Page 105 - Water Loss Control

Evaluating W ater Losses 87

The UARL values in Table 7.2 can just as easily be plotted as a graph of gallons per service
line per day per psi of pressure versus density of service lines, as shown in Fig. 7.3.
In well-run systems worldwide, the greatest annual volume of real losses occurs
from long-running, small- to medium-sized leaks on service connections, except at low
densities of service connections. This is why the IWA Task Forces recommend using
“per service connection” instead of “per mile of mains” as the basic performance indi-
cator for real losses, for connection densities exceeding 32 per mile. Using the previous
calculation example, for the system with 60,000 service connections and 600 mi of mains,
the UARL derived from Fig. 7.3 would be
(a) 0.34 gal/service/d/psi of pressure × 70 psi = 23.8 gal/service/d × 60,000
services = 1.43 mgd (for customer meters 100 ft from the curb stop); or
(b) 0.23 gal/service/d/psi of pressure × 70 psi = 16.1 gal/service/d × 60,000
services = 0.97 mgd (for customer meters 20 ft from the curb stop).
The curved lines in Fig. 7.3 are relatively flat for a wide range of connection densities. In
calculating unavoidable annual real losses, for example, systems with customer meters
50 ft from the curb stop, and connection densities in the range 80 to 200 per mile, an
acceptable simplification from Fig. 7.3 would be to say that the UARL is 0.25 gal/mi/d/
psi of pressure (=±10%).

7.6 Which Performance Indicator? What’s Wrong with Percentages?
Because water utilities are of different sizes, with different characteristics, comparisons of
performance in water loss management need to be made in terms other than volume per year.
Traditionally, several different performance indicators are used by North American utilities to
compare water losses—percent of system input volume
or the metered water ratio, and “per mile of mains per
day” appear to be the most common. But are these reli-
able indicators for comparing performance? Expressing losses as a per-
centage is not the best way
Why do some countries use “per property per
day,” or “per service connection per day,” or “per kil- to compare loss-management
ometer of systems (mains + services length) per day?” performance, as systems with
The IWA Task Force on Water Losses, with nominated lower demands or successful
representation from the AWWA, has been considering customer side conservation
8
best practice internationally, and their conclusions programs will never be able to
strongly suggest that there are more reliable and compete with those with larger
meaningful performance indicators than “percent of
demands. Instead the volume
system input” and “per mile of mains.” of loss per service connection
In emphasizing the importance of the correct
per day should be used.
choice of measuring units, another example from
history is useful. Two thousand years ago, in the
first century A.D., Julius Frontinius Sextus, then
water commissioner for Rome, was spending the whole of his professional career trying
(and failing) to achieve a meaningful balance between the quantities of water entering
and leaving the aqueducts, which served the city. Failure was not due to lack of dili-
gence on his part—he was simply using the wrong measures. The accepted Roman
method was to compare only areas of flow; because they did not take velocity of flow
into account also, their calculations could never be reliable for management purposes.