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Algae can clean sewage without electricity or chemicals – we put it to the test in South Africa

Across Africa, many rural communities face a growing sanitation crisis. Wastewater treatment systems, where they exist, are often old, overloaded, or broken. In some towns, untreated sewage flows directly into rivers, contaminating water sources and harming both ecosystems and public health.

For decades, the global response to wastewater has been to clean the water in large wastewater facilities designed to remove physical, chemical and biological contaminants from domestic wastewater (toilets) or industrial effluent. Wastewater plants produce treated water that is safe to discharge into rivers.

But they’re expensive and energy intensive. They’re also difficult to maintain in rural areas where local government doesn’t get much revenue.

I was part of a team of scientists led by environmental management researcher and professor Paul Oberholster, who set out to look for a much simpler and greener solution in a small town in South Africa’s Limpopo province. Our research found that algae – the same green organisms often dismissed as pond scum – could offer a low-cost, low-tech way to clean domestic sewage.

The team inserted tiny microalgae into the ponds at the Motetema Wastewater Treatment Works in Limpopo. The microalgae removed pathogens without using any chemicals or mechanical equipment that runs on electricity. They cleaned up the sewage from 1,560 homes.

This is a sustainable, low-cost approach to wastewater treatment that can improve public health and the environment in small towns, especially those with limited infrastructure and unreliable electricity. And it’s especially important to find ways of cleaning wastewater that don’t cost much or use electricity because climate change increases water stress and energy costs across the continent.

A nature-based approach to wastewater

Nitrogen and phosphorus are nutrients that are found in human waste, detergents, fertilisers, and polluted water that runs off from big farms into rivers. Untreated sewage is a major source of these nutrients. When they build up in rivers or lakes, they trigger the overgrowth of algae. As the algae die and decompose, they use up oxygen in the water. This makes it difficult or impossible for fish and other aquatic life to survive and leads to “dead zones” where almost nothing can live.

Microalgae can prevent this. Their main energy source is light, and they’re able to grow quickly if they have enough. This helps aerobes – the bacteria that survive and grow only in the presence of oxygen in their environment – to break down organic matter, such as human waste, food residues, and other biodegradable substances present in wastewater.

In other words, the algae provide the oxygen, and the bacteria do the heavy lifting of cleaning up the sewage. Together, they create a natural, low-cost treatment system.

This process is not new. Algae have been used in wastewater treatment ponds in southern Africa, the US, Europe, Central Asia and India for decades.

But what sets the Motetema project apart is the use of two fast-growing algae species (Chlorella vulgaris and Chlorella protothecoides), selected (after testing dozens of strains of algae) for their remarkable ability to absorb nutrients like nitrogen and phosphorus from wastewater.

The team wanted to make sure that they’d have enough of the microalgae to clean all the wastewater. So we grew huge amounts in photobioreactors (transparent tanks designed to provide the best light and conditions for algae to grow). Think of a photobioreactor as a high-tech greenhouse for microscopic plants, where light, carbon dioxide and nutrients are carefully controlled to maximise growth.

The team then inserted the microalgae into the town’s wastewater ponds. The results were remarkable. After one year, the treatment system achieved:

99% reduction in ammonia
83% reduction in orthophosphate
73% reduction in total nitrogen.

Ammonia and nitrogen can be toxic to fish and aquatic life. Phosphorus (measured as orthophosphate) fuels harmful algal blooms. So it was great to see that the microalgae cleaned these out.

Overall, between 73% and 99% of key pollutants were removed by the algae within a year. Before this, the treatment ponds often released effluent into the environment. But with phycoremediation in place, the treated water met environmental safety thresholds and could be released without causing harm.

A solution rooted in rural realities

Motetema is a small, rural town of around 11,000 people. The wastewater system there consists of 12 large treatment ponds. One set of six ponds operates while the other is cleaned.

The system treats around 4.5 million litres of domestic sewage per day. But this amount is nearly double what the treatment ponds were originally designed to handle, and the wastewater system was struggling to cope.

Across South Africa, wastewater treatment plants are outdated, underfunded and rarely enlarged to cope with population increases. Many towns already use pond systems for wastewater treatment. These systems often underperform because of electricity cuts, poor maintenance and limited budgets for the necessary chemicals. There is also a lack of skilled operators.

But algae don’t need salaries or power. With the right strains, simple culturing, and periodic injection into the ponds, these can become effective wastewater systems.

Other rural areas could benefit

Phycoremediation needs land as each pond covers nearly 40,000 square metres. That makes it ideal for rural towns with space available. It also takes at least two to three weeks for the algae to clean wastewater. This means the ponds also need to be large enough to hold all the town’s sewage so that it is fully cleaned by the algae.

Like any natural system, phycoremediation is not without its quirks. In Motetema, the project encountered:

an overgrowth of duckweed – a small, fast-growing floating plant often found on stagnant water surfaces. This blocked the sunlight needed for the algae to do their work.
seasonal wildfires, which damaged bioreactor piping
peak-hour sewage surges, which briefly overwhelmed the system
sludge buildup, which reduced the volume of space in the pond and delayed purification of the wastewater.

These are manageable issues, and highlight that these natural ponds will need some support, including adding fresh, healthy algae from time to time and basic maintenance.

What needs to happen next

This model needs support from municipalities and policymakers. Then it could be replicated across thousands of rural settlements in Africa where conventional wastewater treatment plants are too expensive or difficult to maintain.

Phycoremediation also challenges how we think about wastewater. Rather than treating it as a burden, it becomes a resource, something that can be cleaned and reused to support agriculture, fish farming, or even to recharge groundwater.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Yolandi Schoeman, University of the Free State

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The research at Motetema WWTP was funded by the Department of Science and Technology (DST), Council of Scientific and Industrial Research (CSIR), and the Water Research Commission (WRC), South Africa.

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