Algae: it’s what’s treating wastewater for the City of Slater

In March 2023, pumps fired up inside a greenhouse at the City of Slater’s wastewater treatment plant. Each of the six concrete cells inside the greenhouse filled with wastewater and began to submerge belts of coral colored custom weave polyester. Within a month or two, the belts will begin to host enough algae to effectively remove nutrients from the City’s wastewater.

Story Construction served as the general contractor for much of the work involved in incorporating what’s called a revolving algal biofilm (RAB) system into the City’s existing plant. Work began in 2021, and the system became operational in March 2023.

This RAB system, a patented product from Slater based Gross-Wen Technologies, allows the City of Slater to comply with more stringent municipal wastewater discharge permits.

“We were initially going to put in an SBR (single batch reactor) system, but the SBR system would require a lot more man hours at the plant,” said Mark Estrem, public works director for the City of Slater. “The lower cost associated with operating Gross-Wen’s RAB system tipped the scale for us.”

An added benefit of the RAB system is the ability to harvest and sell the algae that is grown from the process.

“Once algae begins to grow on the material, we visually inspect it to determine when it’s ready to harvest for the first time,” said Jens Dancer, process engineer at Gross-Wen Technologies. “We then start an automatic cycle of scraping off the algae and removing it from the site where it’s dewatered and pelletized. Since the algae is still seeded in the woven fabric, it only takes roughly seven days until it is ready to harvest once again.”

The greenhouse and the new settling basin are located in an area of the site that was previously taken up by the plant’s primary lagoon, which meant that the primary lagoon needed to be drained and desludged for work to commence on the greenhouse and new settling basin. Structural fill material was added where the greenhouse was to be built, and Story crews poured the concrete foundations and basins for the RAB system greenhouse and associated UV disinfection structure. The Story team was also tasked with installing piping and utilities associated with the new process.

“Working with the Story team was a very good experience,” added Estrem. “We learn a lot on projects like this. The whole Story team provided great communication throughout the project, and we couldn’t ask for a better superintendent on site than Joe Christensen.”

A pump station and generator upstream of the system will be upgraded this year to replace aging equipment, completing the work associated with upgrading the City of Slater’s wastewater treatment plant.