Wastewater algae could be used in ice cream

Everett Eustance, ASU assistant research scientist, leads the algae cultivation trial at Mesa's Northwest Water Reclamation Facility. Photo by Andy DeLisle

Arizona State University in the US is leading a project to use greenhouse gases produced through wastewater treatment to generate electricity, create biofuel and even make ice cream — thanks to microalgae.

The three-year project, funded by the Department of Energy, has seen 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.

“That enriched methane can be used to make electricity or it can run your gas stove in your house or fuel natural gas buses,” said Justin Flory, an associate director of research in ASU’s Center for Negative Carbon Emissions, and project manager of the trial.

Meanwhile the microalgae grown from the CO2-rich diet can be used in a wide variety of products.

“We can convert that biomass into many different things. Some microalgae are high in protein and omega three fatty acids and can be used to feed animals or fish. There are even compounds in microalgae that we use today in ice cream," said Flory.

The microalgae can be mashed down into a slurry and be pressured cooked until it becomes a crude biofuel. Biofuels are non-toxic and a renewable source of energy. They also create a closed cycle — the biofuel will still release some carbon dioxide, but it can be eaten by new microalgae grown to make more biofuel.

“Think about how fossil fuels are created — heat and pressure underground over time converts old organic material, like plants, into fossil fuels. That takes millions or thousands of years, but there is technology being developed that can do it in minutes,” Flory added.

Biogas captured from wastewater treatment is delivered to three ponds at ASU's Arizona Center for Algae Technology & Innovation, where it is used to grow algae. Image: Andy DeLisle.

The research has culminated in a six-week trial with the city of Mesa’s Northwest Water Reclamation Plant.

“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,” said Bruce 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.

Wastewater treatment at Northwest Water Reclamation Plant uses anaerobic digestors - large, dome-shaped reactors filled with microorganisms like bacteria that break down organic material. One output of anaerobic digestion is biogas - 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 project the biogas is sent to three, 25-square-metre algae ponds located near the digesters, which were designed by staff at ASU’s Arizona Center for Algae Technology and Innovation.

The biogas is delivered through thin, hollow fibres placed into the ponds. The carbon dioxide diffuses through the membranes into the water, where the microalgae consume it for their photosynthetic growth.

The process delivers carbon dioxide to the microalgae with nearly 100% efficiency — which means nearly all of the carbon dioxide present in biogas is used instead of being released into the atmosphere. The highly efficient carbon dioxide delivery method also increases the microalgae’s growth rate and lowers the cost of its production.

"It seems like there’s a lot of things we call waste that truly are not waste — they're resources.”

Scott Bouchie, City of Mesa

Microalgae requires the right mix of sunlight, water and temperatures to grow properly.

Weather can also affect the microalgae’s growth. In a lab, it is relatively easy to have a consistent environment, but outdoors, weather naturally fluctuates. To address this concern, the team ran the trial through to the end of November, using strains of algae that thrive in cooler weather.

After this project ends, the team is hoping to conduct another trial with the city of Mesa early next year.

Next time the team plans to include another waste stream generated during the wastewater, which contains leftover liquid from the anaerobic digestors and would provide necessary nutrients like nitrogen and phosphorus for the microalgae to grow. This would mean the project would truly be a closed cycle, as every waste produced by the treatment process would be used up.

“Residents of Arizona see a need to really change the way we operate today,” explained Scott Bouchie, the city of Mesa’s director of environmental management and sustainability. “It seems like there’s a lot of things we call waste that truly are not waste — they're resources.”