Enrichment of PAO and DPAO responsible for phosphorus removal at low temperature

A new strategy of enrichment of polyphosphate accumulating organisms (PAO) and denitrifying polyphosphate accumulating organisms (DPAO) at low temperature ranging from 8 °C to 11 °C was demonstrated through two lab-scale reactors operated in sequential anaerobic-aerobic (AO) or anaerobic-anoxic (AA) conditions. It was found that the AO reactor is able to achieve a good phosphorus removal performance after 40 days of operation, while a similar stable phosphorus removal can be obtained in the AA reactor after 80 days. This result suggests that the enrichment of PAO was easier than that of DP AO at low temperature. Through switching batch tests, when DPAO is exposed to aerobic conditions, it can immediately exhibit a good phosphorus removal similar to that under anoxic conditions, while PAO can only present poor phosphorus removal when exposed to anoxic conditions, suggesting that two different types of Accumulibacter were enriched both in AA and AO reactors. Microbial analysis with fluorescence in situ hybridization (FISH) and DAPI (4',6-diamidino-2 -phenylindole) staining revealed that Accumulibacter was dominant both in the two reactors, accounting for 61.6% and 79.3% of all bacteria in AA and AO reactors, respectively. Although the different amount of Accumulibacter was enriched in the two reactors, the similar microbial morphologies were observed by using scanning electron micrograph (SEM), both presenting long-rod morphology. This kind of Accumulibacter may display affinities for sodium acetate used as the carbon sources here. This strategy proposed in this study was shown to be effective in achieving a very high enrichment of Accumulibacter at low temperature by linking chemical analysis with microbial observation.