Odorous substances

Odors are characterized by quaUty and intensity. Descriptive quaUties such as sour, sweet, pungent, fishy, and spicy are commonly used. Intensity is deterrnined by how much the concentration of the odoriferous substance exceeds its detection threshold (the concentration at which most people can detect an odor). Odor intensity is approximately proportional to the logarithm of the concentration. However, several factors affect the abiUty of an individual to detect an odor the sensitivity of a subject s olfactory system, the presence of other masking odors, and olfactory fatigue (ie, reduced olfactory sensitivity during continued exposure to the odorous substance). In addition, the average person s sensitivity to odor decreases with age. 

Substances other than enzymes can be immobilized. Examples include the fixing of heparin on polytetrafluoroethylene with the aid of PEI (424), the controUed release of pesticides which are bound to PEI (425), and the inhibition of herbicide suspensions by addition of PEI (426). The uptake of anionic dyes by fabric or paper is improved if the paper is first catonized with PEI (427). In addition, PEI is able to absorb odorizing substances such as fatty acids and aldehydes. Because of its high molecular weight, PEI can be used in cosmetics and body care products, as weU as in industrial elimination of odors, such as the improvement of ambient air quaHty in sewage treatment plants (428). 

According to the chemical theory of olfaction, the mechanism by which olfaction occurs is the emittance of particles by the odorous substances. These particles are conveyed to the olfactory epithelium by convection, diffusion, or both, and dkecdy or indkectly induce chemical changes in the olfactory receptors. 

Some substances are odorous, others are not. Humans can smell at a distance if one smells the roses in a garden, it is not ordinarUy considered that part of the rose is in contact with the nose. Substances of different chemical constitution may have similar odors. Substances of similar constitution usuaUy have similar odors, eg, in a homologous series nevertheless, even stereoisomers may have different odors. Substances of high molecular weight are usuaUy inodorous and often nonvolatile and insoluble. The quaUty as weU as the strength of odor may change on dUution. 

The sense of smell is rapidly fatigued. Fatigue for one odor does not affect the perception of other dissimUar odors, but wUl interfere with the perception of similar odors. Two or more odorous substances may cancel each other out this compensation means that two odorous substances smelled together may be inodorous. 

Masking. Masking can be defined as the reduction of olfactory perception of a defined odor stimulus by means of presentation of another odorous substance without the physical removal or chemical alteration of the defined stimulus from the environment. Masking is therefore hyperadditive it raises the total odor level, possibly creating an overpowering sensation, and maybe defined as a reodorant, rather than a deodorant. Its end result can be explained by the simple equation of 1 + 1 = >2 (Fig. 2a). 

Absolutes. Absolutes are prepared from concretes by further processing to remove materials that can cause solubihty problems in perfumes. This is done by dissolution in alcohol, filtering, and removal of the solvent, usually at reduced pressures. The resulting products are viscous, oily materials which may be diluted with low odor substances such as diethyl phthalate. 

Essence of Vanilla.—The substance sold under this name is, properly, a spirituous extract of the vanilla bean. Many samples, however, are little more than alcoholic solutions of artificial vanillin, coloured with caramel. Some samples, which cannot be described as adulterated, contain a little coumarin or other odorous substance, added to varj- the characteristic vanillin odour and flavour somewhat. 

Duftstoff, m. odorous substance, aromatic principle, perfume,... 

ODOROUS COMPOUNDS IN SEWER NETWORKS 4.3.1 Occurrence of Odorous Substances in Sewers... 

It is important that odorous substances undergo degradation in the wastewater phase under aerobic conditions. Organic sulfur compounds seem to be fast degradable, whereas this is not the case for nitrogen compounds (Hwang et al 1995). 

In addition to the VOCs produced in sewers under anaerobic conditions, such components may originate from external sources, e.g., industries (Corsi et al., 1995 Olson et al., 1998). Typically, these VOCs are hydrocarbons and similar products. The emission is influenced by a number of sewer system parameters, hydraulic conditions, wastewater characteristics and physicochemical properties for the VOC component. The emission of such incoming VOCs thereby follows the general approach described in this text for odorous substances. 

Inlet structures at treatment plants often increase the turbulence of the wastewater. A corresponding release of hydrogen sulfide and other odorous substances into the air from inflow of anaerobic wastewater may be a nuisance that will require treatment of the gas released. On the other hand, aeration of the anaerobic wastewater may quickly oxidize the sulfide and some of the organic odorous substances. However, theN-containing odorous substances are slowly oxidized and may be a problem whenever they occur in the inflow to the treatment plant (Hwang et al., 1995). 

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. 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

Exchange of volatile compounds across the air-water interface, e.g., oxygen (reaeration that affects aerobic or anaerobic conditions) and release of odorous substances Release of odorous substances to the urban atmosphere and change of reaeration due to a lower atmospheric oxygen concentration Extent of the processes... 

Referring to Table 8.1, it is important to stress that a specific structural change may have an impact on more than just one process, and that it can also affect the total resulting impact of a change in opposite directions. As an example, turbulence in the water phase may increase the reaeration rate of the water phase increasing the emission rate of hydrogen sulfide and odorous substances thereby intensifies the negative effects of anaerobic conditions. 

It is the existence of these organs that many of the Families of Lepidoptera, especially among the moths, owe the continuance of the species. They operate in the association of the sexes, with such an irrestible and far-reaching force as to exceed our conception and to be marvelous to our eyes.. .. Can not chemistry come to the aid of the economic entomologist in furnishing at moderate cost, the odorous substances needed Is the imitation of some of the more powerful animal secretions impracticable ... 

The remarkable similarity of glomerular organization in the first-order central olfactory neuropils of essentially all invertebrates and vertebrates that have a differentiated olfactory system has been noted often (e.g., refs. 9-12). Indeed, Dethier (1) argued persuasively that olfactory systems similar to those of contemporary insects and vertebrates, with comparable glomerular organization, were probably already in place 500 million years ago. Referring to environmental odor substances produced, first by photosynthesis and later by mankind s organic chemistry, in varieties far... 

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure. 

Many materials which have no smell when installed in the olfactometer, may acquire a smell in use. To some odorants plastics may act as a sponge or may be completely permeable. Such combinations should be avoided and since, in industrial situations, it is hard to predict what odorant substances will be encountered, anything less safe than high quality teflon or stainless steel should be avoided in olfactometers. 

The last set of requirements in olfactometry is concerned with the differences between panel members. People vary widely in their sensitivity. A factor of a 100 between the thresholds of two subjects for the same substance is not uncommon. For a number of substances, specific anosmia s or specific hyposmia s are found. In such cases a person has no sensitivity at all or a very high threshold for the given substance, but normal sensitivity to other substances (1). This is an illustration of the fact that sensitivity to odours is specific rather than general. This is also demonstrated by Punter (2, 3) who determined the thresholds of 69 odorous substances for the same group of subjects and calculated the correlations between these thresholds (see figure 2). 

Figure 2. Elistogram of the correlation coefficients between the thresholds for 69 different odorous substances obtained from the same group of subjects (P.EI.Punter).

In order to make the approach practicable it is necessary to introduce the concept of odour potential . By this we mean the propensity of a liquid or slurry to release odorous substances to an atmosphere in other words, a measure of the amount of potential odour... 

The formation of mercaptan is doubly important because of its relation to phytochemical reduction processes. The reaction is analogous to the formation of ordinary alcohols from aldehydes and it also indicates how the plant is capable of synthesizing intensely odorous substances from aldehydes and hydrogen sulfide in a simple manner. The isolation... 

If we consider the influence of gases on conductivity, we can understand how Rosenberg 1391 succeeded in separating of odorous substances by means of a device containing / -carotene. The detection contaminating gases in the atmosphere by organic semiconductors is also a possibility. 

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