Application of Central Composite Design to Optimize Culture Conditions of Chlorella vulgaris in a Batch Photobioreactor: An Efficient Modeling Approach

Microalgae cultivation and their use is a promising approach for integrated CO 2 biofixation, wastewater treatment and renewable energy production. To develop such an important technology, there is a need to optimize the culture conditions, maximizing CO 2 consumption, degrading the nutrients present in the wastewater and maximise the microalgae biomass production. Central Composite Design (CCD) approach was applied to develop quadratic regression models. The developed models were employed separately to estimate optimal sets of three important input parameters (CO 2 concentration, nitrogen-to-phosphorus ratio and culture temperature) for maximizing specific growth rate, biomass productivity and CO 2 biofixation rate. The maximum specific growth rate of 1.93 ± 0.19 d -1 was observed at an optimal set of 34 o C, 4:1 nitrogen-to-phosphorus ratio, and 6 % CO 2 concentration. The maximum biomass productivity of 86.5 ± 20.0 mgL -1 d -1 was obtained at 4.8 % CO 2 , 8:1 nitrogen-to-phosphorus ratio and 28 o C. In addition, the maximum CO 2 biofixation rate was calculated to be 251.9 ± 13.5 mgL -1 d -1 at optimal values of 4 % CO 2 , 1:1 nitrogen-to-phosphorus ratio and 25 o C. Finally, multi-objective optimization method was employed to predict the maximum CO 2 biofixation rate and biomass productivity concurrently. The optimum values of CO 2 biofixation rate (182.84 ± 8.42 mgL -1 d -1 ) and biomass productivity (78.5 ± 10.0 mgL -1 d -1 ) were obtained from operating conditions at 4 % CO 2 , 6:1 nitrogen-to-phosphorus ratio, 25 o C culture temperature. These predicted data were in strong agreement with the experimental values.