Biofiltration: An Innovative Technology for The Future
Biofiltration refers to the biological transformation or treatment of contaminants present in the gas phase, usually air. The fact that air contaminants can be biodegraded by active bacteria has been known for quite some time. However, it is only in the last 10 years, that biofiltration has begun to emerge as an economically viable treatment process. Initially, biofiltration involved the use of naturally bioactive media, such as soil, peat, compost, etc. In naturally bioactive media, microorganisms present in the soil, peat or compost, have been known to biodegrade contaminants, and this has been successfully employed in bioremediation of contaminated sites. However, when contaminated air is passed through soil, peat, or compost, the naturally present microorganisms also begin to biodegrade the air contaminants. This led to the development of soil biofilters, in which soil with low clay and high organic carbon content was packed in a bed and contaminated air was passed through the soil bed to biodegrade the air contaminants. However, as more research was conducted on this simple process, it became clear that the biodegradation rates were low and hence the size of the biofilter bed required to achieve high destruction efficiencies was very large. Since, compost had a higher concentration of microorganisms, compost became the media of choice for biofilters. Major problems encountered were settling of the compost, resulting in increased gas-phase pressure drop, availability of nutrients, such as nitrogen and phosphorus, pH maintenance, and drying of the compost material due to moisture transferring to the flowing gas phase. These problems were countered to some extent by adding wood chips, which provided mechanical support to minimize settling, humidifying the inlet air to maintain proper water content in the compost material, adding lime pellets for pH control, and fortifying the compost with fertilizers containing nitrogen and phosphorus compounds. Further, in compost beds, it was necessary to have shallow beds (height < 1.5 m or 4.5 feet), to prevent compaction of the material and drying of the bed from the top surface. This required the beds to have large cross-sectional areas, and in many cases were simply left completely open from the top. In some cases, powdered activated carbon was also added to buffer the concentration changes, since activated carbon is known to adsorb contaminants. Currently, there are several companies that offer compost biofilters for treatment of odorous and volatile chemicals.