There is a misconception that the presence of autotrophic and heterotrophic bacteria within a biological reactor or biofilter is proof of system capability to effectively remove hydrogen sulphide (H₂S) and other reduced organic sulphur compounds (ROSCs) such as methyl mercaptan (CH₃SH), dimethyl sulphide (DMS), and dimethyl disulphide (DMDS). While the coexistence of these microbial groups confirms biochemical potential, it does not guarantee process efficiency.
The performance of biological odour control systems depends on a combination of engineering and biochemical parameters, including gas–liquid solubility, pH stability, air velocity, nutrient balance, empty bed residence time (EBRT), and media characteristics. These factors collectively determine whether odourants can be physically captured, transferred to the biofilm, and microbial activity can be sustained under conditions that enable the complete oxidation of sulphur compounds into stable, non-odorous end products.
This article outlines how H₂S and ROSCs are removed biologically and lists the range of bioreactor configurations that have been developed to manage these interacting factors. The focus is on understanding the underlying principles rather than comparing or ranking technologies.
1. Introduction
Biological odour control systems—principally biofilters and biotrickling filters (BTFs)—remove volatile sulphur compounds from gas streams through microbial oxidation. These systems depend on a biologically active film attached to a solid medium where the compounds dissolve, diffuse, and are metabolically converted to sulphate, elemental sulphur, water, and carbon dioxide.
System performance is occasionally inferred from the mere presence of autotrophic sulphur oxidisers (Acidithiobacillus, Thiobacillus) and heterotrophic degraders (Hyphomicrobium, Paracoccus, Pseudomonas). However, while these species are essential, their presence alone does not ensure operational success.
Effective odour control results from a carefully balanced combination of physical and biological factors—airflow velocity, odourant solubility, odourant biodegradability, pH, moisture content, nutrient balance, and reactor residence time—which together create the conditions under which conditions for desired system performance these bacteria can function efficiently are met. The following sections describe these fundamental principles in detail.
2. Mechanisms of Sulphur Compound Removal
The biological treatment of reduced sulphur gases involves mass-transfer and biochemical processes that convert volatile sulphur compounds into stable, non-odorous end products such as sulphate (SO₄²⁻), elemental sulphur (S⁰), water, and carbon dioxide. Although the overall reaction sequence is similar, inorganic H₂S and organic sulphur compounds (ROSCs) behave differently because of their distinct physicochemical and biochemical properties.
2.1 Hydrogen Sulphide (H₂S) Removal Mechanism
Hydrogen sulphide is an inorganic reduced gas that is more soluble and rapidly oxidised. Its removal is primarily governed by autotrophic oxidation carried out by acidophilic sulphur-oxidising bacteria, such as Acidithiobacillus thiooxidans and Thiobacillus thioparus.
2.1.1 Absorption
H₂S dissolves more easily into the aqueous film and dissociates according to pH:
2.1.2 Biochemical Oxidation
Oxidation proceeds through two sequential steps:
The final product is sulphuric acid, which acidifies the surrounding medium.
2.1.3 Process Characteristics
- The gas is moderately soluble; mass-transfer limitation is minimal.
- Oxidation rate depends on oxygen diffusion and biofilm thickness.
- Optimal microbial activity occurs under acidic pH (1–3).
- Reaction products cause gradual pH drift that must be buffered to maintain stability.
H₂S is typically the primary odour driver in wastewater and sludge-handling facilities and is removed readily under properly aerated acidic conditions.
2.2 Reduced Organic Sulphur Compound (ROSC) Removal Mechanism
ROSCs—methyl mercaptan (CH₃SH), dimethyl sulphide (DMS), and dimethyl disulphide (DMDS)—are organic sulphur molecules characterised by low solubility and slower oxidation kinetics. Their degradation depends on heterotrophic bacteria capable of cleaving carbon–sulphur bonds, generally under neutral or mildly alkaline conditions.
2.2.1 Absorption
Because of their low Henry’s law solubility, ROSCs are often mass-transfer limited. Improving air velocity, turbulence, or specific surface area enhances contact between gas and biofilm.
2.2.2 Biochemical Oxidation
ROSCs are oxidised aerobically in multi-step reactions:
These reactions yield sulphate and carbon dioxide, often through intermediate compounds such as formaldehyde or elemental sulphur.
2.2.3 Process Characteristics
- Absorption is the limiting step due to low solubility.
- Optimum activity occurs at pH 6–8.
- Requires longer EBRTs (40–60 s) than H₂S.
- Key species: Hyphomicrobium, Paracoccus, Rhodococcus, Pseudonocardia.
ROSCs are typically treated in neutral zones or secondary stages of reactors following H₂S oxidation and contribute to residual odour if not fully degraded.
3. Solubility and Gas–Liquid Mass Transfer
3.1 Solubility Characteristics
The water solubility of each compound determines transfer and its rate of transfer into the aqueous film and, therefore, its availability for microbial oxidation.
Moderate-solubility gases like H₂S are absorbed and oxidised more rapidly, whereas hydrophobic compounds (DMS, DMDS) require enhanced gas–liquid contact through turbulence and increased surface area. Air velocity and media structure both influence the mass transfer coefficient, which defines the rate of gas dissolution into the film.
4. pH Conditions Impact Microbial Function
4.1 Microbial Stratification
Distinct microbial groups thrive optimally at different pH levels, often creating vertical or zonal stratification within media beds.
4.2 pH Stability
H₂S oxidation produces sulphuric acid:
Acid generation gradually lowers pH and can suppress heterotrophic activity unless buffering or recirculation is used. Effective system performance depends critically on maintaining controlled pH conditions. Any variability in gas load, flow rate, or liquid distribution can complicate pH stability, leading to fluctuating microbial activity and reduced overall treatment efficiency.
5. Empty Bed Residence Time (EBRT) and Air Velocity
5.1 EBRT
EBRT represents the mean gas contact time within the bioreactor:
Where L is the bed depth and V is the air velocity. Typical EBRTs for sulphur compound treatment range between 5–20 seconds for H₂S and up to 60 seconds for DMS and DMDS. Sufficient EBRT ensures that absorption and oxidation reactions approach equilibrium.
5.2 Air Velocity
Air velocity is an operational variable linking airflow, residence time, and mass transfer.
- Low velocities (<0.05 m·s⁻¹) may result in poor gas distribution and limited turbulence.
- Moderate velocities (0.08–0.15 m·s⁻¹) provide adequate contact and film renewal without excessive pressure loss.
- High velocities (>0.20 m·s⁻¹) can desiccate biofilms, reduce humidity, reduce capture rate, and increase pressure drop.
Velocity also controls shear stress on the biofilm—an important factor in regulating biofilm thickness and oxygen diffusion. In practice, velocity is optimised to balance EBRT, turbulence, and energy efficiency.
6. Media Properties
6.1 Media Characteristics
6.2 Air Velocity Interaction
The chosen media must sustain design air velocities without compromising uniform flow or humidity. High-velocity operation demands structured or coarse packing to avoid excessive pressure drop, whereas low-velocity systems can use finer or organic media for increased surface area. Reactor cross-sectional area is often adjusted to maintain target velocities while accommodating airflow variability.
6.3 Engineered Enhancements
Modern media designs integrate:
- Hydrophilic coatings for film continuity.
- Hydrophobic surfaces to improve capture of poorly soluble gases.
- Embedded pH buffers, such as calcium carbonate.
- Nutrient reservoirs for sustained microbial growth.
These engineered features improve process resilience under variable flow and contaminant loading.
7. Nutrient and Oxygen Availability
Microbial oxidation requires adequate nitrogen, phosphorus, and trace metals for cell growth and enzymatic activity.
Air velocity and flow distribution also affect oxygen penetration through the biofilm; too low, and oxygen may be consumed at the surface, too high, and desiccation may occur.
8. Integrated Process Dynamics
The overall reactor efficiency is a product of its individual process efficiencies:
These are governed collectively by air velocity, EBRT, pH, moisture, and nutrient balance. Deviations in any parameter influence the others: for example, a change in velocity affects both EBRT and mass transfer, while pH shifts can alter microbial composition and diffusion behaviour. Continuous monitoring of airflow, temperature, and moisture is therefore essential to maintain equilibrium.
9. Conclusion
The biological removal of H₂S and reduced organic sulphur compounds depends on similar fundamental physical, chemical, and biological processes regardless of configuration. Parameters such as solubility, pH, air velocity, EBRT, media characteristics, and nutrient balance collectively determine system performance.
Different bioreactor designs—ranging from biofilters and recirculating or once-pass BTFs to two-stage systems and bioscrubbers—apply these principles in varying ways, each offering distinct advantages depending on the detail of application.
Therefore, when assessing biological odour control systems, due consideration must be given to the underlying science and engineering— consider all the interactive variables that govern mass transfer, biological degradation, and long-term stability—rather than a focus on a particular aspect to the exclusion of other critical variables, which is sometimes evident in vendor claims.
Satisfying a single operating condition does not guarantee system stability; a rigorous evaluation of all interdependent variables remains essential for an informed decision.
