## 3 Results and Discussion
### Offactometric Measurements
For this purpose, gas chromatography coupled to mass spectrometry (GC-MS) is frequently used to identify and quantify key odorous compounds. As suggested by [PERSON] et al. and by [PERSON] et al. [34; 35], panelists should be trained also to recognize \"pig odor\". Although, in this case study, assessors were not trained to recognize the \"pig odor\", they demonstrated consistency in their responses as, during olfactometric sessions, they recognize it at the same dilution. Moreover, it can also be considered that each session was a training for the following one as panelists and odor were always the same for all the experimental periods. Each room was compared to the others to evaluate the odor abatement efficiency (WAS Vs. DF, WAS Vs. CR and DF Vs. CR). In Table 5, the percentage reduction for each comparison is reported. The WAS system presents, for all session, higher odor abatement efficiency compared to DF system, ranging from \(-\)11% to \(-\)77%, with an average of \(-\)39%. In addition, compared to CR, WAS technology reduces the odor concentration by an average of about 16%, while excluding the session 5, when pits were emptied, this value increases to 53%. DF abatement system resulted to be not an effective strategy to reduce odor concentration, even if dust is a carrier of odorous compounds. Compared to CR it presents the highest removal efficiency of 30% in the first session and lower in the following ones. This can be explained by the fact that the propylene filters of the DF system were replaced in July and, therefore, their dust removal efficiency could be affected. Even if biofilters have been demonstrated to be the most effective end-of-pipe technique to reduce odors [9], the use of a DF technology could be an advantage because of the high maintenance requirements of biofilters. Biofilters need an accurate control of the medium moisture of the packing material is required for a correct functionality [36; 37], other than of temperature and nutrients to guarantee the survival of the bacterial population [13]. Moreover, they have the disadvantage of producing N\({}_{2}\)O emissions if nitriifying bacteria are present [22]. The concentration of odor was reduced by WAS technology. The removal of odor-carrying particles might be explained by capture of soluble compounds in the two tanks, \begin{table}
\begin{tabular}{c c c c} \hline
**Session** & **WAS vs. DF** & **WAS vs. CR** & **DF vs. CR** \\ \hline
1 & \(-\)52\% & \(-\)66\% & \(-\)30\% \\
2 & \(-\)77\% & \(-\)74\% & +11\% \\
3 & \(-\)17\% & \(-\)18\% & \(-\)1\% \\
4 & \(-\)11\% & 0\% & +12\% \\
5 & \(-\)36\% & +80\%1 & +182\% 1 \\ \hline \end{tabular} \({}^{\dagger}\)pit evacuated.  
\end{table} 
Table 5: Odor abatement efficiency comparison among the three rooms (WAS, DF and CR). WAS = Wet acid scrubber room; DF = dry filter room; CR = control room. such as N-containing compounds (e.g., NH\({}_{3}\)). It is well known that wet and acid scrubber represent an effective strategy to reduce NH\({}_{3}\) emissions, thanks to the intensive contact between the air and liquid phase, NH\({}_{3}\) is trapped by the acid solution leading to the production of ammonium salt [13]. Considering that the trial was conducted in a naturally ventilated pig facility, an average of 16% odor removal efficiency could be considered a good result. Wet and acid scrubber are normally used in facilities provided with forced ventilation and removal efficiencies are calculated considering the difference in the odor concentration between the inlet and outlet air from the scrubber. An average odor removal of 27% for acid scrubbers in forced ventilation was reported by [PERSON] and [PERSON] [25] and, [PERSON] et al. [22], in their review, indicated removal efficiencies ranging from negative values up to 80%, for all types of air scrubbers. Variations are principally linked to the type of air scrubber and the method chosen for the analyses.