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BiofilterA biofilter is one of several air pollution control technologies that use microorganisms to treat polluted air. 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, immobilised in the biofilm degrade the pollutant. Related technologies include biotrickling filters and bioscrubbers. The technology usually finds application in treated malodorous compounds at low concentrations. Industries employing the technology include food and animal products, off-gas from wastewater treatment facilities, pharmaceuticals, manufacturing etc. Compounds treated are typically single or mixed volatile organic compounds and recent success has been obtained with hydrogen sulphide. Very large airflows may be treated although a large area is then required -- a large biofilter may occupy as much or more land than a football field -- this being one of the principal drawbacks of the technology. One of the main challenges is to prevent the packing media from drying out during operation. The air is normally humidified before it enters the bed and some form of watering system installed. Properly controlled, the packing media may last for a decade or more before it needs replacement. A number of companies offer proprietary packing material although many operators opt for other materials such as compost, peat moss or wood chips. Although widely employed, the research community is still unsure of the physical phenomena underpinning biofilter operation, and information about the microorganisms is only just emerging. Design methods are based on experience or, if the pollutant has not been treated before, pilot-scale testing. A biofilter is a fairly simple device to construct and operate and so often presents a cost-effective solution provided the pollutant is biodegradable, at fairly low concentration and at a reasonable temperature. For high flows of air, a biofilter may be the only cost-effective solution. There is no secondary pollution (unlike the case of incineration) and degradation products (for VOCs) are some additional biomass, carbon dioxide and water. Leached water from the media has very high BOD and must be treated before disposal, although many operators recycle part of it to the watering system to reduce operating costs. Further Reading Biofiltration for Air Pollution Control, Joseph S. Devinny, Marc A. Deshusses and Todd S. Webster, Lewis Publishers, 1998.
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