Assessment and Mitigation of Hydrogen Sulphide Emissions at the Gallipoli Wastewater Treatment Plant

[b]Introduction and Objectives[/b] Hydrogen sulphide (H2S) emissions pose a significant operational and environmental challenge in wastewater treatment plants (WWTPs), particularly where long sewer residence times and anaerobic conditions prevail; under such conditions, sulfate-reducing bacteria produce H₂S, leading to odour nuisance, infrastructure corrosion, and health hazards [1]. Elevated H2S also compromises treatment performance and biogas quality, increasing operational costs and limiting energy recovery potential, making its control a priority in modern WWTP operation [2]. This study presents a feasibility assessment of H2S generation, transport, and emissions at the Gallipoli municipal WWTP (Italy), The objectives were to (i) identify the main sources and pathways of H2S, (ii) quantify emissions under current and future loading conditions, and (iii) evaluate mitigation strategies, including sewer network interventions and in-plant control measures. A further objective was the development of a plant-wide process model to support future optimization and decision-making. [b]Plant Description and Operational Context[/b] The Gallipoli WWTP is designed for approx. 80,000 population equivalents (PE) and experiences strong seasonal variability due to tourism, with influent flows ranging from 5,000–6,000 m[sup]3[/sup]/d in winter to over 12,000 m[sup]3[/sup]/d in summer. The treatment line comprises preliminary treatment, equalization, primary sedimentation, conventional activated sludge with pre-denitrification, secondary clarification, disinfection, and a sludge line with two-stage anaerobic digestion. Although effluent quality consistently meets regulatory limits, energy consumption is relatively high, and biogas utilization limited. The most critical issue is the presence of extremely high H2S concentrations at the inlet works and early treatment units, with liquid-phase sulphide up to 10 mg S/L and gas-phase concentrations reaching several hundred ppm (Figure 1), overwhelming existing odour control systems. [b]Data Collection and Modelling Approach[/b] The analysis relied on operational data (flows, wastewater quality, energy use, biogas production) and targeted measurements of sulphide and H2S in liquid and gas phases. Influent sulphate concentrations, estimated at 300–400 mg SO[sub]4[/sub][sup]2-[/sup]/L through modelling and spot sampling, reflect saline intrusion typical of coastal sewer systems and provide favourable conditions for sulphate-reducing bacteria. A plant-wide model was developed in WEST (DHI A/S), based on ASM2d and extended to include sulphur-related processes such as sulphate reduction, sulphide oxidation, H2S stripping, sulphur-oxidizing bacteria growth, and chemical precipitation [3]. The model covered both water and sludge lines and achieved mass balance closure errors below 0.5%. Calibration focused on reproducing measured H2S concentrations in equalization and primary treatment units, capturing observed magnitudes and temporal trends (Figures 2, 3). [b]Baseline Results[/b] Model results indicate that H₂S generation is dominated by upstream sewer processes. Approximately 94% of influent H2S is released in the equalization tanks and a further 5% during primary sedimentation, while negligible amounts reach the biological reactors. Average H2S emissions from the equalization tanks were estimated at 20–25 kg S/d, with higher values during summer peaks. Existing biofilters were shown to be undersized. High sulphide loads also affect the sludge line, leading to biogas H2S concentrations that may approach 10,000 ppm under peak conditions, preventing direct use in combined heat and power (CHP) units without extensive gas cleaning. [b]Scenario Analysis and Mitigation Options[/b] End-of-pipe air treatment upgrades (e.g. chemical scrubbers or activated carbon filters) could significantly reduce odour emissions but address symptoms rather than root causes and increase operational costs. In contrast, upstream sewer interventions-such as controlled aeration, nitrate dosing, or iron addition-were identified as the most effective measures, reducing influent sulphide loads by over 70% and providing system-wide benefits. Additional process optimizations, including improved aeration control, reduced sludge age, and optimized sludge handling, further contributed to reducing sulphide formation and energy consumption. [b]Energy and Economic Implications[/b] Lower sulphide loads markedly improve biogas quality. Combined sewer-based mitigation and targeted plant upgrades reduce biogas H2S concentrations to below 500 ppm, enabling efficient CHP operation with standard gas treatment. Preliminary economic analysis suggests that the resulting energy savings and increased energy recovery could offset capital investments within 1–2 years. [b]Conclusions[/b] H2S emissions at the Gallipoli WWTP are primarily driven by upstream sewer processes and intensified by seasonal loading variability. Plant-wide modelling proved essential for quantifying emission pathways and comparing mitigation options. While air treatment upgrades provide local benefits, the most sustainable solution lies in controlling sulphide formation in the sewer network. Combined with targeted process optimization, this approach significantly reduces odour emissions, protects assets, improves biogas utilization, and enhances overall plant performance.