Remigi, Enrico, Remigi

Remigi, Enrico

Enrico Remigi is a speaker at WEFTEC 2026.

Titles from this speaker

Description: Assessment and Mitigation of Hydrogen Sulphide Emissions at the Gallipoli Wastewater...

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

Abstract

[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.

This paper was presented at WEFTEC 2026 in New Orleans, Louisiana.

Presentation time

14:30:00

14:45:00

Session time

13:30:00

15:00:00

SessionA Model Odor Control Session

Session locationErnest N. Morial Convention Center

TopicAir Quality and Odor Management, Collection Systems, Facility Operations and Maintenance, Municipal Wastewater Treatment Design, Public Communication and Outreach

Author(s)

Remigi, Enrico, Casile, Andrea, Briano, Jacopo, Polesel, Fabio

Author(s)E. Remigi1, A. Casile2, J. Briano1, F. Polesel4

Author affiliation(s)DHI, 1DHI S.r.l. a socio unico, 1

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160374

Volume / Issue

Content sourceWEFTEC

Word count14

Description: Practical Application Of A Model-Based Digital Twin For Monitoring and Scenario...

Practical Application Of A Model-Based Digital Twin For Monitoring and Scenario Analysis Of A Water Resource Recovery Facility

Abstract

1. INTRODUCTION
The effective management of modern water resource recovery facilities (WRRF) requires a holistic approach that considers multiple objectives, such as process efficiency, energy efficiency, resource efficiency and carbon footprint. Recent developments in the field of digitalization have proven instrumental to achieving these goals. Online sensors enable a detailed monitoring of the systems in real time, while mathematical models of the processes enable to create a virtual replica of the WRRFs which can be utilized to evaluate a variety of operational strategies, prior to their actual implementation [1]. Gruppo CAP manages the integrated water service of the Metropolitan City of Milan (Italy) and has historically been an early adopter of digital solutions to manage operations of its water- and wastewater facilities. This study presents the development and practical use of a Digital Twin (DT) Decision Support System (DSS) for the Bresso-Niguarda WRRF.
2. MATERIALS & METHODS
2.1. Description of the WRRF The Bresso-Niguarda WRRF (Figure 1) treats an average 2,550 m3/h flow from two combined sewers in the northern area of Milan, has a design capacity of 250,000 PE and presently treats a daily load of approximately 210,000 PE. The plant includes the following: mechanical preliminary treatment, primary sedimentation, conventional activated sludge process, tertiary treatment, anaerobic digestion and dewatering of sewage sludge. Typical indicators for operational performance are shown in Table 1. Bresso-Niguarda is one of Gruppo CAP's flagship WRRFs where advanced control strategies are implemented to regulate e.g., biomethane separation, sulfur recovery, low-OPEX impact control logics. The plant is also equipped with nearly 100 sensors and energy meters, which provide a detailed insight in its operation in real-time. Bresso-Niguarda was therefore an ideal candidate for the implementation of a DT that would enable the evaluation of operational alternatives to optimize the process efficiency and energy footprint.
2.2 Digital Twin implementation The DT includes three key components (Figure 2): 1) Data layer: a MIKE OPERATION (DHI A/S) workbench was set up to gather data from the SCADA (present on site); generate input files for the WEST (DHI A/S) model; and to retrieve output data from the WEST model to display on the dashboard. Through the SCADA, data are retrieved in real time, quality checked and pre-processed (e.g., anomaly detection and gap filling), from over 60 sensors, including 18 energy meters which monitor the electricity consumption in key sections of the plant.
2) Process model: a comprehensive model of the plant was implemented in WEST (DHI A/S), including the water line, the sludge line and the control layer. The model was pre-configured with input and output files as well as custom key performance indicators (KPIs) as the main interface handles to the data layer and to the dashboard. The modelling approach is described in detail in section 2.3.
3) Dashboard and performance benchmarking: a dedicated, user-friendly dashboard enables operators to modify the boundary conditions for the underlying process model and to assess a variety of scenarios by comparing process indicators and energy consumption time series throughout the plant, such as: treatment efficiency, energy and chemical consumption for water and sludge treatment, biogas production and valorization and sulfur recovery.
2.3 WEST model The model includes the water, the sludge line and the biogas handling line (gasholder, flare, boiler and engine, sulfur recovery). Specific focus was given to (i) the mechanistic modelling of automatic controllers for chemical dosing, aeration (termed 'Liquicontrol'), sludge recirculation and digesters loading, based on Gruppo CAP's specifications; and to (ii) the accurate simulation of the energy balance over the WRRF. The process model chosen for the water line was ASM2dISS, an extension of the Activated Sludge Model no. 2d (ASM2d, [2]) with extra components and processes to handle inorganic suspended solids (ISS) and sulfur. An extension of the Anaerobic Digestion Model no. 1 was used for the sludge line and included biological sulfur reduction (inspired by [3]) and chemical H2S precipitation.
3. RESULTS
3.1. Preliminary model calibration Prior to its implementation as a DT component, the WEST model was calibrated against historical data (both derived from online sensor and laboratory measurements) for different target variables pertaining to solids balances, nutrient removal, biogas production and composition, and energy balance. Figure 3 presents selected results for solids concentration in process tanks, ammonium concentration in the denitrification effluent and final effluent and biogas flow rate produced in the two parallel digesters. Good agreement is shown between simulation results and measurements, which was the precondition to the incorporation of the model into the DT.
3.2. Application of Digital Twin in daily operation
Currently, the DSS is installed on a working station of Niguarda-Bresso WRRF. Operators, process engineers, as well as the energy management team can run simulations and create scenario analyses with 24-h forecast horizon, to assess the process performance under current or modified boundary conditions (e.g., inlet flow rate, pollutant loads, temperature) and operational settings (e.g., controller set-points). There are 7 key-handles available to the user: the oxygen and ammonia set points of the aeration controller; the mixed liquor recirculation set-point; the reagent dosing set-points; the option to vary flow and enabling/disabling certain sections (e.g., one of the primary settlers, dewatering of secondary sludge) in order to reproduce maintenance and future revamping; the sludge loading schedule to the anaerobic digesters; the biogas utilization (biomethane production, thermal valorization). Examples of scenario analyses are shown in Figures 4 and 5. A six hour-rainfall event, resulting in a constant flow of 5,000 m3/h, while only one mixed liquor pump was operated (Q = 1500 m3/h); and a 50% increase in the sludge loading to digester over 6 hours. The scenario analysis shows a temporary reduction in COD, N and P removal and an increase in biogas production and methane content, as compared to the baseline operation.
4. CONCLUSIONS
A Digital Twin solution was implemented on the Niguarda-Bresso WRRF for the model-based evaluation of the activated sludge aeration system, chemical dosage, mixed liquor recirculation, and sludge treatment. The KPIs focus on reducing energy and chemical consumption, increasing biogas recovery, while ensuring a high process efficiency. Following a successful validation period, this first-generation DT will be upgraded to a full-scale tool for monitoring, decision support and early warning using both scenario analysis and optimization. Furthermore, it is envisaged that the future DT may allow for automated transfer of control actions, e.g. optimal set-points derived from model-based evaluations, back to the plant.
ACKNOWLEDGEMENTS
This project is the first example of Digital Twin applied to wastewater sector in Italy and was financed by 'Industry 4.0' Italian Fund. The system has been proposed for the upcoming release of the white paper in SWAN FORUM-The Smart Water Networks Forum (https://www.swan-forum.com/).

This work presents a model-based Digital Twin realized for the Bresso-Niguarda treatment facility in Italy and that has been used by the operators to monitor the plant performance in real-time and test alternative operations in user-friendly virtual environment. The process model describes the control layer regulating aeration, chemical dosage, sludge recirculation and digesters loading, and implements specific target KPIs required to ensure process performance and minimum energy footprint.

SpeakerRemigi, Enrico

Presentation time

16:25:00

16:40:00

Session time

15:30:00

17:00:00

TopicIntermediate Level, Facility Operations and Maintenance, Intelligent Water, Research and Innovation

Author(s)

Remigi, Enrico

Author(s)Enrico U. Remigi1; Fabio Polesel2; Mirko Flauto3; Luca Spinelli4; Roberto Di Cosmo5; Marco Muzzatti6

Author affiliation(s)DHI A/S, Hørsholm, Denmark1; DHI A/S, Hørsholm, Denmark2; DHI srl, Genova, Italy CAP 3; Gruppo CAP, Milano, Italy4; Gruppo CAP, Milano, Italy5; Gruppo CAP, Milano, Italy6

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Oct 2022

DOI10.2175/193864718825158729

Volume / Issue

Content sourceWEFTEC

Word count19