Compost Odor Control

It now is known that odorous compounds are natural by-products of both aerobic and anaerobic degradations. The feedstock contains a number of components that are precursors to odor formation, including proteins, amino acids and carbohydrates.

Odor compounds are characterized by intensity, mass, and persistence, which determine when and where they are detected. For example, ammonia is extremely intense, but is easily diluted. Its odor may be very strong on-site, and hence mask other odors, but it is rarely an odor problem any distance off-site. Conversely, organic sulfur compounds such as methyl sulfides are very pervasive and may be detected easily even when highly diluted. As a result, these odors, which may not be apparent on-site due to masking by other compounds, are often the source of complaints from neighbors to the compost site.

Minimization of odors emitted from a composting facility may be achieved by one or a combination of physical, chemical, or biological methods. Historically, the most common methods for odor control include dilution, masking, sorption, condensation, oxidization by combustion, chemical scrubbing, and more recently, biofiltration by microbes.

The popularity of biofiltration for odor reduction has increased in recent years. The most desirable characteristics of biofilter media include high-specific surface area, air and water permeability, water-holding capacity, an active microbial population, and relatively low cost.

The basis of the technique is the use of a biologically active, solid-media bed to sorb compounds from the air stream. The sorbed compounds are oxidized by the microbial population in the media. The biofilter is an adaptation of the "soil filter" first used in Germany in 1959 for the control of sewer odors. The trend is to use compost or peat instead of soil as filter media due to the high microbial activity of these media.

The usefulness of biofilters for treatment of odorous composting gases has been subject to some controversy. Some researchers cite studies at a number of European installations that have observed as much as 99 percent removal efficiencies for gases with inlet concentrations as high as 25,000 to 50,000 ED50. (ED50 is the effective dilution at which 50 percent of a panel of people can no longer detect an odor.) However, these studies are contradicted by others reporting insignificant removal of both organic and inorganic sulfur compounds when loading rates are high. Still, there is a consensus that if designed and operated properly, biofilters are extremely effective for reducing odor problems in composting.

The preceding article is extracted from the paper, "Dimethyl Sulphide and Dimethyl Disulphide Emissions from the Caledon Composting Facility," by Lambert Otten and Lindsay Dennison, School of Engineering, University of Guelph, Guelph, ON

Edited by Paul Hersch