Odor is the perception registered by the olfactory nerves. As in the case of taste, there should be no noticeable odor at the point of use of any drinking water. The secondary standard for odor is 3.
Fresh wastewater odor is less disagreeable than stale wastewater odor but, nonetheless, they all have very objectionable odors. Odors are often the cause of serious complaints from neighborhoods around treatment plants, and it is often difficult for inspectors investigating these complaints to smell any odors in the vicinity of the neighborhood. The reason is that as soon as he or she is exposed to the odor, the olfactory nerves become accustomed to it and the person can no longer sense any odor. If you visit a wastewater treatment plant and ask the people working there if any odor exists, their responses would likely be that there is none. Of course, you, having just arrived from outside the plant, know all the time that, in the vicinity of these workers, plenty of odors exist. The effect of odors on humans produces mainly psychological stress instead of any specific harm to the body. Table 2.1 lists the various odorous compounds that are associated with untreated wastewater.
The determination of odors in water was addressed previously under the discussion on taste. Odors in air are determined differently. They are quantitatively measured by convening a panel of human evaluators. These evaluators are exposed to odors that have been diluted with odor-free air. The number of dilutions required to bring the odorous air to the minimum level of detectable concentration by the panel is the measure of odor. Thus, if three volumes of odor-free air is required, the odor of the air is three dilutions. It is obvious that if these evaluators are subjected to the odor several times, the results would be suspicious. For accurate results, the evaluators
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