Their use is sufficiently common in Europe that the Netherlands, a country smaller than the combined areas of New Hampshire and Vermont, has long been home to five large manufacturers of biofilters. Their use in the United States goes back to as early as 1953 when Long Beach, Calif., is reported to
have used a soil biofilter system for odor reduction. Their success in controlling odors, detectible at only a few parts per billion, would make their success in compliance matters almost a foregone conclusion since most air pollution regulations allow emissions of a few parts per million.
Yet, the road to engineering an efficient and dependable biofiltration system has not been smooth. Even the name may have been an impediment. How and why this fascinating technology of nature was termed biofiltration is unknown. It is possible that the early construction techniques contributed to a misconception of the principles involved.
The major component of the traditional biofilter was a large, shallow basin. Usually these were formed of concrete, although some were merely shallow excavations (with or without a liner) in the soil. Perforated piping arrayed across the bottom of the basin was used to deliver noxious air near its floor. The piping was covered with truckloads of compost leveled to a uniform depth of three or four feet above the pipe. Air sparged through the pipe was freed of objectionable odor as it passed up through the compost.
Is it possible that early investigators believed that the compost physically filtered odorous compounds from the air? The prefix “bio” makes clear that they recognized life forms played a role. Were they aware, however, that the process depended on a healthy community of invisible, microscopic life forms; that bacteria and fungi, abundant in compost, were responsible for actual cleansing of the air? Did they understand that, in turn, those life forms depend on a readily available source of carbon-containing material for food and, given the choice, readily choose vapor phase organics because they are more easily metabolized than the solid compost materials?