Kinetics and stoichiometry in the biological phosphorus removal process with short cycle times

Aerobic or denitrifying biological phosphorus removal in a sequencing batch rector (SBR) with short repetitive anaerobic-aerobic (A/O) or anaerobic-anoxic (A2) cycling has been studied. These conditions resemble processes with large internal recycle flows. Under these conditions the fatty acid dosage per anaerobic phase is low compared to a conventional SBR or plug flow process. It was shown that at the same sludge retention time (SRT) the average poly-β-hydroxybutyrate (PHB) content of the biomass decreased, with increasing the number of A/O or A2 cycles. This had no direct effect on the biological phosphorus removal. In the A2 SBR, however, phosphorus removal was deteriorating due to transfer of nitrate to the anaerobic phase. Most mathematical models for biological phosphorus removal processes show a relation between the biomass growth rate and the PHB content. This research shows that such a unique relation does not exist, since at one SRT different “steady state” PHB levels could be obtained, depending on the cycle number and the use of oxygen or nitrate as an electron acceptor. This indicates the need for an alternative kinetic model.