Page 109 - Water Loss Control

Evaluating W ater Losses 91

• ILI: It is a dimensionless ratio between the current annual real losses (CARL) based
on the results of the water balance and the unavoidable annual real losses (UARL)
for a given system.
ILI = CARL / UARL
• UARL: They are calculated as previously described in this chapter, using the IWA
methodology which takes into account average operating pressure, length of mains,
number of service lines, and location of customer meters relative to the curb stop.
The infrastructure leakage index is a relatively new, and potentially very useful, per-
formance indicator. Being a ratio, it has no units, so facilitates comparisons between coun-
tries that use different measurement units (metric, U.S. Customary). The ILI can perhaps
be better envisaged from Fig. 7.5, which shows the
four components of leakage management.
The large square represents the current annual
volume of leakage, which is always tending to The ILI ratio is a great way of
increase, as infrastructure systems grow older. This demonstrating loss management
increase, however, can be constrained by an appro- performance, as each system
priate combination of the four components of a suc- effectively compares the ratio of
cessful leakage management policy.
their individual best possible per-
The small square represents UARL—the lowest formance against how they are
technically achievable value for real losses at the
current operating pressure. The ratio of the current actually performing.
annual real losses (the large square) to the unavoid-
able annual real losses (the small square) is a meas-
ure of how well the three infrastructure management functions—repairs, pipe materials
management, and active leakage control—are being controlled. We will be seeing more
of this diagram in future chapters where we will be discussing some of the hands-on
techniques associated with in the field loss-reduction programs.

Losses flex with pressure Pressure Economic level of real losses
management

Unavoidable
annual real
losses
Speed and quality Active
of repairs leakage control
Potentially
recoverable real
losses

Pipeline and
asset management
Current annual real losses selection,
installation,
maintenance,
renewal,
replacement

FIGURE 7.5 The four components of a successful leakage management policy.
(Source: IWA Water Loss Task Force and AWWA Water Loss Control Committee)