Odour emissions from wastewater systems continue to be a persistent challenge. Biological treatments—such as biofilters, biotrickling filters, and bioscrubbers—are widely applied, yet often misapplied due to gaps in understanding. The result is reduced performance and higher costs. This article reviews reactor types, common design pitfalls, and the process fundamentals that matter most. Effective odour control comes not from a single “best” technology, but from matching system design to the specific characteristics of each site.
- Introduction
Municipalities face growing pressure to control odours from wastewater treatment works, pump stations, and sewer networks. Biological systems are widely applied as sustainable alternatives to chemical treatment, but confusion between biofilters, biotrickling filters, and bioscrubbers often leads to inappropriate choices. This can cause incomplete odour removal, higher costs, and reduced reliability. Effective management requires a clear understanding of each technology’s principles, microbial requirements, and site-specific constraints.
- Classification of Biological Odour Control Reactors
2.1 Biofilters (BF’s)
Type:
Fixed-film, organic or engineered media.
Strengths:
Simple construction and operation, with low energy demand.
Long media life when engineered materials are used.
Well-suited for stable H₂S loads at moderate concentrations.
Weaknesses:
Susceptible to drying and channel formation if not maintained.
Limited flexibility for fluctuating odour loads.
Less effective for hydrophobic VOCs and some reduced sulphur compounds without sufficient EBRT.
Prone to acidification under high H₂S concentrations.
Typical Applications:
Municipal wastewater treatment works (headworks and primary treatment).
Small to medium pumping stations with consistent H₂S emissions.
Landfill gas treatment and composting facilities with steady odour profiles.
2.2 Biotrickling Filters (BTFs)
Type:
Fixed-film with continuous irrigation.
Modes:
Once-through mode: Fresh water or treated effluent passes through, establishing vertical pH gradients.
Recirculated mode: Liquid collected and reapplied, stabilising pH, reducing water demand, and supporting microbial diversity.
Strengths:
Effective for H₂S removal across a wide range of inlet concentrations.
Recirculation mode offers microbial stability and better resilience to load variability.
More adaptable to mixed odour profiles than biofilters alone.
Weaknesses:
Once-through designs can produce unstable pH gradients, reducing reliability under fluctuating loads, as commonly observed in pumping stations.
May require chemical or hybrid polishing where hydrophobic VOCs or certain ROSCs are present.
Typical Applications:
Pump stations and sewerage networks with variable or high H₂S loads.
Municipal and industrial facilities where both sulphides and organic compounds contribute to odour.
Retrofitted systems require resilience against diurnal or seasonal variability.
2.3 Bioscrubbers (BSRs)
Type:
Suspended-growth systems combining an absorber column with a liquid-phase bioreactor.
Applications:
Designed for highly soluble gases such as NH₃ and H₂S.
Strengths:
High efficiency for soluble contaminants at elevated concentrations.
Independent control of mass transfer (absorber) and biodegradation (bioreactor).
Stable operation when contaminant loading is consistent.
Weaknesses:
Larger footprint and higher complexity compared with biofilters and BTFs.
Greater capital and operational costs due to recirculation and nutrient dosing.
Rarely used in municipal odour control because of infrastructure demands and limited suitability for poorly soluble compounds.
Typical Applications:
Industrial facilities with high-strength soluble emissions (e.g., rendering, fertiliser, or food processing plants).
Wastewater treatment plants with concentrated ammonia or sulphide odours.
Sites requiring integration with existing liquid treatment infrastructure.
- Root Causes of Misapplication
Biological odour control failures rarely stem from inherent flaws in the technology. Instead, they reflect systematic oversights in engineering design and microbial process understanding. Three main drivers dominate: mass transfer errors, microbial ecology simplifications, and vendor bias.
3.1 Mass Transfer Oversights
The performance of any biological reactor begins with the gas–liquid transfer step. If odorous compounds are not effectively absorbed into the biofilm, microbial degradation cannot occur, regardless of the biomass present.
Highly soluble species (e.g., H₂S, NH₃)
Rapid absorption into the aqueous phase.
Readily oxidised by acidophilic autotrophs (e.g., Acidithiobacillus spp.) at pH 1–2.
Removal efficiencies >99% routinely documented even at high inlet concentrations (>500 ppmv H₂S) when EBRTs exceed 10–20 seconds
Low-solubility compounds (e.g., dimethyl sulphide [DMS], dimethyl disulphide [DMDS], many VOCs)
Henry’s Law constants are several orders of magnitude higher than for H₂S.
Mass transfer is rate-limiting; it requires a larger surface area, higher EBRT (≥30–60 s), or multistage configurations.
Poorly considered designs tuned for H₂S often experience “breakthrough” of these hydrophobic compounds, leading to residual odour complaints.
Diurnal and hydraulic variability:
Odour loads in sewers fluctuate with pumping cycles, rainfall events, and diurnal usage patterns.
Rapid spikes in H₂S shift pH gradients in once-through BTFs, destablising the removal of compounds that rely on neutral-pH microbial populations.
Recirculation mitigates these fluctuations, maintaining more stable microbial populations.
Key lesson:
Failure to account for hydrophobic ROSCs and VOCs, where present, leads to premature underperformance in real-world applications.
3.2 Microbial Ecology Oversimplification
Engineers often treat microbial populations as a “black box” rather than a dynamic ecosystem. Yet, the balance of autotrophic and heterotrophic consortia is critical:
Autotrophs:
Acidophiles that thrive at low pH (1–2).
Oxidise H₂S to sulphate or elemental sulphur.
Provide rapid and robust treatment for sulphide-rich streams.
Heterotrophs:
Require neutral pH (~6–7).
Responsible for degrading amines, VOCs, and many organic sulphur compounds.
Competition and exclusion
Excessive acidification (e.g., due to continuous H₂S load without buffering) suppresses heterotrophs, leaving amines and VOCs untreated.
Conversely, if systems are buffered too strongly at neutral pH, acidophiles may be inhibited, reducing sulphide oxidation and allowing ROSCs to accumulate.
Key lesson:
Simplifying odour removal to “acidophiles remove everything” or “biofilters are universally neutral” ignores microbial diversity. Successful design requires microbial niche management, not monoculture assumptions.
3.3 Vendor Bias
Misapplication is often driven less by engineering error than by commercial positioning.
Single-technology vendors:
Companies offering only biofilters or only BTFs are incentivised to present their technology as universally applicable.
This results in overselling, unrealistic performance guarantees, and widespread underperformance in field deployments.
Consequences of one-size-fits-all claims:
Once-through BTFs marketed as “superior” often fail under variable load conditions, especially when VOCs and ROSCs dominate.
The bankruptcy of multiple “BTF-only” vendors illustrates the commercial unsustainability of universal claims
Portfolio-based vendors:
Suppliers offering multiple reactor types (engineered biofilters, recirculated BTFs, hybrids, bioscrubbers) are better positioned to tailor solutions.
This aligns with independent literature emphasising that no single biological technology can cover the full spectrum of municipal odours
Key lesson:
Technology selection should be independent of vendor limitations. Engineering decisions must be guided by compound chemistry, EBRT requirements, and microbial niches—not by catalogue constraints.
- Consequences in Municipal Contexts
Operational Costs: Premature media replacement and unnecessary chemical conditioning.
Performance Risks: Breakthrough of poorly soluble ROSCs/VOCs, triggering odour complaints.
Reputational Damage: Failures fuel perceptions that “biology doesn’t work,” pushing municipalities toward chemical scrubbers despite higher lifecycle costs.
- Evidence-Based Design Guidance
Successful odour control relies on aligning system design with site conditions rather than assuming one technology, medium, or configuration is universally superior. Biofilters, biotrickling filters, and bioscrubbers all have roles when applied appropriately. Correct selection depends on understanding the odour environment, operational context, and long-term sustainability.
5.1 Odour Characterisation
Baseline odour assessment is essential, but the level of detail should be proportional to project needs. While comprehensive speciation and variability studies can provide high confidence, they can also be costly. In many cases, a balanced approach—using empirical data, established industry guidelines, and comparable site experience—offers sufficient classification to support sound design without unnecessary expense.
5.2 Technology Evaluation
Each biological reactor type has strengths and limitations that make it more or less suitable depending on the odour profile, site footprint, variability of emissions, and operational resources. No single system is universally applicable. Evaluation should weigh technical performance, operability, and long-term reliability within the specific context.
5.3 Circulation Strategy
Circulation mode influences microbial stability, pH control, and water demand. Both once-through and recirculated systems have valid applications, and the appropriate choice depends on the odour load, variability, and site operating conditions.
5.4 Media Selection
Different media types—organic, mineral, and engineered (structured or random)—each have valid uses. Effective outcomes depend less on theoretical material attributes than on empirically proven performance under operating conditions, including odour removal efficiency and media longevity. Success ultimately comes from selecting the correct odor load, designing an appropriate system, and maintaining consistent operation and maintenance.
5.5 Lifecycle Costing
Capital and operating costs, considered together as lifecycle costs, should be assessed in combination. Over- or under-design can both lead to inefficiencies. Balanced CAPEX/OPEX modelling, supported by real-world operating data, helps ensure systems remain reliable and cost-effective over time.
- Discussion
Biological odour control is not inherently unreliable; failures are traceable to misapplication, oversimplified assumptions, and vendor-driven generalisations. The literature is clear: reactor class distinctions (fixed-film vs suspended-growth), microbial populations, and compound solubility govern performance outcomes. Multistage systems, defined as media separated into distinct chambers, increasingly demonstrate resilience in handling mixed odour profiles, particularly where H₂S co-occurs with ROSCs and VOCs.
- Conclusion
Misapplication of biological odour control technologies remains a systemic challenge in municipal wastewater.
Corrective practice demands appropriate baseline characterisation, portfolio-based technology selection, explicit consideration of the circulation mode, and media tailored to the odour matrix.
Success lies not in universal claims but in the nuanced application of biological principles, underpinned by robust engineering design and microbial ecology.
References
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Stuetz, R., & Frechen, F. (2001). Odours in Wastewater Treatment: Measurement, Modelling and Control. IWA Publishing.
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