Editing Biofilter (section)

==Control of air pollution==
When applied to air filtration and purification, biofilters use microorganisms to remove [[air pollution]].<ref>{{cite book|author1=Joseph S. Devinny |author2=Marc A. Deshusses |author3=Todd S. Webster |name-list-style=amp |title=Biofiltration for Air Pollution Control|publisher=Lewis Publishers|year=1999|isbn=978-1-56670-289-8}}</ref> 
The air flows through a packed bed and the pollutant transfers into a thin [[biofilm]] on the surface of the packing material. [[Microorganism]]s, including [[bacteria]] and [[fungi]] are immobilized in the biofilm and degrade the pollutant. Trickling filters and bioscrubbers rely on a biofilm and the bacterial action in their recirculating waters.

The technology finds the greatest application in treating malodorous compounds and [[volatile organic compounds]] (VOCs). Industries employing the technology include food and animal products, off-gas from [[wastewater]] treatment facilities, [[pharmaceutical]]s, wood products manufacturing, [[paint]] and coatings application and manufacturing and resin manufacturing and application, etc. Compounds treated are typically mixed VOCs and various [[sulfur]] compounds, including [[hydrogen sulfide]]. Very large airflows may be treated and although a large area (footprint) has typically been required—a large biofilter (>200,000 [[acfm]]) may occupy as much or more land than a football field—this has been one of the principal drawbacks of the technology. Since the early 1990s, engineered biofilters have provided significant footprint reductions over the conventional flat-bed, organic media type.

[[Image:CVRD4chamber.air.jpg|thumb|right| Air cycle system at biosolids composting plant. Large duct in foreground is exhaust air into biofilter shown in previous photo]]

One of the main challenges to optimum biofilter 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 will generally last much longer, up to 10 years. Several companies offer these types of proprietary packing materials and multi-year guarantees, not usually provided with a conventional compost or wood chip bed biofilter.

Although widely employed, the scientific community is still unsure of the physical phenomena underpinning biofilter operation, and information about the microorganisms involved continues to be developed.<ref>{{Cite journal |last1=Cruz-García |first1=Blanca |last2=Geronimo-Meza |first2=Andrea Selene |last3=Martínez-Lievana |first3=Concepción |last4=Arriaga |first4=Sonia|author4-link=Sonia Arriaga |last5=Huante-González |first5=Yolanda |last6=Aizpuru |first6=Aitor |title=Biofiltration of high concentrations of methanol vapors: removal performance, carbon balance and microbial and fly populations |url=https://onlinelibrary.wiley.com/doi/10.1002/jctb.5974 |journal=Journal of Chemical Technology & Biotechnology |year=2019 |volume=94 |issue=6 |pages=1925–1936 |doi=10.1002/jctb.5974 |bibcode=2019JCTB...94.1925C |s2cid=104375950 |issn=0268-2575}}</ref> A biofilter/bio-oxidation 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 (increasing residence time = increased size and capital costs), at reasonable concentrations (and lb/hr loading rates) and that the airstream is at an organism-viable temperature.  For large volumes of air, a biofilter may be the only cost-effective solution. There is no secondary pollution (unlike the case of [[incineration]] where additional CO<sub>2</sub> and NO<sub>x</sub> are produced from burning fuels) and degradation products form additional biomass, carbon dioxide and water. Media irrigation water, although many systems recycle part of it to reduce operating costs, has a moderately high [[biochemical oxygen demand]] (BOD) and may require treatment before disposal. However, this "blowdown water", necessary for proper maintenance of any bio-oxidation system, is generally accepted by municipal [[publicly owned treatment works]] without any pretreatment.

Biofilters are being utilized in [[Columbia Falls, Montana]] at Plum Creek Timber Company's [[fiberboard]] plant.<ref>{{cite news|last=Lynch|first=Keriann|title='Bug farm' a breath of fresh air|work=Spokesman Review|date=2008-10-26|url=http://www.spokesman.com/stories/2008/oct/26/bug-farm-a-breath-of-fresh-air}}</ref> The biofilters decrease the pollution emitted by the manufacturing process and the exhaust emitted is 98% clean. The newest, and largest, biofilter addition to Plum Creek cost $9.5 million, yet even though this new technology is expensive, in the long run it will cost less overtime than the alternative exhaust-cleaning incinerators fueled by natural gas (which are not as environmentally friendly).