This paper presents the assessment of real and apparent losses in a District Metering Area of a Portuguese water utility based on the analysis of collected minimum night flow data and extended period simulations of the system. The DMA covers a 0.7 km2 area, is predominantly domestic consumption, with 6200 inhabitants and a large non-domestic consumer; it is a temporarily DMA, isolated by closing ten boundary valves. It has 9.4 km of pipes with diameter from 50 to 300 mm, 589 service connections and 3400 billed consumers. Flow and pressure data have been collected at the DMA input section, during a leak detection survey carried out during an eight-weak period. A brief description of the leak detection survey is presented. Water losses have been assessed before and after the leakage detection survey by means of a bottom-up approach (a six-step procedure) based on minimum night flow analysis and dynamic hydraulic simulation of the system. Minimum night flow data, between 2:00 and 4:00, were used for estimating real losses. Daily leakage pattern has been estimated by means of hydraulic simulations of the system using EPANET. The analysis has shown that the leak detection survey reduced real losses in 27%, which corresponds to an annual water volume of 130 000 m3 (despite the seasonal demand variation of 22%) and apparent losses and unbilled authorised consumption, even after the survey, still represent 33% of total water volume.

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