# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 3 Results and Discussion
Higher concentrations of both ammonia and volatile organic compounds in the case of waste storage (compared to the biogas plant in Biala Podlaska) may result from the waste collection system in the service area and the input to the biogas plant in the form of mixed municipal waste. The general trend for this biogas plant was the highest OAV determined for VOCs. Analyzing Figure 6b,c it can be observed that the highest value of OAV-VOCs parameter accompanies aerobic stabilization of the digestate carried out in the technological yard (ranging from 5.18 to 650.64, depending on the dominant component). In turn, the highest odour concentration  
Figure 6: The results obtained at biogas plant located in Wolka Rokicka. (**a**) Odorant concentrations. (**b**) Odour activity values (based on toluene) and odour concentration at individual measurement points: 1, inside the waste storage plant; 2, mixed waste storage; 3, selectively collected waste storage; 4, mechanical treatment; 5, fermentation preparation; 6, oxygen stabilization; 7, technological wastewater pumping station; and 8, biofilter. (**c**) Range of odour activity values (based on toluene, styrene, and phenol). was recorded at the measuring point, where the highest OAV-NH\({}_{3}\) was also observed--similarly to Promnik biogas plant. This time it was also technological wastewater pumping station, in which ammonia was more responsible for the odour concentration than at other measuring points. In other places of the technological line, the influence of VOCs was dominant. Various correlation coefficients were analyzed while searching for the relationships between odorant concentrations and between OAVs and odour concentration. Table 4 shows the correlation coefficients (r-Pearson's, rho-Spearman's, and taub-Kendall's) at individual biogas plants. When analysing all the results obtained at the tested biogas plants, strong correlations between both VOC and NH\({}_{3}\) concentrations and OAVs and odour concentrations are visible only in the case of Biala Podlaska waste treatment plant. At other installations, the above-mentioned dependencies are not evident for all obtained results, regardless of the correlation coefficients tested. Significant differences in correlation coefficients between individual analyzed biogas plants may be due to different types of feedstock preparation and different fermentation technologies applied. Only at the biogas plant located in Biala Podlaska is the biodegradable waste collected selectively the material directed to the fermentation process. In addition, the digestate is also subjected to single-stage stabilization on the process site. Compared to the other biogas plants analyzed, there were also visible differences in the origin of the technological wastewater. Therefore, it can be assumed that the technological process in such a configuration as applied at the biogas plant in Biala Podlaska affects the observed correlations between the analyzed parameters. This inference is consistent with the results obtained in [49; 50], where it was stated that odours generated by waste treatment depend on the type of raw material, the stage of the decomposition and the operating conditions at the site. Moreover, [PERSON] et al. [51] found that fresh organic waste in the form of the organic fraction of MSW (which is the input to Biala Podlaska biogas plant) were the most influential odorous substrates, due to their high concentration in biodegradable organic matter. The bold values of the correlation coefficients indicate statistically significant relationships. At other biogas plants tested, the analysis of the correlation between NH\({}_{3}\) and VOC concentrations at all measuring points also showed no significant relationship. Strong correlations were only demonstrated at selected sections of the technological process, e.g., in the waste storage section, mechanical treatment, oxygen stabilization, and digestate dewatering [1; 30].