A Technical Explanation of Absorption, Diffusion, and Microbial Oxidation
Hydrogen sulphide removal in biological odour-control systems—Biofilters (BF), Biotrickling Filters (BTF), Bioscrubbers (BS), and hybrid designs—is often described in simplified terms: “H₂S is absorbed in water and bacteria eat it.” While directionally correct, this oversimplification hides the complex physical and biochemical mechanisms that govern real performance.
This article provides a clear, engineering-accurate explanation of the three fundamental stages:
- Gas → Liquid Absorption
- Liquid → Biofilm Diffusion
- Biological Oxidation
Understanding these stages is essential for accurate system design, EBRT selection, media choice, sizing, troubleshooting, and evaluating supplier performance claims.
1. Stage One: Gas → Liquid Absorption
1.1. H₂S Solubility Is Limited—Not “High”
A widespread misconception is that hydrogen sulphide is “highly soluble in water.” It is not. H₂S has low Henry’s Law solubility, meaning only a relatively small fraction transfers into water under typical conditions.
At 20–25 °C:
- Henry’s constant H ≈ 0.1–0.2 mol/(m³·Pa)
- Result: H₂S requires strong driving forces (concentration gradient and/or pH control) to dissolve effectively.
1.2. The Role of pH: Absorption Improves Under Acidic Conditions
H₂S in water exists in two major forms:
- Undissociated H₂S(aq)
- Dissociated HS⁻
pKa₁ ≈ 7.0. This means:
- At pH < 7, the water phase contains mostly H₂S(aq).
- At pH > 7, the equilibrium shifts towards HS⁻, but this shift happens only after dissolution, not before.
Therefore: H₂S must first dissolve into water before any dissociation can occur. Water that is acidic (pH ~2–4) maximises absorption because it prevents premature back-volatilisation.
1.3. Process Implication
A biological system’s ability to remove H₂S is fundamentally controlled by:
- Gas-phase concentration
- Henry’s Law constant
- pH of the trickling water / biofilm
- Contact time (EBRT)
- Available wetted surface area
If H₂S does not adequately dissolve into the liquid phase, downstream biological processes cannot occur, regardless of media type.
2. Stage Two: Liquid → Biofilm Diffusion
Once H₂S is dissolved in the liquid film surrounding the media, it must diffuse into the biologically active layer (biofilm). This step is frequently overlooked, yet it is the rate-limiting step in many reactors.
2.1. Diffusion Depends on Film Thickness and Surface Area
Key controlling parameters:
- Biofilm thickness: Thin, shear-controlled films (typically 50–300 µm) allow faster diffusion. Thick, high-volume biofilms slow penetration and can limit oxidation rates. Thicker biofilms can offer benefits under certain conditions, including greater biomass inventory, improved resilience during high-load events, increased buffering capacity, and the ability to maintain activity during short-term fluctuations in H₂S concentration or operational interruptions.
- Specific Surface Area (SSA) of media: Higher SSA increases gas–liquid contact and shortens diffusion distances.
- Hydration quality: Continuous wetting, without flooding or dry pockets, ensures stable mass transfer.
2.2. Diffusion Limitations Become Critical at High Loads
At elevated H₂S concentrations or short EBRT:
- Dissolved H₂S may accumulate at the outer biofilm layer.
- Oxygen penetration decreases.
- The biofilm develops internal anaerobic zones where oxidation stalls.
- Intermediates (S⁰, polysulphides) accumulate, altering biofilm behaviour and pH.
2.3. Why Media Geometry Alone Cannot Solve This
Structured media improves:
- Distribution
- Wetting
- Predictability
But structured media has:
- Lower SSA
- Lower water retention
- Lower total biofilm volume
Hence the system’s mass-transfer capacity is reduced unless compensated by longer EBRT or lower loading rates.
Random / microporous media offer:
- Higher SSA
- Larger biofilm volume
- Higher retention
but require careful hydraulic design and oxygen management.
3. Stage Three: Biological Oxidation
After diffusion into the biofilm, H₂S is oxidised by sulphur-oxidising bacteria (SOB), primarily autotrophic organisms such as:
- Acidithiobacillus thiooxidans
- Acidithiobacillus ferrooxidans
- Thiobacillus denitrificans
- Thiomonas spp.
These organisms use H₂S as an electron donor, producing sulphur intermediates and ultimately sulphate.
3.1. The Oxidation Pathway
The sequence differs depending on oxygen availability:
High oxygen conditions:
H₂S → S⁰ → SO₄²⁻ + H⁺
Oxygen-limited conditions:
H₂S → S⁰ (accumulates in the biofilm)
Intermediate elemental sulphur is beneficial to a point but can:
- Increase pressure drop
- Change biofilm rheology
- Require periodic flushing in BTFs
- Alter pH progressively (acidification)
3.2. pH Feedback
The oxidation of H₂S to sulphate generates acidity:
H₂S + 2O₂ → SO₄²⁻ + 2H⁺
This means:
- Systems naturally acidify over time.
- Acidic pH improves absorption but requires material compatibility.
- Biofilters may lose buffering capacity if media is exhausted.
- BTFs manage this through recirculation.
3.3. Temperature and Nutrient Dependency
Oxidation rate depends on:
- Temperature (optimal 20–35 °C)
- Macronutrients (N, P)
- Micronutrients (trace metals)
- Dissolved Oxygen concentration
- Biofilm shear balance (not too thick, not too thin)
4. Engineering Implications for Odour-Control Design
4.1. Absorption Dictates the Front-End Design
If H₂S is not absorbed efficiently:
- Media geometry is irrelevant
- EBRT becomes ineffective
- Biological capacity is under-utilised
Hence pH control, wetting, distribution, and hydraulic design are decisive.
4.2. Diffusion Dictates Sizing and Media Selection
Design must ensure:
- High SSA for high loads
- Predictable wetting
- Manageable pressure drop
- Sufficient EBRT for mass transfer, not just biology
4.3. Biological Oxidation Dictates Stability
To maintain long-term performance:
- pH must be monitored
- Alkalinity must be managed
- Nutrients must be supplied appropriately
- Biofilm must be controlled through shear, flushing, or intermittent loading
5. What This Means for Real Systems
Biofilters rely on media moisture retention and natural pH buffering.
They excel at moderate loads but are sensitive to drying, acidification, and shock loading.
Biotrickling Filters offer controlled hydration, pH regulation, and nutrient delivery.
They excel at high or variable H₂S loads and allow fine control over biological conditions.
Bioscrubbers separate absorption and biological oxidation.
They are suitable for very high concentrations, very high operational stability, and controlled chemical environments.
In all systems, the same three principles apply:
If H₂S is not absorbed → it cannot diffuse. If it cannot diffuse → it cannot be oxidised. If it cannot be oxidised → removal efficiency collapses.
Conclusion
Effective H₂S removal is governed by a sequence of interdependent physical and biological processes:
- Absorption into water (governed by solubility, Henry’s Law, pH, EBRT, distribution)
- Diffusion into the biofilm (governed by SSA, hydration, film thickness, oxygen penetration)
- Biological oxidation (governed by microbial pathways, pH feedback, nutrients, and temperature)
When these mechanisms are understood and designed for, biological systems deliver stable, high-efficiency odour removal. When they are assumed, simplified, or ignored, systems underperform and marketing claims fail under real operating conditions.
A sound scientific foundation is what distinguishes robust engineering design from vendor-driven mythology and ensures that odour-control systems perform reliably in the environments where they matter most.
