RNG Directly from Pre-Treated Sewage: New Method Offers Significantly More Efficient Anaerobic Digestion, Cuts Treatment Costs, Reduces CO Emissions, and Produces More RNG

     Researchers at Washington State University recently published in the Chemical Engineering Journal about a new bioreactor that improves the efficiency of converting raw sewage to biogas and processes it into renewable natural gas (RNG). Good News Network reports the very good results obtained when pre-treating the sewage sludge:

“When the researchers pretreated sludge collected from a nearby wastewater facility, they produced 200% more renewable natural gas compared to current practices—and cut the cost of disposal by nearly 50%.”

“This technology basically converts up to 80% of the sewage sludge into something valuable,” said Professor Birgitte Ahring of WSU’s School of Chemical Engineering and Bioengineering, and one of the authors of the paper.

     They explain that wastewater treatment is a major consumer of electricity, making up 3 to 4% of the electricity used in the U.S.  

“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.”

     For their study, the researchers utilized pretreatment of the sewage 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.” That pretreatment step resulted in lowering the cost of sewage treatment from $494 to $253 per ton of dry solids.

     After pretreatment, the researchers utilized a novel bacterial strain and hydrogen to convert CO2 to methane. The result was a gas of high purity at 99% methane. They patented the bacterial strain and are now working on developing a larger demonstration pilot project.

“This approach not only enhances carbon conversion efficiency and methane yield but also enables direct production of pipeline-quality renewable natural gas with minimal CO2 content — addressing two major limitations of existing sludge-to-energy systems into a single, scalable methodology,” said Ahring.

“By successfully bridging advanced pretreatment with biological biogas upgrading, this work provides a new, integrated paradigm for sustainable sludge treatment maximizing energy recovery while contributing to the circular bio-economy.”

     As the abstract notes, the Advanced Pretreatment and Anaerobic Digestion (APAD) processes resulted in increasing the carbon conversion efficiency to 83%, and the RNG output increased by a whopping 200%. 

     CO2 often makes up 35-40% of the biogas stream. Conventional anaerobic digestion pretreatment results in a mere 40% carbon conversion efficiency, so this is a huge improvement. If the process can be perfected and commercialized, it could have huge implications for reducing the costs of wastewater treatment and RNG production at these plants. Of course, it would also cut the carbon emissions of these facilities significantly in two ways. One way is by reducing the energy required for treatment. The other is by converting the CO2 to methane instead of flaring it or venting it into the atmosphere.  

“The biogas yield from conventional anaerobic digestion is often insufficient to justify energy recovery investments, resulting in routine flaring. WWTFs that operate AD systems and want to add biogas to the natural gas grid must first purify the biogas, removing CO2 (35–40%) to meet quality standards for renewable natural gas (RNG). Increased conversion of sludge solids into biogas will reduce the disposal burden of biosolids. Additionally, improving the quality of the biogas will avoid flaring it. If the biogas is not used for electricity production, then it can be upgraded to RNG and injected into the natural gas grid.”

     The schematic and flow chart below show the basic process of the APAD processes.

     The process involves taking dewatered anaerobically digested sewage sludge (DADSS) and pretreating it in the bioreactor, which is shown below. The process is known as Advanced Wet oxidation & Steam Explosion pretreatment (AWOEx). The biogas is upgraded to RNG in a trickle-bed bioreactor. Both processes are combined in the same bioreactor.

     Some figures from the paper are given below showing these stellar results.

     If this process is scaled up, it could result in great savings for wastewater treatment plants, increased RNG production, and a huge reduction in the carbon footprint of the plants.

     The conclusion of the paper, given below, summarizes the results.

 

 References:

 

Researchers Develop Way to Get Natural Gas That’s Renewable Directly From Sewage. Good News Network. April 25, 2026. Researchers Develop Way to Get Natural Gas That’s Renewable Directly From Sewage

Improving anaerobic digestion of sewage sludge to renewable natural gas by the Advanced Pretreatment & Anaerobic Digestion technology (APAD): Pilot testing. Birgitte K. Ahring, Fuad Ale Enriquez, Muhammad Usman Khan, Peter Valdez, Francesca Pierobon, Timothy E. Seiple, and Richard Garrison. Chemical Engineering Journal. Volume 531, 1 March 2026, 173931. Improving anaerobic digestion of sewage sludge to renewable natural gas by the Advanced Pretreatment & Anaerobic Digestion technology (APAD): Pilot testing - ScienceDirect