Biofiltration & Composting

Basics of Biofiltration

• Biofiltration is the process of using a biologically active substance, like compost, soil, peat or mulch, to remove contaminants from waste. The substances used in biofilters must be a moist, organic material able to absorb and then biologically degrade odor-causing compounds. These absorbent materials are often combined with an inert substance, like gravel, to maintain proper porosity and aeration.

Where Biofilters Are Used

• Manufactured composting bins meant for residential spaces often have a basic biofilter to help control odors. However, biofiltration systems are most often used in large-scale composting facilities, as they provide a relatively inexpensive, effective means of reducing almost all offensive smells associated with compost in comparison to alternative methods like chemical stripping and thermal destruction. Biofilter beds in these facilities are ideally between 3 and 4.5 feet deep, according to experts from Cornell University. Any deeper presents moisture consistency problems and any shallower presents problems with circulating the gases properly.

Compost Has A High Absorptive Capacity

• Cured compost makes an excellent material for biofiltration systems, according to experts from Virginia Tech. It has a high absorptive capacity, which means it physically absorbs many materials, including gases that cause odors. It also has a high water holding capacity, which is useful for filtering volatile organic compounds like sulphurs and amines, though scientists are not entirely sure why. In one 1994 study cited by Virginia Tech, 99.9 percent of the hydrogen sulfide gas emitted by a compost pile was removed as long as the water content of the compost used as a filtering agent was between 30 and 62 percent.

Compost Has a High Microbial Population

• Compost filters odors not only by physically absorbing them, but by breaking down odor-causing molecules with its microbes. As a material rich in nutrients like carbon, compost sustains a high microbial population. The microbes -- like bacteria, fungi and actinomycetes -- in turn metabolize, oxide or consume organic compounds that give off offensive smells. The high absorptive capacity and the high population of microbes are the primary reasons well-tended, established compost piles do not emit strong offensive smells.