Nutrient and heavy metal removal from piggery wastewater and CH4 enrichment in biogas based on microalgae cultivation technology under different initial inoculum concentration

Three microalgae-based treatment technologies were applied for removing nutrients in piggery wastewater and CO2 in biogas simultaneously. Chlorella vulgaris (C. vulgaris), Scenedesmus obliquus (S. obliquus), and Neochloris oleoabundans (N. oleoabundans) were selected for mono-cultivation or co-cultivation with fungi or activated sludge. The effects of initial inoculum concentration (low of 62.06 +/- 6.23 mg/L, medium of 121 +/- 9.34 mg/L, and high of 180 +/- 12.78 mg/L) of microalgae/algal symbiont on the pollutant removal efficiency were evaluated. The results showed that cultivation of S. obliquus with activated sludge had a relatively high CO2 and nutrients removal efficiency at a medium concentration of initial inoculum. The highest removal efficiency of total organic carbon, total nitrogen, and total phosphorus in piggery wastewater was 87.29%, 87.26%, and 90.17% by co-cultivation of S. obliquus with activated sludge. The highest CO2 removal of 64.28% could be achieved under co-cultivation of S. obliquus and fungi at medium initial inoculum concentrations. This work will be helpful to promote the study of microalgae-based biogas upgrading and piggery wastewater purification. Practitioner points Nutrient and CO2 were efficiently removed in moderate microalgal initial inoculum. Scenedesmus obliquus-activated sludge cultivation achieved the highest nutrient removal. Co-cultivation of S. obliquus and fungi showed the highest CO2 removal. Zinc was removed efficiently in high microalgal initial inoculum. Chlorella vulgaris/S. obliquus-activated sludge performed economically.