Estimation of Water Balance and Water Losses in Water Utilities – Experiences from the Belgrade Waterworks - page 2

 

Performance Indicators Related To Water Losses

Water utilities differ from one another in many aspects: size, type, operating pressures, number of consumers and other. The IWA Performance Indicators enable utility water loss comparisons (Alegre et al., 2006).

Water balance analyses enables the water utility to determine its Current Annual Real Losses (CARL). Real losses can not be completely eliminated and IWA suggested the following equation for determination of Unavoidable Annual Real Losses – UARL:

UARL = (18×Lm + 0,8×Np + 25×Lp) × p

where: UARL – Unavoidable Annual Real Loss in liters pre day (l/day), Lm – length of pipelines (km), Np – number of service connections, Lp – length of service connection pipes form main pipe to water meter (km), p – average pressure in the distribution network (m). From the equation is clear that that water losses are assumed to be directly proportional to length of pipes (mains and service connections) and average pressure in the distribution network.

The most important performance indicator related to water losses is Infrastructure Leakage Index – ILI (Alegre et al., 2006):

                                

Both CARL and UARL shall be expressed in same units and therefore ILI is dimensionless. The IWA workgroup suggested that indicator ILI should be around 1.0 for the systems with very low water losses and could go above 10.0 for high leaking systems.

Overview of Leakage Control Methods

Water utilities may implement active or passive water loss (leakage) control policies. Passive leakage control addresses visible leakage due to pipe bursts which are reported by customers or inadvertently discovered during regular maintenance. But, visible signs of leakage observed on the ground surface may not represent major water losses, whereas, invisible leaks may go undetected for very long periods of time and may incur significant volumes of water loss. Passive leakage control cannot reduce water losses and therefore water utilities should not exclusively rely on it. On the other hand, active leakage control includes various activities on monitoring and reduction of water losses. The main goal is detection of leaks and bursts that are not visible and the reduction of water loss volume by limiting the duration of leaks. Figure 1 represents four main components of proactive real loss management. Also, activities include measures on detection and reduction of apparent losses.

Implementation of all components in Figure 1 shall lead to reduction of CARL, except for pressure management, where CARL will decrease with pressure decrease but it will increase if pressure in the distribution network increases.

 

Figure 1: Four components of proactive real loss management (Thornton, 2005)