Ученые в Южной Африке создали природную систему очистки сточных вод, удаляющую 99% бактерий

Every day, millions of litres of polluted water flow through many South African rivers and disappear into the sea. In South Africa, the small Stiebeuel River in Franschhoek, about 75 kilometres east of Cape Town, is badly polluted. Sewage and polluted rainwater run into the river because of inadequate sanitation and drainage systems serving the nearby informal settlement of Langrug. About 26,000 people there live in densely populated housing, where ageing and overstretched infrastructure struggles to cope. Read more: How cities can work with nature when droughts take their toll I’m a water researcher at the University of Cape Town’s Future Water Institute and research director at an off-campus research facility called the Water Hub, in Franschhoek. I’m part of a group of water researchers who are working more closely with nature – and learning from it – as a means of treating and reusing water. The research at the Water Hub takes place at an old disused wastewater treatment plant. We are developing this research site into a living laboratory to demonstrate ways of turning highly contaminated water into a safe resource without the addition of chemicals. Read more: Crisis proofing South Africa’s water security In a recent research paper we set out what we discovered in a project where we set up natural water filtering systems to clean the water from the Stiebeuel River. We turned old drying beds, once used to dry sludge from the former wastewater treatement plant at the site, into places to filter the water. We filled these with small stones, biochar (a wood feedstock exposed to temperature of 800 degrees Celsius) and coarse sand. The polluted river water slowly seeps through these filter beds, which removes pollutants such as bacteria and nutrients. After about five days, the water is clean enough to use for irrigation. Our research found this to be very effective. The naturally treated water consistently contained no E. coli bacteria, showing it was no longer contaminated by human or animal waste. It also removed 85% to 95% of the ammonia and phosphorus that pollute rivers and dams. These pollutants are trapped in the filter beds, where naturally occurring microbes feed on and break down much of the pollution. The plants also absorb excess nutrients such as ammonia and phosphorus, helping to clean the water before it flows back into the river or is used for irrigation. Read more: South Africa’s ability to manage wastewater is collapsing: what’s at risk Nature-based systems cannot entirely replace properly functioning wastewater treatment works because of the large volume of water coming from towns and cities. But the findings matter because they show that nature-based water treatment systems that don’t need electricity or expensive equipment can help municipalities that struggle with rising electricity costs, deteriorating infrastructure and limited technical skills. Nature-based wastewater treatment can help communities make better use of limited water supplies. In places affected by drought and climate change, it provides a local source of treated water that can be reused for farming and other purposes. Nature-based treatment of polluted water at the Water Hub The Stiebeuel river that we studied is so polluted that local residents cannot use it anymore. Once a place for bathing in its upper reaches, it has become a dumping ground for solid waste. The river also carries excessive E. coli bacteria. Its E. coli levels are higher than 400,000 colony forming units per 100ml. In South Africa, water quality guidelines say that people should not come into contact with water where E. coli levels are higher 130 counts per 100ml. It also contains high levels of ammonia and phosphorus from household cleaning agents which can trigger large algal blooms. These turn lakes and slow-moving rivers green and starve them of oxygen. On top of this, microplastics and traces of medicines have been found. Read more: Water quality in South Africa: reports show what needs to be fixed, and at what cost The result is a slow but relentless decline in water quality, an increase in treatment costs for the municipality and growing pressure on already scarce freshwater supply. Water that has been polluted by sewage is usually treated by energy-intensive methods, including chemical disinfection. The large-scale biofiltration system that we built doesn’t require this. We pump water from the Stiebeuel River into a 10,000 litre tank and then discharge it to two supply tanks. These feed a biofilter (a natural system of filtering water). The design we’ve used copies the way that healthy wetlands naturally cleanse water. Our research has found that the system: - consistently reduces ammonia and phosphate concentrations by up to 90% - removes 99% of harmful bacteria - achieves over 90% removal of some pharmaceutical compounds and microplastics. Approximately 50,000 litres of contaminated water are treated every day. About 3,000 litres per day are used to irrigate vegetable gardens that are managed by young women from the informal settlement. These gardens provide opportunities for skills development, experience in food production and the development of small-scale enterprise. The rest is discharged back to the river. It plays an important role as clean water gives life support to river systems and plant life so that in turn, they clean the water that flows downstream. What needs to happen next The Water Hub shows how wastewater can be cleaned and reused close to where it is produced, instead of relying on large, central treatment plants. This local approach can be copied in other communities. Decentralising water treatment makes it easier to improve water quality in areas where service delivery by municipalities is collapsing. It is relatively inexpensive to set up a nature-based system like this. The maintenance costs and skills required for the basic operations are minimal. However, operating the system needs people to monitor the water quality regularly and make changes when necessary, such as slowing or speeding up the flow of water or responding to changes in weather or pollution levels. Read more: Wetlands are superheroes: expert sets out how they protect people and places Our research shows that combining science, engineering and community participation can lead to productive assets capable of supporting food production, creating livelihoods and strengthening water security. It shows how decentralised water infrastructure can also support livelihoods at the same time, and that wastewater no longer needs to be seen as just waste but as a valuable resource to be recovered and reused. In African countries, where clean, safe water is becoming one of our most precious resources, learning to work with nature rather than against it may prove to be one of the smartest investments we can make.