An effective strategy for restoring the biofilm system of anammox/endogenous partial denitrification combined with continuous-flow partial nitration from deterioration.

Hydroxylamine supplementation has recently emerged as a potential strategy to persistent sustain partial nitration (PN), but its feasibility in the biofilm system of anammox/endogenous partial denitrification combined with continuous-flow partial nitration (SAEPD-CFPN) systems remains unexplored. Thus, this study assessed the feasibility of hydroxylamine dosing for restoring PN in continuous-flow biofilm reactor and investigated the impact of continuous-flow partial nitration (CFPN) deterioration on biofilm system of SAEPD-CFPN. Results showed that nitrogen removal efficiency (NRE) decreased from 88.86 % to 39.56 %, with the nitrite accumulation rate (NAR) dropping from 87.28 % to 34.33 % due to the deterioration of CFPN. After effective hydroxylamine dosing, the average NRE increased to 89.90 %, while the average NAR rose to 83.83 %. The SAEPD biofilm system could handle insufficient NO-N supply caused by CFPN performance insignificant deterioration; however, its effectiveness diminished when CFPN performance severely deteriorated over the long term due to lower influent COD concentrations. Metagenomic analysis revealed that CFPN recovery was attributed to the effective inhibition of Nitrobacter and NxrAB. The robustness of the SAEPD biofilm reactor against fluctuations in influent NO-N was attributable to its complex microbial community structure. Additionally, intermittent hydroxylamine dosing was proposed as a sustainable strategy to ensure stable SAEPD-CFPN biofilm process operation.