This South African river has a bacteria count off the charts: A natural filter is cleaning it for watering food gardens

Every day, millions of liters of polluted water flow through many South African rivers and disappear into the sea.

In South Africa, the small Stiebeuel River in Franschhoek, 75 kilometers (47 miles) 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 aging and overstretched infrastructure struggles to cope.

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 a 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.

In a recent research paper, we reported what we discovered in a project where we set up natural water-filtering systems to clean water from the Stiebeuel River. We turned old drying beds, once used to dry sludge from the former wastewater treatment plant at the site, into places to filter the water.

We filled these with small stones, biochar (a wood feedstock exposed to a temperature of 800°C [1,472°F]) and coarse sand. The polluted river water slowly seeps through these filter beds, which remove 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.

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 100 mL. In South Africa, water quality guidelines state that people should not come into contact with water where E. coli levels are higher than 130 counts per 100 mL.

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.

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 supplies.

Water 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-liter (2,642-gallon) tank and then discharge it to two supply tanks. These feed a biofilter (a natural system for 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 liters (13,200 gallons) of contaminated water are treated every day. About 3,000 liters (793 gallons) per day are used to irrigate vegetable gardens 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 enterprises.

The rest is discharged back to the river. This treated water plays an important role because clean water supports 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. Decentralized 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 basic operations are minimal. However, operating the system requires people to monitor 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.

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 decentralized 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.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Citation: This South African river has a bacteria count off the charts: A natural filter is cleaning it for watering food gardens (2026, August 12) retrieved 13 August 2026 from https://phys.org/news/2026-08-south-african-river-bacteria-natural.html

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Reporting from Phys.org contributed to this story.