Towards Operational Net-Zero Pathways in Industrial Wastewater Treatment Systems Using Digital Decision Support Tools

• A digital decision support tool (DST) reproduces plant-wide mass/energy balances. • DST accounts Direct (DE), Energy-related (ERE), selected Up&Downstream (UDE), and avoided emissions. • Baseline total operational GHG is 5.6 kg CO 2 -eq/m 3 (DE 39%, ERE 9%, UDE 52%). • Operational net-zero failure could be due to elevated N 2 O/energy/chemicals. • Direct bio-CH 4 injection increases carbon credit ten-fold (net-negative operation). Effective green transition strategies require reliable quantification of greenhouse gas (GHG) emissions and the ability to evaluate alternative operational regimes. In this study, a digital decision support tool (DST) is constructed to assess and meet net-zero targets in water treatment systems (WTS). A set of mathematical models was developed and integrated to replicate the behavior of the largest industrial WTS in Northern Europe, focusing on effluent quality and operational costs. In parallel, a GHG emissions module was constructed to estimate direct emissions (DE, Scope 1 ), indirect energy-related emissions (ERE, Scope 2 ), selected / operation-linked upstream and downstream emissions (UDE, derived from a screened subset of Scope 3 ), and avoided emissions (AE, often referred to as Scope 4 ). Additional biogenic CO 2 emissions (BioE) were quantified within both DE and UDE categories. Four mitigation scenarios were analyzed: (i) carbon refluxing (S1), (ii) carbon recovery and alternative inactivation strategies (S2 and S3), (iii) nitrous oxide (N 2 O) mitigation (S4 and S5), and (iv) biogas upgrading and direct grid injection (S6 and S7). Simulations demonstrate that the proposed approach accurately reproduces COD, N, P, and S mass balances (9.3% deviation), long term and high frequency dynamic performance profiles (11.3% deviation, NRMSE < 0.16), plant-wide energy consumption and production (5.0% and 3.8% deviations respectively), and operational expenditures (OPEX) (4.3% and 3.9% deviations for revenues and costs). GHG accounting results indicate that DE accounts for 39%, ERE for 9%, and UDE for 52% of total operational emissions corresponding to approximately 5.6 kg CO 2 -eq/m 3 . N 2 O is the dominant contributor to DE (88%), while UDE is primarily driven by downstream sludge treatment (74%). AE from fertilizer and downstream natural-gas substitution fully counterbalanced total GHG releases, resulting in an operational net-zero performance of -0.2 kg CO 2 -eq/m 3 within the defined system boundaries. When BioE is accounted, DE is increased by a factor of 3. S1, S6 and S7 achieve operational net-negative performance up to -2.1 kg CO 2 -eq/m 3 by the raised AE from increased substitution of grid energy and mineral fertilizers. S2, S3, S4 and S5 fail to achieve the same net-zero targets due to a significant rise in N 2 O emissions driven by altered COD/N ratios and/or excessive energy (heat) and chemical (NaOH) demands. By integrating comprehensive modelling and GHG accounting, the DST supports evidence-based decision-making for industrial stakeholders seeking to optimize resource use, minimize operation-linked emissions, and guide long-term investments in sustainable wastewater infrastructure. Based on the presented results this tool is now used by the company to handle future optimizations.