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Water Demand and its Types | Fire Demand Formulas |Design Capacities of Water Supply Systems

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.


What is lpcd


lpcd stands for litre/per capita/per day, which is the amount of water required in litres per person per day.


Types of Water Demand


Types of water demand
Types of water demand

1. Domestic water Demand


According to IS1172: 1993, the water demand for Indian towns/cities shall be based on two categories, namely,

  1. Full flushing system / High-income group

  2. Low-income group

Water required by both these groups for different purposes is shown below.

Purpose

High-Income Group (lpcd)

Low-Income Group (lpcd)

Drinking

5

5

Cooking

5

5

Bathing

75

55

Washing cloth

25

20

Washing utensils

15

10

Washing house

15

10

Garden

15

-

Flush

45

30

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


2. Industrial Demand


It depends on the type of industries present in an area/town and usually varies from 50 to 450 lpcd.


3. Institutional and Commercial Water Demand


It is the water required for institutional and commercial establishments like schools, colleges, malls, etc. and it usually varies from 10 to 20 lpcd.


4. Water for Public Uses


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.


5. Fire Demand


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.


6. Water Theft / Lost Adjustment


It is the water which could be lost during transport and is generally taken as 15% of water demand calculated.


Water Demand in India


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,

  1. For High-Income group - 200 (domestic) + 135 = 335 lpcd

  2. For Low-Income group - 135 (domestic) + 135 = 270 lpcd


Per Capita Demand or Average Daily Demand


Per capita demand or annual average daily demand is calculated as (Total yearly demand in litres/(365 * Design Population)). It is measured in lpcd.


Peak Demand of Water


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.

  1. Seasonal variation - high demand in summer, less demand in winter

  2. Daily variation

  3. Hourly variation - high demand in morning and evening hours


Goodrich Formula


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


1. Maximum Daily Demand or Daily Peak


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


2. Maximum Hourly Demand or Hourly Peak

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


3. Maximum Weekly Demand or Weekly Peak

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


4. Maximum Monthly Demand or Monthly Peak


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


Design Capacity of Water Supply Systems


The design capacity of various water supply systems according to IS 1172: 1993 is shown below,

System

Design capacity

Pipeline carrying water from reservoirs to the water treatment units

Maximum daily demand i.e., 1.8 * Average daily demand

Filter units

Daily peak but it is taken as 2 * Average daily demand

Pumping units

Daily peak but it is taken as 2 * Average daily demand

Water distribution network

Based on two criteria,

a) Maximum hourly demand

b) Coincidental demand (Maximum daily demand + Fire demand)

Greater of (a) and (b) is adopted

Practice Problem




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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