Civil Engineering October 2022 | Vol 30 No 9

Civil Engineering October 2022 21 high levels of leakage. Typically, a system with little leakage will have a ratio of around 0.1 to 0.2 and anything higher will suggest some form of leakage problem. In South African systems, any ratio above 0.2 is considered to be an indication of high leakage and any ratio above 0.5 will usually reflect high internal household leakage. Figures in excess of 0.9 have been observed in some areas. Another interesting issue that can be seen in Figure 3 is the refilling spike that occurs each morning when the system is re-pressurised. This spike in flow is caused by the initial volume of water required to fill the empty pipes after a period of zero water pressure. Such sudden and extreme high flow rates can damage the water meter if the peak flow exceeds the upper limit of the meter. In addition to the refilling spike, there is also the problem of air in the pipeline which must escape somewhere. If the air-valves on the pipeline are not functional, the air will create problems either in the reticulation system or in the consumer properties. If intermittent supply is continued for a lengthy period (years), the leakage rates in the reticula- tion system will increase to such an extent that the system will eventually break down completely. Figure 4 shows an example where a water supplier decides to introduce in- termittent supply for 10 hours each night between 20:00 and 6:00. The average daily consumption to the area drops from 17.4 m 3 /hr to 14.0 m 3 /hr which represents a reduction of approximately 20% for a 40% reduction in time pressurised. In this example, the leakage is already very high and yet the benefit achieved is relatively modest. In an area with low leakage, the benefit will be significantly smaller and leads to the question of whether or not such a measure is worthwhile. Recent research undertaken interna- tionally (Charalambous, 2016) also raises questions regarding the water saving ben- efits of introducing intermittent supply. On a large-scale project undertaken in Cyprus, Charalambous (2016) observed the following: Q Q Over 1/3 of water supply systems in Africa operate on intermittent supply Q Q Intermittent supply had a detrimental effect on the integrity of the distribu- tion network Q Q In a specific case study, intermittent supply resulted in a 200% increase in mains bursts and a 100% increase in service connection bursts Q Q Intermittent supply created water quality problems Q Q Intermittent supply caused customer dissatisfaction In the case study, the total water supply initially dropped after introducing inter- mittent supply but within three years had increases to pre-intermittent supply levels, thus eliminating any savings Intermittent supply is not an ap- propriate response to drought or water shortage. To place the issue in perspective, Figure 5 shows the difference that can be expected between intermittent supply and continuous supply in a zone. Obviously to achieve 24/7 pressurised supply after a pe- riod of prolonged intermittent supply will require significant effort and investment due to the damage caused to the network by the destructive action of pressurising and depressurising the system. The example shown in Figure 5 is based on what can be achieved if the system leaks are repaired and the pressure is used as a management tool. As can be seen, the average water use by the zone is 135 m 3 /hr when operating under intermit- tent supply and 95 m 3 /hr when operating under 24/7 pressurised supply. While the figures will naturally change from system to system, the underlying message is clear – intermittent supply will eventually result in greater water use than 24/7 pres- surised supply, assuming that the resource is capable of supporting the demand from the system. CONCLUSIONS Based on real results from systems around the world, the following conclu- sions can be drawn with regard to the use of intermittent supply when used as an intervention by municipalities to save water: Q Q Intermittent supply is considered by many municipalities to be a quick and easy intervention for reducing water use in an area. Q Q Intermittent supply will result in a short-term reduction in water use but this is likely to be offset by increased leakage in the long-term, resulting in an overall increase in water use. Q Q Intermittent supply damages the water reticulation network and increases the number of bursts. If intermittent supply is continued for a lengthy period (years), the leakage rates in the reticulation system will increase to such an extent that the system will eventually break down completely.