Harvesting rainwater can enable industrial companies with sufficient roof space to dramatically reduce their use of potable water for nonpotable applications, easing the strain on municipalities that have to provide ever-increasing volumes of potable water, says specialist precast concrete products manufacturer Rocla.

Rocla is a division of infrastructure and mining products group the Infrastructure Specialist Group (ISG).

ISG sales engineer Justin Kretzmar says the vast use of potable water in South Africa for applications where nonpotable water can be used is worrisome, with many water conservation groups and water engineering firms warning that the country, with its vast majority of arid regions, might soon face water shortages and, by implication, water restrictions. Water rationing has already been implemented in KwaZulu-Natal.

Engineering consulting firm Gibb held a roundtable discussion about water resources last month, during which it suggested that water shortages constituted a looming problem for South Africa, owing to poorly maintained infrastructure resulting in water being wasted.

However, harvesting and storing rainwater in concrete tanks can provide nonpotable, but suitable, water for applications such as washing garbage bins, watering gardens, flushing toilets and laundry services, Kretzmar says.

Municipalities can also use harvested rainwater to water public parks, in street-sweeping applications and for firefighting.

Kretzmar says concrete tanks are the preferred water storage option, as opposed to plastic tanks, owing to the below-ground concrete tanks’ low temperatures and lack of light, which inhibit the growth of bacteria. Lime in concrete tanks also helps to neutralise the acidity of rainwater.

Water Harvesting Examples
Kretzmar says the Moses Mabida stadium, in KwaZulu-Natal, is a noteworthy example of rainwater harvesting, with rain that falls onto the roof of the stadium being collected and used to irrigate the fields.

He notes that some developed nations have embraced water harvesting, with significant benefits. The Australian city of Sydney harvests rainwater from the bulk of the city’s stormwater drainage network, storing the water in large dams and tanks for reuse.

This makes the city more environment friendly as less water is wasted. In general, using harvested stormwater also eases the strain on existing infrastructure, such as treatment plants and pumps, and reduces maintenance, owing to lighter workloads or greater distribution of existing supply capacity.

About 70 projects across the Sydney region are collecting and reusing stormwater. These projects collect more than 1.3-billion litres of water a year, equal to the amount of water contained in 520 Olympic-sized swimming pools.

Further, the evacuation of stormwater into canals or rivers becomes less intensive, owing to the stormwater being stored in purpose-built reservoirs when harvesting rainwater. Subsequently, the risks associated with flash flooding are also mitigated.

Rainfall Attenuation
Another solution to prevent flash flooding and water pooling in urban areas during heavy rainfall is water attenuation.

Increasing urbanisation and paved surfaces, increase the likelihood of flash flooding, owing to stormwater having limited access to seep into underground water tables and rivers through underground means.

Kretzmar notes that a natural area, such as the bushveld, allows for only 2% to 3% runoff when it rains, while the rest of the stormwater seeps through the grass and sand. This is good because it prevents soil and riverbank erosion caused by large volumes of water.

“Once an area has been developed, with buildings that have large roofs and paved and/or surfaced areas, the amount of runoff drastically increases – in some cases by as much as 75%,” he says. This puts strain on stormwater systems and leads to the banks of rivers overflowing and water in dams overflowing.

For this purpose, Rocla provides a number of solutions, including attenuation tanks and permeable paving.

Rocla’s water attenuation tanks are temporary storage tanks that collect rainfall and slowly release the stormwater into the environment of developed areas in a way that is similar to predevelopment levels of release.

The tanks have a large inlet to allow for large volumes of water to enter, with a significantly smaller outlet to restrict and moderate the flow of water into the environment. This allows the discharged water time to seep into the ground without breaking rivers’ and dams’ banks or flooding low-lying areas.

Meanwhile, concrete products company Technicrete’s (also a division of ISG) permeable paving allows for rainfall to seep into the ground, without any compromises of the load-carrying capability of a typical brick.

The paving contains widened joints, which enables the water to enter and, subsequently, find its way into the natural underground water tables. This prevents runoff and dramatically improves the drying time of a paved area after rain.

Shopping centres and taxi ranks can especially benefit from permeable paving as shoppers, pedestrians and passengers can walk on damp paving without having to navigate pools of water that accumulate in low-lying areas of normal paved surfaces.

Kretzmar says the uptake of Rocla’s rainwater-harvesting and attenuation solutions has received some support from industrial clients wanting to improve their green credentials.

He notes that these clients are pursuing these measures on their own accord, with little to no pressure from government on industrial companies to install rainwater-harvesting or attenuation tanks.

Kretzmar adds that municipal authorities do stipulate water attenuation requirements, but says that, in recent instances, the engineers and clients are keen to move away from attenuation dams and, alternatively, towards more elegant and effective systems.