In situ biological CO₂ fixation and wastewater nutrient removal with Neochloris oleoabundans in batch photobioreactor

Microalgae cultivation in wastewater media in phototrophic condition is a promising approach for integrated CO₂ biofixation and wastewater treatment. For this, Neochloris oleoabundans was used to investigate the tertiary treatment of wastewater along with CO₂ biofixation. In this investigation, biomass productivity, CO₂ biofixation rate and percentage of total nitrogen (TN) and total phosphorus (TP) removal from synthetic wastewater are considered under three different operating conditions: temperature, CO₂ feed concentration and nitrogen to phosphorus (NP) ratio in the media. Cultivation of N. oleoabundans was found to be highly temperature sensitive. With the increase of cultivation temperature from 25 to 45 °C, declining trends of biomass concentration, productivity and percentage of TN and TP removal were observed. Cultivation temperature of 25 °C was found to be most favorable in terms of biomass productivity, CO₂ biofixation rate, percentage of TN and TP removal of 92 (mg L^-1 day^-1), 145 (mg L^-1 day^-1), 100% and 32%, respectively. Arrhenius-type kinetic model was used and the model showed good agreement with the experimental findings. Activation energy for the active stage and decay stage was found to be [Formula: see text] = 88.8 kJ mol^-1 and [Formula: see text] = 8.4 kJ mol^-1, respectively. With the increase of CO₂ feed concentration, biomass productivity increased and the maximum biomass concentration and productivity was achieved at 6%. After that with the increase in CO₂, a declining trend was observed. With the increase of NP ratio from 1:1 to 2:1, both the biomass productivity and CO₂ biofixation were increased, but later were subsequently decreased with increase of NP ratio from 4:1 to 8:1. It is interesting that TP removal was increased with NP ratio and 100 percent of TP removal was achieved at 4:1 and 8:1 conditions.