Across the global odour-control industry, classification of biological gas-treatment systems remains inconsistent. Terms such as Biofilter (BF), Biotrickling Filter (BTF), and Bioscrubber (BS) are frequently used interchangeably in supplier literature, consulting reports, marketing brochures, and tender specifications.
This inconsistency arises because:
- Different regions rely on different foundational texts
- Academic terminology varies
- Vendors often blur distinctions in marketing
- Only some jurisdictions have formal engineering standards
To resolve this confusion, this article aims to collate definitions from Germany and the United States:
- Verein Deutscher Ingenieure (VDI), the Association of German Engineers
- Consolidated United States Reference from recognized Academic and Industry sources.
The Association of German Engineers’ Standards documents are technically rigorous and internationally recognised engineering standards for biological waste-gas treatment.
However, because the United States lacks a clear, unified, standardised definition set, and because U.S. academic texts describe these technologies inconsistently, a set of United States Academic & Industry Consolidated Definitions was created for this article.
These U.S. definitions are based on:
- Peer-reviewed research from American journals
- U.S. engineering practice
The consolidated U.S. definitions were aligned with the German VDI standards to form a coherent classification system.
1. Harmonised Definitions
This section states and collates the definitions of the:
- Association of German Engineers (VDI Standards)
- United States Academic/Industry Consolidated Definitions
1.1 BIOFILTER (BF)
1.1.1 Association of German Engineers Definition (VDI 3477)
VDI 3477 defines a biofilter as a stationary fixed bed of biologically active filter material. Waste gas flows through the moist, porous organic media, and pollutants are degraded within microbial biofilms on the material surface. No liquid recirculation is used.
1.1.2 United States Academic/Industry Definition
A biofilter is a fixed-bed biological air-treatment system in which foul air passes through a moist, porous organic medium that supports active microbial biofilms. Odorous compounds such as hydrogen sulfide (Hâ‚‚S), reduced sulfur compounds, and VOCs are absorbed into the moisture layer on the media and biologically oxidised by microorganisms. Biofilters operate without continuous liquid recirculation and are widely applied for odour control at wastewater treatment facilities.(Deshusses, 1997; Williams et al., 1996; Devinny et al., 1999; McNevin & Barford, 2000; Iranpour et al., 2005)
1.2 BIOTRICKLING FILTER (BTF)
1.2.1 European Definition (VDI 3478 Part 2)
VDI 3478 Part 2 defines a biotrickling filter as a fixed-film reactor containing inert packing that is continuously or intermittently irrigated with a recirculating liquid. Microorganisms form attached biofilms on the packing surface. The liquid phase allows pH regulation, nutrient addition, and salt management.
1.2.2 United States Academic/Industry Definition
A Bio-Trickling Filter is a packed-bed, fixed-film bioreactor in which polluted air passes through inert media colonised by microorganisms while a recirculating liquid trickles continuously over the bed. Odorous contaminants such as hydrogen sulfide, ammonia, VOCs, and reduced sulfur compounds are absorbed into the liquid/biofilm and biologically oxidised. The system uses engineered liquid recirculation to maintain moisture, enhance mass transfer, control pH, and stabilise high-load foul air treatment performance. (Gabriel 2004; Dumont 2020; Taha 2022; Rybarczyk 2022; Wiley Review 2005)
1.3 BIOSCRUBBER (BS)
1.3.1 European Definition (VDI 3478 Part 1)
VDI 3478 Part 1 defines a bioscrubber as a two-unit biological gas-treatment system, consisting of:
- An absorber where gas contaminants are transferred into a scrubbing liquid
- A separate bioreactor where dissolved contaminants are degraded by suspended microbial cultures
Liquid recirculation between the absorber and reactor is essential.
1.3.2 United States Academic/Industry Definition
A bioscrubber is a two-stage biological air-treatment system in which odorous gas is first absorbed into a liquid phase and then biologically oxidised in a separate suspended-growth or fixed-film bioreactor. Unlike biofilters and biotrickling filters, gas absorption and biodegradation occur in separate unit processes. Bioscrubbers are particularly effective for water-soluble compounds such as hydrogen sulfide (Hâ‚‚S), reduced sulfur compounds, and selected VOCs, and are widely used in wastewater odour-control applications.(Ockeloen et al., 1996; Humeau et al., 2004; DeHollander et al., 1998; Potivichayanon et al., 2006; Barbosa et al., 2004)
2. Technically consistent alignment
The comparison of German standards and U.S. definitions reveals a clear and technically consistent alignment:
3. Conclusion
By removing ambiguous sources and integrating relevant and trusted sources, this article defines a coherent, aligned definition for BF, BTF, and BS technologies that can be applied.
These definitions aim to eliminate:
- Confusion in naming
- Confusion in specifications and tenders
- Inconsistency during vendor evaluation
- Engineering inaccuracy
- Incorrect sizing
- Incorrect EBRT assumptions
- Inappropriate nutrient/pH strategies
- Misleading vendor comparisons
- Poor long-term biological performance
4. References
4.1 United States Academic/Industry Definition – Biofilter
- Biological waste air treatment in biofilters — M. A. Deshusses (1997)
- Literature review of air pollution control biofilters and biotrickling filters for odor and volatile organic compound removal — R. Iranpour, H.H.J. Cox, M. Deshusses & E.D. Schroeder (2005)
- Odor and volatile organic compound removal from wastewater treatment plant headworks ventilation air using a biofilter — B.M. Converse, E.D. Schroeder, R. Iranpour, H.H.J. Cox & M.A. Deshusses (2003)
- Biofiltration as an odour abatement strategy — D. McNevin & J. Barford (2000)
- Air pollution control biofilters and biotrickling filters: fundamentals to full-scale implementations — (review including wastewater/industrial VOC & odor applications) — various authors (in context of US & international practice)
Consolidated definition
A biofilter is a fixed-bed biological air-treatment system in which foul air passes through a moist, porous organic medium that supports active microbial biofilms. Odorous compounds such as hydrogen sulfide (Hâ‚‚S), reduced sulfur compounds, and VOCs are absorbed into the moisture layer on the media and biologically oxidised by microorganisms. Biofilters operate without continuous liquid recirculation and are widely applied for odour control at wastewater treatment facilities. (Deshusses, 1997; Williams et al., 1996; Devinny et al., 1999; McNevin & Barford, 2000; Iranpour et al., 2005)
4.2 United States Academic/Industry Definition – Biotrickling Filter
- Biotrickling filters for POTWs air treatment: Full-scale experience with a converted scrubber – Gabriel, D., Cox, H.H.J., Brown, J., Torres, E., Deshusses, M.A. (2004).
- Biotrickling filters for the removal of gaseous ammonia from livestock-facility emissions – Dumont, E., et al. (2020).
- Model-based design and operation of biotrickling filters for foul air (e.g. Hâ‚‚S) treatment – Taha, A., et al. (2022).
- Removal of Volatile Organic Compounds (VOCs) from Air — Biotrickling Filtration Review – Rybarczyk, P., et al. (2022).
- Biofilters and biotrickling filters for air pollution control – (Review Article, 2005, Wiley Online Library).
Consolidated definition
A Bio-Trickling Filter is a packed-bed, fixed-film bioreactor in which polluted air passes through inert media colonised by microorganisms while a recirculating liquid trickles continuously over the bed. Odorous contaminants such as hydrogen sulfide, ammonia, VOCs, and reduced sulfur compounds are absorbed into the liquid/biofilm and biologically oxidised. The system uses engineered liquid recirculation to maintain moisture, enhance mass transfer, control pH, and stabilise high-load foul air treatment performance. (Gabriel 2004; Dumont 2020; Taha 2022; Rybarczyk 2022; Wiley Review 2005)
4.3 United States Academic/Industry Definition – Bioscrubber
- Engineering Model for Fixed-Film Bioscrubbers – Ockeloen, H.F., Overcamp, T.J., & Grady, C.P.L. Jr. (1996).
- Optimization of Bioscrubber Performances: Experimental and Modeling Approaches РHumeau, P., Pr̩, P., & Le Cloirec, P. (2004).
- Performance of a Suspended-Growth Bioscrubber for the Control of Methanol – DeHollander, G.R., Overcamp, T.J., & Grady, C.P.L. Jr. (1998).
- Hydrogen Sulfide Removal by a Novel Fixed-Film Bioscrubber System – Potivichayanon, S., Pokethitiyook, P., & Kruatrachue, M. (2006).
- Hydrogen Sulphide Removal by Activated Sludge Diffusion – Barbosa, V.L., Dufol, D., Callan, J.L., Sneath, R., & Stuetz, R.M. (2004).
Consolidated definition
A bioscrubber is a two-stage biological air-treatment system in which odorous gas is first absorbed into a liquid phase and then biologically oxidised in a separate suspended-growth or fixed-film bioreactor. Unlike biofilters and biotrickling filters, gas absorption and biodegradation occur in separate unit processes. Bioscrubbers are particularly effective for water-soluble compounds such as hydrogen sulfide (Hâ‚‚S), reduced sulfur compounds, and selected VOCs, and are widely used in wastewater odour-control applications. (Ockeloen et al., 1996; Humeau et al., 2004; DeHollander et al., 1998; Potivichayanon et al., 2006; Barbosa et al., 2004)
