Characteristics of Chlorella sorokiniana treating wastewater with consideration of HRT (6 d, 16 h, 8 h), hydraulic conditions, light or dark culture were evaluated and compared with activated sludge. Results showed that optimal HRT was 8 h; if longer, effluent chemical oxygen demand (COD) and NH4+-N in the dark began to rebound. Mixing was beneficial to COD removal of algae, while aeration was suitable for nutrient removal. Growth of C. sorokiniana in the light was mixotrophic growth and 1.3–1.7 times more than that of dark heterotrophic growth. The maximum specific growth rate (µmax), productivity, and biomass yields on COD (YCOD), N (YNH4), P (YP) of algae were higher in the light than that in the dark. COD assimilation capacity of algae was similar to activated sludge but with different dynamics. N and P assimilation capacity of algae was 1.4, 1.2–2.5 times more than activated sludge; N and P removal efficiency of algae was 5%–10%, 10%–55% respectively higher than activated sludge. This study confirmed the advantage of algae over activated sludge and reveal why algae could assist the activated sludge process.