| Using water efficiently - Technological options |
|Water use efficiency|
DEFINITIONS. Urban water use encompasses both industrial and domestic activities. The latter includes residential and commercial (services, office buildings, and public parks) uses. Figure 2 illustrates a typical urban water supply system. Similar to the descriptions used in irrigation, conveyance efficiency, Ec, in this setting is defined for systems between water sources and water treatment centers. Distribution efficiency, Ed, which is the main indicator of the overall effectiveness and operation and maintenance performance of an urban water supply system (usually in pipes), is defined for systems between treatment centers and end-users (households, factories, public standbys),
Ed = Vd/Vs
In urban water supply projects, one common measure of Ed is through use of an indicator called unaccounted-for water (UFW), i.e. UFW = Vs - Vd (see Figure 3), therefore,
Ed = (Vs-UFTY)/Vs = 1-UFWr
where: UFWr = UFW/Vs, standing for the ratio of unaccounted-for water.
However, there seem to be different ways of defining Vd in urban sector water use, which has led to the inconsistent use of the term, UFW. Here are some examples from Bank documents.
• The Bank's Working Guidelines on 'The Reduction and Control of Unaccounted-for Water', prepared by the INU Department Jeffcoate, 1987), defines UFW as the difference between the volume of water delivered into a supply system, Vs. and the volume of water accounted for by legitimate consumption, Vd, whether metered or not. As illustrated in Figure 3, by this description UFW consists of two parts: i) physical leakages from distribution pipelines, house connections, valves, and hydrants; and ii) illegal connections (non-physical losses). Since un-metered water is not necessarily lost, legitimate consumption, Vd, includes the amount of water metered, intentionally un-metered for public uses (such as fire service, street cleaning, construction, and public buildings), and the amount of water unrecorded due to meter damage and lapses in reading.
• However, there seems to be some ambiguity about including un-metered public water uses as part of unaccounted-for water. The Working Guidelines also state that the UFW includes "water consumed but not recorded by consumer's meters or otherwise accounted for by government/public use".
• A Planning Manual published by the Bank (Okun, 1987) defines UFW as the difference between the measured produced water and the metered water used.
• A recent OED report (1992) defines UFW as 'the difference between the measured volume of water input into a system and the amount of water sold'.
This inconsistency in the definition of unaccounted-for water may lead to non-comparable evaluations of efficiencies in urban water supply projects. A generally agreed definition would avoid such problems.
Unlike the field efficiency in irrigation, end-user efficiencies in the urban sector are classified into: industrial consumptive use, Eic; domestic consumptive use, Edc; and overall urban sector use, Eu. Figure 4 illustrates the concepts with simple numerical examples. Consumptive use of water in industry includes evaporation losses (such as cooling processes in thermal, steel and manufacturing industries), the amount used in products (such as food processing and beverage industries), and unaccounted-for losses (such as leakage). Although the leakage losses should be differentiated from consumptive uses, it is usually difficult to separate them out because the estimate of consumptive water use is usually obtained from the amount of water supplied less the amount discharged into the sewers or rivers.