New technique radically boosts biogas yields from sewage sludge

Could one man's trash truly be another's treasure? Well, scientists have unveiled a technique that dramatically increases fuel extraction from one of humanity's most abundant byproducts – sewage sludge, a.k.a. poop – while cutting disposal costs.

The researchers from Washington State University say they have developed a pretreatment step and a novel bacterial strain that together can triple the amount of extracted biogas from biowaste water, while slashing disposal costs by half.

According to the study published in the Chemical Engineering Journal, wastewater treatment facilities account for 3% to 4% of total electricity consumption in the United States. For context, the US Energy Information Administration reports that electric vehicle charging accounted for roughly 0.5% of total US electricity consumption in 2025! Conventional treatment processes also add about 21 million metric tons of greenhouse gases to the atmosphere annually.

The study notes that roughly half of the United States' nearly 15,000 wastewater treatment facilities use anaerobic digestion, a process in which microbes break down sewage sludge in oxygen-free conditions to reduce waste volume while producing biogas. Some facilities use this biogas to offset part of their electricity demand, helping lower the massive energy consumption associated with wastewater treatment.

However, conventional anaerobic digestion remains relatively inefficient, with the process typically converting less than 40% of the sludge's carbon into usable gas. Large amounts of carbon-rich residual sludge, known as biosolids, remain after digestion and often end up in landfills, where they can generate additional greenhouse gas emissions. Meanwhile, the biogas itself contains large amounts of carbon dioxide – often around 35% to 40% – reducing its direct usefulness as a fuel and requiring additional upgrading before it can be injected into natural gas infrastructure as renewable natural gas.

To address these limitations, the researchers developed a two-stage system called the Advanced Pretreatment and Anaerobic Digestion (APAD) process. Rather than discarding the residual sludge left behind after conventional anaerobic digestion, the system subjects it to an additional treatment stage known as Advanced Wet Oxidation and Steam Explosion (AWOEx).

During this process, the digested sludge is exposed to high temperatures, pressure, and controlled amounts of oxygen, helping to break apart resistant organic compounds that conventional digestion struggles to fully decompose. The treated sludge is then fed into a secondary anaerobic digestion stage, allowing microbes to extract additional biogas from material that would otherwise remain as waste.

In a separate downstream stage, the resulting biogas is upgraded into higher-purity renewable natural gas using a trickle-bed bioreactor containing a novel methanogenic bacteria strain, Methanothermobacter wolfeii BSEL (we know you didn't pronounce that). Fed with hydrogen, the microorganism biologically converts carbon dioxide in the biogas into additional methane, increasing the fuel quality of the final gas stream.

“This (bacterial strain) bug doesn't need anything – it is a workhorse,” says Birgitte Ahring, WSU professor and corresponding author on the paper. “It doesn't need organic additives or a lot of nursing. It does well with water and a vitamin pill.”

The researchers report substantial gains from the system. By extracting far more usable material from sludge that would normally remain as waste and by converting much of the carbon dioxide in the resulting biogas into additional methane, the integrated process increased renewable natural gas output by 200% compared to conventional anaerobic digestion alone.

The system also achieved an overall carbon conversion efficiency of 83%, meaning a much larger portion of the sludge's carbon was converted into usable fuel rather than remaining trapped in residual waste. The researchers additionally report that the pretreatment stage reduced sludge treatment costs from $494 to $253 per dry ton by significantly lowering the volume of leftover biosolids requiring disposal. Smells like money.

“This technology basically converts up to 80% of the sewage sludge into something valuable. If we can replicate this work on other organic materials, we'll have a waste treatment technology that is world-class when it comes to efficiency,” says Ahring.

The team has patented the bacteria strain and is now looking to develop a larger-scale version of the project with an industrial partner.

Technically speaking, if successfully scaled nationally and if the underlying economics hold up in real-world deployment – among many other “ifs” – the researchers' modeling suggests the pretreatment portion of the system could potentially translate into billions of dollars in annual savings across US sewage sludge treatment operations. The work was funded by the US Department of Energy Bioenergy Technologies Office, a clear indication that the researchers are definitely onto something.

Source: Washington State University