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 CO2 biofixation, wastewater treatment and renewable energy production. To develop such an important technology, there is a need to optimize the culture conditions, maximizing CO2 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 (CO2 concentration, nitrogen-to-phosphorus ratio and culture temperature) for maximizing specific growth rate, biomass productivity and CO2 biofixation rate. The maximum specific growth rate of 1.93 ± 0.19 d-1 was observed at an optimal set of 34oC, 4:1 nitrogen-to-phosphorus ratio, and 6 % CO2 concentration. The maximum biomass productivity of 86.5 ± 20.0 mgL-1d-1 was obtained at 4.8 % CO2, 8:1 nitrogen-to-phosphorus ratio and 28oC. In addition, the maximum CO2 biofixation rate was calculated to be 251.9 ± 13.5 mgL-1d-1 at optimal values of 4 % CO2, 1:1 nitrogen-to-phosphorus ratio and 25oC. Finally, multi-objective optimization method was employed to predict the maximum CO2 biofixation rate and biomass productivity concurrently. The optimum values of CO2 biofixation rate (182.84 ± 8.42 mgL-1d-1) and biomass productivity (78.5 ± 10.0 mgL-1d-1) were obtained from operating conditions at 4 % CO2, 6:1 nitrogento-phosphorus ratio, 25oC culture temperature. These predicted data were in strong agreement with the experimental values.