Reveal the impacts of temporary recirculating effluent on a continuous-flow anaerobic/oxic/anoxic (AOA) system

Intracellular carbon storage and endogenous denitrification are important for post-anoxic nutrient removal in anaerobic/oxic/anoxic (AOA) systems. Although additional sludge recirculation has been used to strengthen post-anoxic treatment, its effects are coupled with increased biomass loading. In this study, clarified effluent was temporarily recirculated to the post-anoxic zone of a continuous-flow AOA system operated with a single conventional sludge return. The effluent recirculation ratio (R₂) was sequentially adjusted to 0, 50 %, 100 %, and 0 to examine the treatment and biological responses during its introduction and subsequent withdrawal. At R₂ = 100 %, effluent TIN and PO₄^3--P concentrations averaged 2.97 and 0.54 mg/L, corresponding to removal efficiencies of 93.7 % and 86.9 %, respectively, while the calculated COD_intra rate and specific endogenous nitrate-reduction rate were 97.6 % and 0.93 mg N/(g VSS·h), respectively. After effluent recirculation was withdrawn, TIN removal recovered to approximately 93.7 %, while PO₄^3--P removal remained at 86.7 %; the specific endogenous nitrate-reduction rate further increased to 1.17 mg N/(g VSS·h). Microbial-community analysis showed that nitrifying populations remained at relatively low abundances, whereas PAOs and representative denitrifying functional groups were more abundant in the later operational phases. In Phase IV, PAOs reached 4.92 %, while the representative DPAO Denitratisoma and DGAO Candidatus Competibacter reached 1.77 % and 3.58 %, respectively. Overall, temporary effluent recirculation was associated with improved nutrient removal and biological changes that partly persisted after its withdrawal, providing an experimental basis for further evaluation of this operational approach in continuous-flow AOA systems.