Determination of water demand is indispensable when it comes to the design of a proper water work project. An accurate estimation of water demand helps to determine the quantities of water and moments when the water will be used therefore generating various demand patterns. The demand arises mainly for residential, institutional, industrial and public uses.
The different types of water demands and their variations are briefly described in this article.
Types of Water Demands
Water demands can be classified into:
- Domestic Water Demand
- Industrial Water Demand
- Institutional and Commercial Water Demand
- Demand for Public
- Fire Demand
- Waste and Theft
1. Domestic Water Demand
Domestic water demand accounts for 55 to 60% of the total water consumption. As per IS 1172-1983, the domestic consumption in India accounts for 135 lpcd (liters/capita/day)
2. Industrial Water Demand
The per capita consumption of industries is generally taken as 50 lpcd.
3. Institutional and Commercial Water Demand
On an average, per capita demand of 20 lpcd is required to meet institutional and commercial water demand. For highly commercialized cities, this value can be 50 lpcd.
4. Public and Civil Use
The per capita consumption for public and civic use can be taken as 10 lpcd. This water is used for road washing, public parks, sanitation etc.
5. Fire Demand
Per capita fire demand is ignored while calculating the total per capita water requirement of a particular city because most areas have fire hydrants placed in the water main at 100 to 150 meters apart. The fire demand is generally taken as 1 lpcd.
6. Waste and Thefts
This consumption accounts for 55 lpcd. Even if the waterworks are managed with high proficiency, a loss of 15% of total water consumption is expected.
Per Capita Demand (q)
Per capita demand is given by the relation:
Per capita demand is dependent on the following factors:
- Water supply system
- Cost of water
- Climatic Conditions
- Size of the city
- Quality of water supply
- Pressure in the water distribution system
- Metering policy and charging method
- Industrial and commercial activities
- Development of sewerage facilities
Variations in Water Demand
There are different variations in water demands which are calculated for the specific design of pipe mains, service reservoirs, source of supply, distribution system and pumps.
1. Maximum Daily Consumption
Maximum Daily Consumption = 180% of Average Daily Demand = 1.8q
Maximum daily consumption is the design water consumption for source of supply and pipe mains.
2. Maximum Hourly Consumption
Maximum hourly consumption = 150% of avg. hourly demand of max.day
= 1.5 x (Maximum daily demand/24)
= 1.5 x (1.8q/24) = 2.7 x (q/24)
Maximum hourly consumption = 2.7 x Annual Average hourly demand
3. Maximum Hourly Demand of maximum day
Maximum Hourly Demand of maximum day = 2.7q
4. Coincident demand or Coincident draft
Maximum daily demand plus fire demand gives the coincident draft. This design water consumption is used for distribution system.