Engineers turn sewage sludge into fuel and feed

With a new solar-powered method that converts sewage sludge into green hydrogen fuel and protein-rich animal feed, researchers have literally turned trash into treasure.

The method recovers more than 90% of the organic carbon in the solid waste and completely removes harmful heavy metals. It is also more efficient and has a smaller environmental footprint than conventional sludge management techniques such as anaerobic digestion, which relies on bacteria breaking down organic waste to produce biogas and nutrient-rich residue.

Sewage sludge is the semi-solid material left over after wastewater treatment. The world produces millions of tons of it every year. In the US, nearly half of the country’s sludge is spread on farmlands as fertilizer, with the rest going to landfill or incinerated. This has raised concerns about harmful chemicals and contaminants leaching from sludge into soil and into water sources.

Researchers at Nanyang Technological University set out to tackle both the waste and environmental issues related to sludge. They devised a three-step solar-powered process that integrates mechanical, chemical, and biological processes to treat the waste material.

Protein for animal feed derived from sewage sludge using a new solar-powered method. Credit: NTU Singapore

As described in the journal Nature Water, the researchers start by first mechanically breaking down the sludge. A chemical treatment step then removes heavy metal contaminants from organic materials such as proteins and carbohydrates.

Next, the researchers use a solar-powered process that relies on special electrodes to transform the organic materials into valuable products. The process gives hydrogen, a clean energy fuel, and acetic acid, a key ingredient for food and pharmaceutical industries. Finally, they use light-activated bacteria to convert nutrients into proteins that are suitable for animal feed.

The “mechano-electro-bioprocess” efficiently concentrates and stabilizes heavy metals, “while simultaneously producing green hydrogen at an impressive efficiency and rate,” the researchers write in their paper. The process converts sunlight to hydrogen energy with an efficiency of about 10%, producing over 13 liters per hour of hydrogen.

The team also did a comprehensive life-cycle and technoeconomic analysis to show that the process brings “substantial environmental and economic benefits”. It reduces carbon emissions by 99.5% and energy use by 99.3% compared with conventional anaerobic digestion.

Source: Hu Zhao et al, Solar-driven sewage sludge electroreforming coupled with biological funnelling to cogenerate green food and hydrogen, Nature Water, 2024.