Phosphorus Removal and PAOs Populations at a Full-Scale Integrated Fixed-Film Activated Sludge (IFAS) Plant

Integrated Fixed-Film Activated Sludge (IFAS) has been shown to be successful for the enhancement of nitrification and denitrification in conventional activated sludge systems, however only a few pilot scale studies have included Enhanced Biological Phosphorus Removal (EBPR) in their IFAS set up. This study reports for the first time an evaluation of the performance of biological phosphorus removal, including kinetics, in a full scale plant that was designed as an IFAS-BNR; moreover this study gives some insight on the identification and quantification of the polyphosphate accumulating organism (PAO) populations and their distribution on the fixed-film (media) and in the suspended activated sludge. In contrast to the general expectation that PAOs only reside in the suspended mixed liquor in an IFAS system, PAO presence and activity was observed in biofilms on the media. However, only a small percentage of the total PAO activity was associated with the media and the measured activity seemed to be associated with thicker biofilm that probably created diffusion limitation along the film depth. Observation of PAOs and Accumulibacter–like PAOs indicated that Accumulibacter were present in both biofilm and mixed liquor from stage 1 and stage 2 aeration basins. Other PAOs that did not appear to be related to Accumulibater were present in the system but they have not yet unidentified. Negative results were obtained with fluorescent in situ hybridization probes targeting Competibacter, but tetrad forming organisms were observed using staining techniques.