Sewage treatment and removal of micropollutants in suspended and attached growth systems combining Chlorella sorokiniana and activated sludge

Background Algal‐bacterial consortia represent a promising symbiotic system with high potential for removing major pollutants and micropollutants from municipal wastewater. Four continuously stirred sequential batch reactors (SBRs) were used to investigate the combination of Chlorella sorokiniana and activated sludge for municipal wastewater treatment in both suspended and attached‐growth systems under three different hydraulic retention times (HRTs). The reactors were configured as follows: SBR1 contained C. sorokiniana alone, SBR2 included C. sorokiniana and activated sludge, SBR3 combined C. sorokiniana with biocarriers, and SBR4 incorporated C. sorokiniana , activated sludge, and biocarriers. Results During Phases A (HRT = 3 d) and B (HRT = 2 d), all SBRs achieved dissolved chemical oxygen demand (COD) removal rates above 80%, with the highest removal observed in SBR4 (85%). Similarly, NH4‐N removal exceeded 98% across all SBRs. In Phase C, the reduction of HRT to 1 d stressed the ability of SBR3 and SBR4 to efficiently remove COD and NH4‐N as the existing biomass could not produce enough oxygen for maintaining aerobic conditions. PO4‐P removal remained partial (<55%) in all SBRs, regardless of the HRT. Biomass collected from the different systems was analyzed for protein, starch, and lipid content, with the highest levels observed in SBR2, measuring 43.6%, 10.9%, and 34.1%, respectively. Experiments conducted during Phase C assessed the removal of four benzotriazoles and 2‐hydroxybenzothiazole (2‐HBT), demonstrating that these reactors were also capable of partially removing these micropollutants. The highest removal was observed for 2‐HBT in SBR1 (93%) while dissolved oxygen appeared to be critical for micropollutants removal. Conclusions Suspended and attached‐growth algal‐bacterial systems can be used for municipal wastewater treatment. Future research should focus on improving the understanding of biomass settleability and the mechanism of micropollutants removal.