# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 4 Conclusions  
The main conclusions of this research can be summarised as follows:  
1. The article presents a practice, in the form of a chemical sensor method, and a tool, in the form of a portable gas detector, for monitoring odorant emission from biogas plants processing municipal waste. 2. Measurement research by means of the analyzed method and device can facilitate future activities related to the minimization of odour emissions and can aid in finding \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Biogas Plant Location**} & \multicolumn{3}{c}{**VOC-NH3**} & \multicolumn{3}{c}{**OAV-NH\({}_{3}\)-\({}_{\text{od}}\)**} & \multicolumn{3}{c}{**OAV-VOCs-\({}_{\text{od}}\)**} \\ \cline{2-10}  & **r** & **rho** & **taub** & **r** & **rho** & **taub** & **r** & **rho** & **taub** \\ \hline Biala Podlaska & **0.995** & **0.737** & **0.617** & **0.999** & **0.934** & **0.890** & **0.995** & **0.885** & **0.794** \\ Promnik & 0.149 & \(-\)0.160 & \(-\)0.154 & **0.927** & 0.613 & 0.490 & 0.415 & 0.461 & 0.34 \\ Wolka Rokicka & \(-\)0.307 & \(-\)0.220 & \(-\)0.189 & **0.990** & 0.544 & 0.459 & \(-\)0.307 & \(-\)0.346 & \(-\)0.276 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Correlation coefficients between VOC and ammonia concentrations, OAV-NH\({}_{3}\) and \({}_{\text{od}}\), and OAV-VOCs-\({}_{\text{od}}\) at individual plants. the relationships between odorant emissions and the type of treated waste, technological processes, and technological regime. 3. In the analyzed studies, the highest concentrations of odorants were associated with oxygen stabilization of digestate (both VOC and NH\({}_{3}\) concentrations) and with technological wastewater generated at biogas plants (mainly NH\({}_{3}\) concentrations). For this reason, special attention should be paid to the encapsulation of biological processes and all wastewater-related operations, and proper exploitation of the pumping station. 4. The portable gas detector can be used to control technological processes in municipal waste biogas plants by measuring the odorant concentrations and calculating the odour activity value (OAV). Thanks to this result, it is possible to plan the activities aimed at minimizing the odour nuisance related to the presence of specific compounds (groups of compounds) in the mixture of process gases. In all analyzed biogas plants, the highest OAV level was determined for VOCs. This indicates the need for VOC monitoring in line with the requirement of Best Available Techniques for waste treatment [52]. The OAV for VOCs can be determined taking into account the predominant components in the VOC mixture. In the case of process gases emitted at the analyzed biogas plants processing municipal waste, the dominance of toluene is very clear, followed by phenol and styrene. 5. At the analyzed waste biogas plants, the highest odour concentration concerns the places for which the highest OAV values were determined--either for VOCs, or for NH\({}_{3}\), or both. These places were the technological wastewater pumping station and the oxygen stabilization process. 6. The analysis of the correlations between VOC and NH\({}_{3}\) concentrations shows only a slight connection between these parameters. In many parts of the analyzed technological lines, VOC and NH\({}_{3}\) emissions occurred alternately and the concentrations of both the dominant odorants changed. This was confirmed by the OAV analysis, which indicated that only at the technological wastewater pumping station was a similar contribution of NH\({}_{3}\) and VOCs to the overall odour of a complex odour mixture observed. 7. An unquestionable disadvantage of the analyzed device representing the chemical sensor method is the relatively small range of chemical compound determination. For low odorant concentrations, the use of the device may be unreliable. The device is also sensitive to high humidity.