Revolving Algal Biofilm (RAB) demonstration unit

The Revolving Algal Biofilm (RAB) demonstration unit at the Metropolitan Water Reclamation District of Greater Chicago is made of two separate conveyor belts. Full-size modules contain eight. 

Across the Midwest, towns are struggling with the costs of meeting wastewater discharge requirements.

Much like the nutrient loss reduction strategies affecting farm practices, entities that discharge treated wastewater into rivers or streams must meet DNR permits that limit nitrogen and phosphorus.

Excesses of these nutrients, among a host of other problems, are responsible for algae blooms in regional lakes and streams and contribute to the 8,000-square mile “dead zone” in the Gulf of Mexico.

Many small towns are considering ways to finance mechanical sewage treatment plants or retrofit current lagoon systems to meet stricter regulations. But a new technology developed in Iowa may help, and algae — typically considered part of the problem — is the solution.

In 2012, Zhiyou Wen and Martin Gross of Iowa State University developed a system to grow algae on a series of conveyor belts rising out of a basin of water. It’s called the Revolving Algal Biofilm system, or RAB. They initially designed the system as a way to efficiently harvest algae for fertilizer or biofuels.

“Our focus has always been how to take a problem or waste and make something valuable out of it,” Gross said.

In 2014, they founded Gross-Wen Technologies to develop the RAB for commercial application. Then, they were approached by the Municipal Water Reclamation District of Greater Chicago, which wanted to research ways to remove nutrients from wastewater with algae.

“They saw our algae system we developed to make algae biofuels and told us it would be perfect to treat wastewater,” Gross said.

The Chicago pilot, with additional pilots in the Iowa communities of Dallas Center, Ames and Cresco, provided data to prove the system’s effectiveness. In May 2018, the Iowa DNR gave regulatory approval for cities to use the RAB to meet permit requirements.

Max Gangestad, director of business development at Gross-Wen Technologies, said using algae to treat wastewater isn’t a new idea, but the RAB is the first algae-based system to be commercially viable.

“People have known you can treat wastewater with algae … but it’s never been economical to do so,” Gangestad said.

Each RAB module is about 1/10th the size of a football field. It contains eight conveyor belts inside a greenhouse structure. As it grows, algae is scraped off and collected. Depending on how long wastewater is exposed to the “biofilm,” the system can reduce total nitrogen and total phosphorus by 90 percent or more. However, most municipal permits don’t require such high nutrient reduction.

Compared to a mechanical sewage treatment plant, the RAB also has lower energy costs and requires less skilled labor.

The city of Slater, Iowa, will be the first to use the RAB to meet their wastewater permit requirements with three RAB modules. It’s expected to cost about $4 million and will undergo construction this summer.

Jennifer Davies, administrator and clerk for the city of Slater, said cost was a leading factor in choosing the RAB system. The alternative was a sequencing batch reactor, which breaks down sludge through aeration and would have cost the city of 1,500 people about $6.6 million to build, plus significant ongoing labor costs.

But the RAB had other draws, Davies said.

“We also really liked that this was a more green concept, being able to use algae,” Davies said.

She added that though the city is only required to remove ammonia with their current wastewater discharge permit, the RAB removes other nutrients, which may allow the city to adapt if discharge permits are made stricter in the future.

Gross-Wen anticipates marketing the algae to greenhouses and golf courses as fertilizer. As the supply grows, the method shows promise for recapturing nutrients for row crop use.