Odor maskers and neutralizers for air treatment

The problem of odors and how they are perceived is a growing one. Some studies have shown that odor pollution is the second most common reason for complaint after noise, and this importance given to odor by local residents is linked to the fact that it is very often associated with the notion of toxicity, even if this is without much foundation, since the concentrations perceived by humans are far below the exposure limit values existing for toxic gases [1] . The following general rules on the acceptability of odor sensations can be formulated [2] :

• any pleasant smell becomes unpleasant at very high concentrations;

• whether an odor is pleasant or unpleasant depends largely on its context;

• odor aversions are more persistent than preferences;

• tolerance of unpleasant odors differs considerably from one individual to another.

While scientific and technological advances have made it possible to improve the quality of the water we consume and discharge into the environment, they have also led to a significant increase in odour nuisances associated with wastewater collection and treatment. On the one hand, older wastewater treatment plants are being overtaken by the urban fabric; on the other, new plants are being forced to locate ever closer to built-up areas. In addition, changes in the structure of sewage networks (increasingly long), and sometimes in the quality of effluent (e.g., mixing of industrial effluents), are conducive to the formation of odorous compounds in pipes, which are then released at treatment plants. In addition to the problem of wastewater treatment plants, other sources of odours are of concern to the authorities. Industries, as well as farms, poultry farms, open-air landfills and other types of structures, generate unpleasant odors for the surrounding population. Even if, unlike VOCs (volatile organic compounds), there is no specific legislation on odor emissions to date, there is nonetheless an incentive to move towards purified discharge, in order to improve neighborhood comfort. Indeed, according to the French Environment Code, odor pollution occurs if the odor is perceived as an excessive olfactory nuisance. The decree of February 2, 1998 on discharges from installations classified for environmental protection (ICPE) defines the level of an odor as the dilution factor to be applied to an effluent so that it is no longer perceived as odorous by 50% of the people making up a population sample. This decree does not set limit values for odor pollution,...