Page 73 - Water Loss Control

Steps and Components of a W ater Loss Contr ol Pr ogram 55

General DMA setup

Source Reservoir

Closed valves
WTP
DMA meters

DMA meter
Closed valves

FIGURE 5.2 General DMA setup. (Source: Adapted from IWA Water Loss Task Force)

The MNF in urban situations, usually occurring between 2 a.m. and 4 a.m., is the
most meaningful piece of data as far as leakage levels are concerned. During this period,
authorized consumption is at a minimum and, therefore, leakage is at its maximum
percentage of the total flow. The estimation of the leakage component at minimum
night flow is carried out by subtracting an assessed amount of legitimate night-time
consumption for each of the customers connected to the mains in the zone being stud-
ied. Typically, in European and North American urban situations, about 6% of the pop-
ulation will be active during the minimum night-time flow period. This activity is
typically to use a toilet and the water use is almost totally related to the flushing of WC
cisterns, although it can include substantial amounts of irrigation at certain times of the
year. Analysis of minimum night-time flows therefore also requires the use of sophisti-
cated techniques to determine legitimate night use. If it is known that there is signifi-
cant or non-normal night use, otherwise known also as exceptional night use, within
the zone, then this must also be estimated or measured by for example carrying out
meter reading tests during the minimum night period.
The result obtained from subtracting the assessed night use and exceptional night
use from the minimum night-time flow is known as the net night-time flow (NNF) and it
consists predominantly of physical losses from the distribution network.
After completing these three initial components of a water loss control program it is
now necessary to transfer volumes into values in order to determine the economic opti-
mum volume of leakage.

5.3 Determine the Economic Optimum for Your Water Losses
Water loss management is an economic issue. Utilities should aim to manage losses in
order to minimize overall operating costs. With any water loss reduction strategy, the
lower the level of water losses achieved, the higher the cost of reducing water losses
further. For this reason, it is never economic for a utility to remove all water losses. The
economic optimum is the economic balance point at which the value of water lost (real
or apparent losses), plus the cost to reduce the volume of real or apparent losses, is at a
minimum. It is certainly best practice to determine the economic optimum point for