## 3 Results and Discussion
### Trace Compounds in Biogas
Instead, the average annual concentration of several terpenes, such as limonene, \(\alpha\)-pinene, \(\beta\)-pinene, and p-cymene, was higher compared with that estimated in other FW AD plants. [PERSON] et al. [7] measured an average terpene concentration of about 187-298 mg/Nm\({}^{3}\) (limonene: 158-187 mg/Nm\({}^{3}\)), while the value obtained by [PERSON] et al. [27] was 1.5 mg/m\({}^{3}\). However, in the case studied by [PERSON] et al. [27], most of the terpenes were eliminated in the hydrothermal pre-treatment of FW before they underwent the AD process. As in the study reported by [PERSON] et al. [31], carbonyl compounds were mainly represented by acetone and methyl-ethyl-ketone, with an average concentration of 22 mg/Nm\({}^{3}\) and 77 mg/Nm\({}^{3}\), respectively, which was also in line with the values reported by [PERSON] et al. [7]. The amounts of some other VOCs in biogas were different from those reported in other studies. The concentrations of aliphatic and aromatic hydrocarbon compounds were 28 mg/Nm\({}^{3}\) and 112 mg/Nm\({}^{3}\), respectively. These values were found to be higher than those measured in other full-scale FW digesters [17]. Unlike other studies carried out on biogas produced by FW full-scale digesters [7; 17; 27], no halocarbon compounds were quantified. Gaseous ammonia annual average concentration, which was 36 mg/Nm\({}^{3}\), was higher than that recorded in similar FW AD plants (0.2-6.5 mg/m\({}^{3}\)) [7]. The mean concentration of N\({}_{2}\) (0.2%vol), water vapor (0.3%vol), HCl (17 mg/Nm\({}^{3}\)), and siloxanes (0.6 mg Si/Nm\({}^{3}\)) also fell within the range of typical FW AD biogas values. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Compound** & \multicolumn{3}{c}{**December–May**} & \multicolumn{3}{c}{**June–November**} & **Annual** \\ \cline{2-7}
**(mg/Nm\({}^{3}\))** & **Min** & **Max** & **Mean** & **Min** & **Max** & **Mean** & **Mean** \\ \hline Total VOCs & 582 & 1425 & 1149 & 432 & 746 & 661 & 895 \\ limonene & 199 & 670 & 449 & 58 & 128 & 94 & 272 \\ \(\alpha\)-pinene & 113 & 237 & 173 & 26 & 71 & 42 & 108 \\ \(\beta\)-pinene & 118 & 218 & 152 & 37 & 80 & 44 & 98 \\ p-cymene & 89 & 262 & 178 & 14 & 85 & 38 & 108 \\ \(\gamma\)-terpinene & 0 & 1 & 0 & 0 & 15 & 4 & 2 \\ mycrene & 0 & 0 & 0 & 0 & 4 & 1 & \textless{}1 \\ \(\Delta\)-3-carene & 0 & 1 & 0 & 0 & 10 & 3 & 1 \\ propane & 0 & 8 & 2 & 6 & 40 & 27 & 15 \\ butane & 0 & 10 & 3 & 3 & 45 & 28 & 15 \\ pentane & 0 & 2 & 1 & 0 & 5 & 2 & 1 \\ hexane & 0 & 6 & 1 & 8 & 25 & 17 & 9 \\ heptane & 0 & 7 & 2 & 10 & 32 & 24 & 13 \\
2-methyl-pentane & 0 & 1 & 0 & 0 & 3 & 1 & \textless{}1 \\ benzene & 2 & 32 & 23 & 26 & 112 & 42 & 33 \\ toluene & 1 & 24 & 17 & 22 & 34 & 25 & 21 \\ ethyl-benzene & 1 & 26 & 22 & 22 & 39 & 26 & 24 \\ xylene a  & 7 & 41 & 26 & 37 & 82 & 51 & 39 \\ mesilytene & 0 & 5 & 1 & 0 & 1 & 0 & 1 \\ cumene & 3 & 40 & 28 & 34 & 72 & 43 & 36 \\ acetone & 5 & 14 & 9 & 32 & 64 & 35 & 22 \\ methyl-ethyl-ketone & 18 & 38 & 32 & 79 & 147 & 122 & 77 \\ hexanal & 1 & 12 & 8 & 0 & 3 & 1 & 3 \\ heptanal & 1 & 10 & 4 & 0 & 1 & 0 & 1 \\ octamethylcyclotetrasiloxane & 0 & 1 & 0 & 0 & 2 & 1 & \textless{}1 \\ decamethylcyclopentasiloxane & 0 & 2 & 0 & 0 & 3 & 1 & \textless{}1 \\ \hline \end{tabular}
\end{table} 
Table 2: VOCs speciation (raw biogas).