Characterization of odors and other volatile emissions

Abstract The phenomenon of olfaction is complex. Several models have been proposed to explain the observations relative to odor perception and identification, yet none of the models is entirely satisfactory. Psychological aspects of odor evaluation compound the difficulties in making objective measurements of either odor strength or odor quality. Correlations between the chemical composition of air and its odors have proven more difficult than anticipated. Anaerobic decomposition of manure is a stepwise process in which complex organic compounds are degraded to successively smaller, less complex molecules. Since, at a given moment, any or all of these intermediate compounds may be present, the observed odor represents the sum of the individual contributors. Data indicate that the total odor may not represent the simple summation of indiduaual contributors but that extensive interaction is occurring. Research to identify the chemical compounds presents in odorous air from animal waste degradation has yielded about 45 compound to date. Even though this list i s undoubtedly incomplete, it indicates the complexity of the situation and forewarns of the difficulties to be encountered in odor control. The amines, mercaptans, organic acids, and heterocyclic nitrogen compounds are generally regarded as being of greatest odor signif icance. Chemical techniques exist for measuring the concentration of many odorous compounds produced by the anaerobic decomposition of manure. The low concentration at which these compounds are odorous in air, however, frequently exceeds the sensitivity of existing analytical techniques. Thus, extensive modification of traditional analytical techniques is necessary. Ammonia, hydrogen sulfide, mercaptans, and volatile organic acids have been quantitatively measured in the air-volatilized material from manure storagetanks. Although certain odorous gases are known to be toxic to both humans and livestock, the primary concern is one of annoyance or nuisance to humans. Rules and regulations relative to livestock odors are based primarily on the concept of nuisance. Whenever a neighboring property owner feels the odor from an animal production unit isunreasonably interfering with the use and enjoyment of his property, he has the right to initiate legal action to recover damages or to seek an injuction to halt or modify an operation. Both private and public nuisance have involved major expenses for the livestock producer and, in a few instances, required that a producer cease operation to a more appropriate location. Ammonia has been the most widely studied odorous gas being evolved by anaerobic manure decomposition. The evalution of ammonia is of interest not only because of its odor but also because the potential for reabsorption by nearby water bodies would lead to the possibility of aquatic enrichment. The atmosphere near livestock production units has been measured in enough locations to demonstrate a significant increase in ammonia concentrations in these areas compared to residential or other agriculturalareas. The volatilization rate of ammonia is a function of temperature, pH, and ammonia content of the material from which it would escape. Various chemical treatments have been explored for reducing the odor of stored animal manure. Chlorine, lime, potassium permanganate, hydrogen peroxide, and paraformaldehyde have been applied to manure for their characteristics of inhibiting anaerobic bacterial activity and their reaction with known odorous compounds. These chemicals have been demonstrated to be temporarily effective when added in sufficient concentration. There are several proprietary odor control chemicals being sold for odor reduction or masking. Their performances have received only limited study and the published results are highly variable.