It is the annual average amount of daily water required by one person and it includes the domestic use, industrial and commercial use, public use, wastes, thefts etc.
Mathematically;
Table 1 Rate of Demand for Various Uses
|
Sl.
No. |
Type
of Use |
Rate
of Demand in lpcd (Litre per Capita per Day) |
|
1 |
Domestic Use |
200 |
|
2 |
Industrial Use |
50 |
|
3 |
Commercial Use |
20 |
|
4 |
Civil or Public Use |
10 |
|
5 |
Waste and Theft, etc. |
55 |
|
|
Total |
335 |
(As per IS 1172-1983, the domestic consumption in India accounts for 135 lpcd (liters/capita/day) without full flushing system. The value is 200 lpcd with full flushing system as indicated in the Table 1.)
Multiplying this value of 335 liters/person/day by the projected population at the end of the planning period gives the city’s annual average daily water demand. Multiply this by 365 to get your annual water requirement in liters.
Factors Affecting Water Demand or Percapita Demand
The average annual water demand (per capita demand) varies greatly from city to city. It is generally 100-360 liters/person/day by Indian standards. These variations in total water use in different cities and communities depend on a variety of factors that need to be thoroughly investigated and analyzed before determining the per capita requirements for planning and design purposes. Total water demand is affected by following factors.
1) Size of the City
Demand increases with size of city. Larger cities generally have a higher per capita demand than smaller cities. Large cities require large amounts of water to maintain a clean and healthy environment. Similarly, large cities generally require more commercial and industrial activity and require more water. The wealthy living in air-cooled homes can also increase city water use.
The population indirectly affects the size of a city. Because even small cities can have high water consumption if they are fully industrialized or have industries that require huge amounts of water or are inhabited by wealthy people. On average, the per capita demand in Indian cities varies by population, as shown in the table below.
Table 2 Variation in Per Capita Demand (q) with population in India
|
Sl.
No. |
Population |
Per Capita Demand in lpcd (Litre per Capita per Day) |
|
1 |
Less than 20,000 |
110 |
|
2 |
20,000 - 50,000 |
110 - 150 |
|
3 |
50,000 - 200,000 |
150 - 240 |
|
4 |
200,000 - 500,000 |
240 - 275 |
|
5 |
500,000 - 1,000,000 |
275 - 335 |
|
6 |
Over 1,000,000 |
335 - 350 |
2) Climatic Conditions
At hotter and dry places, the consumption of water is generally more, because more of bathing, clearing, air-coolers, air-conditioning, 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 of water, while poor slum-dwellers consume very little. Thus, water consumption is directly dependent on the consumer’s economic status.
4) Industrial and Commercial Activities
The pressure of industrial and commercial activities at a particular place increase the water consumption by large amount. Many industries require very large amounts of water (much more than households need), which greatly increases the demand for water. As mentioned earlier, the demand for industrial water is not directly related to population or city size, but generally, there is more industries in big cities, increasing the per capita demand in big cities. However, for well-planned and zoned cities, estimating industrial and commercial needs separately can help to predict water needs more accurately.
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 Systems
If the pressure in the distribution pipes is high and sufficient to make the water reach at 3rd or even 4th storage, water consumption shall be definitely more. This water consumption increases because of two reasons.
- People living in upper storage will use water freely as compared to the case when water is available scarcely to them.
- The losses and waste due to leakage are considerably increased if their pressure is high. For example, if the pressure increase 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 percent.
7) Development of Sewerage Facilities
The water consumption will be more, if the city is provided with ‘flush system’ and shall be less if the old ‘conservation system’ of latrines is adopted.
8) System of Supply
Water may be supplied either continuously for all 24 hours of the day or may be supplied only for peak period during morning and evening. The second system, i.e. 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.
Water may be supplied continuously for 24 hours a day or only during morning and evening peak hours. Supplying the water only during the peak hour (morning and evening) can lead to saving in water consumption due to losses occurring in a shorter time and consumers paying more attention to their water consumption. However, in many locations, intermittent delivery fails to provide greater savings than continuous delivery for the following reasons.
- In intermittent supply systems, water is generally stored by the consumer in tanks, barrels, utensils, etc. for the time it is not being supplied. This water, even if not used, is discarded when fresh supplies are restored. This greatly increases rejections and losses.
- People usually tend to leave the faucet open during off-hours so that they know when the supply is restored which leads to waste.
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
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 group, leading to unhygienic conditions. 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 a certain fixed monthly flat rate.
In the second case, i.e. when the delivery is not counted and the fee is fixed, people think that they only need to pay a fixed amount regardless of how much water they use, so generally doesn’t save water. Therefore, they generally consume water and on multiple occasions, their taps are left unclosed. All this leads to a lot of waste and a lot of water consumption. Moreover, meters put unnecessary hindrances 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.
Factors Affecting Losses and Waste
The various factors on which losses depend and the measure to control them are given below.
- Water tight joints
- System of supply
- Unauthorized connections
Fluctuations in Rate of Demand
Average Daily Per Capita Demand (q)
If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations. The variations in water demand is listed below.
1) Seasonal Variation
The demand peaks during summer. Fire breakouts are generally more in summer, increasing demand. So, there is seasonal variation.
2) Daily Variation
It depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.
3) Hourly variations
These are very important as they have a wide range. During active household working hours i.e. from six to ten in the morning and four to eight in the evening, the bulk of the daily requirement is taken. During other hours the requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply.
Variation in Demand
Smaller towns have more variation in the demand. The shorter the period of draft, the greater is the departure from the mean.
(A) Maximum Daily Consumption
Maximum daily consumption = 1.8 x Avg. daily consumption
=1.8 q
(B) Maximum hourly Consumption
This is taken as 150% of its average.
Maximum hourly consumption of maximum daily Peak Demand
= 1.5 x Maximum daily consumption
Coincident Draft
For general community purposes, the total draft is not taken as the sum of maximum hourly demand and fire-demand, but is taken as sum of maximum daily demand and fire demand or the maximum hourly demand, whichever is more. The maximum daily demand when added to the fire demand is known as the ‘Coincident Draft’.
Example Question
A water supply scheme has to be designed for a city having a population of 1,00,000. Estimate the important kinds of drafts which may be required to be recorded for an average water consumption of 250 lpcd. Also record the required capacities of the major components of the proposed water works system for the city using a river as the source of supply. Assume suitable data.
Solution
(i) Average daily draft
(per capita average consumption in litre/person/day) x population
Average daily draft = 250 x 1,00,000 litres/day
= 250 x 105 litres/day
= 25 MLD
(ii) Maximum daily draft
It maybe assumed as 180% of annual average daily draft
= 45 MLD
(iii) Maximum hourly draft of the maximum day
It may be assumed as 270 percent of annual average
(iv) Fire flow may be worked out from
= 41733 litre/min
where P = population in thousands
= 61 MLD
Coincident draft = maximum daily draft +fire draft
= 45 +61
=106 MLD
which is greater than the maximum hourly draft of 67.5 MLD
Hence ok.
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