Steam. Pressure. Oxygen. And Presto — More Biogas.

Pretreatment of biosolids with steam and pressure shows promise for boosting biogas production and cutting solids volume at clean-water plants

Steam. Pressure. Oxygen. And Presto — More Biogas.

Birgitte Ahring, Ph.D., with the biodigesters used in her pilot project for pretreating wastewater treatment solids with steam and pressure.

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A pilot project significantly boosted biogas yield by pretreating biosolids with steam, pressure and oxygen before anaerobic digestion.

The process converted 80-85% of the organic material to biogas, versus the typical 50% without pretreatment, says Birgitte Ahring, Ph.D., a professor and biofuels researcher in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering at Washington State University – Tri Cities.

“What is new in this study is that we took the biosolids and exposed it to high temperature and pressure,” Ahring says. “By doing that we made it far more available for digestion. This is a short process that happens in a pressure vessel.”

In the pretreatment, the material is heated with steam to 165-190 degrees C (329-374 degrees F) in a pressure vessel that reaches 10-13 bar (145-188 psi) for 10 to 30 minutes.

More gas, less volume

The steam and pressure break down cell walls and large molecules. “Sludge under these conditions starts reacting,” Ahring says. “Some of the reactions are by hydrolysis. It breaks up the material into smaller pieces, and parts of it react with the oxygen to hydrolyze even more. You open the materials that are recalcitrant, meaning difficult to digest.”

The pretreatment has two key benefits. It produces more biogas for use as fuel. It also reduces the volume of undigested biosolids, which could minimize resources needed for dewatering, transportation and beneficial use or landfilling.

Ahring has conducted the project in a former auto repair facility. Sludge is delivered in buckets from the Walla Walla wastewater treatment facility. Pretreatment occurs in a steel vessel that is built into one of the pits originally used for oil changes. After pretreatment, the material is pumped to a storage tank and then to two 400-liter digesters.

Although pretreatment kills the bacteria in the material, Ahring says that is not a problem because the digesters are fed continuously with small portions, ensuring an overall retention time of days where the active microbes can break down the organics and produce biogas. “It’s like when you have a sourdough starter,” she says. “Something is in the bioreactor, so the sludge doesn’t need to have microbes in it. There are plenty of microbes in the bioreactor to do the work.”

Food waste, too

Wastewater treatment plants that take in food waste to boost methane production might also benefit from the pretreatment process.

“It depends on the food waste,” Ahring says. “If you’re dealing with the peels from potatoes or with all the residues from cut veggies, then it is very, very good to pretreat. If you’re dealing with materials from meals, you don’t need to pretreat it. But one thing we know from this process is that it improves all the polymers in the sludge, and that includes lipids and proteins.”

Based on good results from the pilot project, Ahring began working with Clean Vantage, a company looking to develop the commercial potential of the process, and with the city of Walla Walla, which wants to put the technology to work in an operating wastewater treatment plant: “We are working with them on getting the design ready for implementation.”

Rooted in renewables

The pretreatment process is rooted in biofuels research that has been conducted at Washington State for many years. The process was found to be helpful in converting materials such as straw and wood chips to aviation fuels and cellulosic ethanol.

“That’s what we started on,” Ahring says. “I came in 2008, and we worked a lot on these types of materials. That’s actually what showed this pretreatment technology was successful. But we were making liquid transportation fuel. When we went to sewage sludge, we were surprised to see that it also works very well on that.”

Although the pretreatment uses energy and adds an extra biosolids treatment step, Ahring thinks the cost of pretreatment will be more than offset by more methane production and lower biosolids volume: “The techno-economics show that it’s really beneficial. Those two things make up a really good business case.”

The next phase of the research is to install pretreatment equipment at an operating plant. If that goes as planned, Ahring sees big potential for the widespread use of the technology: “What I see is there’s a huge market here.”


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