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Water demand is the amount of water required to satisfy the needs of a population present or to be present (forecasted) in an area. Before planning any water supply projects for an area it becomes absolutely necessary to calculate the total demand for water in that area so as to design the water supply systems with such capacity. Unit for water demand measurement and types of demand are discussed further.
lpcd stands for litre/per capita/per day, which is the amount of water required in litres per person per day.
According to IS1172: 1993, the water demand for Indian towns/cities shall be based on two categories, namely,
Full flushing system / High-income group
Low-income group
Water required by both these groups for different purposes is shown below.
Water Demand for High-Income group and Low-Income group
Totalling the above values, we get water demand for each group as,
High Income group = 200 lpcd
Low Income group = 135 lpcd
It depends on the type of industries present in an area/town and usually varies from 50 to 450 lpcd.
It is the water required for institutional and commercial establishments like schools, colleges, malls, etc. and it usually varies from 10 to 20 lpcd.
It is the water required for public usage such as gardening, fountains, etc. and it completely depends on the area where the water supply system is being developed.
It is the amount of water required for fire fighting purposes if in case a fire breaks out in an area. This water is required to be available at a pressure of about 100 to 150 kN/m^2 or 10 to 15m head of water. Fire demand is not calculated for smaller towns where the population is less than 50,000. For larger cities, fire demand is calculated using various formulas which are discussed further.
Fire Demand Formulas
1. Kuichling's Formula
Q = 3182 * √P,
where,
Q - water required in litres/minute
P - population in 1000s (i.e., if population is 1,00,000 then P = 100)
2. Freeman's Formula
Q = 1136 * [(P/10)+10],
where,
Q - water required in litres/minute
P - population in 1000s
3. National Board of Fire / Underwriter's Formula / National Fire Insurance of America
Q = 4637 * √P * [1 - (0.01*√P)],
where,
Q - water required in litres/minute
P - population in 1000s (valid for population of less than 2,00,000)
4. Buston's Formula
Q = 5663 * √P,
where,
Q - water required in litres/minute
P - population in 1000s
5. According to Indian Standard IS 9668: 1990
Q = 1800 litres/minute,
For every 50,000 population up to 3,00,000 population,
Above 3,00,000 population, extra water shall be 1,800 lit/min for every 1,00,000 population.
It is the water which could be lost during transport and is generally taken as 15% of water demand calculated.
Summing it all, per capita demand for an Indian city is about 335 lpcd. Further, according to IS 1172: 1993, the total quantity of water supplied shall be,
For High-Income group - 200 (domestic) + 135 = 335 lpcd
For Low-Income group - 135 (domestic) + 135 = 270 lpcd
Per capita demand or annual average daily demand is calculated as (Total yearly demand in litres/(365 * Design Population)). It is measured in lpcd.
Annual average demand or per capita demand represents only the average amount of water required. But the water demand varies (as shown below) and it becomes important to consider these variations in designing the system.
Seasonal variation - high demand in summer, less demand in winter
Daily variation
Hourly variation - high demand in morning and evening hours
Goodrich formula is used to calculate the per cent of the annual draft that is required as a peak factor in meeting the variations in water demand. It's given by,
p = 180 * (t)^-0.1,
where,
p - per cent in the annual draft
t - time in days
Daily peak factor, p ( according to Goodrich) = 180 * (1)^-0.1 = 180%
Maximum daily demand = Daily peak factor * Average daily demand
Maximum daily demand = 1.8 * Average daily demand
Maximum hourly demand = 1.5 * Average hourly demand on the max day
Maximum hourly demand = 1.5 * (Maximum daily demand day/24)
Maximum hourly demand = 1.5 * 1.8 * (Average Daily Demand/24)
Maximum hourly demand = 2.7 * Average hourly demand (based on annual basis)
or in simple terms,
Maximum hourly demand = 1.5 * Maximum daily demand
Weekly peak factor, p ( according to Goodrich) = 180 * (7)^-0.1 = 148%
Maximum weekly demand = Weekly peak factor * Average weekly demand
Maximum weekly demand = 1.48 * Average weekly demand
Monthly peak factor, p ( according to Goodrich) = 180 * (30)^-0.1 = 128%
Maximum monthly demand = Monthly peak factor * Average monthly demand
Maximum monthly demand = 1.28 * Average monthly demand
The design capacity of various water supply systems according to IS 1172: 1993 is shown below,
For more insights please do refer to the attached video lecture (below) on fire demand and coincidental draft. Also, check out the full environmental engineering course at APSEd.
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