Biofiltration is an air pollution control technology, primarily found in Europe, in which VOC's are oxidized into carbon dioxide and water using microorganisms. Process air is passed through a pre-treatment humidifier to saturate the gas stream before it enters the biofilter. Humidified air then flows through the biofilters, where the VOC's are absorbed into an aqueous layer surrounding the filter material. The microorganisms, contained in the filter material, use the VOC's as their primary food (carbon) source and convert the VOC's to CO2 and water.
The most attractive feature of biofiltration is the low operating and maintenance costs relative to other emission control technologies. The recirculating pump for the humidifier and the fan that moves the air through the system are the only two energy sinks in the system.
An additional benefit of biofiltration over other oxidation technologies is the lack of secondary contaminants. Biofilters produce none of the nitrogen (NOx) compounds found in the effluent from thermal or catalytic oxidizers. In ozone non-attainment areas, the reduction or elimination of these compounds is important and makes the biofilter an attractive alternative.
A biofilter, in its simplest form, consists of a hole in the ground, filled with filter media, ranging from dirt, to wood chips, bark, peat moss or compost. These types of biofilter designs are inexpensive and easy to construct, but are not adequate to meet the standards required for VOC destruction efficiency regulatory requirements for a number of reasons. These types of systems have difficulty in demonstrating compliance, due to the complications in measuring system performance. These systems also are affected by ambient conditions that make it difficult to maintain consistent, reliable performance.
Sophisticated biofilters have a fully instrumented design, including a totally enclosed biological reactor, a prehumidification system and a moisture control system with data-logging capabilities. This type of system makes demonstrating compliance a straightforward process. Compliance can be demonstrated by simply measuring inlet and outlet concentrations of VOC, or by relating historical logs of system parameters to the current performance of the system.
Biofiltration is not applicable to all gas streams, but in situations where it does apply, the economics indicate that it should be seriously considered as an alternative to other control technologies.
Biofiltration has been an accepted form of VOC and odor control in Europe for a number of years. In the first half of the 1990s, a number of different industries in the United States have accepted biofiltration as an alternative to thermal and sorptive VOC control technologies. These industries include the wood products industry, the flavor and fragrance industry, and a number of different solvent-using industries, such as film processing and screen printing.