BACKGROUND: The application of Chlorella vulgaris for simultaneous CO2 biofixation and nutrient removal has been optimised using response surface methodology (RSM) based on Box Behnken design (BBD). Experimental conditions employed comprised CO2 concentrations (C-c,C-g) of 0.03-22% CO2, irradiation intensities (I) of 100-400 E, temperatures of 20-30 degrees C and nutrient concentrations of 0-56 and 0-19mgL(-1) nitrogen and phosphorus, respectively, the response parameters being specific growth rate , CO2 uptake rate R-c and %nutrient removal. RESULTS: Over 10 days the biomass concentration reached almost 3gL(-1) for C-c,C-g of 5% CO2, with corresponding values of 0.74gL(-1) day(-1) and 1.17day(-1) for R-c and , respectively, and 100% nutrient (N and P) removals. At 22% CO2 the R-c and decreased by around an order of magnitude, and nutrient removal also decreased to 79% and 50% for N and P, respectively. CONCLUSION: Optimum values 5% CO2, 100 E and 22 degrees C were identified for C-c,C-g, I and T, respectively, with and R-c reaching 1.53day(-1) and 1gL(-1) day(-1), respectively, along with associated nutrient removal of 100%. Regression analysis indicated a good fit between experimental and model data. (C) 2016 Society of Chemical Industry