In odour control, precision in terminology is critical. The assumption that bioscrubbers are simply biotrickling filters (BTFs) with recirculated water and random media packing continues to circulate in industry discussions, design briefs, and even tender documents.
At first glance, the systems appear similar—each includes a vessel, packing media, and circulating liquid. Yet beneath the surface, they are fundamentally different reactor classes governed by distinct microbial and process dynamics.
Confusing them is more than a matter of semantics; it leads to design errors, performance failures, and loss of confidence in biological odour control technologies.
1. Fixed-Film vs Suspended-Growth: The True Distinction
The fundamental distinction between these systems lies in where and how the microorganisms live and operate:
- Biotrickling Filters (BTFs) are fixed-film systems. Microbes grow on the surfaces of a solid medium—organic, mineral, or synthetic—forming stable biofilms that metabolise pollutants as the gas passes through.
- Autotrophic species dominate acidic zones, oxidising hydrogen sulphide (H₂S).
- Heterotrophic species thrive in neutral zones, degrading mercaptans and volatile organics.
- Bioscrubbers, in contrast, are suspended-growth systems. Microorganisms remain freely suspended in a liquid phase, not attached to media. They consist of two vessels:
- An absorber, where pollutants are transferred from gas to liquid, and
- A separate aerated bioreactor, where the dissolved compounds are biologically degraded.
This distinction aligns bioscrubbers with the activated sludge family of processes, while BTFs belong to the attached-growth reactor class. Equating the two overlooks a fundamental process boundary defined by microbial kinetics.
2. The Role of Packing Media: A Matter of Application, Not Preference
In odour control design, media selection is not a competition between “structured” and “random” packings, but an exercise in matching physical and biological function to the application.
- Random media—such as lava rock, ceramic pellets, plastic spheres, or foam cubes—provide irregular surface area and diverse flow paths. They can encourage biofilm diversity and moisture retention, but also require careful hydraulic design to prevent channeling or blockage under high solids loading.
- Structured media, by contrast, are engineered with defined geometries that enhance gas–liquid distribution and allow predictable pressure loss. They are often used in applications where consistent airflow, maintenance access, and performance predictability are priorities.
Both media types can perform well when correctly paired with the chemistry, load profile, and operating philosophy of the system. No single medium is universally “better.” What matters is the balance between surface area, void fraction, wetting, and longevity for the specific duty.
In bioscrubbers, it’s important to note that the packing inside the absorber is not a biological surface at all—it functions purely as a mass-transfer interface. The microorganisms are active in the downstream bioreactor, not on the packing material itself. Treating absorber packing as equivalent to biotrickling media is one of the most common technical misconceptions in odour control design.
3. Why Misclassification Matters
When bioscrubbers are treated as “recirculated BTFs,” the resulting systems often fail to meet their design intent. The consequences are practical, not theoretical:
- Design Shortfalls: Without the second vessel, the liquid phase becomes saturated with pollutants, and degradation is incomplete.
- Performance Limits: Bioscrubbers are efficient for soluble gases (e.g., H₂S, NH₃) but cannot reliably handle hydrophobic organics or reduced sulphur compounds such as; Methyl Mercaptan (MM), Dimethyl Sulphide (DMS) and Dimethyl Disulphide (DMDS).
- Operational Demands: Bioscrubbers require continuous aeration, nutrient dosing, and pH control, increasing OPEX relative to fixed-film systems.
- Stability Issues: BTFs, with their immobilised biomass and buffering capacity, better accommodate variable load and intermittent operation typical of municipal environments.
These differences are well documented in literature and field experience, where early bioscrubber systems were often retrofitted or replaced with biotrickling filters or biofilters to achieve stable, predictable odour control.
4. Classifying the Technologies Correctly
From an engineering standpoint, the classification boundary between these systems is unambiguous:
Understanding these parameters ensures that each technology is selected and designed for what it does best, not what it happens to resemble.
5. The Engineering Takeaway
Odour control design succeeds when technologies are selected based on odour chemistry, solubility, kinetics, and operational context—not on visual similarity or vendor generalisation.
The choice between structured or random media, once-through or recirculated flow, or fixed-film versus suspended growth should always begin with one guiding question:
What biological and physical environment does this odour mixture require for complete, stable oxidation?
When that principle drives design, both bioscrubbers and biotrickling filters find their rightful place—and each performs reliably within its intended envelope.
6. Conclusion
Bioscrubbers are not biotrickling filters with recirculation and random packing. They are distinct systems based on suspended microbial kinetics, dual-vessel design, and specific application boundaries. Similarly, within the world of biotrickling filters, media type is a matter of selection, not superiority—the right medium for the right chemistry, at the right loading, defines long-term success.
Getting these distinctions right is what transforms biological odour control from a misunderstood specialty into a credible, high-performance engineering discipline.
References
- Deshusses, M.A. (1997). Biological waste air treatment in biofilters and biotrickling filters. Biotechnol. Prog., 13(3), 194–200.
- Kennes, C., & Thalasso, F. (1998). Waste gas biotreatment technology. J. Chem. Technol. Biotechnol., 72(4), 303–319.
- Kennes, C., & Veiga, M.C. (2001). Bioreactors for Waste Gas Treatment. Kluwer Academic Publishers.
- Stuetz, R., & Frechen, F. (2001). Odours in Wastewater Treatment: Measurement, Modelling and Control. IWA Publishing.
- Nielsen, P.H. et al. (2019). Microbial communities in wastewater treatment plants. Nat. Rev. Microbiol., 17, 89–102.
