The effect of residual substrate utilization on sludge settling in an enhanced biological phosphorus removal process

The simultaneous process requirements of phosphorus removal and bulking control were demonstrated in study via a unified explanation along with an introduction of the parameter q_re, i.e., specific residual substrate utilization rate. Pilot-plant experiments using synthetic wastewater were performed at different SRTs (5,10,15) and anaerobic:aerobic volume ratios, V_an:V_a (1:9,1:4,1:2.3). These results indicated that more than 75% of phosphorus removal and 94% of COD removal can be achieved during experimental periods. However, sludge settling properties in terms of SVI varied significantly, and a higher phosphorus removal did not necessarily imply a better sludge settling. Nevertheless, SVI-values highly correlated with q_re - regardless of different SRTs and V_an:V_a ratios. This correlation would imply that the specific utilization of residual substrate left for filaments in oxic zone is the primary factor influencing SVI. Furthermore, eliminating the access of filaments to substrate plays a more influential role on SVI than favouring the poly-p microorganisms: the main floc-formers in EBPR process. Based on these findings we can infer that when in the design and/or operation stage of the EBPR process, the carbon sequestration by poly-p microorganisms in anaerobic zone can be as much as possible so as to obtain a lower q_re in the oxic zone. That is, if phosphorus removal and bulking control is achieved simultaneously.