A recent publication in the industry suggested that structured media offers inherently better odour-removal performance in Bio-Trickling Filters (BTFs) than random or microporous media—an oversimplification that warrants closer examination.
This claim sounds intuitive—thin-film flow, uniform channels, and predictable hydrodynamics should mean better performance. But when we examine what actually governs odour removal in Bio-Trickling Filters (BTFs), the science tells a different story.
Why Clear Definitions Matter
In odour-control engineering, terminology is often used loosely, which weakens technical debate and enables marketing language to replace scientific clarity. For discussions about media performance to be meaningful, it is essential to define each concept precisely. The following definitions serve as a reference point for the rest of this article.
Key Media Definitions
Structured Media
Engineered plastic modules formed from corrugated sheets or extruded profiles that create straight, uniform macro-channels for gas and liquid flow.
Characteristics: thin-film wetting, low pressure drop, predictable hydraulics, low SSA, low water retention.
Random Media
Loose-fill elements such as rings, foam squares, saddles or hollow spheres that pack irregularly, creating a highly variable macro-void structure.
Characteristics: higher SSA, complex flow paths, improved moisture retention, thicker and stratified biofilms, and greater buffering capacity.
Microporous Media
Foams, porous composites, ceramics, or cellular materials with extremely high SSA and interconnected micro-pores forming a continuous internal matrix. Characteristics: exceptional water retention, thick and diverse biofilms, high sulphur-storage capacity, and high total biological treatment potential.
Macro- vs Micro-Structure
Media performance is shaped at two structural scales:
- Macro-structure: governs airflow, pressure drop, and global wetting behaviour.
- Micro-structure: controls SSA, water retention, capillary behaviour, and biofilm attachment.
In simple terms: Random media may appear “random” at the large scale, but its micro-surface can be structured and uniform—particularly in open-cell foams or engineered porous materials—giving it both complexity and high biological efficiency.
1. Odour Removal Is Controlled by Biology and Mass Transfer, Not Channel Geometry
The efficiency of any BTF depends inter alia on a combination of:
- Specific (media) surface area (SSA)
- Biofilm volume and thickness dynamics
- Media surface characteristics
- Oxygen transfer and diffusion depth
- Water retention and bulk-liquid reactions
- Sulphur accumulation tolerance
- System resilience to load variability
Structured media provides predictable flow paths, but it also has:
- Lower SSA
- Lower biofilm volume
- Limited water retention
- Limited buffering capacity
These limitations mean that even with high specific mass-transfer rates, structured media may not offer optimum odour removal capacity.
2. Real Odour Systems Do Not Operate Under Stable Conditions
Structured media excels in controlled environments where:
- Odour loads are stable
- Flows are constant
- Irrigation is uninterrupted
- Temperatures are consistent
- No ROSCs are present
But real municipal and industrial odour sources behave very differently:
- Pump stations produce sharp H₂S spikes
- Rising mains discharge high-strength bursts
- Irrigation systems cycle on/off
- Wastewater conditions are subject to diurnal cycles
- Presence of elevated concentrations of ROSCs (CH₃SH, DMS, DMDS) is common in biosolids processing
- Temperatures fluctuate
Under these dynamic conditions, thin-film systems lose moisture, saturate quickly, and recover slowly. High-SSA random and microporous media remain hydrated, offer superior buffering capacity, and maintain stable performance through high-load events.
3. Water-Phase Oxidation Matters
One of the most overlooked mechanisms in BTFs is bulk-water oxidation:
- H₂S and methyl mercaptan dissolve in retained water
- Suspended biomass in recirculated systems oxidises dissolved sulphide
- Water retention buffers shock loads
- Moisture acts as a thermal and biological stabiliser
Random/microporous media retain significantly more water, enabling 15–40% additional H₂S removal through water-phase reactions. Structured media, designed to shed water, simply cannot access this mechanism.
4. ROSC Removal Favourably Biases High-SSA Media
Structured media struggles to treat reduced organic sulphur compounds (ROSCs), which are:
- Poorly soluble
- Mass-transfer limited
- Sensitive to pH and hydration stability
High-SSA media support the microbial diversity and layer thickness required to treat these compounds effectively.
5. The Real Advantage of Structured Media?
Structured media is valuable where:
- loads are stable
- H₂S concentrations are moderate
- Operational control is strong
- EBRT is sufficient
- Thin-film behavior can be maintained
This equates to predictable hydraulics, but predictable hydraulics do not equate to superior biological capacity.
Summary
There is limited scientific or engineering justification to the claim that structured media is inherently superior for odour removal in Bio-Trickling Filters.
- Structured media = predictable flow, low pressure drop, high specific kinetics.
- Random/microporous media = higher SSA, thicker biofilms, better water retention, increased buffering capacity, and greater stability under real-world variability.
In most real odour applications, high-SSA media provides equal or better total odour removal—and significantly better long-term resilience.
Media selection should therefore be based on load variability, ROSC presence, water solubility, SSA, pressure loss, flushing characteristics, EBRT, water management, and operational risk—not on a blanket assumption of geometric superiority.
