Nitrogen Removal Performance of an IFAS System under Low C/N Municipal Wastewater Conditions

To overcome the limited total nitrogen removal and dependence on external carbon sources in conventional treatment of low C/N municipal wastewater, a continuous-flow integrated fixed-film activated sludge (IFAS) reactor was established to investigate its nitrogen and carbon removal performance and underlying mechanisms under different organic loading conditions. The reactor was operated for 203 days with progressively increased influent COD, combined with dissolved oxygen regulation, intermittent aeration, and hydrazine-assisted inhibition. The results showed that nitrogen removal gradually improved after start-up. Under higher organic loading, the reactor achieved an average total nitrogen removal efficiency of about 80%, with a maximum of 89%, and showed good recovery after a 45-day starvation period. Activity tests suggested the coexistence of anammox, partial nitritation-anammox, and partial denitrification-anammox pathways. Overall, the stratified microenvironment in the IFAS system promoted synergistic interactions among functional microorganisms, indicating its potential for efficient and stable nitrogen removal from low-C/N municipal wastewater.