Rainwater harvesting guide – background, scope and purpose

By Water Research Commission

South Africa has changed its water legislation to usher in reforms in the water sector and ensure the sustainable use of increasingly scarce water resources.

Photo4Water collection in rainwater harvesting tanks at a low-cost housing project in Kleinmond, Western Cape.
Image credit: Water Research Commission

While water-related legislations that provide a proper enabling environment for the integrated management of water resources are in place, their implementation still proves to be a challenge.

The South African water experience is unique since, until 1994, access to water by the vast majority of the population was restricted by the apartheid regime. To convolute the situation further, acid mine drainage from defunct and flooded underground mines severely threaten South Africa’s scarce water resources (Naicker et al., 2003; Hobbs and Cobbing, 2007; and Oelofse, 2008).

Due to the escalating economic costs of supplying water via centralised supply systems, the ever-increasing demand for water, and the decreasing quality of water bodies, there is a renewed interest in decentralised water supply infrastructures. Rainwater harvesting (RWH) is an unconventional water source that has been receiving a lot of attention over the past two decades. In the Sharm El-Sheikh declaration, the African Ministers’ Council on Water (AMCOW), which also includes the South African minister of water affairs, committed to increase RWH’s share of total water supply to 10–15%.

Figure 1 Number of households using rainwater harvestingFigure 1: Number of households using rainwater harvesting tanks as main water source in each of the nine provinces of South Africa (DWS, 2014).

Rooftop RWH systems are already a source of drinking water in rural areas (Figure 1), especially in the Eastern Cape and KwaZulu-Natal (Mwenge Kahinda et al., 2010).

RWH presents many benefits for urban sustainability and is quickly emerging as a key strategy in order to cope with water scarcity in urban and rural South African settlements. The Department of Water and Sanitation supports a national RWH programme, which has a narrow but important focus on the construction of above- and below-ground rainwater storage tanks by rural households for food gardens and other productive water uses. The intention, as highlighted in Chapter 4 of the National Water Resource Strategy 2 (NWRS 2), is to extend the programme to RWH in both rural and urban households and office buildings.

Although RWH is a well-recognised source of water, its nationwide adoption and implementation (particularly in urban settlements) have been at a much slower pace. This has been largely due to lack of specific user and programme implementation guidelines.


Rainwater harvesting’s nationwide adoption and implementation have been at a much slower pace, largely due to lack of specific user and programme implementation guidelines.


To develop general or national guidance for RWH, several factors must be considered. While potable use is possible for harvested rainwater, necessary on-site treatment and perceived public health concerns will likely limit the quantity of rainwater used for potable demands. Irrigation and the non-potable uses of water closets, urinals and heating, ventilation, and air conditioning make-up are the end uses that are generally the best match for harvested rainwater. A lesser amount of on-site treatment is required for these uses and, as seen from the usage statistics (Figure 2), these uses constitute a significant portion of residential and commercial demand.

Focusing harvested rainwater on irrigation and selected non-potable indoor uses can significantly lower demand while allowing a balance and a public comfort level between municipal potable water and reused rainwater.

Figure 2 Typical water use in South African homesFigure 2: Typical water use in South African homes (DWAF, 2003).

There have been broad suggestions on a project to project basis as to how to ensure optimum quality of harvested rainwater. The Department of Water Affairs and Forestry (DWAF) released a document concerning the criteria guidelines required for the design of an RDP rural water supply (DWAF, 1997).

Pertaining to RWH, the following were suggested:

  • Animals and people should be prevented from contaminating rainwater collection surfaces. House roofs are generally the preferred collection surface.
  • Rainwater collection surfaces should be constructed from inert materials and should be well maintained and cleaned (particularly at the end of the dry season) to prevent contamination.
  • A ‘first flush’ system should be incorporated into the rainwater collection system, to remove as much contamination as possible before the storage tank starts to fill.

National RWH guidelines, which give clear direction as to the routine water analysis and monitoring that needs to be undertaken to ensure constant quality of rainwater, do not currently exist in South Africa. This document is therefore a first attempt to address this need.

SCOPE AND PURPOSE OF THE RWH GUIDE

Scope
The South African rainwater harvesting resource guideline is a manual that provides guidance for the design, installation, and management of domestic rainwater harvesting systems.

Purpose
The South African rainwater harvesting guideline is the primary source of information and decision-support to all stakeholders. The guidelines contain similar information to what is available in the international literature and a number of guidelines; however, the information here is tailor-made to South African conditions.

The ultimate purpose of the guidelines is to enhance water security by mainstreaming domestic rainwater harvesting into the development and management of water resources in South Africa.

Users of the guidelines
The South African rainwater harvesting guidelines are being developed as the primary information resource for stakeholders such as homeowners, engineers, architects, contractors, developers, regulators, as well as members from municipal, provincial, and national levels of government.

Ongoing review
The South African rainwater harvesting guidelines will be periodically reviewed. The purpose of the reviews is to incorporate into the guidelines all relevant new information from international and local sources as they become available.

Overview
The South African rainwater harvesting guidelines will be divided into the following information sets:

  • An overall introduction and discussion on domestic rainwater harvesting within the South African context.
  • The current existing enabling environment: legal framework and institutional arrangements, or their lack thereof.
  • The factors that can affect rainwater quality and suggestions on how these risk factors can be mitigated through appropriate design, installation of rainwater harvesting systems, and use of appropriate household water treatment.
  • The first point of contact for rainfall, the catchment area, and conveyance systems required to transfer the rainwater collected on the rooftops to the storage tanks.
  • The issues that must be considered when selecting, sizing, and installing a storage tank. It therefore provides guidance on how to optimise the tank size and maximise system collection efficiency.
  • A synopsis of the various components that comprise a dual system; guidance on how to assess and then mitigate the risks associated with cross-connection and backflow.
  • Guidance on the selection, installation, and management of the appropriate type of overflow-handling system.
  • Financial assessment.

To be continued in future editions of Plumbing Africa.


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