Corrected response surface methodology for microalgae towards optimized ammonia nitrogen removal: A case of rare earth mining tailings wastewater in Southern Jiangxi, China

Using microalgae for rare earth elements wastewater treatment has been proved to be effective, however, the technology is challenged by the unstable performances caused by the changeable wastewater qualities and natural conditions. In this study, a corrected response surface methodology was first used to provide an optimized operation strategy for the application of wastewater treatment using microalgae, by investigating the effects and interrelations of three main factors including temperature, illumination intensity and microalgae concentration, taking rare earth elements wastewater in Southern Jiangxi as an example. Nine models were obtained using nine types of wastewater. The models were then modified by categorizing wastewaters into two classes (error range <12%) according to their initial ammonia nitrogen concentrations. The optimal conditions for ammonia nitrogen removal in class I and class II wastewater were as follows: temperature of 21.0 and 23.6 °C, illumination intensity of 46.8, 45.3 LX %, microalgae concentration of 1.64 and 1.67 g dry weight/L. The results are confirmed by stable operation of a continuous-flow photobioreactor for more than 122 days. The amount of microalgae required to treat wastewater under natural conditions is 1.5–1.8 g dry weight/L estimated by the corrected model. It implies that nearly 16,710 kg and 46,520 kg of ammonia nitrogen could be removed from two typical wastewaters with optimal microalgae dosage in 2020. The results demonstrated that corrected response surface methodology provides a powerful approach for optimizing the operational conditions for wastewater treatment by microalgae.