Partitioning of Unaccounted for Water for Zomba City Water Supply System in Malawi

Abstract

Urban water supply utilities in developing countries are faced with challenges of low service coverage and high unaccounted for water (UFW). UFW reduces the water available to customers, and results in loss of revenue for the water utility. In Southern Africa, UFW ranges from 11% to 60% and in Malawi it is between 28% and 48%. Zomba has UFW of 30%. Most studies on UFW have not focussed on its partitioning into real and apparent loss components. Crude UFW does not allow the water utility to identify appropriate priority areas and actions for UFW reduction. It is against this background that a study was carried out on the Zomba City water supply system to partition UFW. The water supply system is owned and operated by Southern Region Water Board (SRWB). The study analysed UFW, partitioned it into its components and assessed major factors affecting the components. Four water supply zones (Airwing, Malonje, Sadzi and Mtiya) were selected based on their unique characteristics. Historical data on water production and consumption were used to assess overall UFW. Bulk meter readings for each zone were taken for three months alongside the corresponding consumptions. Flow and pressure loggers were installed to obtain Minimum Night Flows (MNF) and assess pressures respectively. Water balances were carried out. Meter inaccuracy tests were also carried out to assess the impact of meter errors on apparent losses. Records of pipe bursts, faulty meters and queries on high bills were also used in the study. The average UFW from 1999 to 2008 was estimated to be 27.5% of which real losses represented 71% while apparent losses were 29%. The average UFW for the four study zones was 13% for Airwing, 62% for Malonje, 51% for Sadzi and 6% for Mtiya. Real losses in Airwing estimated at 73% of UFW, Malonje had 75%, Sadzi had 40% and Mtiya had 23% as real losses. The remaining percentages represented apparent losses for each study site. It was also established that real losses in Airwing were due to pipe bursts mainly as a result of pipe age while in Malonje and Sadzi; the real losses were affected by bursts due to pipe age and high pressures. It was found out that more than 58% of the pipe network in the whole distribution system was over 60 years old. Malonje and Sadzi areas had operational pressures above 50 m and experienced frequent pipe bursts. It was also found out that 53% of customer meters tested were over 5 years old. Meter inaccuracy tests also showed that all the meters under-registered and that 46% of the meter errors were above the allowable tolerance of -5% at low flow rates and -2% at high flow rates. It was concluded that the average unaccounted for water was above the 20% recommended for Southern African urban water supplies. Partitioning of UFW showed that different areas had different levels of real and apparent losses and gave a hint on UFW components hence is an important tool towards proper planning of priority areas in UFW control strategies. Real losses in Airwing were affected by distribution network age, while in Malonje this was affected by pressure and distribution network age. Sadzi had a high level of apparent losses as the number of connections in the zone was very high. It is recommended that routine water audits be done for the other remaining areas to come up with a comprehensive UFW control strategy. It is also recommended that feasibility of pressure management for UFW reduction be investigated in Malonje area. Distribution network replacement is recommended in Airwing. UFW control strategy for Sadzi should focus more on reducing the high levels of apparent losses.