What is lpcd in water supply?
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.
Water demands can be classified into:
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)
The per capita consumption of industries is generally taken as 50 lpcd.
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.
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.
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.
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 is given by the relation:
Per capita demand is dependent on the following factors:
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.
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.
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
Each additional use has health and other benefits, but with decreasing urgency (see Figure 1). This is often measured in litres per person (capita) per day (Lpcd).
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,
Water required by both these groups for different purposes is shown below.
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.
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)
Q = 1136 * [(P/10)+10],
where,
Q - water required in litres/minute
P - population in 1000s
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)
Q = 5663 * √P,
where,
Q - water required in litres/minute
P - population in 1000s
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,
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.
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.
What topics do you want us to cover? Do fill out the form below and let us know!
There are many variable factors affecting water consumption. So it is tough to assess the public water demand precisely. To determine the quantity of water certain thumb rules and empirical formulas are available. Each formula is available for a particular use. Various types of demand are
Domestic Water demand includes the water required for drinking, cooking, bathing, lawn sprinkling, gardening, sanitation purpose, etc. It depends upon habits, social status, and climatic conditions of people. As per IS 1172-1993, the domestic consumption varies from 135 lpcd- 200 lpcd depending upon the economic situation and full flushing system. Minimum Domestic water consumption for Indian towns and cities with full flushing system and the Weaker section as per IS 1172-1993
It represents the need of industries, either existing or likely to start in the future. This quantity will thus vary with the number and types of industries present in the city. For small-scale industries, in general, it is 50 lpcd. In industrial cities, the per-capita water requirement is 450 lpcd.
It includes the use of institutions, such as hospitals, hotels, restaurants, schools and colleges, railway stations, offices, etc. This quantity will certainly vary with the nature of the city and with the number and types of commercial establishments and institutions present in it. It varies from 20 lpcd to 50 lpcd.
This includes Watering in a public park, gardening, washing, sprinkling on roads, use in a public fountain, etc. In India, generally, public demand is zero. It is less than $5%$ of total consumption. Demand for public use is 10 lpcd
It is the amount of water required for firefighting purpose in case of a fire break out in an area. This water is required to be available at a pressure of about 100 to 150 $text{kN/m}^2$or 10 to 15m head of water. If the population is less than 50000 fire demand is not calculated. For larger city fire demand was calculated. there are various formulas available to calculate the fire demand
This includes water loss in leakage due to bad plumbing or damaged meters, stolen water, and other losses and wastes. It is taken as 15% of the total consumption.
Water demand is represented as per capita. per capita water demand= (Total yearly requirement of the city in liters/ (365 times Design population)). The per capita water demand is 335 lit/ hour/ day.
