Performance of piggery wastewater treatment and biogas upgrading by three microalgal cultivation technologies under different initial COD concentration

Three treatment technologies (mono-cultivation of microalgae, co-cultivation of microalgae with fungi or activated sludge) including two selected strains Chlorella vulgaris (C. vulgaris) and Scenedesmus obliquus (S. obliquus) were cultivated in the piggery wastewater (PWW) with different COD concentration of 800-2000 mg L−1. The effect of different COD concentration on pollutant removal of PWW, CO2 removal of raw biogas and the optimum strategy selection with light/dark ratio of 12 h:12 h was investigated. The results showed that co-cultivation of microalgae with activated sludge exhibited better performance than with fungi or mono-cultivation. The medium COD level of 1200 mg L−1 was proved to be the optimal dilution. The highest removal efficiencies of COD, TN, TP and CO2 were attained in co-cultivation of C. vulgaris with activated sludge mode under diluted COD of 1200 mg L−1, which were 79.86 ± 6.11%, 80.23 ± 6.31%, 89.37 ± 6.58% and 63.48 ± 4.27%, respectively, with the highest CH4 content of 83.24 ± 3.84%, and the heavy metal lower than the threshold of 0.6 mg L−1. The results confirmed the superiority of cultivation of microalgae with activated sludge in PWW for removal of nutrients, heavy metals and simultaneous biogas upgrading at reduced cost.