Literature Review

To have a better understanding of why and how rainwater harvesting systems should be implmented research was conducted to investigate where the techniques of rainwater harvesting come from, how is it defined and what is South Africa doing to implement rainwater harvesting. To further understand why it is important to implement rainwater harvesting, research was carried out to identify what the limitations of freshwater supplies are globally, in South Africa and in Cape Town . Research was also conducted to indentify what the benefits and disadvantages are of rainwater harvesting systems, how rainwater can be collected and what the various components of rainwater harvesting are.

Rainwater harvesting was defined as ‘the collection, storage and use of rainwater on site’ and has been around since 4000 BC. In South Africa rainwater harvesting has mainly been used in areas that lack access to water reticulation systems.

Globally only 0.0008% of the available water on the planet can be utilised by humans. Research by the United Nations Environment Programme (UNEP) (2002, p. 2) has concluded that increased population growth, industrialisation, urbanisation, agriculture intensification and water-intensive lifestyles has resulted in a “global water crisis”. The current supply of freshwater is limited by cost of production and is becoming more and more polluted, which has resulted in water security becoming a major national and regional concern (UNEP, 2002, p. 2).

Compared to global standards South Africa has very scarce and limited water resources. The average rainfall of South Africa is about 450mm which is below the global average of 860mm (RSA GCIS, 2012, p. 657). Variable rainfall distribution and characteristics have resulted in uneven distribution of water resources whereby 60% of river flow comes from 20% of the land area.

If in 2010 the City of Cape Town had a freshwater supply of 556 million m3 and demand of 511 million m3, with a growth rate of 2% per annum it is estimated that by 2014 the demand for freshwater would be higher than the available supply.

There is a wide range of benefits to using rainwater harvest systems. Some of these benefits include:

Providing a free source of water where the only costs are those which the user paid for the system to be installed;
The ability to provide a source of water when there is no other water source available;
The recharge of groundwater supplies using techniques of soil infiltration;
The ability to provide good quality water if the groundwater is contaminated or unacceptable; and
A method of replacing or complementing conventional water sources if their cost of supply is too high. This includes providing a good source of landscape irrigation, water for non-potable and potable indoor uses after the rainwater has undergone the appropriate treatment.

Even though rainwater harvesting has numerous advantages it also has disadvantages some of which include:

The small catchment areas and storage volumes are not always able to cope with the variation of weather patterns. Thus in times of drought the storage tank can dry up;
The maintenance of rainwater harvesting systems and quality of collected rainwater can be difficult for some users to comprehend;
A reduction of income to public water systems due to large scale implementation of rainwater harvesting; and
The lack of clear guidelines for users and developers to follow since rainwater harvesting systems are not usually part of the building codes.

Rainwater can be harvested using 3 techniques which are roof surface runoff, ground surface runoff and groundwater recharge.

The components of a rainwater harvesting system include:

Catchment area: such as roof and ground surface areas;
Conveyance to storage: such as gutters, downpipes, swales and stormwater pipes;
Pretreatment: such as leaf screens, first flush diverters, roof washers and sand filters;
Storage: such as tanks mades from various marterials or ground water;
Delivery System: such as gravity fed systems if the storage is raised or water pumps;
Final Treatment: such as cartridge filters, UV light, Ozone, Membrane filtration, chlorination, boiling and buffereing.