Deep-level nutrient removal and denitrifying phosphorus removal (DPR) potential assessment in a continuous two-sludge system treating low-strength wastewater: The transition from nitration to nitritation

Abstract In a continuous two-sludge denitrifying phosphorus removal (DPR) process of anaerobic anoxic oxic - moving bed biofilm reactor (AAO - MBBR), nitritation was practicable through the combined regulation of high temperature (T: 30–32 °C), short hydraulic retention time (HRT: 8 h) and low dissolved oxygen (DO: 1.0–1.5 mg/L). The system lasted for 90 days with stable nitrite accumulation ratio (NAR > 60%), and the total inorganic nitrogen (TIN) removal was 7% higher than complete nitrification. Ammonia oxidizing bacteria ((AOB) 6.18–9.41%) responsible for nitritation showed a clear relationship with NAR, but Nitrospira (2.11% → 2.35%) gradually outcompeted Nitrobacter (1.19% → 0.31%) under higher temperature. During the transition from nitration to nitritation, the DPR potential (characterized by ΔPO43−/ΔNOx−) increased by 11.90% while the energy requirement of poly-β-hydroxyalkanoates (PHA) and glycogen (Gly) decreased by 12.58% and 14.50%, respectively, contributing to higher TIN (84.83%) and TP (97.45%) removals. DPR batch tests using different electron acceptors (NO3− .vs. NO3− + NO2−) revealed that removing 1 mg PO43− only consumed 7.12 ± 0.25 mg PHA via NO3− + NO2− (.vs. 8.50 ± 0.12 mg PHA via NO3−) and 16% carbon source was saved although the DPR capability was suppressed as NO2− concentration exceeded 15 mg/L. Based on the achievement of nitritation, the feasibility of integrated DPR - Anammox in the AAO - MBBR system for deep-level nutrient removal was discussed.