Bruce Rittmann is a researcher at Arizona State University leading a project using the greenhouse gases produced through wastewater treatment to generate electricity, create biofuel and possibly even make ice cream — all thanks to microalgae. The three-year project, funded by the Department of Energy, is culminating in a six-week trial with the city of Mesa’s Northwest Water Reclamation Facility in Sloan Park near the Salt River.
“The city of Mesa has anaerobic digesters, is located close to ASU, is always eager to try out advanced technologies, and has been a great research and development partner with our center for some years,” says Dr. Rittmann, a Regents Professor in the School of Sustainable Engineering and the Built Environment and director of the Swette Center for Environmental Biotechnology at the Biodesign Institute.
Dr. Rittmann has been involved in research using gas-transfer membranes — the basis for this project — for 20 years and has been using this technology with microalgae for over a decade. By pairing up with the city and getting access to their anaerobic digesters, which treat wastewater, his team has been able to scale the project up massively.
Not only does wastewater smell bad, it’s also a major problem for the environment. Wastewater treatment produces biogas made up of methane and carbon dioxide (CO₂), the biggest contributors to climate change.
Typically, wastewater treatment facilities burn the biogases they produce. This eliminates the methane, which converts to carbon dioxide in the burning process. “So now you’re just emitting CO₂. Of course, I think we’re all familiar now with CO₂ being a problem,” says Justin Flory, an associate director of research in ASU’s Center for Negative Carbon Emissions and project manager of the trial. “It’s less of a problem than methane, but it’s still a problem.”
People are not going to stop producing wastewater. In fact, as the population grows, we’re probably only going to produce more of it. But the very problem that wastewater treatment creates could provide value instead.
This project proposes a better solution with no burning required. The researchers take the carbon dioxide produced by treating wastewater and feed it to microalgae, which can then be turned into a variety of products. The process also takes the methane produced and generates purer biomethane, which is a high-value product.
Capturing greenhouse gases with microalgae
If the heart of this project is to convert greenhouse gases into useful products, microalgae is the stomach.
Wastewater treatment at Northwest Water Reclamation Facility involves anaerobic digestors. These are large, dome-shaped reactors filled with microorganisms — like bacteria — that break down organic material in the biosolids produced as part of wastewater treatment. One output of anaerobic digestion is biogas, which is a mixture of methane and carbon dioxide.
The water reclamation facility captures and compresses those gases for storage in tanks. Normally it is burned to generate heat, which emits carbon dioxide into the atmosphere.
For the ASU project, they deliver some of that biogas to three, 270-square-foot algae ponds located near the digesters. The ponds were designed by staff at ASU’s Arizona Center for Algae Technology and Innovation and fabricated by students with the assistance of Everett Eustance, an assistant research scientist in the Swette Center who is also the lead researcher running the cultivation trial.
The biogas is delivered directly through thin, hollow fibers placed directly in the ponds. The carbon dioxide diffuses through the membranes of the fibers into the water, where the microalgae consume it for their photosynthetic growth. The process can deliver carbon dioxide to the microalgae with nearly 100% transfer efficiency, which means that nearly all of the carbon dioxide present in the biogas is used instead of being released into the atmosphere. The carbon dioxide delivery also increases the microalgae’s growth rate and lowers the cost of its production.
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