Comparison of microalgae cultivation modes for advanced wastewater purification and pollutant-resource conversion: A pilot study

Microalgae absorb CO2 and nutrients (e.g., nitrogen, phosphorus) for biomass growth, which has high potential for wastewater purification and pollutant-resource conversion. However, current studies mainly focused on short-term effect on stimulated water in lab. In this study, the feasibility of pilot-scale microalgae cultivation in real wastewater with low pollutant content and no sterilization was evaluated, and the optimal culture mode was proposed. It was found that microalgae could remove >95 % of nitrogen and phosphorus during the batch cultivation, proving the strong purifying ability of wastewater with low level of nitrogen and phosphorus. However, the high ratio of nitrogen to phosphorus led to long-term phosphorus shortage in the semi-continuous cultivation mode. The fluctuation of wastewater quality/quantity broke the balance of suspended microalgae growth and wash-out in the continuous cultivation mode. Besides, the aggregation of microalgae at the bottom inhibited the sustainable wastewater purification in both semi-continuous and continuous cultivation modes. Finally, continuous cultivation mode of attached microalgae (CCMA) was proposed with stable and efficient performance of wastewater treatment, and the process of microalgae-water separation could be omitted. Under this mode, the average removal of COD, TN, and TP reached 39.33 %, 21.27 % and 88.10 %, respectively, which could be further enhanced by optimizing the carrier density. Therefore, CCMA was a promising approach for scale-up low-pollutant wastewater treatment.