The viability of domestic rainwater harvesting in the residential areas of the Liesbeek River Catchment, Cape Town

Abstract

By 2030 South Africa (SA), a developing country, is predicted to be severely impacted by physical water scarcity. In order to avert a future water crisis, the country needs to find ways to reduce its reliance on conventional surface water schemes based on impoundments on rivers. Rainwater harvesting (RWH) is an alternative water resource. To date, the viability of domestic RWH within an urban setting has not been adequately considered in SA. The purpose of this study was thus to address this omission through the detailed modelling of a representative catchment. The Liesbeek River Catchment in Cape Towncomprising some 6 200 domestic properties in 6 suburbs covering an area of around 1 300 ha -was chosen for this purpose; and a new computational tool, the Urban Rainwater/Stormwater Harvesting model (URSHM), was developed to take best advantage of the available data. The analysis showed that: RWH was only economically viable for a minority of property owners; climate change is likely to have limited impact on the performance of RWH systems; and -contrary to some claims -RWH is an unreliable means of attenuating peak stormwater flows.

Supplementary household water sources to augment potable municipal supply in South Africa

“…Fisher-Jeffes et al (2017) confirmed that rainwater use, in the Western Cape study area investigated by the team, was not financially viable and underlined the need for future research to better understand the viability of rainwater harvesting in different climatic regions of South Africa.…”

Improving the Viability of Stormwater Harvesting through Rudimentary Real Time Control

“…This was expected as Fisher-Jeffes [4] showed that SWH systems are most likely to be economically viable when supplying areas with a high water demand concentration (i.e., water demand/area) as this limits the extent of the dual water distribution network required-thereby keeping the infrastructure costs down. Each of the three options that modelled WDA 2 indicated a cost per cubic metre to supply harvested stormwater comparable to the typical maximum tariffs (15.81, 23.51 or 29.03 ZAR/m 3 depending on the demand; the CoCT uses a 'rising-block' tariff scheme) that residents within the catchment were being billed by the CoCT for their potable water usage at the time of the study.…”

“…Chlorination, as a method of disinfection, was avoided as chlorinated water can have harmful effects on plants if the water is used for irrigation purposes. Since the required dose of UV disinfection is dependent on the UV transmittance (UVT) of the water, the harvested stormwater would require filtration beforehand to ensure a consistent quality [4]. The inclusion of slow sand filters-a low cost treatment device requiring relatively low levels of maintenance that can tolerate wide fluctuations in flow range whilst still effectively removing organic matter and mild turbidity [56,57]-was assumed for this purpose.…”

“…The study focused on non-potable indoor residential demand because in areas that experience winter rainfall, such as Cape Town, using non-potable water such as stormwater for indoor water demand improves the economic viability during periods in which there is no irrigation demand [2]. Treating the water to potable standard and distributing through the existing system was ruled out because reliable on-site advanced treatment was considered too expensive [4] whilst that the cost of pumping the water to the nearest municipal water treatment works, a considerable distance away, was also considered to be too high.…”

“…This, together with the additional threat of climate change, has resulted in a growing interest in alternative water resources to supplement the existing supplies. Stormwater, in particular, has been identified as a viable supplementary water resource owing to its relative abundance in urban areas, but a practical obstacle to Stormwater Harvesting (SWH) is long-term storage [2,4]. Since stormwater flows are intermittent, SWH systems require facilities that can store large enough volumes of the collected runoff to ensure that they can deliver a reasonably reliable water supply during dry periods [5,6].…”