Performance and microbial community dynamics of an internal circulating photogranular membrane bioreactor for synthetic mariculture wastewater treatment under different hydraulic retention times

Two internal circulating photogranular membrane bioreactors (IC-PMBRs) were constructed and operated for three months under different hydraulic retention times (HRTs) (R1: 48 h, R2: 24 h). The R1 system with a longer HRT was conducive to maintaining structural stability and enhancing the settleability of photogranules, whereas the photogranules in the R2 system experienced severe disintegration. Both systems proved effective in removing nitrogen (N) and phosphorus (P) from mariculture wastewater. Both systems achieved complete NH₄⁺-N removal, with TN removal efficiencies of 51.07 ± 9.22% (R1) and 57.87 ± 7.64% (R2). Additionally, PO₄^3--P removal efficiencies were 34.92 ± 3.94% (R1) and 33.61 ± 7.23% (R2). The membrane fouling cycle in the R1 system was significantly longer than that in the R2 system (R1: 40 d, R2: 11 d). High-throughput sequencing analysis revealed that substantial community succession occurred in both systems. The R1 system exhibited higher abundances of functional bacteria, such as Proteobacteria and Bacteroidota, compared to the R2 system. Furthermore, the microalgal community structure in the R1 system was more diverse, exhibiting stronger mutualistic symbiotic relationships with bacteria. These characteristics created a more favorable microenvironment for photogranules in the R1 system. This study offers novel insights and a theoretical basis for the widespread application of photogranules in the MBR treatment of mariculture wastewater.