Optimization of environmental factors affecting tertiary treatment of municipal wastewater by Chlorella protothecoides in a lab scale photobioreactor

In the present study, a lab scale externally illuminated photobioreactor was used to study the potential of a freshwater microalga Chlorella protothecoides (SAG-211-10C) in tertiary treatment of municipal wastewater. The effect of parameters such as: light intensity, photoperiod, pH, concentration of carbon dioxide in air, temperature and aeration rate on nutrient removal efficiencies of wastewater was analyzed. Low irradiance and low photoperiods (2 klux and 8 h:12 h) resulted in poor treatment efficiency (17.97% Chemical oxygen demand,29.44% total nitrogen, 23.91% total phosphorous). Alkaline pH (8,9 and 10) reduced the COD and TN removal efficiency, but improved total phosphorous removal rates. Higher carbon dioxide concentration in air adversely affected COD removal rates (27.97% at 8% CO2 v/s 69.95% at 2% CO2). The nutrient removal efficiencies could not be increased by operating the PBR under a non-optimum lower temperature(20°C) or higher temperature (30°C). Under optimum conditions (Light intensity-6 klux, photoperiod-16 h:8 h, pH-6.8, concentration of carbon dioxide in air −6%, temperature-25 °C and aeration rate-3 lpm), highest removal of COD (78.03% on 10th day), 100% removal of TN (on 7th day) and 100% removal of TP (on 6th day) was observed. The highest biomass concentration under optimum conditions was 1.96 g/L.