This paper presents the investigation results of retrofitting an anoxic selector to an anaerobic selector through stepwise reduction of air supply in a full-scale activated sludge process with a focus on enhanced biological phosphorus removal (EBPR). The process experienced gradual shift from a Ludzack-Ettinger (LE) to an anaerobic-anoxic-oxic (A2O) process and subsequently, an anaerobic-oxic (A/O) process. The major findings are: (i) the average influent-based PO43−‐P release in the anaerobic selector compartment was 16.3 mg P l−1 and that in the secondary clarifier was 1.7 mg P l−1. 75% of the SCOD and 93% of the acetic acid in the primary effluent were taken up in the anaerobic selector compartment, respectively; (ii) PO43−‐P uptake contributed by both aerobic and denitrifying phosphorus accumulating organisms (DPAOs) occurred mainly in the first and second aerobic lanes together with simultaneous nitrification and denitrification (SND) while there was not much contribution from the last aerobic lane; (iii) The average PO43−‐P concentration of the final effluent was 2.4 mg P l−1 corresponding to a removal efficiency of 85%; (iv) the SVI was satisfactory after retrofitting; and (v) the increase of NH4+‐N in the final effluent from the commencement to the completion of the retrofitting resulted in an approximate 40–50% reduction in oxygen demand and a significant aeration energy saving was achieved.

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