Wastewater treatment plants are among the most complex sources of odorous emissions to manage in both industrial and environmental contexts due to the very nature of the treatment process: different operational stages, variable organic loads, open areas and enclosed spaces coexisting within the same facility, each with distinct emission characteristics.
As a result, there is no single solution capable of addressing all critical issues. Every area of the plant, from pre-treatment units to the sludge line, from oxidation tanks to sludge handling rooms and the vents of residual sludge storage silos, requires a specific analysis and a solution tailored to actual operating conditions.
This article describes the four critical areas typically found in these facilities, the emission-related issues associated with each of them, and the principles that guide the selection of the most appropriate mitigation measures.
The emission complexity of a wastewater treatment plant does not depend on a single source, but on the combination of biological, chemical and physical processes that occur simultaneously in different areas of the facility.
The main compounds responsible for odorous emissions are:
These compounds are generally emitted simultaneously, in proportions that vary according to process conditions. This highlights the need for a treatment system capable of acting simultaneously on compounds with very different chemical characteristics.
Another critical factor is the variability of emission loads over time: these depend on incoming organic loads, weather conditions, operational phases and seasonal variations. A system designed on the basis of average values may prove inadequate during peak events, which are often the most critical periods in terms of external odour complaints.
Added to this is a fundamental distinction in the physical nature of emissions, which influences the entire design approach. Point-source emissions, originating from vents, local extraction systems or enclosed spaces, can be captured and treated using dedicated filtration systems. Fugitive emissions, typically associated with open or semi-open areas such as screening channels and equalisation basins, cannot be contained through conventional extraction systems and require perimeter-control or enclosure solutions. Defining the operating context is therefore essential for the correct design of environmental control systems.
Oxidation and denitrification tanks are often a source of odorous emissions within wastewater treatment plants.
Odours mainly originate from insufficient oxygenation conditions or from the anaerobic degradation of organic matter:
The compounds most frequently responsible for unpleasant odours are:
The distinguishing feature of this area is the fugitive nature of the emissions. These tanks are often open, making it technically impractical to install conventional extraction and ducting systems without significant structural modifications to the existing facility. Emissions disperse into the atmosphere before they can be effectively captured.
In these situations, the most appropriate approach is not air treatment through extraction, but the containment of emission dispersion by means of an osmogenic barrier. The system generates a continuous curtain of microdroplets containing neutralising agents that intercept odorous compounds before they spread into surrounding areas or beyond the plant perimeter. This solution does not interfere with plant operations and does not require modifications to existing infrastructure.
This was the case in a project we carried out at a wastewater treatment plant in northern Italy, where emission issues were concentrated around the oxidation and denitrification tanks. The project involved the design, supply and installation of an osmogenic barrier consisting of four HPS EVO systems equipped with 300 nozzles and 1,200 metres of dedicated piping. The activities also included the optimisation of existing extraction systems and the verification of capture flow rates, with the aim of integrating the perimeter control system with improved containment measures in the most critical areas.
The sludge line is generally the most critical section of a wastewater treatment plant from an odour-control perspective. Thickening, dewatering and storage processes concentrate the organic compounds degraded during treatment, generating high-intensity emissions of ammonia and H₂S.
In thickeners, sludge is retained for a certain period, allowing solids to settle at the bottom (gravity thickening), or it is concentrated through mechanical systems. The thickened sludge is then collected and sent to subsequent treatment stages, while the separated water is generally recirculated to the head of the plant.
Odours are mainly generated because sludge contains high levels of biodegradable organic matter and, during its retention in the thickener, oxygen-deficient conditions (anaerobic conditions) may develop. Under these conditions, anaerobic bacteria break down organic matter, producing odorous compounds.
Odour intensity generally increases when:
For this reason, thickeners are often considered among the main sources of odorous emissions in wastewater treatment plants and may be enclosed and equipped with air extraction and treatment systems.
In one project we carried out, on-site assessments identified:
The solution involved the design and installation of an air extraction and treatment system equipped with a DKFil dry filter, sized for an airflow rate of 9,000 Nm³/h. The activities included:
Commissioning results confirmed an odour removal efficiency of over 90%.
The selection of a dry filtration system in this application followed a specific rationale: the DKFil operates through selective physical and chemical adsorption and is particularly effective in treating high concentrations of H₂S and ammonia, even under highly variable emission conditions, without requiring the management of liquid effluents typically associated with wet scrubber systems.
The sludge handling room is the area of the plant where final sludge treatment operations take place, including mechanical dewatering (using centrifuges, filter presses or belt presses), temporary storage and handling prior to recovery or disposal.
In this section, sludge coming from thickeners or digesters is treated to further reduce its water content.
The sludge handling room is one of the most critical areas from an odour-control perspective because sludge handling and treatment operations promote the release into the air of volatile compounds already present in the material or generated by its biological degradation.
Odour emissions are favoured by:
For these reasons, sludge handling rooms are often maintained under negative pressure through air extraction systems. The extracted air is then treated using biofilters, chemical scrubbers or activated carbon filters before being released into the atmosphere.
In one project we carried out, preliminary assessments highlighted:
The solution involved an extraction system serving the sludge handling room, combined with a hybrid treatment plant consisting of an acid wet scrubber, an alkaline wet scrubber and a horizontal DKFil unit, designed for an airflow rate of 20,000 Nm³/h. The activities included:
The selection of a three-stage hybrid system reflects the chemical nature of the emission mix: the acid scrubber is primarily used for ammonia removal, the alkaline scrubber targets sulphur compounds, while the DKFil unit treats residual VOCs and compounds not removed during the previous stages. Each stage is sized according to the pollutant concentrations entering the treatment process.
In storage tanks, the loading phase can generate abnormal emissions of sulphur compounds, creating both environmental and operational issues.
The activities carried out included:
For this application, a vent extraction system was installed, combined with a hybrid treatment unit (wet scrubber + DKFil), designed for an airflow rate of 1,000 Nm³/h to treat emissions containing H₂S.
The project enabled the optimisation of vent management, addressing point-source emissions characterised by high concentrations of pollutants and odorous compounds, as well as significant emission variability. By capturing and conveying these emissions to a dedicated treatment system, an effective environmental control measure was implemented to mitigate their impact.
The correct approach therefore begins with a preliminary assessment phase that should precede any engineering decision:
Only after this phase is it possible to correctly size the treatment system, avoiding interventions that address symptoms rather than the root causes.
The project made it possible to optimise the management of high-concentration point-source emissions, improving vent capture and conveyance while increasing the overall efficiency of the treatment process.
The four examples described above illustrate technically different interventions applied to different areas of the same type of facility. This variability in solutions is not random; it reflects an assessment process that always precedes the selection of a treatment system and takes into account variables specific to each operating context.
The main factors that determine the choice of technology are:
None of these variables can be properly assessed without direct emission characterisation. Sizing a system based solely on literature values or previous experience with similar installations may provide a useful starting point, but it cannot replace actual measurements. For this reason, our projects systematically include a preliminary chemical and olfactometric assessment phase, carried out in collaboration with the Gesteco Analysis Laboratory and the LOD Olfactometric Laboratory, both part of the Luci Group. This collaboration enables accurate emission characterisation, identification of the compounds responsible for odour nuisance and quantification of the odour load that must be removed to achieve compliance with prescribed limits.
The same approach applies to existing facilities where treatment systems are already installed. In many cases, wastewater treatment plants operate with outdated systems or units that are undersized compared to current operating conditions. Our revamping service begins with a detailed assessment of the existing situation, identifies the causes of inefficiencies and proposes targeted upgrades capable of restoring performance without necessarily replacing the entire system. A consultancy service is also available for preliminary evaluations.
Every wastewater treatment plant presents a unique combination of emission-related issues that is unlikely to be replicated exactly elsewhere. This specificity makes a section-by-section approach essential, even before selecting the most appropriate treatment technology.
If you operate a wastewater treatment plant and are evaluating a new odour control system or assessing the performance of an existing one, contact us for a technical site inspection. We will analyse the critical issues affecting your facility and propose the solution best suited to your operating conditions.
For a practical example of an intervention, including emission analysis, system design and commissioning results, you can refer to the case study we published on a municipal wastewater treatment plant: Odour abatement in a wastewater treatment plant.