A Biofilter for control of air pollutants consists of one or more beds of biologically active materials, primarily mixtures based on compost, peat or soil. Contaminated off-gas is vented through the filter. With sufficient time, the air contaminants will diffuse into the biofilm which surrounds the filter particles.

Aerobics degradation of the target pollutants will occur in the biofilm if micro-organisms, mainly bacteria, are present that can metabolize them. End products from the complete biodegradation of air contaminants are carbon dioxide and water, and microbial biomass. The oxidation of reduced sulphur compounds and chlorinated organic compounds also generates inorganic acids.

The components needed for preconditioning of the off-gas, its transport to and distribution in the filter bed, account for the other main elements of a biofilter system. Heat exchangers to cool hot off-gases or filters for the removal of particulates may be required for certain types of emissions.

Radial blowers are generally used to overcome the back pressure caused by the filter. The off-gas must also be saturated with water, since it would otherwise remove moisture groom the filter material, resulting in the drying of the bed, the death of most micro-organisms and a total loss of control efficiency. Spray nozzles usually provide the required humidity in an humidification chamber. Additionally, automatic irrigation of the filter bed from the top is also used in some systems to maintain the required moisture content in the filter material.

The off-gas is vented, usually through slotted concrete slabs or concrete blocks with distribution canal and air nozzles, into the bottom of the filter bed. Down-flow systems have also been used in several recent installations.

Mineralization of the organic matter in biofilters will, over time, lead to compaction of the filter material and a corresponding increase in back pressure. In open filters, the filter material is typically turned over after two years in order to increase its porosity, and discharge after a further year or two.

For enclosed systems, a useful life for filter material of up to five years can be achieve. Maintaining the porosity of the compost by turning it over, and/or replacing it entirely, once spent, are the two major maintenance requirements for biofilters with compost-based filter material.