Mass-flow control enhances nitrogen removal in a three-stage partial nitritation, anammox and partial denitrification/anammox system treating food waste digestate: Performance and N2O release characteristics.

The partial nitritation/anammox process is a promising technology for treating food waste digestate (FWD). However, its nitrogen removal efficiency is limited to about 89%. To overcome this constraint, a three-stage system integrating partial nitritation, anammox, and partial denitrification/anammox (PN-A-PDA) was established. The challenge of realizing stable continuous operation was addressed through mass-flow regulation, achieving a total inorganic nitrogen removal efficiency (TINRE) of 97.6 ± 1.4%. The effluent NO2--N/NH4+-N ratio in PN unit was the key control variable for overall mass-flow regulation, which was maintained at 1.15 - 1.23 by adjusting the aeration coefficient (AC) to 311 - 372 m3 air/kg N and pH feedback ranges to 8.25 - 8.31. The NO3--N/NH4+-N ratio in the anammox effluent increased exponentially with influent NO2--N/NH4+-N ratio and was maintained at 1.2 - 3.3, corresponding to a TINRE of 91.4 ± 2.9%. TINRE in PDA unit was maintained at 69.5 ± 9.5% by regulating HAc-COD/NO3--N within 2.1 - 2.8 based on periodic quantitative carbon source dosing mode. Moreover, the three reactors exhibited distinct N2O release and consumption characteristics. The PN reactor dominated N2O production, with 97.8% released to the atmosphere, equivalent to 2.75% of influent nitrogen. The remaining 2.2% entered the downstream reactors as dissolved N2O, with stage-specific removal efficiencies of 73.3% and 58.8% in the anammox and PDA reactors, respectively. These findings demonstrate the feasibility of the mass-flow controlled PN-A-PDA system and clarify its specific N2O release and consumption behavior during food waste digestate treatment.