Multistage Scrubber

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    Air Treatment System for Sludge Dryer Emissions – WWTP Application

    Technical Summary

    Application: Sludge drying
    Industrial sector: Wastewater treatment
    Air flow rate: 6,000 Nm³/h
    Air characteristics:     humid process air
    Main pollutants: H₂S, ammonia, sulfur compounds, VOCs
    Treatment lines: 2

    This sludge dryer odour control system is designed for wastewater treatment plants handling complex industrial emissions.

    Project Overview

    Sludge drying processes used in wastewater treatment plants generate complex gaseous emissions containing hydrogen sulfide, ammonia and various odorous organic compounds released during thermal treatment of sewage sludge.
    For the sludge drying units, two dedicated air treatment systems were engineered and installed to ensure reliable odour abatement.
    Each system was designed to treat process air extracted directly from the sludge dryer and combines multiple treatment stages to address the complex chemical composition of the emission stream.

    Engineering Challenges


    Air emissions generated by sludge dryers present several challenges for air pollution control systems:

    • variable pollutant composition
    • high humidity and condensable vapours
    • presence of aerosols and fine droplets
    • coexistence of sulfur compounds, nitrogen compounds, VOCs and dust

    These conditions require a multi-technology treatment approach, capable of handling different pollutant classes within the same emission stream.

    Sludge dryer odour control System Configuration

    Each treatment line integrates several sequential technologies.

    Venturi Scrubber + Packed Column

    The first treatment stage consists of a wet scrubber system composed of a Venturi section followed by a packed column.
    The Venturi section improves gas–liquid contact and promotes removal of dust and soluble compounds.
    The packed column increases gas–liquid contact time and allows efficient absorption of odorous pollutants.
    Main design parameters:

    • Air flow rate: 6,000 Nm³/h
    • Pressure drop: < 1,500 Pa
    • Contact time: 2 s
    • Column diameter: up to 2,000 mm
    • Column height: up to 7.5 m
    • Construction material: polypropylene
    industrial sludge dryer odour control system in wastewater treatment plant

    Biofiltration Stage

    Downstream of the chemical scrubber, the air stream is treated through a biofilter designed to biologically oxidize residual odorous compounds.
    The biofilter uses wood chips as filtering media, providing a suitable environment for microbial populations capable of degrading sulfur compounds.

    Main characteristics:

    • Filter media volume: 75 m³
    • Media type: wood chips
    • Moisture range: 55–85 %
    Biofiltration stage used for odour control of sludge dryer emissions in a wastewater treatment plant.

    Air Conditioning System

    To maintain optimal operating conditions for the biofilter, the system includes an air conditioning section with electric heating.
    This stage stabilizes temperature and humidity before biological treatment.

    Air conditioning stage installed upstream of the biofilter for treatment of sludge dryer emissions in a wastewater treatment plant.

    Activated Carbon Polishing Filters

    The final stage consists of activated carbon adsorption filters designed to remove trace pollutants remaining after biological treatment.
    Each filter contains approximately 2,500 kg of activated carbon, complemented by impregnated alumina for enhanced removal of sulfur compounds.

    Results

    The sludge dryer odour control system ensures stable performance and reliable odour abatement over time.

    The installed systems provide stable treatment of emissions generated by sludge drying operations, ensuring reliable odour abatement and compliance with environmental regulations.

    The integration of chemical scrubbing, biofiltration and adsorption allows progressive removal of pollutants with different chemical properties, increasing operational robustness.

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    Air Pollution and Odour Control for Category 3 Animal By-Products Processing

    Technical summary

    Flow rate: 8,000 m³/h
    Industry: Animal by-product ABP transformation
    Technology: 1 Acidic + 1 Alkali/oxidative PC-1 single staged scrubbers
    Target pollutans: Odour, ammonia, VOC
    Process air temperature: 80°C

    Read More “Air Pollution and Odour Control for Category 3 Animal By-Products Processing”
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    Odour Control for Biological Sludge Composting

    Technical summary

    Flow rate: 88,000 m³/h
    Industry: Biological sludge composting (municipal wastewater sludge + green waste)
    Technology: Multiventuri scrubber + two-stage packed column scrubber (acid + alkaline/oxidative)
    Target pollutants: Odour, ammonia, hydrogen sulphide, VOC
    Process air condition: Intermittent emissions with high odour load during loading/unloading phases

    Process-driven design for intermittent high-load emissions (88,000 Nm³/h)

    Application context

    Odour Control for Biological Sludge Composting requires a clear understanding of how emissions are generated during the composting of municipal wastewater biological sludge mixed with green waste.

    The plant treats approximately 88,000 Nm³/h of extracted air from:

    • Sludge unloading areas
    • Mixing and structuring phases
    • Composting tunnels
    • Mechanical handling operations

    The composting objective is to stabilise organic matter and produce agronomically valorisable compost through controlled aerobic degradation.

    When properly managed, composting itself does not necessarily generate severe continuous odour levels. The dominant issue lies elsewhere.

    Intermittent peak emissions during sludge unloading

    In biological sludge composting, emissions are predominantly intermittent rather than continuous.

    The highest odour peaks occur during:

    • Sludge unloading
    • Initial mixing with structuring green waste
    • Mechanical agitation of fresh material

    Fresh biological sludge may contain anaerobic microenvironments formed during storage and transport. When exposed to oxygen during unloading, rapid release occurs of:

    • Ammonia (NH₃)
    • Hydrogen sulphide (H₂S)
    • Reduced sulphur compounds
    • Volatile organic compounds
    • High humidity and bioaerosols

    Designing Odour Control for Biological Sludge Composting based on average conditions would underestimate these peak events.

    Engineering challenge at 88,000 Nm³/h

    The system had to manage:

    • High ammonia concentrations
    • Hydrogen sulphide spikes
    • Variable aerosol loads
    • Near-saturated humidity
    • Short-duration high-intensity emission events

    The primary design parameter was therefore peak load management, not nominal airflow alone.

    Multiventuri pre-conditioning stage

    Multiventuri scrubber for Air Pollution and Odour Control in biological sludge composting plant showing independent venturi stages and recirculation piping
    Multiventuri stage designed to manage intermittent high-load odour peaks during biological sludge unloading.

    A multiventuri stage was installed upstream of chemical scrubbing as structural gas conditioning.

    In Odour Control for Biological Sludge Composting, this stage provides:

    • Aerosol reduction
    • Gas homogenisation
    • Initial contaminant absorption
    • Peak concentration damping

    By distributing airflow across multiple venturi throats, the system avoids localised overload during unloading events and stabilises downstream chemical treatment.

    Compact dual chemical scrubbing system

    Following pre-conditioning, the gas enters two vertical scrubbers operating in sequence:

    Acid stage

    Dedicated to Ammonia neutralisation.

    Basic-oxidative stage

    Dedicated to hydrogen sulphide and reduced sulphur compound oxidation.

    Separating chemical environments ensures:

    • Stable pH control
    • Independent reagent dosing
    • Controlled reaction kinetics under variable load

    This sequential architecture is central to effective Odour Control for Biological Sludge Composting at high airflow rates.

    Environmental outcome

    The installation was implemented to mitigate odour impact affecting surrounding residential areas.

    By addressing the intermittent and peak-driven nature of emissions, the system achieved stable odour mitigation despite variability in sludge composition and seasonal factors.

    Engineering insight

    In Odour Control for Biological Sludge Composting, understanding the biological origin of emissions is essential.

    The most critical emissions are:

    • Linked to anaerobic-to-aerobic transition
    • Triggered by sludge exposure
    • Concentrated during short operational windows

    Effective treatment at 88,000 Nm³/h requires architecture capable of absorbing shock loads and stabilising highly variable gas streams.

    Process-driven engineering, rather than average-based design, determines performance.

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    Two-Stage Packed Column Scrubber for Pet Food Air Pollution Control

    Technical Summary

    Flow rate: 30,000 m³/h
    Industry: Pet food production
    Technology: PC-2 Two-stage packed column wet scrubber (acid stage + caustic-oxidative stage)
    Target pollutants: Amines, organic vapours, fatty acids, oil aerosols
    Process air characteristics: Emissions from extrusion, drying, coating and cooling phases

    Acid and caustic-oxidative stages designed around operational maintenance windows

    Air Pollution Control for Pet Food Production requires alignment between scrubber architecture and factory operating schedule.

    Air Pollution Control for Pet Food Production – two-stage packed column scrubber 30,000 m³/h
    Two-stage packed column scrubber installed for pet food air pollution control. Designed for 30,000 m³/h with maintenance-based operation strategy.

    Application context

    This Air Pollution and Odour Control system was designed to treat up to 30,000 m³/h of process air generated by a dry pet food production line operating on a single 8-hour shift.

    Air is extracted from the complete production cycle:

    • Extrusion
    • Drying
    • Coating
    • Cooling

    The emission profile includes:

    • Amines
    • Organic vapours
    • Fatty acids
    • Oil aerosols from coating phases

    Although airflow is relatively stable, contaminant concentration varies according to recipe formulation and coating intensity.

    Why a packed-only solution was selected

    Packed column scrubbers are often considered structurally limited in pet food applications because of oil mist deposition on the first stage packing.

    However, this limitation becomes critical only when production schedules do not allow regular maintenance.

    In this specific case, the production line operates on a single daily shift, providing predictable downtime. This makes it possible to:

    • Schedule periodic inspection
    • Perform cleaning when required
    • Manage progressive packing fouling as part of normal operation

    The engineering decision was therefore based on operational rhythm, not on catalogue standardisation.
    In this case, Air Pollution Control for Pet Food Production is based on predictable maintenance cycles rather than continuous elastic operation.

    System architecture

    The installed configuration consists of two separate packed columns, operating sequentially:

    First stage – Acid

    Designed to neutralise amines and stabilise odour load.

    Second stage – Caustic-oxidative scrubber

    Designed to complete neutralisation and improve robustness against residual organic compounds.

    The physical separation between stages allows:

    • Independent hydraulic and aeraulic control
    • Clear chemical segregation
    • Targeted maintenance planning

    This architecture remains structurally simple while ensuring staged treatment.

    This configuration was selected as a structured Air Pollution Control for Pet Food Production strategy aligned with the plant’s operating rhythm.

    Operational reality: fouling as a managed parameter

    In pet food production, oil aerosols generated during coating inevitably deposit on internal surfaces.

    In packed towers, the first stage becomes the primary deposition surface.

    Consequences over time may include:

    • Gradual pressure drop increase
    • Reduced mass transfer efficiency
    • Risk of channel formation
    • Foam generation

    In this installation, these effects are not treated as unexpected faults but as predictable phenomena integrated into the maintenance strategy.

    Because the factory does not operate continuously, the system remains viable and economically rational.

    Engineering positioning

    A two-stage packed column system can be an effective Air Pollution and Odour Control solution for pet food production up to 30,000 m³/h — provided that:

    • The plant does not operate continuously
    • Maintenance intervals are structurally available
    • Operational discipline is maintained

    Where these conditions exist, a packed-only architecture remains technically sound.

    Where production shifts increase or variability intensifies, reaction dynamics and fouling management require a different approach — which led to a more efficient evolution.

    When to choose a two-stage packed column scrubber

    Air Pollution Control for Pet Food Production can effectively rely on a two-stage packed column architecture when production operates on limited daily shifts and structured maintenance windows are available. In these conditions, oil mist deposition on the first stage packing can be managed through scheduled cleaning without compromising long-term stability. Where continuous three-shift production is required, reaction dynamics and fouling accumulation demand more elastic configurations.

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    Industrial Odour Control System for Dry Pet Food Production

    Technical Summary

    Flow rate: 120,000 m³/h (two parallel lines, 60,000 m³/h each)
    Industry: Pet food production
    Technology: Variable-throat Venturi scrubber + downstream wet scrubbing stages
    Target pollutants: Organic vapours, fatty acids, amines, oil aerosols
    Process air sources: Extrusion, drying, coating and cooling

    120,000 m³/h – Designing for variability, not nominal flow

    Air Pollution and Odour Control in Pet Food Production requires full integration with thermal processes, airflow stability, and emission variability management.

    Air pollution and odour control in pet food production kibble

    Application context – Air emissions in pet food production

    This case study concerns the design of an Air Pollution and Odour Control in Pet Food Production plant, with a total extracted air flow rate of approximately 120,000 m³/h.

    The production process includes:

    • extrusion
    • drying
    • coating
    • cooling

    Air emissions are characterised by:

    • organic vapours
    • fatty acids
    • amines
    • oil aerosols

    In pet food production, emission profiles are not constant. Raw material composition and coating formulations significantly influence contaminant concentration and aerosol load.

    For this reason, air pollution control in pet food plants cannot be designed based on nominal flow rate alone.

    The real engineering constraint: no interference with the drying process

    The odour control system had to be integrated without disturbing:

    • the thermal balance of the oven
    • the internal pressure profile of the dryer
    • the process airflow stability

    Any unstable downstream pressure condition would have:

    • altered the internal oven temperature
    • affected moisture control
    • increased methane consumption
    • reduced product quality consistency

    In pet food production, maintaining thermal stability is directly linked to product performance and energy efficiency.

    The air pollution control system therefore had to adapt to the production process — never the opposite.

    Variable throat Venturi scrubber for emission variability

    Given the fluctuating contaminant load and presence of oil aerosols, a Venturi scrubber with variable throat section was selected as the primary stage of the odour control system.

    The Venturi operates on inertial impaction principles: particle and aerosol removal efficiency is directly related to gas velocity and pressure drop (ΔP).

    The variable throat configuration allows:

    • adjustment of gas velocity according to real operating conditions
    • controlled pressure drop
    • stable capture efficiency across variable loads
    • avoidance of efficiency loss during low-load operation
    • prevention of excessive ΔP during peak conditions

    Unlike fixed-throat systems, the variable geometry maintains the required impact energy while adapting to process variability typical of pet food air emissions.

    The Venturi stage was dimensioned based on required separation efficiency, not solely on nominal air flow rate.

    Controlled fan operation and process stability

    A key design principle of this Air Pollution and Odour Control system was that downstream fans must never interfere with the process fans of the oven.

    The tail-end extraction fans were installed downstream of the treatment plant and equipped with variable frequency drives (VFD).

    Their function is to:

    • continuously adjust rotational speed
    • maintain the required negative pressure
    • compensate for system pressure losses
    • preserve stable depression across the treatment unit

    This ensures that:

    • the internal pressure of the dryer remains unaffected
    • the thermal equilibrium of the oven is preserved
    • process airflow conditions remain stable

    The odour control system behaves as a controlled hydraulic load, not as an active disturbance to the production process.

    Pressure drop and separation efficiency

    In Venturi scrubbers, separation efficiency is directly related to gas velocity and pressure drop. Reducing ΔP in order to minimise energy consumption without understanding the required impact energy inevitably compromises particle and aerosol removal efficiency.

    In this application, the variable throat configuration allows optimisation of ΔP according to real operating conditions, ensuring stable performance without unnecessary energy penalties.

    Energy and quality impact

    Maintaining stable oven conditions in pet food production results in:

    • consistent drying performance
    • stable product temperature profile
    • reduced methane consumption
    • improved product quality consistency

    The air pollution control system therefore contributes not only to emission abatement, but also indirectly to energy optimisation and process reliability.

    Engineering insight

    In dry pet food production, effective Air Pollution and Odour Control requires full integration with process airflow and thermal dynamics.

    Designing only for flow rate leads to instability.

    Designing for:

    • emission variability
    • aerosol load
    • pressure management
    • process integration

    ensures predictable performance.

    Air pollution control in pet food production is not an add-on system.
    It is a process-integrated engineering solution.