CHAPTER – III QUANTITY OF WATER 3.1 Per Capita Demand of Water It is the average quantity of water required by a person in a day. The unit is lpcd (litres per capita demand of water).
The unit of total water demand is litres/day. 3.2 Design and Base Periods i.
Survey Year: It is the year in which survey is carried out.
ii.
Base Period: It is the period between survey year and base year during which the works of survey, design and construction are completed. Base Period is generally taken as 2 to 3 years.
iii.
Base Year: It is the year in which implementation is done. Base Year = Survey Year + Base Period
iv.
Design Period: Any water supply project is planned to meet the present requirements of community as well as the requirement for a reasonable future period (up to service year). This period between Base Year and Service or Design Year is taken as Design Period. It is generally 15 to 20 years. This period is taken 15 years in communities where the population growth rate is higher and 20 years in communities where population growth rate is comparatively lower.
v.
Design/Service Year: It is the year up to which water demand is to be fulfilled. Service Year = Survey Year + Base Period + Design Period = Base Year + Design Period
3.2.2 Selection Basis Design Period is selected based on the following:
Useful lives of the component considering obsolescence, wear, tear, etc.
Expandability aspect.
Anticipated rate of growth of population including industrial, commercial developments and migration-immigration.
Available resources.
Performance of the system during initial period.
Suppose, r = growth rate of population If r ≥ 2, design period is 15 years and if r < 2, design period is 20 years. 3.3 Types of Water Demand 3.3.1 Domestic Demand
Water demand required for domestic purposes. Required for both urban and rural areas. Depends upon the habit, social status, climatic conditions, living standard, etc.
S.N.
Types of Consumption
Water Demand (lpcd)
1 2 3
Private Connection and Fully Plumbed System Private Connection and Partly Plumbed System Public Stand Post
112 65 45 (can come down to 25)
3.3.2 Livestock Demand
Quantity of water required for domestic animals and livestock including birds. Generally considered in rural water supply. Livestock demand should not be greater than 20% of domestic demand.
S.N.
Types of Consumption
Water Demand (lpcd)
1 2 3
Big animals >> cow, buffalo Medium animals >> goat, dog Small animals >> birds
45 20 0.2
3.3.3 Commercial/Institutional Demand
Quantity of water required for commercial institutions like schools, colleges, hospitals, offices, etc. For commercial and institutional purpose, 45 lpcd can be taken.
Institutions a. Hospitals/Health Posts/Clinics i. With Bed ii. Without Bed b. Schools i. Boarders ii. Day Scholars c. Hotels i. With Bed ii. Without Bed d. Restaurants/Tea Stall e. Offices i. Unclassified ii. Resident iii. Non resident
Demand Urban Area
Rural Area
500 l/bed/day 2,500 l/day
325-500 l/bed/day 1600-2500 l/hospital/day
65 lpcd 10 lpcd
42-60 lpcd 6.5-10 lpcd
200 l/bed/day 500-1000 l/day 500-1000 l/day
200 l/bed/day 500-1000 l/day 200-500 l/day
500-1000 l/day 65 lpcd 10 lpcd
325-1000 l/office/day 65 lpcd 10 lpcd
3.3.4 Public/Municipal Demand
Considered only in urban areas for municipal purposes e.g. cleaning roads, for public parks. We adopt criteria by Indian Government. i. Street Washing = 1 to 1.5 l/m2 of surface area of road/day ii. Public Parks = 1.4 l/m2/day iii. Sewer Cleaning = 4.5 l/person/day 3.3.5 Industrial Demand
Normally considered in urban areas. Quantity of Water required for various industries and factories. Generally taken as 20 to 25% of total demand.
3.3.6 Fire Fighting Demand Authority 1. National Board of Fire Underwriters Formula 2. Freeman's Formula 3. Kuichling's Formula 4. Buston's Formula 5. Indian Water Supply Manual Formula
Formula (P in '000, Q in l/min) Q = 4637 √P (1 - o.01 √P) Q = 1136 (P/5 + 10) Q = 3182 √P Q = 5663 √P Q = 100 √P, Q in cubic meter/day
3.3.7 Loss and Wastage
15% of total demand is considered to be loss and wastage.
Loss or wastage of water can occur due to defective pipe joints, cracked and broken pipes, faulty valves and fittings, unauthorized connection (theft), allowance for keeping tap open, etc. Loss and wastage is about 40% in Kathmandu Valley. Considered only for urban areas.
3.3.8 Total Demand Total Demand = Domestic Demand + Livestock Demand + Commercial Demand + Municipal Demand + Industrial Demand + Fire Fighting Demand + Loss and Wastage 3.4 Variation in Demand of Water
If this average demand is supplied at all the times, it will not be sufficient to meet all the fluctuations. There are three types of variations in demand of water.
Seasonal Variation: The demand peaks during summer. Fire breaks out generally more in summer, increasing demand. So, there is seasonal variation. Maximum seasonal consumption is 140% and minimum seasonal consumption is 80% of average daily per capita demand. Daily Variation: Daily variation is due to the variation in activities. People draw out more water on holidays and festival days, thus increasing demand on these days. Daily variation may also occur due to climatic condition (rainy day or dry day) and the character of the city (industrial, commercial or residential). Maximum daily consumption is 180% of average daily per capita demand. Hourly Variation: Hourly variations 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 variation in requirement is negligible. The maximum hourly consumption is 150% of average daily per capita demand.
3.5 Peak Factor Maximum demands at all these variations are expressed in terms of percentage of average annual daily consumption (AADC) or ‘Qav’. AADC or Qav = P x q, where P is the population and q is per capita demand. Peak Demand is the maximum hourly demand on the day of maximum demand of the season of maximum demand. Peak Demand = PFH x PFD x PFS of AADC Where, PFH = Peak Factor of Hourly Variation PFD = Peak Factor of Daily Variation PFS = Peak Factor of Seasonal Variation Hence, Peak Demand = 1.5 x 1.8 x 1.4 x AADC = 3.93 x AADC
Generalizing, Peak Demand = Peak Factor x AADC Peak Factor is normally taken 3 in Nepal.
3.6 Factors affecting Demand of Water a. Size of the City: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have mostly private connection in every house with fully plumbed system. b. Presence of Industries c. Climatic Conditions: If a community is located in hot climate, water use will be increased by bathing, lawn sprinkling and use in parks and recreation fields. In extreme cold climates, water may be wasted at the faucets to prevent freezing of pipes, resulting in increased consumption. d. Standard of Living: The higher the standard of living is, the higher the demand and greater the variation in demand. e. Quality of Water: If water is aesthetically and medically safe, the consumption will increase as people will not resort to private wells, etc. f. Pressure in the Distribution System: Higher pressure results in increased use while lower pressure results in decreased use. g. Efficiency of water works administration: Leaks in water mains and services and unauthorized use of water can be kept to a minimum by surveys. h. Cost of Water i. Policy of metering and charging method: Water tax is charge in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate.
3.7 Population Forecasting – Necessity and Methods A particular method is to be adopted for a particular case or for a particular city. The selection is left to the discretion and intelligence of the designer. Sample Problem:
Year
Population
Increase in Population
% increase in Population
Incremental increase in Population
Decrease in % increase of Population
1981 1991 2001 2011
8000 12000 17000 22500 Total Average
4000 5000 5500 14500 A = 4833
50 41.67 32.35 124.02 G = 41.34
1000 500 1500 I = 750
8.33 9.32 17.65 D = 8.82
Present Population, P = 22500 A = average increase per decade = 4833 G = average % increase in population per decade = 41.34% I = average incremental increase per decade = 750 D = average decrease in % increase of population = 8.82 3.7.1 Arithmetical Increase Method
Assumption: The increase in population from decade to decade is assumed constant. This method is suitable for larger and old cities which have practically reached their maximum development (i.e. cities which have reached their saturation population).
Pn = future population at the end of ‘n’ decades n = number of decades P = present population From above example, P2021 = 22500 + 1 x 4833 = 27333 P2027 = 22500 + 1.6 x 4833 = 30233 3.7.2 Geometrical Increase Method or Uniform Percentage Growth Method
Assumption: The percentage increase in population from decade to decade is constant.
This method is suitable when the city is young and rapidly increasing. This is the most common method used in Nepal. (
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Pn = population after ‘n’ decades G = average % increase per decade
Gives high result than arithmetical increase method – so, much safer result. (
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3.7.3 Incremental Increase Method
This method combines both the above two methods – gives value between the above two methods. (
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3.7.4 Decreased Rate of Growth Method Year 2011 – 2021 2021 – 2031 2031 – 2041 2041 – 2051
% increase 32.35 – 8.82 = 23.53 23.53 – 8.82 = 14.71 14.71 – 8.82 = 5.89 -ve (so zero – constant)
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