Enhanced removal of nitrate and phosphate from wastewater by Chlorella vulgaris: Multi-objective optimization and CFD simulation

Abstract

To enhance the efficiency of wastewater biotreatment with microalgae, the effects of physical parameters need to be investigated and optimized. In this regard, the individual and interactive effects of temperature, pH and aeration rate on the performance of biological removal of nitrate and phosphate by Chlorella vulgaris were studied by response surface methodology (RSM). Furthermore, a multi-objective optimization technique was applied to the response equations to simultaneously find optimal combinations of input parameters capable of removing the highest possible amount of nitrate and phosphate. The optimal calculated values were temperature of 26.3 °C, pH of 8 and aeration rate of 4.7 L·min− 1. Interestingly, under the optimum condition, approximately 85% of total nitrate and 77% of whole phosphate were removed after 48 h and 24 h, respectively, which were in excellent agreement with the predicted values. Finally, the effect of baffle on mixing performance and, as a result, on bioremoval efficiency was investigated in Stirred Tank Photobioreactor (STP) by means of Computational Fluid Dynamics (CFD). Flow behavior indicated substantial enhancement in mixing performance when the baffle was inserted into the tank. Obtained simulation results were validated experimentally. Under the optimum condition, due to proper mixing in baffled STP, nitrate and phosphate removal increased up to 93% and 86%, respectively, compared to unbaffled one.