Weekend Argus (Saturday Edition)
Calls to rethink rainwater harvesting
Cities can’t rely on accustomed weather patterns for scant supplies
NOT A drop of rainwater falling on Cape Town should enter the storm-water system and be discharged into the sea and every drop of rainwater thus saved must be used twice.
If this is not the summary of a new official injunction – as Cape Town’s water supply falls steadily beyond the most critical levels experienced in a century – it should be.
So says Llewellyn van Wyk, former Cape Town councillor and architect, and now the principal researcher at the Built Environment Unit of the Council for Scientific and Industrial Research (CSIR) in Pretoria.
As cities grow, and demand for resources such as water mount, Van Wyk argues, urban planners are challenged to rethink water catchment and, sooner rather than later, effect dramatic changes.
No more can cities rely on accustomed patterns of weather, or customary programmes of dam-building, to meet the need for freshwater, one of the globe’s scarcest resources.
The argument for a new approach was set out in a chapter Van Wyk wrote for the 2015 Green Infrastructure Handbook.
He said this week: “There are two objectives.
“The first is that not a drop of rainwater falling on the urban area should enter the storm-water system and be discharged to the sea.
“This requires a threetiered system of water catchment: rainwater harvesting off all roofs; rainwater harvesting off all paved surfaces, and rainwater harvesting off all open spaces.
“This water can be stored in a system comprising rainwater tanks, and controlled recharging of small and large aquifers (natural underground reservoirs).
“The second objective is that every drop of rainwater must be used twice.
“South Africa’s national rainwater average is 464mm, which is well below the global average of 860mm.
“However, reusing the water once takes the average to above the global average.”
New technologies were available – taking toilets “off the grid”, and reusing grey water for non- potable, or non-drinking, purposes – for general application in the building sector.
Both objectives, however, were predicated on adopting a wholly new approach to water, the subject of Van Wyk’s chapter in the Green Infrastructure Handbook.
It points out that dam build- loop must be closed by minimising disposal of surface water to the sea; multiple use must be made of available water through recycling and reuse, and demand must be reduced through the application of water-efficient and water-free fixtures, including sanitation systems”.
The second key eco-engineering option was harvesting rainwater, recognising rooftops across the city as “catchment areas”.
Rainwater harvesting technologies were simple to install and operate and could be undertaken and managed by the local community with local materials.
“Rainwater harvesting is convenient as water is stored at the point of use with the owner of the system having full control over how and when it gets used thereby reducing operation and maintenance costs.
“The range of collection devices has a lower environmental impact than the traditional collection devices such as dams and reservoirs. Rainwater harvesting can provide a continuous source of water supply especially for the poor and rural populations as water collection and storage capacity can be adjusted to suit the local climatic conditions and programmatic requirements.”