Turning (More) Fat and Sewage Into Natural Gas

Anaerobic digesters, like those pictured here, can be used to convert sewage sludge and fatty waste into natural gas. (Image: Rachel Schowalter. Shared by the Massachusetts Clean Energy Center under a Creative Commons license)
Anaerobic digesters, like those pictured here, can be used to convert sewage sludge and fatty waste into natural gas. (Image: Rachel Schowalter. Shared by the Massachusetts Clean Energy Center under a Creative Commons license)

North Carolina State University researchers have developed what is, to date, the most efficient means of converting sewage sludge and restaurant grease into methane. After treating sewage, wastewater treatment plants are left with solid sludge, called biosolids.

For years, utilities have treated biosolids with microbes that produce methane. In recent years, utilities have been adding grease interceptor waste (GIW) into the mix. Grease interceptors are used to trap fat, oil, and grease from food service establishments so that they don’t clog up sewers.

(Image: Screenshot / YouTube)

For years, utilities have treated biosolids with microbes that produce methane. (Image: Screenshot / YouTube)

By adding GIW in with their biosolids, utilities can produce more methane, making the entire operation more efficient. But there are challenges. Francis de los Reyes, a professor of civil, construction, and environmental engineering at NC State and lead author of a paper on the work, said:

The researchers determined that increasing the amount of GIW they fed into the digester a little at a time allowed them to increase the amount of GIW in the mix to the point where it made up 75 percent of the overall volatile solids, or feedstock. De los Reyes said:

This allowed the researchers to achieve the highest methane yield reported to date for lipid-rich waste: 0.785 liters of methane per gram of volatile solids put into the digester. De los Reyes said:

The researchers were also able to identify a suite of microbes that appear to be particularly important in converting lipid-rich waste into methane. The researchers are following up with studies on other types of food waste, such as meat and fruit/vegetable waste.

(Image: Screenshot / YouTube)

The researchers were also able to identify a suite of microbes that appear to be particularly important in converting lipid-rich waste into methane. (Image: Screenshot / YouTube)

They are also looking at fundamental microbial ecological theories to explain how the needed microbial species come to dominate and persist in the ecosystems found inside the waste digesters. The paper, Increased loading stress leads to convergence of microbial communities and high methane yields in adapted anaerobic co-digesters, was published in the journal Water Research.

Provided by: North Carolina State University [Note: Materials may be edited for content and length.]

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