Page 125 - Water Loss Control

Identifying Economic Interventions against W ater Losses 107

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derived then a curve is fitted. This may assume a given shape to the curve. The diffi-
culty with this approach is that the current position on the curve represents a static situ-
ation of the balance between average leakage over a number of years at a constant
resource level. It may take a number of years to reach stability when detection resources
are changed. It is therefore a long process to develop accurate estimates of a number of
points on the curve.
Alternatively, a theoretical approach using component loss modelling methodolo-
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gies can be used to define the ALC curve, but this will require a number of assump-
tions, such as burst flow rates, although attempts can be made to calibrate these from
actual data. A compromise is to establish the ALC curve by building a component loss
model of the system and then to calibrate this such that it passes through the current
operating position established by analysing the actual cost of operations. The economic
intervention period can then be found by direct differentiation of this curve or by
numerical methods.

9.3.2 Background Leakage and Backlog Removal
Background leakage is generally defined as the leakage below the level of detection (with
current technology). The level of background leakage can be assessed using a number
of methodologies. However, the level of background leakage is a function of the extent
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and method of leakage detection employed, which itself will have different operating
costs associated with different levels of leakage. Therefore, a matrix of leakage detection
costs versus level of background leakage can be derived, from which a view can be
taken on the appropriate economic method of detection, and the associated level of
background leakage.
Background levels of leakage have been related to system characteristics. 9,10 Such as
pressure, length of mains, and number of connections. From these, unit background
losses at standardized pressure have been estimated. These can then be related to asset
type, material, age, and condition. From this work it is possible to provide an estimate
of the background level of leakage that might be expected in an area. By comparing this
to the actual minimum achieved on that area, a view can be taken as to whether back-
ground levels have been achieved or whether it is likely that there are leaks on the area
that would be possible to find.
These leaks will have gradually accumulated on the system over a number of years,
and are essentially hidden in accepted minimum historic night flows. The leaks may be
on parts of the network that are not normally checked for leaks, for example, large
industrial complexes, mains which are believed to have been abandoned, private sup-
ply pipes, complex road junctions. The number of backlog leaks and hence the associ-
ated repair bill can be substantial, but they are one-off costs and the cost benefit can be
readily assessed. However, it may be appropriate to take other action, for example,
pressure reduction (described later) to reduce the frequency at which the system is
breaking in order to allow for this backlog to be reduced over a period of time within
the current repair budget. Alternatively, it is possible that these could be considered as
a one-off capital cost depending on local accountancy rules.

9.3.3 Transition Costs
Once an economic level of leakage has been established, then a company should move
toward this ELL. However, as this is likely to be at a lower level of leakage than the cur-
rent level, moving to this point will involve one-off costs. As each point on the ALC