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.
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.
Polypropylene scrubbers are widely used in industrial air pollution control systems due to their excellent chemical resistance and relatively low cost. A Polypropylene scrubber is typically used in wet scrubbing applications where corrosive gases, odours or pollutants must be removed from air streams. However, in real industrial environments, Polypropylene scrubbers are often selected based only on chemical compatibility. This is where most design mistakes begin.
Why Polypropylene Scrubbers Are So Common
The success of scrubbers made in Polypropylene comes from three main factors:
High chemical resistance
Cost-effectiveness compared to stainless steel
Ease of fabrication
For many standard applications, a polypropylene scrubber is a reliable and efficient solution.
But this is only part of the story.
The Real Limit of Polypropylene Scrubbers
The main limitation of scrubbers made in Polyptopylene is not chemical resistance.
It is mechanical behaviour under temperature.
Polypropylene has a high thermal expansion coefficient:
0.15–0.20 mm/m·°C
In industrial scrubbers, this means that even small temperature variations can generate significant structural movement.
Thermal Expansion in Polypropylene Scrubbers
Thermal expansion is often underestimated during design.
In real operation, it leads to:
Misalignment between scrubber and piping
Stress on flanges and connections
Progressive deformation of the structure
Long-term mechanical fatigue
These effects are not immediate, but they become critical over time.
In constrained layouts, like long duct runs with fixed supports, thermal expansion cannot dissipate. It accumulates.
Why Polypropylene Scrubbers Fail in Operation
When scrubbers made in Polypropylene fail, the cause is rarely chemical.
Typical failure mechanisms include:
Rigid piping connected to the scrubber
Lack of expansion compensation
Poor mechanical support design
Temperature fluctuations not considered
In many cases, the problem is attributed to installation errors, while the real issue is design-related.
Polypropylene vs Other Materials
Polypropylene is often compared with other materials such as HDPE or stainless steel.
Each material behaves differently:
Polypropylene scrubbers → excellent chemical resistance, high thermal expansion
HDPE scrubbers → better mechanical resilience, similar expansion issues
Stainless steel scrubbers → low expansion, but limited chemical resistance in aggressive environments
There is no universally “best” material. The correct choice depends on the process conditions.
When Polypropylene Work Well
Polypropylene perform reliably when:
Temperature varies significantly
Large structures are mechanically constrained
The system is rigid and over-constrained
Long-term dimensional stability is critical
In these cases, alternative materials or different design approaches should be considered.
Conclusion: Understanding Polypropylene Scrubbers
Polypropylene scrubbers are not a bad choice. They are often a misunderstood one.
The key is not only to evaluate chemical resistance, but to understand how Polypropylene scrubbers behave under real operating conditions.
In industrial air treatment, material selection is not a chemical decision. It is an engineering decision.
Choosing the right scrubber recirculation pump is one of the most critical decisions in wet scrubber design.
the actual liquid-to-gas ratio (L/G)
the stability of the system
the effectiveness of mass transfer
the long-term behavior under fouling, salts, and process variability
Most scrubbers do not fail because of incorrect flow rate. They fail because the recirculation system becomes unstable over time. And the pump is the core of that system.
Types of scrubber recirculation pumps
In industrial wet scrubbers, three main types of wet scrubber recirculation pumps are commonly used:
Vertical centrifugal pumps in Polypropylene (PP)
Horizontal centrifugal pumps in Stainless Steel (AISI 316)
Magnetic drive (seal-less) pumps
These technologies are often treated as interchangeable. In reality, they behave very differently under real operating conditions.
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 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.
In many industrial wet scrubbing systems, foam formation is often dismissed as a secondary or cosmetic issue. In reality, persistent foam is almost always the visible symptom of a deeper process inconsistency. This insight originates from real operational evidence: a recirculation tank completely filled with stable foam, generated during normal plant operation.
What foam really indicates
Foam inside a scrubber is not random. It is the result of a combination of physical and chemical conditions:
When these factors align, foam transitions from transient to persistent. At that point, it stops being a nuisance and becomes a process variable.
Why Foam Is a Problem
The consequences are rarely limited to aesthetics:
Loss of effective liquid–gas contact
Carryover of droplets and contaminants
Unstable pressure drop
Reduced mass transfer efficiency
Risk of downstream fouling
Cavitation of recirculation pump
In extreme cases, the scrubber continues to operate, but no longer performs its intended function.
The Typical Reaction (and Why It Fails)
The most common response is the addition of antifoam agents. This approach is appealing because it is immediate and simple. However, it rarely addresses the root cause. Antifoam acts on the symptom, not on the mechanism that generates foam. As a result:
Consumption increases over time
Performance becomes inconsistent
Operating costs rise without solving the instability
Engineering Perspective: Treat the Cause, Not the Symptom
Foam must be interpreted within the broader process context. The correct approach requires asking fundamental questions:
What compounds are actually present in the gas phase?
Is the chemical regime (acid/base/oxidative) coherent with those compounds?
Is the liquid recirculation accumulating by-products?
Is the hydraulic design promoting local imbalances?
Only by reconnecting the scrubber to the upstream process can the issue be solved.
Design Implications
Persistent foam often reveals deeper design limitations:
Systems designed for steady-state conditions operating under variable loads
Lack of pollutant speciation during the design phase
Oversimplified assumptions about liquid chemistry
Inadequate purge and make-up strategies
In other words, the problem is not the foam itself. The problem is that the system was never designed to handle the real process conditions.
Key Takeway
A scrubber does not fail when foam appears.
It fails when foam is treated as an isolated issue.
Understanding and controlling the origin of the emission is the only way to restore performance.
Final note
Visible phenomena in industrial systems are rarely superficial. They are signals. Ignoring them — or masking them with quick fixes — only delays the real engineering work.
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