The olfactory system can distinguish thousands of odors. Olfactory information travels to the limbic system of the brain which governs emotions, behavior, and has memory storage. The information from odors also goes to the brain's cortex, or outer layer, where conscious thought occurs. Therefore, odor is very subjective and difficult to regulate due to the varied sensitivities and the differing emotions, and memories which an odor elicits. Some odors are generally considered unpleasant by most people: odors containing ammonia are pungent; those which have hydrogen sulfide smell like rotten eggs; and odors with cadaverine have a rancid smell. However, even an "agreeable" odor may produce different responses in separate individuals. For example, the smell of cut grass may bring pleasant memories of a picnic to one person while the same odor may trigger a painful memory of a lawn mower crash in another person and become a disagreeable odor.

Components of Odor from Composting

If an odor from the composting process occurs, the odor characteristics may assist in identifying the source and possibly remediating the problem. The composting process breaks down feedstock and utilizes the nitrogen and carbon. The best mix is about one part nitrogen to 30 parts carbon. Woody wastes contain lots of carbon and green wastes contain lots of nitrogen. Too much carbon may cause the pile to break down too slowly, while too much nitrogen may cause odor. Carbon provides energy for the microbes, and nitrogen provides protein. Ammonia contains nitrogen and an ammonia smell may indicate that a compost pile has too much nitrogen for the amount of carbon present. Ammonia may indicate that the carbon pieces are too large (not chipped enough) and therefore are not available for the biological processing with the nitrogen. Hydrogen sulfide and mercaptans (rotten eggs) signify that an anaerobic condition exists which may have resulted from too much water and/or not enough air spaces. If an odor occurs it may not be related to the composting operation but to other operations located nearby such as publicly owned treatment works (POTWs), rendering plants, dairies, etc.

Some of the common components in the raw emissions from composting are typical odorous compounds (see below). The chart is based in part on the Olfactory Response to Mushroom Composting Emissions as a Function of Chemical Concentration, by R. Noble, P. J. Hobbs, A. Dobrovin-Pennington, T. H. Misselbrook, and A. Mead.

Typical Odor Causing Compounds from Composting
Compound NameChemical FormulaPrimary Odor Characteristic
AcetaldehydeCH3CHOPungent
AmmoniaNH3Urine, pungent
Butyric acidCH3CH2CH2COOHRancid, sour
Diethyl sulfideC2H5C2H5SGarlic
Dimethyl amineCH3CH3NHFishy
Dimethyl sulfideCH3CH3SFoul, decayed cabbage
Ethyl mercaptanC2H5SHDecayed cabbage
FormaldehydeHCHOPungent
Hydrogen sulfideH2SRotten eggs
Indole Fecal
Methyl mercaptanCH3SHFoul, decayed cabbage
PhenolC6H5OHMedicinal
Propyl mercaptanC3H7SHUnpleasant
Sulfur dioxideSO2Pungent
Trimethyl amineCH3CH3CH3NFishy, ammonical
Valeric acid CH3CH2CH2CH2COOHBody odor

Compost "Odor wheels" are also useful for describing odors in the field.

Odor Dimensions

There are four common odor dimensions; detectability, intensity, character or quality, and hedonic tone. Some enforcement programs have used odor intensity to determine a violation.

  1. Detectability refers to the odor threshold. It is the concentration of an odor where the population is aware of the odor. The human nose tends to be the most sensitive detection instrument available and varies greatly from individual to individual.
  2. Intensity refers to the perceived strength or magnitude of the odor sensation. Intensity increases as a function of concentration. Some enforcement programs may use an intensity scale to determine an odor violation. For example, if an intense odor is determined to be a 4 or a 5 on a scale of 1 to 5, then a violation would be issued. A method of measuring odor intensity is (ASTM Method E-544-10).
  3. Odor quality or character is expressed as a description of the odor, like sweet, pungent etc. A description of an odor by a complainant may assist investigators in identifying an odor source.
  4. Hedonic tone is a judgment call on the degree to which an odor is acceptable or not. It may range from “very pleasant” (high score, positive) to “unpleasant” (low score, negative). The same odor may be unpleasant to one person and very agreeable to another person. For example, an individual may savor the smell of chlorine because it reminds them of their days as a champion swimmer and practicing in pools treated with chlorine. The same smell of chlorine may be very disagreeable to another person because it reminds them of being ill in the hospital since chlorine was used to clean the surfaces in the hospital.

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