Membrane bioreactor vs settler–ultrafiltration: effect of seasonal carbon to nitrogen ratio on performances, carbon footprint and membrane fouling

This study focuses on comprehensive monitoring of two membrane lines connected to an intermittently aerated Integrated Fixed-Film Activated Sludge (IFAS) reactor: a membrane bioreactor (MBR) and a Settler-Ultrafiltration (UF) unit. The system was analysed under natural influent carbon-to‑nitrogen ratio (C/N) shift from 2.5 ± 0.2 to 5.0 ± 0.5 g COD g⁻¹ N. Using real domestic wastewater, the comparison focused on carbon and nutrients removals, nitrous oxide (N₂O) emission, carbon footprint (CF) assessment and fouling characterisation via resistance in series model. Increasing C/N markedly enhanced nitrogen removal: ammonium removal increased from 29 to 42% to 88–91%, and total nitrogen removal from 31 to 44% to 78–83% across the two lines, while total COD removal remained consistently high (>90%). UF showed the highest gaseous N₂O flux (3.0 ± 1.7 to 2.4 mg N₂O-N m⁻² h⁻¹) while MBR roughly halved (1.4 ± 0.4 to 0.73 mg N₂O-N m⁻² h⁻¹). Despite this, the EF increased as nitrification improved with higher C/N (MBR line: 0.33% to 0.72%; UF line: 0.37% to 0.72%). CF decreased markedly and became dominated by indirect emissions: Settler-UF 2.49 to 1.09 kg CO₂e d⁻¹; MBR 2.18 to 0.75 kg CO₂e d⁻¹. Fouling rates dropped sharply (UF 0.38 to 0.04 bar d⁻¹; MBR 0.28 to 0.07 bar d⁻¹) in the C/N 5 g COD g⁻¹ N period, while UF exhibited higher irreversible resistance. Overall, the results demonstrate that influent carbon quality strongly governs nitrogen removal, N₂O pathways, carbon footprint, and fouling behaviour, and that emission factors and carbon footprint may diverge under improved biological performance.