Page 153 - Water Loss Control

Modelling W ater Losses 131

amount of apparent loss that a water utility would suffer in spite of enacting all reason-
able and effective apparent loss control activities. Similar to the infrastructure leakage
index (ILI) that assesses real loss standing as the ratio of current real loss volume over
the unavoidable annual real losses (UARL) (See Chap. 7) an apparent loss index (ALI)
would be the ratio of the current apparent loss volume in the water audit over the
UAAL.
However, in lieu of a reliable UAAL measure, the team’s interim recommendation
is that 5% of the metered consumption be assumed as a reference value for apparent
losses. The authors feel that this may be high for water utilities in developed countries,
which typically have good customer meter management and where buildings do not
have roof tanks which present the opportunity for very low flows that pass unregis-
tered through many water meters. (See Sec. 12.4.) These utilities typically also have
reasonable policies and safeguards that prevent exorbitant unauthorized consumption.
Therefore the 5% assumption may be high in developed countries, but reasonable for
developing countries.
The Water Loss Task Force is actively engaged in work to develop a set of apparent
loss performance indicators and further information will be made available by the IWA
team as research progresses.

10.3.1 Modeling Customer Meter Accuracy
Customer meters have been called the “cash register” of the utility and are responsible
for ensuring an equitable distribution of water volume and income throughout various
different customer types within a utility. It is therefore extremely important to assess
the accuracy of the meters on a regular basis and make repairs or replace groups of
meters to keep the customer meter population at an overall high level of accuracy.
Accurate metered consumption data is also necessary for engineering functions such as
hydraulic models, evaluation of water conservation programs, and sizing of infrastruc-
ture for water resources development. The reader should also refer to Chap. 16 which
provides detailed information on meter performance, as well as procedures for meter
accuracy testing.
It is necessary to model average weighted meter accuracy for the entire customer
meter population and include it in the water audit. The water balance calculations are
used to deduct the volumes of apparent losses from the total volume of losses in order
to arrive at a top-down approximation of the annual volume of real losses.
In attempting to quantify the volume of apparent loss due to customer meter inac-
curacy in the water audit, it is important to recognize that three primary occurrences
cause a meter population to become inaccurate, namely

• Eventual decline of the inherent (mechanical) accuracy of a meter population
through wear.
• The meter or the meter reading device may fail or “stop” altogether.
• Meters may not be of the proper size or type to accurately register the full range
of water flows encountered in a given customer supply.
It is necessary to disaggregate, or separate, the activities of the water utility’s
meter management in these three occurrences in order to properly construct a repre-
sentative picture of the annual volume of apparent loss attributed to customer meter
inaccuracy and the reasons for each disaggregated volume. In this way, planning can