Relevant odorants

It remains an issue, that odor-relevant compounds exist in much lower concentration in most cases than the less odorous compounds, thus they are also less influential on the sensor signal. Much research effort has yet to be undertaken to apply the electronic nose in the future. 

The concept of mole fraction of a component used in Equation (4.1) is a convenient measure of concentration when dealing with trace quantities and dilute solutions, often experienced in environmental systems. This is especially the case with transport phenomena and equilibrium between phases, where it results in simple quantitative expressions. The phenomena of interest when dealing with the exchange of odorous compounds and oxygen between wastewater and a sewer atmosphere are, in this respect, relevant examples. 

As previously mentioned, the simple equilibrium approach requires that the relevant volatile compounds exist in a nondissociated molecular form in the water phase. For several substances, this is not the case. Hydrogen sulfide is, as an odorous compound, an important example, with sulfide chemical species related according to the following equilibrium ... 

As can be seen from Table 4.1, all three situations are relevant for odorous compounds. Concerning the air-water oxygen transfer (reaeration), resistance primarily exists in the liquid film. 

Group 1 (C02) indicates that microbial degradation of wastewater organic matter takes place in the sewer. In terms of odor, the other groups (2-4) are relevant. In spite of the fact that the investigation did not include the sources of the components found in the sewer atmosphere, group 2 probably is a result of... 

In addition to odor, several human health-related problems are potentially associated with the occurrence of hydrogen sulfide at concentration levels relevant for sewer networks. In this respect, it is interesting to compare values from Table 4.5 with Figure 4.2. The levels indicated in Table 4.5 depend on human sensitivity and time of exposure. 

The three main types of sulfide control procedures give an overall subdivision, although some overlap between the three groups may exist. Some of these procedures may be relevant not just for the control of sulfide but also for the reduction of other odorous substances. An overview of the methods emphasizing the process aspects will be given. The descriptions of the methods will not aim at giving information on detailed design principles. 

The odor threshold, 0.58 ppm to 5.0 ppm (Amoore and Hautala 1983 Ruth 1986) is low compared with irritant or toxic concentrations. No acute exposures were located resulting in mild effects in humans. Three monitoring studies, involving no symptoms to mild symptoms during chronic occupational exposures of adult males, are relevant to development of AEGL-1 values. The symptoms and blood concentrations of cyanide in the monitoring study of Chandra et al. (1980) indicate that the workers may have been exposed at higher atmospheric concentrations than those reported. 

Chemical analysis of odorants in ambient air is hampered by the presence of a plethora of volatile organic compounds, which do not contribute to the odour. Nevertheless application of either powerful separation and identification techniques, such as the GC-MS combination, or specific GC-detection or absorption procedures allow qualitative and quantitative determination of odourants. Improvements are under way to achieve the sensitivity necessary for relevant immission concentrations, which go down to 0.1 ppb for some odorants. 

When comparing the dust-borne concentrations of butyric acid and p-cresol with the odour thresholds it seems that the concentrations are too small to be relevant for an odour nuisance. However, if the dust is removed from the gas phase of the air from animal houses the odour disappears (39), (40), (14). This supports the opinion of HAMMOND et al. (40) that the odor is concentrated on the dust particles. The authors conclude from their data that the concentration of the two odorants butyric acid and p-cresol is about 4TO7 greater on an aerosol particle than it is in an equal volume of air. Thus, an aerosol particle deposited on the olfactory organ carries odour equivalent to a much greater volume of air (40). These considerations indicate that dust from animal houses should be taken into account in connection with odour emission/immission measurements not only by chemical analysis but by sensory evaluations using olfactometers without dustfilters, as well. 

Physical and Chemical Properties. The physical and chemical properties of heptachlor and heptachlor epoxide are sufficiently well defined to allow assessments of the environmental fate of the compounds to be made (ACGIH 1986 Chapman 1989 HSDB 1990a MacKay 1982 OHM/TADS 1985a, 1985b). Some physical and chemical properties of heptachlor epoxide that are not relevant to environmental fate are lacking. Knowledge of these properties, such as odor, flashpoint, and flammability limits, would be useful for workers involved in the manufacture, use, or clean-up of heptachlor and heptachlor epoxide. 

A complex biologically relevant odor, such as that of a fruit for fruit bats, may require fewer molecules overall for detection than each of its single components alone (Laska 1990 Laska and Hudson, 1991). Vampire bats (P/asmodws... 

Some biologically relevant smells may have two functional thresholds, the higher one resulting in a qualitatively different odor. For instance, geosmin in high concentrations smells like musty basement or soil, in low concentration like beets. ... 

Given how sensitive many birds are to odors, breeders of endangered bird species may be able to use food and environmental odors to imprint young birds on relevant cues of their future habitat (Nevitt, in Malakoff, 1999). 

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