For maintaining high phosphorus removal efficiency,
it is necessary to eliminate nitrate in the RAS
[12]. In order to protect the anaerobic conditions forsufficiently available carbon source uptake and phosphorus
release by PAOs, pre-denitrification and
post-denitrification are always adopted to remove the
remaining nitrate in the effluent and the RAS [4]. In
the former process, the RAS is first denitrified in a
separate anoxic tank and then transferred into the anaerobic
zone; while in the latter, the mixed liquor of
the system is finally denitrified in an anoxic tank for
further reduction of remaining nitrate. However, these
processes always require exogenous carbon source.