Apparent partial denitrification and anammox contribution to nitrogen removal in full-scale multistage anoxic–oxic system treating food waste digestate

Food waste digestate (FWD) contains high NH 4 + -N and total chemical oxygen demand (COD), but limited bioavailable carbon, challenging conventional nitrification–denitrification. This study quantified pollutant removal contributions across a full-scale multistage anoxic–oxic (A/O) system consisting of five reactors, A1, O1, O2, O3 and A2, and examined the factors underlying spatial functional differentiation. 15 field sampling campaigns were integrated with apparent nitrogen pathway partitioning, batch kinetic tests, dissolved organic matter (DOM) characterization, enzyme assays, and microbial community analysis. Pollutant removal was concentrated in A1/O1, whereas O2, O3, and A2 provided limited additional removal. Across A1, total nitrogen (TN) decreased from 148.12 to 76.60 mg/L, NH 4 + -N from 90.46 to 36.38 mg/L, and NO 3 − -N from 22.26 to 0.90 mg/L. Apparent stoichiometric partitioning attributed approximately 75.5% of the identifiable nitrogen-removal contribution in A1 to partial denitrification/anammox. Batch tests showed that carbon-source type regulated NO 3 − -N reduction, NO 2 − -N turnover, and NH 4 + -N involvement, while O1 maintained stable nitrification. DOM characterization further showed that bioavailable carbon was depleted in the front-end units, leaving humic-like, low-reactivity fractions downstream. Enzyme patterns determined by enzyme-linked immunosorbent assay (ELISA) and microbial community profiles supported the functional differentiation between A1 and O1. Thus, limited downstream removal was associated with substrate depletion rather than insufficient hydraulic retention. Effective treatment of food waste digestate requires strengthening the front-end reaction zones and regulating carbon bioavailability, rather than relying on uniformly distributed treatment functions across the multistage configuration.