Distribution and transformation of phosphorus-containing substances in a combined oxidation ditch-membrane bioreactor

Abstract This work aimed at revealing the distribution and transformation of phosphorus (P) -containing substances in a combined oxidation ditch (OD)-membrane bioreactor (MBR). During the operation, it was found that P mainly occurred in microbial cells, followed by extracellular phase and the least in aqueous phase. The primary P fraction in the bioreactor was inorganic phosphorus (IP), in which non-apatite inorganic P (NAIP) accounted for the major ratio, comprising 57–73% of IP. The values of P uptake and release rate attained a low value after operating for 40 days with no sludge discharged. The relative proportion of Poly-P identified by 31P NMR declined obviously after long-term operation without discharging sludge, revealing lower bioactivity of PAOs in the OD-MBR. In terms of maintaining a stable P-removal efficiency, an MBR should discharge excess sludge at a suitable time point by comprehensively considering the relative values of PUR&PRR, and the proportion of Poly-P.