Waste management is one of the major environmental issues that rapidly growing urban centers have to contend with as a challenge, yet it has opportunities within, especially wastewater and sewage sludge. The growth of cities and industries increases sewage sludge, which is difficult to process and dispose of due to its complex structure, composition, and contaminants such as heavy metals and pathogens.
The United Nations estimates that around 2.5 billion more people will be living in cities by 2050. This statistic, coupled with the UN-Habitat’s estimation of more than 100 million tons of sewage sludge generated globally each year, shows that the amount of waste is likely to increase.
Nairobi’s sewer network covers about 208 square kilometers, which is less than 30% of the city’s total area, highlighting the stark nature of the crisis. However, can cities turn their waste into a resource instead of a burden?
Traditional methods such as incineration or landfill have proven to cause more environmental pollution and are time-consuming and energy-inefficient. To tackle this compounding crisis, researchers from Nanyang Technological University, Singapore (NTU Singapore), have developed an innovative solar-powered method to transform sewage sludge and wastewater into green hydrogen for clean energy and single-cell protein for animal feed.
The researchers discovered that the process is more efficient than conventional methods as it recovers significantly more resources, completely removes heavy metal contaminants, has a smaller environmental footprint, and offers better economic feasibility.
Professor Zhou Yan from NTU’s School of Civil and Environmental Engineering (CEE) and Nanyang Environment and Water Research Institute (NEWRI) said, “Our solar-powered process demonstrates how we can tackle multiple challenges at once, turning a difficult waste product into clean energy and nutritious protein. By integrating mechanical, chemical, and biological approaches, our method has successfully addressed pollution and resource scarcity, offering a new sustainable strategy in wastewater management.”
The process begins by mechanically breaking down the sewage sludge. A chemical treatment separates harmful heavy metals from organic materials, including proteins and carbohydrates.
Next, a solar-powered electrochemical process uses specialized electrodes to transform the organic materials into valuable products, such as acetic acid, a key ingredient for food and pharmaceutical industries, and hydrogen gas, a clean energy source.
Finally, light-activated bacteria are introduced to the processed liquid stream. These bacteria convert nutrients into single-cell protein suitable for animal feed.
Lead researcher Associate Professor Li Hong from NTU’s School of Mechanical and Aerospace Engineering (MAE) and the Energy Research Institute said that the method exemplifies the circular economy and contributes to a greener future.
The solar-powered method achieves an energy efficiency 10 per cent more than the conventional method by generating up to 13 litres of hydrogen per hour, reduces carbon emissions by 99.5 per cent and energy use by 99.3 per cent.
According to Dr. Stefan Diener, an expert in resource recovery from waste, “Emerging technologies that transform fecal sludge in valuable products, such as biofuels or animal feed, present a significant opportunity for sustainable urban waste management.”
He further highlights that these innovations can alleviate the environmental burdens associated with traditional waste disposal methods.
Cities globally are increasingly adopting new technologies in wastewater management and other wastes. Facilities like T-PARK in Hong Kong treat sewage sludge by incineration, generating energy in the process. Moreover, the city is implementing solar energy projects in sewage treatment works to harness renewable energy.
First author, Dr Zhao Hu, Research fellow at School of MAE, said, “We hope that our proposed method shows the viability of managing waste sustainably and shift how sewage sludge is perceived — from waste to a valuable resource that supports clean energy and sustainable food production.”
However, the researchers added that despite the promising findings, more studies are needed to determine if the process can be scaled up. The cost of using an electrochemical process to completely break down organic materials and extract all heavy metals from waste has been highlighted as a key challenge. Moreover, designing a complex system for a wastewater treatment facility adds to the difficulty.
