A project by Cape Town researchers cleans river water using stones, wood and sand, making it available for irrigating the gardens of a growing community.
Researchers from Cape Town, in South Africa, have discovered a natural way to purify the polluted waters of the Stiebeuel river, turning them into a resource for irrigating the gardens of a community settled along the waterway.
As the Guardian reports, launched in 2018, the project only managed this year to reduce bacterial load and pathogens to levels that allow the water to be used for irrigation, though not for human consumption. The removal of sewage, chemicals and pharmaceuticals takes place without the use of any chemical products, relying instead on stones, thermally treated wood and sand, along with renewable energy. The system removes around 99 per cent of toxins and is capable of purifying 36,000 litres of water per day. Most of it is returned to the river, while a portion is retained for irrigation.
"We take water from the river and treat it daily; about 3,000 litres are used to grow vegetables to feed children and people in need who live in the Water Hub informal settlement in Langrug, near Stellenbosch, in the Western Cape province," explains Kevin Winter, one of the project's researchers. "It would be really hard to tell it apart from water treated by the municipal network — the kind that goes through a treatment plant and is chlorinated."
Since 2006, the population of Langrug has grown from 3,000 to over 26,000 inhabitants, a phenomenon driven primarily by demand for seasonal labour on farms and in the affluent town of Franschhoek. This growth has left residents without access to clean water or sewage facilities, and local water infrastructure has reached the limit of its operational capacity.
The researchers worked on an abandoned wastewater treatment plant, transforming the old drying beds into a network of filtration cells. The plant is off-grid and uses solar energy to pump water from the river into the cells, where it is slowly filtered over the course of five days. Large stones are used to trap particles and chemicals; next, a layer of biochar — wood carbonised at 800°C — filters out traces of pharmaceuticals, drug residues and ammonia; finally, layers of smaller stones and sand absorb any remaining toxins and pollutants.
The filtration system is not designed to replace traditional large-scale treatment plants serving towns and cities, but it is capable of providing clean water to rural communities. For the researchers, over the next 30 years the climate will change and it will be necessary to cope with prolonged periods of drought, marked by food scarcity and limited access to water. Ensuring water availability for food production and food security, as part of a climate change adaptation strategy, is of crucial importance at a global level. And this project could serve as a model.
