A pilot study of a new method for treating sewage sludge from a wastewater treatment plant efficiently created renewable natural gas while reducing the cost of the treatment.
Wastewater treatment facilities use large amounts of electricity to clean up municipal wastewater, using between 3% and 4% of the total electricity demand in the U.S. Their treatment processes also contribute to global warming, adding about 21 million metric tons of greenhouse gases to the atmosphere annually.
About half of the approximately 15,000 wastewater treatment plants in the U.S. use anaerobic digestion to reduce sewage waste and make biogas but the process, in which microbes break down the waste, is inefficient and struggles to break down all the complex molecules in the sludge. The biogas composed of carbon dioxide and methane has limited use, and the leftover sludge, called biosolids, most often ends up in landfills.
In their study, published in Chemical Engineering Journal, the researchers added a pretreatment step, treating the sludge at high temperature and pressure with oxygen added before the anaerobic digestion process. The small amount of oxygen under high-pressure conditions acts as a catalyst to break down the long polymer chains in the material. The researchers showed that their pretreatment resulted in reduced cost to treat the sewage from $494 to $253 per ton of dry solids.
The team then used a novel bacterial strain they discovered and isolated to upgrade the biogas, converting carbon dioxide with hydrogen into methane or renewable natural gas. The researchers analyzed and verified the renewable gas, showing that it was 99% pure methane.
This work could help communities sustainably clean up waste while generating renewable natural gas for their energy needs.
The researchers have patented the bacterial strain. They are now working with an industrial partner to develop a larger scale project.
Data from Washington State University