Characterization of the denitrifying fraction of phosphate accumulating organisms in biological phosphate removal

Results of experimental investigations are presented that strongly support the hypothesis that PAO from activated sludge systems consist of two groups: a) denitrifying PAO (DNPAO) capable of using oxygen and nitrate and b) non-denitrifying PAO (non-DNPAO) only able to use oxygen. Batch experiments were performed in which activated sludge obtained from a pilot scale BiodeniphoTM was submitted to a sequence of anaerobic/anoxic/aerobic, anaerobic/aerobic or anaerobic/anoxic conditions while monitoring the course of NOx-N, NH4-N, PO4-P, PHB and PHV. Several methods for the determination of the two fractions of PAO are performed and compared. This study extends on previously reported results (Kern-Jespersen and Henze, 1993) in that the pH was controlled to around pH 7 to assure that phosphate precipitation was minimal, and in the measurement of PHB and PHV. With regards to the latter, the paper also examines the influence of the size of the internal PHA pool on the anoxic phosphorus uptake rate. Simulations implementing existing models for the growth of non-DNPAO and DNPAO are used to confirm the experimental results and to gain a better understanding of some of the observations.