Page 129 - Water Loss Control

Identifying Economic Interventions against W ater Losses 111

at a high frequency, whilst other parts of the network will burst at a much lower fre-
quency. In order to have the greatest impact on leakage one would try to identify those
pipes with a high frequency of failure and replace these first. The benefit of replacing
further sections of pipe will then be less. Again the law of diminishing return applies,
and a point will be reached when it is not economic to replace pipes. A similar curve
will exist for the distribution of service pipe bursts across the network.
It has been suggested that there will also be a distribution of background leakage,
which will not necessarily be the same as that for burst frequency. Those mains with
high burst frequencies may have a low background leakage level and vice versa. This is
because background leakage is primarily driven by leakage at joints on service pipes
rather than mains themselves. Therefore, network rehabilitation should be targeted at
burst and background leakage separately.
To find the economic point, the following calculations are performed:

• The benefit of replacing a section or group of essentially similar pipes in the
same locality in terms of reduction in burst frequency and/or background
leakage is assessed.
• The cost of replacing these pipes is estimated.
• The reduction in leakage is estimated using component loss modelling.
• The savings in costs in inspections, repairs, and active leakage control are
assessed.
• The marginal cost/benefit is assessed as the cost less the sum of the savings
divided by the leakage saving.

All the schemes with a cost benefit lower than the cost of water would be deployed.
This will establish the economic level of network rehabilitation and the associated
leakage level.

9.4.3 Sectorization
It is common practice in some parts of the world to split the water network into sectors and
monitor flows into and out of these sectors at night. Data about the flows into sectors pro-
vides information to be able to locate leaks faster and therefore improve leakage detection
efficiency. However the introduction of sectorization involves costs in the following areas:
• One-off cost of construction of meter chambers
• Cost of meter and replacements and/or refurbishment
• Cost of data logging equipment
• Ongoing cost of data retrieval (either manual or by telemetry)
The benefit of introducing sectorization in terms of leakage will be a function of the
natural rate of rise of leakage in the sector. Not all sectors will have the same rate of rise,
and so again there will be a curve showing diminishing returns. Other factors affecting
costs will be the environment, the complexity of the network, and the degree to which
sectorization has already been established. The calculations are similar to the ones
described above for pressure management and rehabilitation. They can be carried out
to establish an economic breakpoint that would give the economic level of sectorization
and the optimum size of sectors.