## 1 Organic Waste Treatment Plants and Gaseous Emissions
### VOC and Odors  
The presence of odors is the main concern associated with VOC emissions and it has been investigated by a wide number of researchers. 2-butanone, \(\alpha\)-pinene, tetrachloroethylene, dimethyl disulfide, \(\beta\)-pinene, limonene, phenol and benzoic acid were included in the study of [PERSON] _et al_. [28] as representative compounds of some important classes related to high odor impact. [PERSON] _et al_. [12] found that odorimetric tests (olfactometry measurements) were in agreement with GC-MS analysis of VOCs, especially for terpen when VOCs and odors from MSW composting were monitored. [PERSON] _et al_. [20] concluded that the relationship between chemical (GC-MS measurements) and odor concentration (olfactometry) is specific for each type of odor and cannot be generalized. These authors established a good linear relationship between odor concentration and total VOC concentration at a biofilter output during the composting of vegetable, fruit and garden wastes. Odor concentration was also well correlated with esters and ketones content. However, in an animal rendering plant, VOCs emitted from the biofilter were a poor indicator of the odor level. In this case, the best relationship was found for compounds containing organic sulphur. [PERSON] _et al_. [22] determined ammonia, amines, dimethyl sulfide and acetic acid as the responsible for most odors in food waste composoting plants compared to numerous VOC. These authorsidentified 29 compounds in the odor from this type of waste treatment plants although no correlation attempt between chemical analyses and odor concentration measurements was reported. [PERSON] _et al_. [21] investigated the relationship of the critical odorants from food waste composting plants with their human olfactory effect (olfactometry results). These authors found six critical odorants: ethylbenzene, dimethyl sulfide, trimethylamine, _p_-cymene, ammonia and acetic acid. Correlations found were different when low or high concentrations of these compounds were considered. For ethylbenzene, dimethyl sulfide, trimethylamine and _p_-cymene, which presented a very low olfactory threshold (0.002 ppm), a linear relationship between concentration and odor was only found for concentration values within the 0.25-100 ppm range. A linear correlation was also found for odors and acetic acid (in the 0.1-50 ppm range), while it was not possible for ammonia when concentrations within 5 and 100 ppm were considered. On the other hand, [PERSON] _et al_. [29] and [PERSON] _et al_. [19] tried to correlate VOC emissions with odors measured by an electronic nose and also with the biological activity during the biological treatment process. [PERSON] _et al_. [29] found a good correlation between odor molecules detected by the electronic nose and the Dynamic Respiration Index (DRI), used as a measure of the biological activity during a food-waste composting process at pilot scale. Even if an adequate oxygen concentration was maintained during the biological treatment process, anaerobic conditions were developed during the highest microbial activity stage resulting in a high level of sulphur compounds, methane and hydrogen in the outlet gas stream. [PERSON] _et al_. [19] studied odors (measured by the electronic nose and by olfactometry) and VOC concentration (using GC-MS) also relating them to the biological activity (measured using aerobic and anaerobic indices) in an anaerobic digestion treatment plant processing the organic fraction of municipal solid waste. These authors state that as the biological stability increases (during the entire anaerobic digestion process represented by the sampling points, _i.e_., not digested waste, digested waste and post-digested waste) odor emissions measured by olfactometry decrease, although no correlation between total VOC concentration and olfactometry measurements could be established.