Nutrient removal and biogas upgrading by integrating freshwater algae cultivation with piggery anaerobic digestate liquid treatment

An integrated approach that combined freshwater microalgae Scenedesmus obliquus (FACHB-31) cultivation with piggery anaerobic digestate liquid treatment was investigated in this study. The characteristics of algal growth, biogas production, and nutrient removal were examined using photobioreactor bags (PBRbs) to cultivate S. obliquus (FACHB-31) in digestate with various digestate dilutions (the concentration levels of 3200, 2200, 1600, 1200, 800, and 400 mg L −1 chemical oxygen demand (COD)) during 7-day period. The effects of the level of pollutants on nutrient removal efficiency and CO 2 removal process were investigated to select the optimum system for effectively upgrade biogas and simultaneously reduce the nutrient content in digestate. The treatment performance displayed that average removal rates of COD, total nitrogen (TN), total phosphorous (TP), and CO 2 were 61.58–75.29, 58.39–74.63, 70.09–88.79, and 54.26–73.81 %, respectively. All the strains grew well under any the dilution treatments. With increased initial nutrient concentration to a certain range, the CO 4 content ( v / v ) of raw biogas increased. Differences in the biogas enrichment of S. obliquus (FACHB-31) in all treatments mainly resulted from variations in biomass productivity and CO 2 uptake. Notably, the diluted digestate sample of 1600 mg L −1 COD provided an optimal nutrient concentration for S. obliquus (FACHB-31) cultivation, where the advantageous nutrient and CO 2 removals, as well as the highest productivities of biomass and biogas upgrading, were revealed. Results showed that microalgal biomass production offered real opportunities to address issues such as CO 2 sequestration, wastewater treatment, and biogas production.