Bio-filtration is a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants. Common uses include the micro biotic oxidation of contaminants in air. When specifically applied to air filtration and purification, bio-filters use microorganisms to remove air pollution. The air flows through a packed bed and the pollutant transfers into a thin biofilm on the surface of the packing material. Microorganisms including bacteria and fungi are immobilized in the biofilm and degrade the pollutant. Trickling filters and bio-scrubbers rely on a biofilm and the bacterial action in their recirculating waters.

The technology finds greatest application in treating malodorous compounds and water-soluble volatile organic compounds (VOCs). Industries employing the technology include food and animal products, off-gas from wastewater treatment facilities, pharmaceuticals, wood products manufacturing, paint and coatings application and manufacturing and resin manufacturing and application, etc. Compounds treated are typically mixed VOCs and various sulphur compounds, including hydrogen sulphide. Very large airflows may be treated although a large footprint area will be required.

One of the main challenges to optimum bio-filter operation is maintaining proper moisture throughout the system. The air is normally humidified before it enters the bed with a watering (spray) system, humidification chamber, bio-scrubber, or bio-trickling filter. Properly maintained, a natural, organic packing media like peat, vegetable mulch, bark or wood chips may last for several years but engineered, combined natural organic and synthetic component packing materials, such as leca, will generally last much longer.

A bio-filter system is a fairly simple device to construct and operate and offers a cost-effective solution provided the pollutant is biodegradable within a moderate time frame, at reasonable concentrations, and that the airstream is at an organism-viable temperature. For large volumes of air, a bio-filter may be the only cost-effective solution. There is no secondary pollution and degradation products form additional biomass, carbon dioxide and water.

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