Introduction to Biological Air Treatment Technologies

In addition to conventional physical and chemical air treatment methods, biological processes have become highly effective technologies for controlling air pollution. Among these systems, the conventional biofilter is one of the oldest and most widely used biological reactors for polluted air treatment. Biofilters have been applied for decades, initially for odor removal in facilities such as wastewater treatment plants, composting sites, and municipal waste processing units.

Biofiltration systems are especially suitable for treating low to moderate concentrations of gaseous pollutants over a broad range of airflow rates. In general, lower gas flow rates are preferred when pollutant concentrations are high because excessive contaminant loads can inhibit microbial activity. Most conventional systems are recommended for pollutant concentrations below approximately 5–6 g/m³ of air, although recent laboratory-scale developments have improved the treatment capacity for higher concentrations and loading rates.

Since the 1980s, biofiltration technologies have expanded beyond odor control applications and are now extensively used for removing volatile organic compounds (VOCs) and inorganic contaminants from industrial waste gases. During this period, new reactor configurations such as the biotrickling filter were developed and optimized specifically for air pollution control. Over the past decades, additional advanced systems including membrane bioreactors and rotating biological contactors (RBCs) have also been investigated. While some of these technologies remain at the experimental or pilot scale, they represent promising alternatives for future large-scale industrial applications.

Principles of Biodegradation in Air Pollution Control

In theory, any biodegradable air pollutant can potentially be treated biologically. However, the overall efficiency of a biological air treatment system depends not only on microbial activity but also on several physical and chemical factors. For example, carbon monoxide is biologically degradable, yet its low solubility in water makes its removal from air streams difficult in conventional bioreactors. Similarly, parameters such as humidity, temperature, nutrient availability, and gas transfer properties strongly influence biodegradation performance.

Historically, many industrial contaminants particularly xenobiotic compounds such as chlorinated hydrocarbons were believed to be non-biodegradable because they do not naturally occur in the environment. However, extensive microbiological research since the late 1970s has demonstrated that many bacteria and fungi possess the enzymatic capability to degrade a wide variety of volatile organic and inorganic pollutants.

Biological degradation generally converts contaminants into harmless end products such as carbon dioxide, water, sulphate, or biomass. Under high loading conditions, intermediate metabolites may temporarily accumulate, but complete biodegradation is typically achieved under optimized operating conditions.

Aerobic Biodegradation of Volatile Organic Compounds

Most volatile pollutants treated in bioprocesses contain carbon and hydrogen atoms, such as methane or toluene. Some compounds also contain oxygen, including methanol and formaldehyde. During aerobic biodegradation, microorganisms oxidize these pollutants into carbon dioxide and water while simultaneously producing microbial biomass.

The generalized biodegradation reaction can be represented as:

a VOC+bO2→cCO2+dH2O

When microbial growth occurs, nutrients such as nitrogen and phosphorus are required for biomass synthesis. Microbial biomass is commonly represented by the empirical formula:

C5H7NO2​

Consequently, biological reactors often require supplementation with macro- and micronutrients to maintain efficient microbial activity.

A common example is the aerobic biodegradation of toluene:

C7H8+9O2→7CO2+4H2O

When ammonium chloride is used as the nitrogen source, hydrochloric acid can be produced, resulting in acidification of the reactor medium. Therefore, pH control becomes essential unless acid-tolerant microorganisms such as fungi dominate the system.

Biodegradation of Halogenated Compounds

Halogenated volatile organic compounds, especially chlorinated pollutants, are common contaminants in industrial waste gases. Examples include chlorobenzenes, dichloromethane, and trichloroethylene (TCE). During biodegradation, these compounds generate acidic by-products such as hydrogen chloride.

The general biodegradation reaction for chlorinated compounds is:

R−Cl+O2→CO2+H2O+HCl

Unlike non-halogenated compounds, the degradation of chlorinated pollutants significantly decreases pH levels inside the bioreactor. As a result, efficient pH regulation is necessary to prevent inhibition of microbial activity.

For example, monochlorobenzene degradation can be expressed as:

C6H5Cl+7O2→6CO2+2H2O+HCl

Many microorganisms capable of degrading chlorinated compounds are highly sensitive to acidic conditions, making biotrickling filters more effective than conventional biofilters for these applications.

Treatment of Inorganic Sulphur Compounds

Hydrogen sulphide (H₂S) is one of the most frequently treated inorganic sulphur pollutants in biological air treatment systems. Its biodegradation generates elemental sulphur or sulphuric acid depending on oxygen availability.

Partial oxidation occurs under oxygen-limited conditions:

H2S+0.5O2→S0+H2O

Complete oxidation under sufficient oxygen supply produces sulphuric acid:

S0+H2O+1.5O2→H2SO4​

This acid formation can lower reactor pH considerably. Fortunately, many sulphur-oxidizing bacteria are naturally acid tolerant and remain active even under highly acidic conditions.

Unlike VOC degraders, many H₂S-degrading microorganisms are autotrophic, meaning they use carbon dioxide instead of organic carbon as their carbon source.

Important Operating Parameters in Bioreactors

The performance of air pollution bioreactors is generally evaluated using several operational parameters.

Volumetric Loading Rate (VLR)​

Mass Loading Rate (MLR)​​

Elimination Capacity (EC)​

Removal Efficiency (RE)

Carbon dioxide production is also frequently monitored to evaluate biodegradation completeness.

Main Bioreactor Configurations for Air Pollution Control

Conventional Biofilters

Conventional biofilters are packed-bed reactors containing natural or inert packing materials colonized by microorganisms. Polluted air passes through the packed bed where contaminants are biodegraded by microbial biofilms.

Natural packing materials such as compost, peat, and soil provide nutrients and indigenous microorganisms. However, these materials gradually compact and degrade over time, causing increased pressure drop and reduced reactor efficiency.

Inert materials such as perlite, lava rock, polyurethane foam, and plastic packings are increasingly used because they offer better structural stability and airflow characteristics. These systems require periodic nutrient and moisture addition to maintain microbial activity.

Biofilters are particularly effective for treating hydrophobic and poorly water-soluble pollutants, but they are less suitable for contaminants producing acidic metabolites.

Biotrickling Filters

Biotrickling filters are advanced packed-bed reactors containing inert support materials continuously irrigated with a nutrient-rich liquid phase. The presence of this circulating liquid allows:

• Better pH control
• Easier nutrient distribution
• Removal of inhibitory metabolites
• Improved temperature regulation

These systems are especially effective for treating chlorinated compounds and hydrogen sulphide because acidic by-products can be neutralized and washed out continuously.

Common packing materials include:

• Plastic rings
• Polyurethane foam
• Activated carbon
• Perlite
• Lava rock

One major challenge in biotrickling filters is excessive biomass accumulation, which can cause clogging and pressure drop increases. Techniques such as backwashing, air sparging, nutrient limitation, and mechanical agitation are often applied to control biofilm thickness.

Bioscrubbers

Bioscrubbers combine a gas absorption unit with a suspended-growth bioreactor. Polluted air first passes through a scrubber where contaminants are transferred into water. The polluted liquid is then treated biologically in a separate reactor.

This technology is most suitable for water-soluble pollutants with low Henry’s constants. Bioscrubbers are widely used for:

• Hydrogen sulphide removal
• Methanol treatment
• Biogas desulphurization
• NOx and SOx control

Unlike packed-bed reactors, bioscrubbers do not suffer from clogging problems because microorganisms grow in suspension rather than attached biofilms.

Membrane Bioreactors for Air Treatment

Membrane bioreactors separate the gas phase from the aqueous microbial phase using membranes. Pollutants diffuse through the membrane and are biodegraded by microorganisms growing on the opposite side.

These systems offer several advantages:

• Efficient pH regulation
• Easy nutrient supply
• Reduced microbial emissions into the air stream
• High treatment efficiency at low residence times

Both dense and microporous membranes are used. However, membrane fouling, biofilm overgrowth, and high operational costs remain major limitations for large-scale industrial implementation.

Rotating Biological Contactors and Rotating Drum Systems

Rotating biological contactors (RBCs) contain rotating discs partially submerged in nutrient solution. Biofilms grow on the disc surfaces and alternately contact air and nutrients during rotation.

Advantages include:

• High oxygen transfer efficiency
• Reduced clogging problems
• Stable biofilm growth
• Good pollutant removal efficiency

Related systems such as rotor biofilters and rotating drum biofilters also improve mixing, moisture distribution, and biofilm control.

Two-Liquid Phase Bioreactors

Two-liquid phase bioreactors contain both water and an immiscible organic solvent such as silicone oil. The organic phase acts as a pollutant reservoir, especially for hydrophobic VOCs.

This technology offers important benefits:

• Improved treatment of poorly soluble pollutants
• Reduced microbial inhibition
• Better resistance to shock loads
• Higher elimination capacities

Silicone oil is commonly used because it is non-toxic, non-biodegradable, chemically stable, and has low volatility.

Hybrid and Multi-Stage Biological Systems

Modern air pollution control systems increasingly combine multiple reactor types to improve performance. Examples include:

• Biofilter + biotrickling filter systems
• Bioscrubber + biofilter combinations
• Multi-stage reactors for mixed pollutant removal

These hybrid systems allow optimization of pH, nutrient conditions, and microbial populations for different pollutants within the same treatment process.

Conclusion

Bioprocesses have become essential technologies for sustainable air pollution control. Conventional biofilters and biotrickling filters remain the most widely applied systems for industrial gas treatment due to their high efficiency, low operational cost, and environmental compatibility.

Recent advances in membrane bioreactors, rotating biological contactors, bioscrubbers, and two-liquid phase systems have significantly expanded the range of treatable pollutants and operating conditions. Current research focuses on improving the treatment of highly hydrophobic, toxic, and recalcitrant contaminants while reducing residence times and increasing reactor stability under industrial operating conditions.

As environmental regulations continue to tighten, biological air treatment technologies are expected to play an increasingly important role in industrial emission control and sustainable environmental management.