Chapter Four: Water Demand Assessment
Contents
1 Chapter Four: Water Demand Assessment
1.1 1.1 WATER DEMAND ASSESSMENT
Water demand is the quantity of water that a source must produce to meet all the water requirements of a project. These include water delivered to the system to meet the needs of consumers, water supply for firefighting, system flushing, water required for the operation of treatment facilities and amount of water lost due to leakages in the infrastructure.
In planning and designing any water supply project, water demand assessments for current and future needs are of prime importance. Engineering decisions are required to determine the area and the population, industries, institutions and other existing and emerging consumers to be served, design period, the per capita consumption of various categories of consumers’ pressure zones, amount of water likely not to be charged (NRW) and other needs of water in the area.
In addition, demand assessment may assist in determining the nature and location of various facilities to be provided such as the source of water and capacity of water storage facilities (FAO AQUASTAT, 2011 and MoW Design Manual, 2009). For effective determination of water demand, designers need to critically assess the components of water demand for the planned water supply system. The following are the main components that should be considered when conducting water project demand assessment:
- Water demand for domestic use
- Institutional Water Demands
- Industrial Water Demands
- Water Requirements for Energy Cooling Systems
- Commercial Water Demand
- Livestock Water Demand
- Water for Fire Fighting
- Operational Demands
- System water losses
- Non-Revenue Water
- Net Water Demand (revenue water)
1.2 1.2 GENERAL FACTORS AFFECTING WATER DEMAND ASSESSMENT
Demand assessment is the most critical element in project planning for short, medium or long-term water projects (Water Mission Technical Handbook of 2019 and MoW Design Manual of 2009). The complexity of water demand assessment to meet various socio-economic needs in a community may be brought about by many factors influencing individual water consumption patterns which include:
- Religion,
- Social economic status -cooking and health practices,
- Climatic conditions,
- Cultural, habits of the community,
- Age and education,
- Availability of alternative water sources,
- Level of service, technological process,
The combination of these factors may contribute to over or under estimation of the water demand in the given project area. Over-estimation of demand may justify a project that should not have been built. This then leads to unnecessary costs, over-estimation of intended revenue and premature implementation of a project. It is recommended that a thorough community survey be conducted for the project area to determine the magnitude of factors influencing water consumption pattern as described above.
The fundamental components of calculation of water demand are by determining the total population and the average daily consumption of individuals. Apart from individual based water consumption pattern, institutions surrounding the community to be served have greater influence on the determination of the overall water demand in the project area. In view of this, designers of the water supply project must take into consideration the different water consumption patterns of institutions when calculating the total water demand in the project area. For each type of institution, specific requirements should be obtained from the institutions concerned.
Water requirements for each institution must be computed separately and aggregated into the overall water demand assessment. In the rural areas, allowance should also be given for connection to schools, dispensaries, health centres, offices etc. Losses should always be determined from the gross total water requirements and not from the net. For the best conceptualization of water demand, water requirement of each sector or sub-sector must first be computed separately and later aggregated with other computed segmental demands to obtain the overall project water demand. The formula below provides general consumers whose water demand may be required:
It is worth noting that, the above formula is just a guide and should be adjusted according to the area in question. Additionally, in any assessments, demand assessment should aim at obtaining the design and operational information to optimise the net benefits to the community.
1.3 DETERMINATION OF WATER DEMAND FOR DIFFERENT USES
The foregoing section presented the components of water demand in a project. However, before establishing the project water demand, there is need to establish the water consumption of each consumer either individually or as an institution. Water consumption is the quantity of water that is directly utilized by the consumer. Water consumption is initially split into domestic and nondomestic components, non-domestic use includes Commercial Use, Institutional Use and Industrial Use, Fire Fighting and System Water Consumption.
To establish a project water demand, it is important to determine the prevailing
water project consumption categories and their water requirements. The
following steps detail a procedure to be followed when determining the project
water demand:
Step1: Establish Domestic water consumption
Step 2: Establish Institutional Water Consumption
Step 3: Establish Industrial Water consumption
Step 4: Establish Water Requirements for Energy Cooling Systems (if they exist)
Step5: Establish Commercial Water consumption
Step 6: Establish Livestock Water consumption
Step 7: Establish Net Water Demand
Step 8: Establish Fire Fighting Water consumption
Step 9: Establish Operational water consumption
Step 10: Establish System water losses
Step 11: Establish Non-Revenue Water
Step 12: Establish Overall Water Demand
Step 1: Establish Domestic water consumption
Domestic water consumption is water utilised for household chores, such as
bathing, cooking, washing, drinking, laundry, dishwashing, gardening, car washing
and other less water intensive or less frequent purposes. Individual water use
in rural, peri-urban or urban setting tends to differ considerably depending on
levels of service.
Levels of Service for domestic consumers:
The amount of water consumed depends on the level of service provided, the
following are five categories of level of service in domestic water use categories:
Category one : Low income using public taps or kiosks,
Category two : Low income multiple houses with yard tap, multiple
houses served from a single yard tap,
Category three : Low income single household with yard tap,
Category four : Medium Income Household, Medium income group
housing, with sewer or septic tank.
High Income
Household: High income group housing, with sewer or septic tank
Water use is at its lowest when water is distributed through water points (category
one level of service-public taps or kiosks) with some walking distance from the
home. When water is brought into the house by piping (regardless of whether
the location is rural or urban), the consumption increases considerably. It is
necessary to also note that there is no incentive for the consumer to utilize less
water when water is supplied at a flat monthly rate. With waterborne sanitation
and high standard of in-house installations (category five-bath, washing machine
etc) the per capita consumption may be ten times more than from a public tap.
Considerations when estimating domestic water consumption:
The proposed figures for water consumption rates as given in Table 4.1 are a guide
for designers. However to make an accurate estimate of the water requirements
it is necessary to consider the following:
- Decide upon the use of different per capita consumption rate if water requirement survey data from the project area justifies,
- Divide the population to be served into consumption groups as per levels of service obtained through social-economic survey data of the project area(combination of percentage of the population to be served from each service category),
- Use population density of residential plots if the data is available,
- Adjust the community water requirement if survey of alternative sources of water are available.
Establish the domestic water requirement by summation of individual domestic consumer category population multiplied by respective per capita consumption.
Step 2: Establish Institutional Water Consumption
When designing and planning for a water project, water requirements for
present and anticipated institutions in the project area have to be computed
using institutional water requirement data obtained in community survey. Public
and private institutions include: Schools, Hospitals, Administration Offices,
Army, Police, Missions, Churches and Mosques, Prisons, etc. In Table 4.2, some
guiding figures for institutional water unit consumption are given. For water
requirements for staff working in institutions see section 4.3.1. If large demand
units are included in the scheme, such as Universities, major hospitals, boarding
schools etc., a special study of their water requirements is recommended instead
of using the average figures given in Table 4.2:
Additionally, water supply requirements for railway stations, bus stations, bus terminals, sea ports, airports including provisions for waiting rooms and waiting halls have to be considered separately in the planning and design of a water supply scheme. Table 4.3 provides general guidance for water requirements considerations in stations and ports. The number of persons should be determined by average number of passengers handled by the station daily and seasonal average peak requirements should be taken into consideration during design.
Establish the institutional water requirement by summation of individual institutional consumer category population multiplied by respective per capita consumption.
1.4 1.4 Variations in Water Consumption
1.4.1 1.4.1 Definitions
Though water demand is normally calculated according to the average requirements, actual consumption varies from hour to hour and from day to day. Due to this non-uniformity of water demand, provision needs to be made in different units of the water supply system to cater for these variations. In order to evaluate the importance of variation in water consumptions, the following definitions are relevant: average daily consumption of the individual.
- Average Annual Demand (Qaa) - The total volume of water delivered to the system in a full year expressed in cubic meters. When demand fluctuates up and down over several years, an average is used.
- Average Daily Demand (Qda) - The total volume of water delivered to the system over a year divided by 365 days. The average use in a single day expressed in cubic meter per day.
- Maximum Month Demand (Qmmax) - The cubic meter per day average during the month with the highest water demand. The highest monthly usage typically occurs during a summer month.
- Peak Weekly Demand (Qwmax) - The greatest 7-day average demand that occurs in a year expressed in cubic meter per day.
- Maximum Day Demand (Qdmax) - The largest volume of water delivered to the system in a single day expressed in cubic meter per day. The water supply, treatment plant and transmission lines should be designed to handle the maximum day demand.
- Peak Hourly Demand (Qhmax) - The maximum volume of water delivered to the system in a single hour expressed in cubic meter per day. Distribution systems should be designed to adequately handle the peak hourly demand or maximum day demand plus fire flows, whichever is greater. During peak hourly flows, storage reservoirs supply the demand in excess of the maximum day demand.
Summary of relationships between patterns
Kd = Qdmax / Qda = peak day factor (4.8) Kh = Qhmax / Qdmax = peak hour factor (4.9)
For design purposes, the peak factor shall be selected by giving consideration to the size and kind of the scheme and services required. Generally, an intake and the supply main from the intake to the treatment works is dimensioned to meet the peak day ultimate demand. For gravity schemes this means a main designed for a flow during 24 hours, while for pumping main the design flow is according to the pumping hours decided upon. This gives the minimum size of pipe required and where the water is treated, the required ultimate dimensions of the treatment units.
1.4.2 1.4.2 Variation in the Rate of Consumption
The average-rate of supply per capita is in fact the mathematical average taken over an average year. Thus if Q is the total Quantity of water supplied to a population 'P' for 365 days, then the average rate of daily consumption 'q' is given by the following equation:
q = Q/(P x 365) litres/capita/day (4.10)
The types and nature of variations of demand of which 'q' is an average are given below.
1.4.2.1 1.4.2.1 Diurnal variation in demand
The consumption of water is not uniform throughout the day. Generally two peak periods of demand are observed, one in the morning and one in the evening. The maximum intensity of demand which occurs in the morning varies between about 1.5 and 2.4 times the average demand for the day depending upon the size of the population served.
Large impounding reservoirs are designed to cater for the variations in demand and to ensure a steady delivery of water for a period from one drought year up to three consecutive dry years. For the supply channels and mains for the conveyance of this water to the treatment works, a figure of between 1.35 and 1.50 times the average demand rate ‘q’ is used. The pumping stations and treatment units are usually designed by taking 1.35 to 1.5 times the average demand rate since they are to meet the peak seasonal requirements. If the pumping is for less than 24 hours, then the above rate must be multiplied by the ratio of 24 hours to the hours of pumping. Transmission mains are designed to meet peak daily demand although distribution mains in the supply area are required to meet peak period demand and in urban areas are designed for up to 2.5 times the average rate of demand.
1.4.2.2 1.4.2.2 Peak Factors
Though peak factors are necessary for calculating the actual peak period demand, application in individual cases is difficult. The population data such as the number of people using each water point, number of hours per day, people’s habits in collecting water is difficult to obtain and again may vary by season or by school calendar. Hence use of peak factors to calculate an individual’s demand is impracticable and the figures given in Tables 4.9 and 4.10 are simply a rough guide. Peak factors however, must be adopted for the dimensioning of the various components in a water distribution network. For every scheme, a study is required to establish the appropriate water demand and for large schemes it must be done thoroughly.
For small rural water supply schemes, a simpler method may be adopted to derive peak flows beyond the last storage tank with the peak flow for human consumption being given by the expression:
(Average daily demand × 4) / 24 (4.11)
That assumes the peak demand is four times the average hourly demand or in other words the daily demand is drawn over 6 hours during the two peak periods of the day.
For livestock, it is usual to work on the basis of a ten-hour period such that the peak flow may be taken as:
Daily livestock demand / 10 (4.12)
Thus in small rural water supply schemes providing water both for human consumption and livestock, the peak flow used for dimensionless of the distribution mains is the sum of the peak flow for human consumption and the peak flow for livestock consumption.
In many urban areas and particularly in those that have experienced past restrictions in water supply, a number of consumers and especially those in high cost areas and institutions etc., will have constructed their own ground level or below ground storage, often well in excess of their daily requirements. In such instances, the actual peak hour flow can be noticeably less than 2.5 indicated in Table 4.9. In the absence of information to the contrary, peak factors that may be used for different consumer categories are as given in Table 4.9:
Table 4.9: Consumers’ Peak Factors
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(Source: MoW 3rd Design Manual 2009)
When it is impossible to separate consumers into categories as given in the previous table, but the total population of the area is known Table 4.10,can be used as a guideline. Table 4.10: Peak factors for known population in the area
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1.4.3 1.4.3 Predicting Demand
It is advisable to project demand for short, medium and long term periods and to update this at least every five years. The Water Mission Handbook, 2019 provides guidance on means of computing the water demand in various situations. It categorizes demand predictions into three levels namely:
i. Maximum Demand After Commissioning This is calculated by assuming that 100% of the members of the population will each use full individual consumption amount.
Therefore MDAC = Total population Served x Maximum individual consumption
ii. Anticipated Demand After Commissioning (ADAC) This is computed based on the assumption that not all 100% of population in the community will use the maximum amount of water. So predictions of the percentage of the population that will use the water need to be estimated. Additionally, it may be decided that individual consumption may be less than full amount determined in the time period just after commissioning of the system.
ADAC=Total pop.served XAnticipated % of pop. /100 X Anticipated Individual Consumption
iii. Maximum Future Demand This type of water demand is computed to determine the system sustainability and capacity to meet the future demand growth. It is worth to note that the future demand can be hard to predict. However, inputs like population growth, migration, trade, urbanization and any significant changes in the national policies can be used to facilitate determination of projected future demand in the project area using data from the National Bureau of Statistics (NBS).
The population growth rate (r) is among the factors, which are predominantly used to facilitate the computation of the future demand. The future population in a project area may be computed using the following formula.
Pn = PO (1 + r / l00)n (4.14)
Where, Pn = population after n years, PO = present population, and r = annual growth rate (%)
Once the predicted future population is determined, it can be multiplied by the anticipated population to be served and individual consumption to determine the maximum future demand (MFD).
MFD =Future Pop. X Anticipated % Pop./100 X Anticipated individual consumption
1.4.4 1.4.4 Design Demand
All three calculated demands will have an impact on the safe water system design. It is recommended that the system be initially designed to meet the Anticipated Demand After Commissioning. Once that design is completed it can be compared to the Maximum Demand After Commissioning and the Maximum Future Demand. If the design can meet all the three calculated demands, this increases the confidence level of the design. However, if the design does not meet the other two demands, this does not necessarily mean that the design should be changed. Instead, the design engineer can look in what changes could be made to meet the other two demands. If meeting these would take a minimal amount of additional equipment and cost, then doing so would be appropriate. However, if meeting the other two demands would take a large amount of additional equipment and cost, then it may not be economical or practical. Under such situations, decisions will have to made after discussing with the authority or the client. The first consideration should be considered to increase the storage capacity of tanks.
1.5 1.5 References
AmitKohli and Karen Frenken (2011). Cooling water for energy generation and its impact on national-level water statistics:http://www.fao.org/3/a-bc822e.pdf
FAO AQUASTAT Report, 2011 MoW 3rd Edition Design Manual, 2009 Marco, Farina et al (2011). Water consumptions in public schools. Available online at www.sciencedirect.com
Reynolds, C., Steedman, J.C. and Threfall, A.J. (2008). Reinforced concrete Designers Hand Book. Francis and Taylors.
Water Mission Technical Handbook of 2019: Version 1.2
Water Sustainability in Prisons: Water Efficiency: The Journal for Water Resource Management.http://digital.waterefficiency.net/display_article.php?id=2349297&view=284543. Retrieved on 29thDecember, 2019
