## 1 Organic Waste Treatment Plants and Gaseous Emissions
However, in both cases it must be considered that the C/N ratio should be related to the biologically available carbon and nitrogen forms, a point that is not often considered [10]. At the laboratory scale, total VOC concentration in exhaust gases from composting processes of different wastes has been also studied and it was concluded that the highest concentrations of VOCs were emitted during the first 48 h of the process [11]. These authors also stated that total VOC emissions could not be correlated with the biological activity of the process as measured by respirometry. [PERSON] _et al_. [12] found the same emission pattern (main VOC release during the first days of process), although the possible correlation with the biological activity was not studied. [PERSON] _et al_. [13] and [PERSON] _et al_. [14] studied VOC emissions from the composting process considering the possible negative effects on plant workers and the nearby population. [PERSON] _et al_. [13] stated that VOC concentrations detected at a composting plant treating source-separated catering waste were much lower than the national threshold limit values, thus avoiding primary health effects, although many of the compounds exceeded their threshold odor concentrations; thus secondary symptoms as nausea and hypersensitivity might be expected from exposure to unpleasant odors. In fact, the effect of workers' simultaneous exposure to several VOCs is not very well known since synergic effects should be also considered. On the other hand, [PERSON] _et al_. [14] found various xenobiotic VOCs in gaseous emissions during laboratory scale composting of different MSW components. VOC emissions in biological treatment processes have also been studied as environmental loads related to these processes. Regarding this, VOC emissions have been reported as emission factors (_i.e_., amount of VOC emitted per weight of waste treated) or from a Life Cycle Assessment (LCA) perspective, since their contribution is mainly related to the Photochemical Oxidation Potential (POP). Emission factor values of 1.70 and 0.59 kg of VOC Mg\({}^{-1}\) of OFMSW treated were reported by [PERSON] and [PERSON] [15] and [PERSON] [3] from studies at laboratory scale, while [PERSON] _et al_. [16] found 0.2 and 7.3 kg VOC Mg\({}^{-1}\) of OFMSW treated in two different full scale composting plants using different technologies. In LCA studies, emissions of VOC to the atmosphere are expressed in kg ethylene equivalent/kg of emission and included in the POP category [17]. The presence of VOCs in gaseous emissions from waste treatment plants has been reported as total VOCs emitted [16-19], as concentration of different families of VOCs [14,19] or as individual VOC concentrations [3,20]. Table 1 summarizes some values reported for VOC emissions in biological treatment processes of organic wastes by different authors. Some values in Table 1 are reported as a range of VOC concentrations obtained in a same plant during different sampling days or as different values obtained by the same authors in different plants. As seen in Table 1, values of VOC concentration are very variable, even for the same facility and the same type of waste. In the case of total terpen, for instance, values reported in Table 1 for an effluent of a biofilter in a composting plant range between 20 and 12,350 \(\upmu\)g/m\({}^{3}\), while the same type of compounds for an anaerobic digestion plant show concentrations between 683 and 4,750 ppbv for non-treated OFMSW and 414 and 1,151 ppbv for the anaerobically digested residue. As a general trend observed from the wastes and treatments reported, the compounds with the highest presence in the emissions are terpen and alcohols followed by ketones.