Dosing alum to remove phosphorus (P) from wastewater is a common practice. However, the dosing-location and quantity of alum required to meet P discharge limits are vaguely defined. As such, utilities overdose alum to avoid noncompliance, but this leads to wastage and costs. This study aimed to address this issue through a long-term evaluation of an alum-assisted full-scale intermittently decanted extended aeration (IDEA) plant. Specifically, the effects of relocating alum dosing from a low P containing IDEA-tank to a bioselector containing elevated P concentrations were examined. The plant is fitted with two IDEA-tanks, each retrofitted with a bioselector at the inlet end. Over 359 d, key parameters (dissolved oxygen (DO), NH4+-N, NO2−-N, NO3−-N, PO43–-P) were quantified to account for the effects of switching alum-dosing into the bioselector and varying dosages (429, 643, 1072 and 1286 g-Al3+ per treatment cycle). Results indicated a 52% reduction of alum usage with no impact on discharge limit (≤0.85 mg-P/L). As expected, a failure to maintain DO setpoint (1.6 mg/L) reduced both NH4+-N and PO43–-P removal. Increasing alum dosage simply could not alleviate this problem, but maintenance of DO at least 85% of setpoint enabled effective rectification. This 15% DO buffer zone offers operators an opportunity to rectify imminent operational failures related to DO, prior to escalating alum dosage. An operational framework to manage DO related failures is proposed. Overall, this study offers insights on how to cost effectively apply alum and manage DO failures to achieve P discharge limits in IDEA plants.