Phosphorus recovery in the alternating aerobic/anaerobic biofilm system: Performance and mechanism

To balance the high phosphorus concentration in recirculated solution and the stability of biofilm system, this study explored the performance and mechanism of phosphorus uptake/release for recovering phosphorus from sewage when the phosphorus content in biofilm (P_biofilm) changed. The results showed that the maximum phosphorus concentration in the concentrated solution reached 171.2 ± 2.5 mg·L^-1 in harvest 1st-5th stages. Polyphosphate accumulating organisms (PAOs) performed a metabolic shift from glycogen accumulation metabolism (GAM) to polyphosphate accumulation metabolism (PAM) when P_biofilm increased at each phosphorus enrichment stage, and more phosphorus was absorbed/released by PAOs. Nevertheless, the release of poly-phosphate from PAOs was inhibited after phosphorus concentration stabilized, and PAOs were unable to absorb phosphorus from wastewater as it reached the phosphorus saturation stage. To maintain the stability of the system, phosphorus had to be harvested so that the saturated phosphorus in PAOs was easily released in a new recirculated solution, resulting in adequate storage space for PAOs to absorb phosphorus. Meanwhile, the ³¹P NMR analysis demonstrated that phosphorus was stored in EPS and cell of PAOs, whereas EPS played a significant role than cell at the anaerobic phase. Particularly, ortho-phosphate was the major component of phosphorus release by EPS and poly-phosphate was the major part of phosphorus release by cell. Furthermore, the change of P_biofilm had no impact on biofilm characteristics and microbial communities, whereas some PAOs would be enriched, and others that were not suitable for this process would be inhibited with repeated cycles of alternating aerobic/anaerobic operation.