Odor is perhaps the most common problem associated with composting, and the failure to adequately address it has led to numerous neighbor complaints and the closure of many large scale facilities. Fortunately, for the most part odors can be controlled, but proper management can take time and money.
At many composting sites odors originate with the incoming ingredients, which may have been stored anaerobically (without oxygen) for a week or more before transport to the site. Once these ingredients are incorporated into the composting system, subsequent odor problems are usually a result of low oxygen or anaerobic conditions. Anaerobic odors include a wide range of compounds, most notoriously the reduced sulfur compounds (e.g. hydrogen sulfide, dimethyl sulfide, dimethyl disulfide, and methanethiol), volatile fatty acids, aromatic compounds and amines. Ammonia is the most common odor that can be formed aerobically as well as anaerobically, and thus has its own set of managment options.
If organic feedstocks are already anaerobic and odorous when they arrive at the site, they need to be brought to an aerobic state as quickly as possible. Usually this means combining them with a coarse, dry bulking amendment to increase the porosity and allow oxygen penetration. Experiences vary as to the optimum frequency of subsequent turning, which depends on how thoroughly materials are mixed initially, as well as the porosity of the pile. If the porosity is adequate, it may make sense for the material to sit for a few days or weeks to get through the initial period of high oxygen demand.
Oxygen is the obvious compound to add when the source of the odors is anaerobic metabolism. Forced aeration systems provide a way to mechanically introduce oxygen, and are common at facilities composting materials like biosolids (sewage sludge) with a high potential to generate odors. These systems require relatively uniform pile shapes and porosity to reduce the potential for air to short-circuit along the path of least resistance. In passively aerated systems, which depend on diffusion and natural convection, adequate porosity is essential to reduce the resistance to oxygen movement. The pile or windrow dimensions must also be appropriate for both the mix of ingredients and stage in the composting process, so that the oxygen diffusing into the pile is not entirely consumed before it reaches the center.
In addition to these traditional approaches to aeration, oxidizing chemicals like hydrogen peroxide, potassium permanganate, and chlorine are used by the wastewater treatment industry for odor control. These compounds will chemically oxidize anaerobic odors, but may kill the composting microorganisms as well. This is especially true for chlorine. In low concentrations, evenly incorporated in a pile, a compound like potassium permanganate could be effective, but would probably also be expensive.
Odors can also be biologically oxidized after they have formed, and this is probably tremendously important for most composting systems. Odorous anaerobic products produced in the low oxygen center of a pile usually pass through an aerobic zone on the way out. Microorganisms will then degrade the odors aerobically. This process probably occurs on both a macro scale (the pile as a whole) and a micro scale (within individual particles or clumps), essentially providing in situ biofiltration. When turning an anaerobic compost pile this advantage is lost, which is why frequent turning is not the best way to deal with an odor problem, and instead often makes the problem worse. In a windrow system, it is far better to address the fundamentals of porosity and pile size to insure adequate passive aeration (diffusion and convection) throughout the compost pile.
Catalysts and Innocula
Catalysts purport to degrade odorous compounds, usually via biologically generated enzymes. A catalyst facilitates a reaction without itself being permanently changed by the reaction, and thus each enzyme can act on many molecules of an odorous compound before it is eventually degraded. Enzymatic catalysts are normally applied either on the surface of a compost pile or in the airspace above it. A number of products are on the market, but very little independent research has been done to verify their effectiveness.
Preventing excessive odors requires consistent management of the composting process, starting with prompt attention to incoming ingredients. Wet materials should be mixed with a porous bulking amendment to provide the necessary pre-conditions for oxygen transport, and then must be aerated or turned as required during the active stages of the composting process. Understanding the factors leading to anaerobic conditions, including the interactions of particle size, porosity, moisture, and oxygen transport, will provide additional insights into odor prevention. While the emphasis should always be on prevention, odor treatment may also be required, particularly in sensitive neighborhoods.
Helpful reviews and discussions related to this document were provided by James Gossett, Nancy Trautmann, Daniel Cogan, and Cary Oshins.
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