Water Journal : Water Journal December 2012
refereed paper water DECEMBER 2012 95 non-revenue water Abstract Understanding elements of non-revenue water (NRW) has become more critical in an era where urban water has a rapidly increasing value. Meter under- and over-registration of flows has received considerable research attention over the years, but there has been limited focus on non-registration, particularly for domestic water meters over their life cycle. This paper presents empirical estimates on non- registration for typically installed domestic water meters and emphasises that both under- and non-registration should be considered for any meter replacement assessments and associated policies. Economic modelling showed that domestic water meters should be replaced at lower throughput volumes when considering both of these meter inaccuracy categories. The key value of the paper is that it provides a methodology for determining non- registration for a fleet of domestic meters. Introduction Most Australian utilities have meter replacement programs, but the criteria for replacement vary. It is argued that different operating conditions, pipe materials, the condition of the network and water quality affect meters differently. However, there is still limited understanding of meter accuracy performance with in-situ age, particularly when considering its starting or minimum registration level (Qs). Theoretically, water with a flow rate less than the Qs flow rate of a meter (< 15L/hr) cannot be measured and represents lost revenue. In this paper the term non-registration refers to the failure of the meter to register accurately the volume of water passing at flow rates below the meter’s specified minimum registration rate (Qs). The significance of this unread volume is not yet well understood. In the past, much work has been done on meter accuracy, with utilities testing batches of meters in accordance with the NMI R49-2 (2009b). However, the low flows at which the meters are tested are usually not low enough to measure the actual minimum registration level for each meter. So traditionally under-registration at the various standard flow rates has been assumed to be the primary contributor to non-revenue water attributable to meter inaccuracy. The research reported here will show that non- registration could be more significant than under-registration and should be considered together with under-registration when estimating the non-revenue water (or the unregistered volume) and for determining replacement intervals for a fleet of meters. Research has also shown that a fleet of water meters can become less accurate with age and usage (i.e . under-registration of flow) and, as will be shown, non-registration of the meter can also increase with usage, resulting in higher volumes of unaccounted for water (Arregui et al., 2005; 2006). This paper provides a summary of the results of analysing the impact of meter usage and age on the starting registration flow of meters, albeit based on a small sample size. It proposes a methodology (based on the available research) for calculating the non- registration volume of a suite of meters and scheduling the replacement of a meter based on non- and under-registration calculations. The study was conducted on a common brand of 20mm meter used in Australia. Customer Meter Inaccuracies Component of Non-Revenue Water As illustrated in Figure 1, customer- metering inaccuracies, together with unauthorised consumption, account for the apparent losses when undertaking a water balance of the system input volume (Alegre et al., 2006). Since this value cannot be precisely measured or determined, the Water Services Association of Australia (WSAA) has provided a default value for the purposes of preparing the water balance. The WSAA (2010) default value for average non-revenue water based on an under-registration of residential meters is 2.0% of recorded residential metered volume. In earlier versions of the WSAA Handbook (2006), the default value for residential meters was 1.5% for under- registration, with an additional 0.5% being added to the residential meter error to account for meter non-registration. This change results in a simplification of the sources of meter error and, therefore, lacks focus on non-registration as being a key source of non-revenue water, as is demonstrated in this paper. This paper advocates that the unregistered consumption of domestic customer meters comprises the sum of the non-registered and under-registered volumes and should be expressed as: unregistered volume = non-registered volume + under-registered volume The total unregistered volume is significant when estimating apparent losses for a meter, and the fleet of meters should, therefore, be disaggregated into the registration groups when calculating the water loss due to meter error, as will be further shown in this paper. Understanding Non-Registration of Domestic Water Meters To gain a better understanding of non- registration of water meters, this research combines three components. The first estimates the extent of non-registered water at low flows for new and aged P Mukheibir, R Stewart, D Giurco, K O’Halloran Implications for meter replacement strategies UNDERSTANDING NON-REGISTRATION IN DOMESTIC WATER METERS System Input Volume Authorised Consumption Billed Authorised Consumption Billed Metered Consumption Revenue Water Billed Unmetered Consumption Unbilled Authorised Consumption Unbilled Metered Consumption Non- Revenue Water (NRW) Unbilled Unmetered Consumption Water Losses Apparent Losses Unauthorised Consumption Customer Metering Inaccuracies Current Annual Real Losses (CARL) Leakage on Transmission and Distribution Mains Leakage and Overflows at Storage Tanks Leakage on Service Connections, up to Customer Meter Figure 1. IWA Water Balance (Alegre et al., 2006).
Water Journal February 2013
Water Journal November 2012-1