Researchers Uncover 70% More Efficient Method to Convert Sewage to Biogas

A novel pretreatment technique involving high-pressure,oxygenated steam is proving to significantly increase efficiencies in breaking down conventional sewage sludge into natural gas and residual waste. Photo: Image by kubinger from Pixabay

The resultant fuel can be used for direct production of electricity or as a source of renewable natural gas for transportation and as part of the existing natural gas grid.

Current wastewater treatment techniques typically use anaerobic digestion to break down sewage, a mostly unusable by-product of human civilization. Anerobic digestion methods using microbes break down the waste, with high amounts of biosolids left behind. Those biosolids usually end up being dumped in landfill.

Even under the best conditions, the conventional approaches also require a considerable amount of electricity to make them work. Further, even in the best cases where at least part of the sludge is converted to natural gas, the conversion efficiency is limited to less than 50%.

A team of researchers led by Birgitte Ahring, a professor in Washington State University’s Gene and Linda Voiland School of Chemical Engineering and Bioengineering, decided to look into whether there was anything which could be done to improve those efficiency numbers.

After reviewing the problem, what the researchers came up with was adding a pretreatment step to the biosolid breakdown process. That step involves the injection of high-pressure oxygen-rich steam into sewage sludge which was also raised to higher temperatures prior to it proceeding through the rest of the treatment steps. They found that even small amounts of oxygen added under high-pressure conditions and heat allowed it to act as an efficient catalyst in breaking down far more of the biosolid polymers present at this early stage.

The methodology used in this catalyst-injection step was based on a pretreatment technique the same research group had been investigating for some time. They had already used it to accelerate the breakdown in straw and wood-based materials common to local industries industry. While it worked there, they were unsure until they tried whether it would make as big a difference in biosolid breakdown, mostly because those materials have a much higher percentage of lipids and proteins.

“This is not a very high-tech solution,” said research lead Ahring. But the key to its success was that it is so highly efficient that it “can be useful at a very small scale,” she explained.

Once the pretreatment step was complete, the rest of the wastewater treatment method followed conventional means. With the head start of the steam-injection breakdown pretreatment in place, however, the research team found they were regularly able to produce net biogas conversion rates of 85%. That represents a 70% productivity increase in the process.

Study lead Ahring, who happens to serve as a member of the faculty for the WSU Tri-Cities Bioproducts, Sciences, and Engineering Laboratory, to reflect that the possibilities unleashed by this straightforward innovation could be huge.

“If [communities using this technique] could make their own electricity or for some of the large plants, make renewable natural gas and add it to the natural gas grid,” she said, “then they can reduce the use of fossil fuels. Here we are beginning to move into the idea of the circular economy.”

Ahring’s team at Washington State University is currently partnering with a Richland, Washington, based cleantech startup named Clean-Vantage which has been pioneering other pretreatment technologies, to commercialize it. Together they are coming up with approaches to scale up the process and provide subsystems for even small communities to utilize to release more natural gas from the biosolids in sewar sludge and other wastewater.

Pacific Northwest National Laboratory (PNNL) is doing the financial and technology production modeling analysis to back up the business side of this effort.

A detailed technical paper describing the work of Ahring and her team, “Improved valorization of sewage sludge in the circular economy by anaerobic digestion: Impact of an innovative pretreatment technology,” by Nalok Dutta, et. al., was published in the December 2022 issue of the peer-reviewed journal Waste Management.