Identification of Microalgae, Evaluation of Water Purification Efficiency, and Optimization of Large-Scale Production Conditions Based on Recirculating Aquaculture Systems

Water purification is the most crucial process for recirculating aquaculture systems (RASs), while microalgae exhibit significant potential for nutrient removal from aquaculture wastewater. Four microalgal species with removal capacities for ammonia nitrogen (NH4+-N), nitrite nitrogen (NO2-N) and total phosphorus (TP) were isolated and identified from the land-based RASs, namely, Kirchneriella cornuta HY-1, Desmodesmus abundans HY-1, Coccomyxa sp. HY-1 and Oocystis sp. HY-1. The growth kinetics were characterized through measurements of cell density, chlorophyll content, and dry biomass in the in situ wastewater. Based on zeta potential, the flocculation efficiencies of seven agents were evaluated. Preservation parameters were optimized under nutrient-supplemented conditions with protective agents. Under large-scale culture conditions, over a 7-day period, the highest NH4+-N removal efficiency of 87.20 ± 0.56% was achieved by C. sp. HY-1, the highest NO2-N removal efficiency (85.63 ± 2.56%) was obtained by O. sp. HY-1, and the maximum TP removal efficiency (71.19 ± 3.35%) was exhibited by K. cornuta HY-1. Moreover, the harvest period could be reached after about 14 days of cultivation. FeCl3 was identified as the optimal harvesting flocculant for all four species. Optimal preservation conditions were determined as 25 °C and 31.25 μmol·m−2·s−1 for D. abundans HY-1, C. sp. HY-1, and O. sp. HY-1, and 25 °C and 18.75 μmol·m−2·s−1 for K. cornuta HY-1. This work provides theoretical guidance for large-scale microalgal production and confirms the technical feasibility of microalgae-driven in situ wastewater remediation.