Achieving nitrogen removal in the integrated upper fixed-biofilm activated sludge reactor without recirculation: Differential protein and metagenomic analysis.

The Integrated Fixed-film Activated Sludge (IFAS) system emerges as an advanced nitrogen removal technology, particularly effective for treating high-nitrogen wastewater due to its sophisticated configuration. This research introduces an enhanced Integrated Upper Fixed-film Activated Sludge (IUFAS) reactor featuring a two-stage series design. By strategically positioning carrier media in the upper compartment and implementing controlled influent distribution with aeration in the lower section, the system achieves functional compartmentalization within a single reactor without liquid and sludge recirculation. Experimental results under influent conditions of chemical oxygen demand/total nitrogen (C/N) ratio (4-5) and hydraulic retention time (10 h) confirmed effective nitrogen removal, evidenced by effluent total nitrogen consistently below 7 mg N/L and removal efficiency exceeding 87%. Notably, the optimized IUFAS configuration achieved functional zoning by establishing a pronounced dissolved oxygen gradient between the upper (0.1 ∼ 0.7 mg/L) and bottom compartments (0.3 ∼ 3.6 mg/L). This oxygen stratification facilitated distinct nitrogen removal pathways, including stable anaerobic ammonium oxidation (Anammox) as evidenced by successful Candidatus Brocadia enrichment in the secondary reactor's upper zone. Microbial analysis further indicated potential modulation of electron flow by sulfate-reducing bacteria and sulfur-driven denitrifying bacteria, whose synergistic activity optimized electron transfer pathways and enhanced denitrification efficiency. Additionally, microalgae reduced aeration demand, lowering energy consumption. These findings propose novel strategies for optimizing carbon source allocation in nitrogen removal processes, supporting the development of energy-efficient wastewater treatment systems.