Repurposing commercial anaerobic digester wastewater to improve cyanobacteria cultivation and digestibility for bioenergy systems

Abstract

Wastewater was sourced directly from an industrial anaerobic biodigestion system to assess its potential to serve as a growth medium and improve the digestibility of microalgae in a closed-loop bioenergy system. Cyanobacteria (Oscillatoria sp.) were cultured in the wastewater and further explored as a feedstock for biomethane production via methanogenesis. Promisingly high levels of nitrate (1180 mg L⁻¹) and phosphate (61.5 mg L⁻¹) were found in the nitrification-treated, NO₃-rich wastewater stream. While cyanobacteria were not observed to grow directly in this NO₃-rich wastewater stream, a modest dilution of the wastewater by a factor of two produced denser cultures than could be grown in standard f/2 media, which was made by enriching seawater. Growth in wastewater to improve the digestibility of the cyanobacteria resulted in significantly more methane from cultures in both the NO₃-rich and the untreated, NH₄-rich wastewater streams.