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Researchers repurpose wastewater treatment greenhouse gases to grow algae, make useful products

Wastewater treatment plants produce the greenhouse gases methane and carbon dioxide, which they typically squander by burning.

Now, researchers are partnering with a Mesa plant to study how to repurpose such biogas byproducts to grow algae and make useful products.

"If it really works, they could then enhance the quality of the biogas and even generate more value by growing the microalgae," said Bruce Rittmann, director of the Swette Center for Environmental Biotechnology at ASU's Biodesign Institute, who leads the team. 

Manufacturers can convert microalgae into fuels, feeds and food additives. But the microorganisms consume lots of carbon, and pulling it from the air takes time.

So Rittmann's team boosts the process, using hollow, hair-like membranes to deliver CO2 to their algae ponds. The process works though osmosis: The imbalance created as the microalgae consume carbon dioxide pulls more CO2 from the membranes into the ponds.

The team works with the city of Mesa's Northwest Water Reclamation Plant to harvest the biogas, which anaerobic bacteria emit while devouring organic solids during wastewater treatment. 

"You're creating a virtuous cycle here: The CO2 is cycling around and around and around between the digester and the microalgae ponds. And every time it cycles around, it's gathering energy from the sun to make more more biomass to put back in the digester," said Rittmann. 

Rittmann said the researchers also concentrate methane for resale.

"We have a great team of about half a dozen people who work on this membrane coordination project. Dr. Everett Eustance, the guy who designs, builds and operates the field pilots, is doing a really wonderful job and deserves special recognition," he said.

The team will wrap up its current six-week project in January but plans to continue its long collaboration with the city of Mesa. 

“We're demonstrating this at a pretty large scale out there at the Mesa wastewater treatment plant because they have digesters, they have the biogas – and they’re really interested in this,” said Rittmann.

"The city of Mesa is super cooperative, and they're also really interested in innovation," Rittmann said. 

Nicholas Gerbis was a senior field correspondent for KJZZ from 2016 to 2024.