Waste is everywhere. All the cities in the world invariably produce sewage sludge — the thick, organic mess left over from wastewater treatment. This sludge is piling up faster than ever. More than 100 million tons of this dry sludge accumulate globally every year, clogging treatment facilities and costing billions to process.

But what if this waste wasn’t just waste? What if it could be food or fuel? Scientists have just developed a system that does exactly that. In a breakthrough study published in Nature Water, researchers have created a solar-powered process that turns sewage sludge into two valuable resources: single-cell protein, which can be used as animal feed, and green hydrogen, a clean fuel.

Sludge: A Hidden Goldmine

We’re not talking about finding actual gold in sludge (though that’s also a worthwhile idea), but about turning sludge into something useful.

Yes, sewage sludge, the sticky, smelly byproduct of wastewater treatment plants is packed with useful things. It has organic matter, nutrients like nitrogen and phosphorus, and a mix of heavy metals. Currently, this causes a problem: it’s too thick to break down quickly and too contaminated to use easily. Burning it releases toxic chemicals, and composting it takes ages.

This is why the new study comes in so clutch. The team of researchers led by Nanyang Technological University in Singapore figured out a way to take sewage sludge and break it down into useful components with almost zero waste. It’s not a simple process, it uses a combination of mechanical grinding, electrochemical reactions, and bacteria; but with this approach they recover nearly all of the carbon and nutrients, transforming them into valuable products.

The process works in three stages.

The first step takes care of the sludge’s complex mechanical structure. Scientists put the sludge through an intense grinding process with a bit of alkaline catalyst (like potassium hydroxide). This shreds the sludge at the molecular level, making it easier to extract useful components while also trapping heavy metals in solid form for safe disposal.

Then, the dissolved organic material is sent through an electrolysis system powered by renewable solar energy. This is where much of the magic happens. Instead of simply breaking down the sludge, the system turns it into something useful. At the anode, the organic matter undergoes electrochemical oxidation, producing volatile fatty acids — especially acetic acid, a key building block for microbial food production. At the cathode, water is split to generate green hydrogen — a clean energy source that could help power industries or fuel vehicles.

Lastly, the acetic acid and other resulting components are fed to microbes (specifically, purple phototrophic bacteria). These bacteria turn the acid into protein that can then be used for animal feed.

The Numbers Are Impressive

It’s not the first time the idea of using sludge for something useful has been touted. But in this case, the results seem striking. Over 91% of the total organic carbon in the sludge is recovered and converted into useful products, and 63% of the carbon is turned into single-cell protein. CO₂ emissions are slashed by 99.5% compared to conventional techniques of processing sludge, like anaerobic digestion.

Of course, the big question is can this be scaled up? First author Dr Zhao Hu believes so. “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.”

The technology is designed to be modular and adaptable. Solar-powered electrolysis and microbial fermentation can be integrated into existing wastewater treatment plants, reducing the need for major infrastructure overhauls. It can fit into smaller or bigger cities.

As urbanization is skyrocketing, and with it, the amount of sewage sludge we produce, this could be a game changer. Most of this sludge is either dumped in landfills, burned, or left to rot in treatment plants. This new method transforms waste into resources, reducing environmental burdens while closing the loop on waste production.

If successful at scale, this innovation could transform the way cities handle their waste — turning sewage into a sustainable resource rather than a growing liability. The future is clear: waste is not the enemy. It’s the key to a cleaner, greener, and more sustainable world.

Journal Reference: “Solar-driven sewage sludge electroreforming coupled with biological funnelling to cogenerate green food and hydrogen” by Hu Zhao, Ziying Sun, Chenchen Li, Dan Wu, Li Quan Lee, Dan Lu, Yunbo Lv, Xiang Chu, Ying Li, Wenguang Tu, Ovi Lian Ding, Jin Zhou, Zhigang Zou, Yan Zhou and Hong Li, 1 November 2024, Nature Water. DOI: 10.1038/s44221-024-00329-z