Microalgae Can Clean Domestic Wastewater Without Chemicals or Electricity: Study

Scientists have discovered that the tiny green organisms often referred to as “pond scum” offer a low-tech, low-cost method of cleaning domestic sewage in place of expensive wastewater treatment facilities.

All over the world, water is cleaned using large wastewater plants designed to remove chemical, biological and physical contaminants from domestic wastewater or industrial effluent. The water produced by these facilities can then safely be discharged into rivers. However, the energy needed to operate these treatment plants makes them expensive and hard to maintain for rural governments without much revenue.

“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,” wrote co-author of the study Yolandi Schoeman, an ecological engineer and post-doctoral fellow at University of the Free State, in The Conversation.

South Africa’s rural wastewater treatment plants release polluted water if they aren’t maintained properly. Microalgae that purify the water could be a solution. theconversation.com/algae-can-cl… #EnvironmentAndEnergy

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In search of a simpler, greener solution to wastewater treatment, the research team traveled to South Africa’s Limpopo province, where they inserted the microalgae into ponds at the Motetema Wastewater Treatment Works. The algae successfully removed pathogens and cleaned the sewage of 1,560 homes without the use of mechanical equipment or chemicals.

“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,” Schoeman wrote.

Phosphorus and nitrogen are found in human waste, fertilizers, detergents and polluted water runoff from big farms that end up in waterways. When these nutrients build up in lakes or rivers, an overgrowth of algae develops. As the algae die off and decompose, they deplete the water’s oxygen, making it hard or impossible for aquatic life to survive and creating “dead zones.”

Microalgae prevent this by creating oxygen through photosynthesis, which helps aerobes — bacteria who need oxygen to grow and survive — break down organic matter like food residues, human waste and other biodegradable substances found in wastewater, creating a natural and inexpensive water 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,” Schoeman explained. “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.”

In order to make sure the team had enough microalgae, they grew large amounts in transparent tanks called photobioreactors. They then placed the microalgae into the community’s wastewater ponds.

After one year, the natural treatment system — called phycoremediation — resulted in a 99 percent reduction in ammonia, 83 percent less orthophosphate and a 73 percent reduction in nitrogen.

Before the treatment, effluent was frequently released into the environment by the ponds. But the treated water not only met environmental safety standards, it could be released without harm to the community.

Motetema is a rural town of approximately 11,000 people. Its wastewater system consists of a dozen large treatment ponds, six of which remain in operation while the others are cleaned.

The town’s system treated roughly 1.19 million gallons of domestic sewage daily — almost twice what the ponds were designed to handle — putting a strain on the wastewater system.

“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,” Schoeman wrote. “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.”

Phycoremediation requires land and time to work, as each treatment pond covers almost 430,556 square feet, and the algae need a minimum of two to three weeks to clean wastewater.

With support from policymakers and municipalities, the model could be replicated and used across thousands of settlements in rural Africa, where conventional treatment plants are too difficult or expensive 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,” Schoeman wrote.

The study, “Is Africa Ready to Use Phycoremediation to Treat Domestic Wastewater as an Alternative Natural Base Solution? A Case Study,” was published in the journal Phycology.