Upstream inputs and sludge line recirculation drive dissolved organic nitrogen accumulation in industrial wastewater treatment systems

As effluent total nitrogen (TN) limits become increasingly stringent, dissolved organic nitrogen (DON) is emerging as a structural capacity constraint in highly optimized industrial wastewater treatment systems because conventional nitrification–denitrification primarily targets inorganic nitrogen species. The challenge is particularly acute in the biotech industries, where DON could originate from residual proteins in fermentation broths, downstream purification losses, biomass washing, and alkaline stabilization. This study develops and validates an integrated modeling framework to predict effluent dissolved organic carbon (DOC) and DON under full-scale industrial conditions while preserving COD, N, P, and metal (S, Al, Ca, Mg, Na, K) mass balance consistency. The largest industrial wastewater treatment system in Northern Europe was simulated using an upstream–plantwide mechanistic framework integrating anaerobic digestion (ADM), activated sludge (ASM), and physicochemical (PCM) models. Process extensions representing alkaline-induced solubilization and conservative transport of soluble inert DOC/DON fractions were implemented. Calibration used a dedicated short-term measurement campaign (DataS) and reconciled plant-wide mass balances. A complementary multiple linear regression (MLR) layer, developed from 22 years of operational data (DataH), was integrated to strengthen long-term predictive robustness. Ten operational scenarios (S0–S9) were evaluated, including upstream DOC/DON redirection in ultrafiltration/reverse osmosis (UF/RO) retentate and/or spent biomass, and sludge-line management to avoid DOC and DON recirculation. The integrated framework reproduced plant-wide mass balances with < 13 % deviation across treatment units. Model predictions show that DOC and DON discharge patterns are strongly controlled by upstream and biosolids-line reconfiguration, with concentrations decreasing from 200 to 50 mg COD/L for DOC and from 11 to 4 mg N/L for DON across scenarios. The regression layer further confirmed prediction reliability with R² > 0.79 between DOC/DON effluent concentrations and key operational drivers, including flow rates, COD loads, and alkaline stabilization intensity. Economic assessment indicates that avoiding DOC/DON recirculation can reduce operational expenditures (OPEX) by up to 13% (S8), and external retentate treatment may increase costs by up to 90%. Sustainability indicators further show that operational reconfiguration reshapes plant-wide resource use and environmental performance. In total, DON represents a critical structural constraint on the expansion of industrial wastewater treatment capacity. Unless DON formation is reduced through upstream or sludge-line reconfiguration, compliance constraints (DON < 5 mg N/L) will limit the plant's effective treatment capacity and narrow it to a point where capital-intensive tertiary treatments such as granular activated carbon (GAC) polishing may become unavoidable. Such measures could increase OPEX with a factor of three and add approximately 1000 tons CO₂-eq/year GHG emissions. The proposed framework provides a robust decision-support tool for designing expansion strategies that balance regulatory compliance, economic performance, and environmental sustainability.