In biofilters, microorganisms, usually bacteria or fungi, destroy VOC (Volatile Organic Compounds) emissions in a natural biological manner.
In biofiltration, microorganisms oxidize the organic compounds into carbon dioxide and water. The microorganisms live on a thin water layer on the surface of a substrate known as the biofilm. Because the microorganisms live on a water layer, keeping a wetted surface is essential.
The substrate provides structural support and elemental nutrients for the microorganisms. The biofilter media shown below is made of a polypropylene and calcium carbonate ribbon and is used in ponds to rapidly secure and develop bacteria.
The schematic shown here is of a typical biofilter. The gas is first pretreated, then is fed into the biofilter, where it travels through filter beds containing microorganisms that break down the VOCs, producing water and carbon dioxide. The temperature and humidity are well controlled in this region. The water produced by the oxidation is often recycled back to the filter bed area. The stream, now up to 90% free of VOCs, is passed to a column where it can exit to the atmosphere.
Biofilters can be installed above or below ground. Below-ground systems that are built directly into the ground use the existing native soil as the biofilter's contaminment walls, or they can be enclosed with cement. Above-ground biofilters are typically made of cement or fiberglass.
VOC removal by biofilters is used in a variety of industries such as chemical manufacturing and food processing. Biofilters are most economical when used for gas streams that have low-concentrations of VOCs and are oxygen-rich. The in-ground biofilter shown below is installed at a food processing plant for treatment of off-gas from water treatment.
Perry, Robert H., and Don W. Green. Perry's Chemical Engineers' Handbook. 7th ed. New York: McGraw-Hill Inc., 1997: 25-5, 25-8, 25-22 - 25-57. Print.