Characterization of the EcoRecover Process for Intensive Microalgal Cultivation and Tertiary Nutrient Recovery from Wastewaters

Mixed community microalgal wastewater treatment technologies have the potential to advance the limit of technology for biological nutrient recovery while producing a renewable carbon feedstock, but a deeper understanding of their performance is required for system optimization and control. In Fall 2020, a 568 m3·day-1 (150,000 gal·day-1) Clearas EcoRecover microalgal treatment system was installed at the Village of Roberts (Wisconsin, USA; latitude 45.0°N) wastewater treatment plant to meet their pending effluent permit of 0.04 mg·L-1 total phosphorus (TP; 6-month average). Here we report data and analyses from continuous on-line system monitoring, long-term on-site monitoring, and on-site batch experiments, with particular focus on a 3.5 month winter period with limited outside influences (e.g., no major upstream process changes). Across this period of intensive monitoring, effluent TP concentrations were consistently below 0.03 mg-P·L-1 for 3 months. Core microbial community taxa included Chlorella spp., Scenedesmus spp., and Monoraphidium spp., and key indicators of stable performance included near-neutral pH, sufficient alkalinity, and a diel rhythm in dissolved oxygen. By tracking elemental and biochemical composition of the biomass, we also demonstrated the importance of carbohydrate storage (in photobioreactors) and mobilization (in a dark mix tank) in achieving reliable nutrient recovery and algal growth.