a day’s consumption of the locality served by it.