Observation of Struvite in the Mixed Microalgae Biofilm Matrix of a Rotating Algal Biofilm Reactor During Nutrient Removal from Municipal Anaerobic Digester Filtrate

Central Valley Water Reclamation Facility (CVWRF) in Salt Lake City is the largest municipal wastewater treatment plant in Utah and must meet new and rigorous nutrient effluent standards–over 95% reduction in phosphorus output by 2025. Filtrate from CVWRF anaerobic digesters contains high levels of nitrogen, phosphorus, and magnesium. Supersaturation of these constituents leads to nuisance struvite precipitation that clogs belts, pumps, and pipes downstream of anaerobic digesters. Struvite is a mineral precipitate composed of equimolar magnesium, ammonium, and phosphate. Controlled precipitation of struvite helps prevent clogging and scaling, removes phosphate from wastewater, and generates a marketable fertilizer product. Struvite was observed within the microalgae biofilm matrix of an outdoor, pilot-scale rotating algal biofilm reactor (RABR) designed to remove nitrogen (N) and phosphorus (P) from anaerobic digester filtrate. East/west biofilm orientation and biomass harvesting interval influence struvite content within the biofilm matrix. Despite RABR influent component ion molar ratios with potential for various magnesium and calcium precipitates, microalgae biofilm provides pH, temperature, and nucleation sites favorable to struvite precipitation. The RABR system removed N and P through biofilm growth and through struvite precipitation. Microalgae biofilm can be harvested and pelletized into fertilizer, and the struvite content will add fertilizer value to the product. More research is needed for optimization and scalability of P removal through combined microalgae biofilm and struvite precipitation