The microbiology of denitrifying enhanced biological phosphorus removal systems has been a subject of much debate. The question has centred on the affinities of different types of Candidatus Accumulibacter PAOs, type I and type II, towards different electron acceptors such as oxygen, nitrate and nitrite. This study used a propionate anaerobic/anoxic/aerobic lab-scale sequencing batch reactor where a microbial culture was successfully enriched in Accumulibacter type I organisms (approx. 90%). The culture was able to take up phosphorus using nitrate, nitrite and oxygen as electron acceptors, although experiments with oxygen led to the fastest P removal rate. The phosphorus uptake to nitrogen consumed ratio (P/N ratio), when using both nitrate and nitrite, was shown to be affected by pH in the range of 7–8.2, achieving higher values for lower pH values (7.0–7.5). The effect of pH on P removal seems to follow a similar trend for both nitrate and nitrite. To our knowledge, this is the first study where the impact of pH in the phosphate removal stoichiometry using the three most significant electron acceptors is shown for such a high enrichment in Accumulibacter type I.

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