Page 101 - Water Loss Control

Evaluating W ater Losses 83

The detected problems have to be quantified and a best estimate of the annual vol-
ume of this component has to be calculated.

7.4.5 Calculation of Real Losses
Defi nition: The annual volumes lost through all types of leaks, breaks, and overflows on mains, service
reservoirs, and service connections, up to the point of customer metering.
The volume of real losses is calculated by subtracting the volume of authorized con-
sumption and the volume of apparent losses from the total system input volume.

7.4.6 Calculation of Nonrevenue Water
Defi nition: The difference between system input volume and billed authorized consumption.
Nonrevenue water is the portion of the water that a utility places into the distribution
system that is not billed and, therefore, recovers no revenue for the utility. Nonrevenue
water consists of the sum of unbilled authorized consumption (metered and unme-
tered), apparent losses, and real losses.
It is recommended as best practice by the IWA and AWWA WLCC that the assess-
ment of real losses using a “top-down” water balance should be complemented by at
least one of the following two methodologies:
• Component analysis of real losses, a technique which models leakage volumes
based upon the nature of leak occurrences and durations (see Chap. 10)
• “Bottom-up” analysis of real losses using district metered area (DMA) and
minimum night-time flow (MNF) analysis (see Chap. 16)

Both methodologies add increased refinement and confidence in the calculated vol-
ume of real losses and are described separately in this manual.

7.5 Unavoidable Annual Real Losses—Unavoidable Water
Losses and Discovered Leaks and Overflows
Nowadays it is well understood among water loss practitioners that every system has
a certain volume of real losses occurring, that is unavoidable. Even newly commis-
sioned sections of the distribution network will have some volume of real losses.
Since the last century, it has been common practice in North America to estimate,
using various formulae, the unavoidable leakage from pressurized pipework systems—
those small leaks which are believed to be undetectable, or which are considered uneco-
nomic to repair. The original intention of this was presumably to try to define a baseline
or lower limit for leakage management, below which it is uneconomic to attempt fur-
ther leakage control. An outline of the various methods previously used in North
America can be found in Ref. 7. The system-specific predictions based on an auditable
7
8
component-based equation proposed by the IWA Task Force on Water Losses, described
later in this chapter, can be regarded as a natural progression of previous North Ameri-
can efforts to predict unavoidable losses.
Because of the simplified nature of some of the formulae previously used in North
America, or the very generous allowances given for old pipework (particularly cast-
iron pipes), the effect of the unavoidable leakage calculation has in practice often resulted