Abstract Summary
1. Introduction Hydrogen sulfide (H₂S) poses significant challenges in sewer systems, contributing to odor, corrosion, and safety risks. Traditional chemical treatments, while effective, often rely on costly and non-sustainable materials. This study explores an innovative circular economy approach for sulfide control by utilizing a thermochemically-regenerated multifunctional coagulant using drinking water sludge (DWS) as feedstock to simultaneously achieve odor control in sewer systems and phosphorus control in primary clarifiers. The benefit of introducing a multi-mineral DWS (enriched in calcium, iron, aluminum, silica as main key constituents ) also arguably extends further downstream, e.g. by enhancing activated sludge settleability and anaerobic digestion, as reported in some (isolated) previous studies. In our work, we compared this novel coagulant against nitrate as alternative treatment agents for sewers and against iron as alternative coagulant for chemically-enhanced primary clarification. The research was conducted using a novel sewer physical twin (SPT) system that replicates real-world sewer conditions, bridging the gap between lab-scale concepts and practical applications. By focusing on biofilm dynamics, wastewater characteristics, and economic viability, this study aligns with key themes of circular water systems, climate resilience, and stakeholder engagement. 2. Materials and Methods The study utilized three parallel SPT systems to evaluate the effects of DWS, a commercial iron coagulant, and nitrate on sulfide control, biofilm behavior, and wastewater properties. DWS, rich in ferric iron and aluminum, and the commercial ferrous iron product, were dosed during the first stage of experiments. In a subsequent stage, nitrate was introduced following iron treatment to investigate synergistic effects on biofilm disruption and sulfide control. The systems were operated under dynamic conditions for over 300 days, simulating real sewer hydrodynamics and including recovery periods between treatments to evaluate system resilience. 3. Results and Discussion The results demonstrate the effectiveness of DWS in reducing sulfide and phosphorus levels, with up to 63% inhibition of sulfate reduction and 67% soluble phosphorus removal, outperforming the commercial iron product under similar conditions. The systems exhibited rapid recovery following iron treatment, with sulfide levels returning to baseline within five days, demonstrating the reversible nature of sulfide inhibition by DWS and the commercial iron product. In contrast, recovery from nitrate treatment was slower, with sulfide concentrations remaining suppressed for over 30 days due to biofilm disruption and iron release. The application of DWS also increased inert suspended solids, which, while presenting sludge handling challenges, offers opportunities for enhanced CEPT (via ballasted flocculation in the primary clarifier) and methane generation in downstream anaerobic processes. Nitrate addition following iron treatment resulted in effective sulfide control, with dissolved sulfide concentrations dropping to 1 mg/L, while sulfate levels increased due to oxidation of iron sulfides and elemental sulfur. This sequential treatment strategy disrupted biofilms, releasing iron and sulfur compounds that contributed to prolonged sulfide removal even after nitrate addition ceased. 4. Conclusions This study underscores the potential of circular economy solutions in wastewater management by demonstrating the dual benefits of resource recovery and operational efficiency. The reuse of DWS, a waste stream from drinking water treatment, exemplifies a sustainable alternative to conventional chemical treatments. Sequential application of DWS and nitrate not only enhances sulfide control but also promotes biofilm disruption, offering long-term benefits for sewer systems. These findings provide a practical pathway for scaling up circular water solutions while fostering climate resilience, economic viability, and ecosystem protection. Engaging stakeholders and integrating these approaches into policy frameworks will be critical for transitioning to sustainable and efficient wastewater management systems. Cost analysis revealed that multi-mineral DWS treatment was the most economical option, while nitrate-only treatment was significantly more expensive. Combined iron and nitrate treatments with recovery periods offered a cost-effective strategy for biofilm destabilization and long-term sulfide control, highlighting the importance of optimizing treatment sequences.
