# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 3 Results and Discussion  
Table 3 shows the results of microclimate parameters in particular biogas plants. These are the mean values of inside (T\({}_{\text{i}}\), RH\({}_{\text{i}}\)) and outside (T\({}_{\text{o}}\), RH\({}_{\text{o}}\)) parameters. From a measurement viewpoint, the outside and inside conditions did not generate a significant difference due to the stable temperature and humidity conditions of the biogas plants and the measurement results at individual locations. Figures 4-6 show the results of measurements of chemical compounds and odour concentrations and OAVs at analyzed biogas plants at particular measurement points. Figures 4a, 5a and 6a present the results of odorant concentrations, Figures 4b, 5b and 6b indicate the odour activity values (based on toluene) and odour concentration and Figures 4c, 5c and 6c show the range of OAV-VOC values. Figures showing the relationship between OAV and cod (Figures 5b and 6b use a logarithmic scale (values converted with natural logarithm) on the value axis (Y-axis) to make figures easier to read. The presented  
Figure 3: Measurement points locations at biogas plant in Wolka Rokicka. \begin{table}
\begin{tabular}{c c c c c} \hline
**Biogas Plant** & **T\({}_{\text{i}}\)** (\({}^{\circ}\)**C**)** & **T\({}_{\text{o}}\)** (\({}^{\circ}\)**C**)** & **RH\({}_{\text{i}}\)** (\%)** & **RH\({}_{\text{o}}\) (\%)** \\ \hline Biala Podlaska & 21.3 & 20.1 & 47.8 & 50.2 \\ \hline Promnik & 20.1 & 16.6 & 63.1 & 56.6 \\ \hline Wolka Rokicka & 26.0 & 25.3 & 49.3 & 40.7 \\ \hline \end{tabular}
\end{table} 
Table 3: The results of microclimate parameter measurements. results are an arithmetic mean of five-minute measurements, for which the standard deviation was less than 5%. Hydrogen sulphide and methanethiol were not determined at any of the studied plants at the indicated measurement points due to the limit of compound detection. Therefore, the graphs do not include these compounds. Figure 4: _Cont._  
At the biogas plant located in Biala Podlaska, the source of the highest VOC emission was the oxygen stabilization of the digestate (16.22 ppm). At this point, the highest ammonia emission (49 ppm) was simultaneously observed. At the analyzed plant, this process is carried out in the technological field. These results are related to the type of treated waste and the processing technology used. Only at this biogas plant (among the plants included in the research), is biodegradable waste collected selectively at source the feedstock for the fermentation process. In addition, no dewatering of the digestate is performed in the process line of this biological treatment plant. After the fermentation process is completed (21 days), the digestate is transported to the technological field where it undergoes a single-stage aerobic stabilization. Emissions of hydrogen sulphide and methanediol were not detected at any measurement point. In turn, the lowest emission of odorants at this biogas plant was observed in the case of the technological wastewater pumping station. The measurement results may have been influenced by the nature of wastewater being pumped--both sewage from a social and office building and wastewater gravitationally flowing from the technological field. Analyzing Figure 4b,c, it is interesting to observe that the highest values of the OAV parameter are also related to the aerobic stabilization of the digestate, but contrary to Figure 5a (bigger ammonia concentration), they are mainly related to OAV-VOCs (ranging from 1.52 to 345.11, depending on the dominant component), which is due to the low value of OT for main constituents. The highest OAVs correspond to the highest values of odour concentration. The highest odour concentration was observed in the case of oxygen stabilization, where at the same time the highest OAV for both VOCs and NH\({}_{3}\) was determined. At this measurement point, ammonia is more responsible for the  
Figure 4: The results obtained at biogas plant located in Biala Podlaska, (**a**) Odorant concentrations.