Chapter Four: Water Demand Assessment

1 Chapter Four: Water Demand Assessment

1.1 1.1 Water Demand Assessment

Water demand is the quantity of water that the source must produce to meet all project water requirements. These include water delivered to the system to meet the needs of consumers, water supply for firefighting, system flushing, water required for operation of treatment facilities and amount water lost due to leakages in the infrastructure. In planning and designing of any project, water demand assessments of current and future needs are of prime importance. Engineering decisions are required to determine the area and the population, industries, institutions and other 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 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 either 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 the 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 community,
• Age and education,
• Availability of alternative water sources,
• Level of service, technological process,

The combination of these factors may attribute 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 thorough community survey be conducted to the project area to determine the magnitude of factors affecting water consumption pattern described above.

The fundamental components of calculating water demand are by determining the total population and the average daily consumption of the individuals.

Apart from individual based water consumption pattern, the 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 particular 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 segment demands to obtain the overall project water demand. The formula below provides general consumers whose water demand may be required:

Total water demand = Domestic + Institutional +Industrial +Commercial +Livestock + Fire fighting + other uses (Operational demand) + NRW (4.1)

It is worth to note that, the above formula is just a guide and should be adjusted according to the area in question. Additionally, in whatever assessments, demand assessment should aim at obtaining the design and operational information to optimize the net benefits to the community.

1.3 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 non-domestic components, non-domestic use include Commercial Use, Institutional Use and Industrial Use, Fire Fighting and System Water Consumption.

To establish 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;

Step 1: 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 exist)
Step 5: 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 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 to domestic water use categories;

Category one: Low income using public tap or kiosks,
Category two: Low income multiple house with yard tap, multiple houses served from single yard tap,
Category three: Low income single house hold 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 capital 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 considering the following;

• Decide upon use of different per capital consumption rate if water requirement survey data from the project area justify,
• Dividing 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 of population density of residential plots if the data is available,
• Adjusting the community water requirement if survey of alternative sources of water are available.