How Biofilters Clean the Air: An Essential Tool for Pollution Control

Air pollution from industrial facilities, wastewater plants, and agricultural operations releases a constant stream of volatile organic compounds (VOCs), odorous gases, and other harmful emissions into the atmosphere. Conventional control technologies like incineration or activated carbon adsorption can be expensive, energy-intensive, or generate secondary waste. Biofilters offer a fundamentally different approach – they use living microorganisms to break down pollutants directly. First documented by German scientist Bach in 1923 and adopted widely since the 1950s, biofiltration has grown into a mainstream pollution control strategy valued for its low cost, low energy demand, and clean end products.

What are biofilters? 🔗

A biofilter is a bed of packing material – organic or synthetic – on which microorganisms colonize and form a thin, moist biological layer called a biofilm. Biofiltration is classified as a fixed-film process, meaning the microbial community is anchored to the media rather than suspended in liquid. The biofilm community typically includes bacteria, fungi, yeast, protozoa, and other microorganisms, all working together to metabolize contaminants passing through.

The core function of a biofilter is straightforward: contaminated air is humidified and then pushed through the packed bed. Pollutants adsorb onto the biofilm surface, where immobilized microbes degrade them into harmless end products – primarily carbon dioxide, water, and additional microbial biomass. Unlike incineration, this process generates no secondary pollutants such as NOx or additional CO₂ from fuel combustion. The packing media itself plays a dual role: it provides physical support for the biofilm and supplies nutrients that sustain microbial growth.

Common packing materials include compost, peat, wood chips, perlite, diatomaceous earth, pelletized ceramics, and granular activated carbon (GAC). Each material has specific surface area, porosity, and water-retention characteristics that influence how well the biofilm establishes and how effectively air distributes through the bed. Biofilters are particularly well-suited to treating volatile organic compounds (VOCs), hydrogen sulfide (H₂S), ammonia (NH₃), and a broad range of odorous industrial emissions at low-to-moderate concentrations.

Types of biofilters and how they work 🔗

Biofilters come in several configurations, each suited to different operational contexts and pollutant types. The major distinctions are between open and closed designs, and between horizontal and vertical flow orientations.

Open vs. closed biofilters 🔗

Open biofilters consist of a bed of organic material spread over a perforated air distribution system, with the top surface exposed to the atmosphere. Air is blown upward through the media from below. These systems are relatively inexpensive to build and are well-suited to treating large volumes of air with moderate pollutant concentrations – they are common at composting facilities and wastewater treatment plants. Their main limitation is sensitivity to weather: rain can oversaturate the media, while dry or cold conditions can reduce microbial activity.

Closed biofilters are enclosed within a housing structure that allows for precise regulation of temperature, humidity, and airflow. This controlled environment enables consistent treatment performance regardless of external weather conditions and makes closed biofilters appropriate for industrial applications – pharmaceutical manufacturing, chemical processing, food production – where emission standards are strict and pollutant concentrations may be higher.

Horizontal vs. vertical flow configurations 🔗

In horizontal biofilters, contaminated air flows laterally through the filter bed. This configuration tends to create a longer contact time between the air stream and the biofilm, which can improve removal efficiency for certain compounds. Horizontal systems are commonly used in open-bed outdoor installations at agricultural and composting operations.

Vertical biofilters direct air either upward (most common) or downward through a deeper bed. Upflow vertical designs are typical in both open and enclosed tower-type systems. In a standard vertical upflow biofilter, contaminated air enters through the lower section of the filtering bed, rises through the porous media, and clean gas exits from the top – where it can be released directly into the atmosphere. Vertical tower configurations generally have a smaller physical footprint than horizontal systems, making them practical where space is limited.

Fixed-bed and trickle-bed variants 🔗

Fixed-bed biofilters (FBBs) use either synthetic or organic media as a static support for the biofilm. Because there is no continuous liquid phase flowing through the bed, fixed-bed systems are particularly suitable for treating pollutants that are poorly soluble in water. However, they typically require a larger footprint than trickle-bed systems.

Trickle-bed biofilters add a continuously recirculated liquid stream flowing over an inert packing material. This liquid supplies nutrients to the biofilm, removes acidic metabolic by-products, and helps regulate pH. Trickle-bed systems are preferred for water-soluble compounds and offer tighter process control. They have a smaller footprint and are easier to monitor, though their mechanical complexity is higher than that of simple fixed-bed designs.

Operational factors that determine biofilter performance 🔗

A biofilter is ultimately a biological system, and its performance depends on keeping the resident microbial community healthy and active. Virtually any factor that affects the biofilm – moisture, temperature, pH, nutrients, residence time, and toxic substances – can affect the overall operation of the biofilter. Getting these parameters right is the difference between a system achieving over 90% removal efficiency and one that fails to meet regulatory targets.

Airflow rate and residence time 🔗

Empty bed residence time (EBRT) – the time polluted air spends in contact with the biofilm – is one of the most critical design variables. If air moves through the bed too quickly, pollutants do not have sufficient contact time with microorganisms for adequate degradation. If flow is too slow, the system may become anaerobic and lose efficiency. The biofiltration process begins with diffusion and transfer of contaminants from the gas phase into the water phase of the biofilm, governed by Henry’s Law – compounds with low Henry’s Law constants dissolve more readily into the biofilm and are generally easier to remove. Research has shown that as VOC inlet concentrations increase, removal efficiency declines, underscoring the importance of matching flow rates to actual pollutant loads.

Humidity and moisture content 🔗

Moisture is essential for biofilm survival and function. If the media dries out, the biofilm shrinks or dies; if it becomes waterlogged, anaerobic conditions develop and the wrong microbial communities dominate. A moisture content of 40-60% in the media helps maintain buffer capacity and supports a neutral pH range of 6-8, which is optimal for the survival and metabolism of resident microorganisms.

In practice, incoming air is typically pre-humidified before entering the biofilter to prevent the media from drying out. Biofilters require consistently high humidity, and factors including surface irrigation, weather, and the temperature of entering air all affect moisture levels. Outdoor open-bed systems face particular challenges in arid climates or during summer, often requiring overhead sprinkler systems to maintain adequate moisture. Closed systems use automated humidity controls to keep moisture within the target range year-round.

Temperature 🔗

Microbial metabolic rates are temperature-dependent. The temperature of the biofilter should generally be maintained between 15°C and 30°C to keep mesophilic bacteria – the dominant degraders in most biofilters – operating efficiently. Below this range, metabolism slows significantly; above approximately 40°C, thermophilic organisms begin to dominate, and many of these are less capable of degrading the target VOCs. Scrubbing efficiency drops sharply above approximately 55-65°C. Temperature management is particularly challenging for outdoor biofilters in cold climates, where insulation or heating systems may be needed to prevent efficiency losses during winter.

pH control 🔗

pH directly affects microbial community composition and metabolic activity. For the decomposition of organic compounds, the pH of the media should ideally be maintained between 7 and 8; when the biofilter becomes acidified, efficiency drops. Acidification is a common operational problem: as microbes degrade certain pollutants – particularly sulfur compounds and VOCs – they produce acids as metabolic by-products, which progressively lower the pH of the media.

Monitoring and correcting pH is therefore a routine maintenance task. Different pH levels also shift which compounds are most effectively removed – slightly acidic media scrubs ammonia better, while mildly alkaline media treats organic acids more efficiently. Buffering agents such as limestone or calcium carbonate are commonly added to the media or irrigation water to counteract acidification and stabilize pH within the optimal range. Trickle-bed systems have a natural advantage here, as the recirculating liquid stream can be directly pH-adjusted.

Nutrients and media longevity 🔗

Beyond carbon from the pollutants themselves, microorganisms require nitrogen, phosphorus, and trace minerals for cell growth and maintenance. Microbial growth in biofilters requires nutrients in the form of nitrogen, phosphorus, and trace elements. Organic packing media like compost and peat naturally supply many of these nutrients, but over time these reserves deplete. Synthetic or inert media typically require periodic nutrient dosing through the irrigation water. Filter media also gradually compacts and degrades, increasing pressure drop across the bed and reducing airflow distribution. Filter media generally needs to be replaced every four to six years, depending on pollutant load and operational conditions.

Why biofilters matter for air pollution control 🔗

Biofilters occupy an important niche in the air pollution control toolkit. Compared to physico-chemical techniques, biological off-gas treatment methods – including biofilters – offer lower investment and operational costs and can handle gas flow rates exceeding 200,000 m³ per hour. Well-designed and properly managed biofilters can reduce odors and hydrogen sulfide by as much as 95% and ammonia by around 80%. There is no secondary pollution – the process does not require chemical reagents or fuel, and the degradation products are carbon dioxide, water, and additional biomass rather than hazardous residues.

Their limitations are real but manageable. Biofilters are most effective for biodegradable VOCs and odorous gases at low-to-moderate concentrations. Compounds that are highly toxic to microbes, or gases present at very high concentrations, may require pre-treatment before biofiltration. The biological nature of the system also means it takes time – sometimes weeks – for the microbial community to establish and acclimate to a new pollutant stream. But for the wide range of industrial, agricultural, and municipal applications where these conditions are met, biofilters represent one of the most sustainable and cost-effective air treatment technologies available.

What do you think? As biofilters depend on living microbial communities, how should facility operators balance the need for consistent pollutant removal with the inherent variability of biological systems – particularly when pollutant loads change seasonally or suddenly? And given that open biofilters are vulnerable to weather extremes, at what point does the higher capital cost of a closed, climate-controlled system become the more practical long-term investment?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC9585892/
2. https://en.wikipedia.org/wiki/Biofilter
3. https://pubs.acs.org/doi/10.1021/acsengineeringau.2c00020
4. https://condorchem.com/en/biofilters/
5. https://ppcair.com/products-services/biofiltration/biofiltration-factors
6. https://www.ijirset.com/upload/2022/july/29_A%20Review_NC.pdf
7. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/biofiltration
8. https://compostsystems.com/systems/biofilters/
9. https://pca-air.com/en/blog/how-does-biofilter-work
10. https://compostsystems.com/biofilter-theory-design-operation/
11. https://link.springer.com/article/10.1007/BF00695975
12. https://www.thepoultrysite.com/articles/biofilters-for-odour-and-air-pollution-mitigation