## 3 Results and Discussion
### Trace Compounds in Biogas  
The results related to the analyses carried out on raw biogas are reported in Table 1. As biogas production and composition depend on many factors, such as operational parameters of the AD process and seasonal variability of FW characteristics, the mean, minimum, and maximum values of the parameters measured over a year are reported in Table 1.  
As demonstrated [32], the characteristics of the FW used by the plant to feed the anaerobic digester were different in the summer and winter. In particular, in the warmer months, total volatile solid (TVS) content in FW was higher (TVS = 23.1%) than in the colder months (TVS = 21.0%), while pH value was higher in the winter (pH = 5.7) than in the summer (pH = 4.5). Also, the temperature of the AD process reached values of 44-45 \({}^{\circ}\)C in the summer and 42-43 \({}^{\circ}\)C in the winter. The variation in both FW composition and AD operational parameters led to different trends of trace contaminant formation in biogas. The varying levels of H\({}_{2}\)S concentration measured in biogas during the examined year were linked to the variable amount of sulfur-containing FW constituents. It is known that AD of FW made up of vegetables such as potatoes, lettuce, and tomatoes produces no measurable quantity of H\({}_{2}\)S [35]. In the months from April to November, the H\({}_{2}\)S content in the biogas was lower (mean H\({}_{2}\)S concentration: 833 ppmv) than in the other months (December-March, mean H\({}_{2}\)S concentration: 1581 ppmv) due to greater consumption of low-sulfur-containing vegetables, such as lettuce, tomatoes, zucchini, cucumbers, eggplants, carrots, potatoes, and fruits, including peaches and watermelons, widely produced in this area. In the colder months (December-March), on the other hand, the higher H\({}_{2}\)S content in biogas was due to the AD of a larger amount of protein-containing waste, mainly composed of meat remains. Mean, minimum, and maximum values of H\({}_{2}\)S measured in biogas were comparable to those observed by other similar full-scale AD plants treating FW [7]. Total VOC content in biogas also strongly depended on FW characteristics. From December to May (Table 2), a higher mean concentration of VOCs (1149 mg/Nim\({}^{3}\)) was measured compared with the one detected in the other months (June-November, mean VOCs concentration: 661 mg/Nim\({}^{3}\)). The higher VOC concentration corresponded to the period of production and consumption of citrus fruits, such as oranges and tangerines, of which the area is an important producer. In fact, from speciation analysis of the VOCs (Table 2), it was observed that the amount of terpen in biogas and, in particular, of limonene, \(\alpha\)-pinene, \(\beta\)-pinene, and p-cymene, which citrus peel is rich in, considerably increased in the period between December and May, while the concentrations of aromatic, carbonyl, and hydrocarbon compounds barely varied throughout the year. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mean a** & **SD b** & **Min** & **Max** \\ \hline CH\({}_{4}\) (\%vol) & 59.4 & 1.0 & 58.1 & 60.5 \\ CO\({}_{2}\) (\%vol) & 40.1 & 0.9 & 39.2 & 40.8 \\ H\({}_{2}\)S (ppmv) & 1207 & 421 & 613 & 1740 \\ VOCs (mg/Nim\({}^{3}\)) & 895 & 304 & 432 & 1425 \\ H\({}_{2}\)O (\%vol) & 0.3 & 0.2 & 0.1 & 0.6 \\ N\({}_{2}\) (\%vol) & 0.2 & 0.1 & 0.1 & 0.3 \\ HCl (mg/Nim\({}^{3}\)) & 17 & 4 & 12 & 20 \\ NH\({}_{3}\) (mg/Nim\({}^{3}\)) & 36 & 7 & 30 & 46 \\ Siloxanes (mg Si/Nim\({}^{3}\)) & 0.6 & 0.2 & 0.2 & 0.8 \\ \hline \hline \end{tabular} a  
NDFOOTNOTE]  
\end{table} 
Table 1: Raw biogas composition. A similar trend was observed in the study of a full-size FW AD [27], in which the greatest presence of terpenes in biogas, mainly represented by limonene, was measured in the spring period. The average concentration of limonene and p-cymene was lower than that reported by [PERSON] et al. [31], while that of \(\beta\)-pinene was similar.