Civil Engineering October 2022 | Vol 30 No 9

20 October 2022 Civil Engineering in either direction. If the groundwater surrounding the pipes is contaminated in any way, there is a risk that contaminated water will be supplied to the property when the system is re-pressurised. In cases where there is insufficient chlorine in the potable water to neutralise the contaminants, this can result in certain water borne diseases. In such cases, diar- rhoea will be the first symptom to occur, but typhoid and even cholera are likely, both of which can be life-threatening and create huge problems for over-stretched healthcare services, as well as damage any tourism and the economy. REDUCING CONSUMPTION THROUGH INTERMITTENT SUPPLY The main purpose of introducing intermittent supply is to reduce water consumption. Figure 3 depicts a graph of the flow into a residential area in South Africa in which intermittent supply has been introduced by the municipality. This is an interesting and informative logging result from an area which was experiencing high levels of household leakage. In order to save water, the municipality adopted a water supply regime of 16 hours on and 8 hours off. In order to assess the level of leakage, the water supplier was asked to maintain the system pressurised for two days in order to observe and measure the minimum night flow (MNF) (WRC, 1999) which was estimated to be 165 m 3 /hr. Over the same two-day period, the average daily demand (ADD) was measured to be 200 m 3 /hr. In such cases, the ratio of the MNF to the ADD is a useful indicator of leakage in the area. In this case the MNF:ADD ratio was found to be 0.83, or over 80%. While this calculation is not always a reliable indicator (since not all of the nightflow is leakage), it is generally a useful indicator in areas experiencing Figure 4 Water savings through intermittent supply Flow (m 3 /h) 50 40 30 20 10 0 11 June Raw values, at device native intervals, between 2016-06-10 12:11 and 2016-06-17 12:11 12 June 13 June 14 June 15 June 16 June 17 June Consumption (m 3 ) 1 000 800 600 400 200 0 Average flow = 17.4 m 3 /hr Average flow = 14.0 m 3 /hr Figure 3 Typical flow logging for a zone with intermittent supply Flow (m 3 /h) 450 400 350 300 250 200 150 100 50 0 Mon 00:00 Tue 00:00 Wed 00:00 Thu 00:00 Fri 00:00 Sat 00:00 Sun 00:00 • Rapid refilling of empty pipes • No supply during night • MNF = ± 165 m 3 /hr • ADD = ± 200 m 3 /hr • MNF/ADD = ± 83% Flow