# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 2 Materials and Methods
### Study Methodology
Three different correlation coefficients were calculated, i.e., [PERSON]'s, [PERSON]'s and [PERSON]'s. [PERSON]'s correlation coefficient is used to study linear relationships between data. It is a standardized covariance. Pearson correlation coefficient is calculated when both variables are measurable and have a distribution close to normal and the relationship is rectilinear. [PERSON]'s rank correlation coefficient, like [PERSON]'s, is used to study the relationship between data, but unlike it, it allows you to study any monotonous dependencies, not just linear relationships. Ranks determine the position at which a variable is located when the data is organized. Where two equally observed values occur, they shall be assigned an arithmetic mean, which shall be calculated from their position(s). [PERSON]'s tau factor is based on the difference between the probability that two variables are in the same order (for the observed data) and the likelihood that their order of is different [43; 44]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**PID Sensor** & **Range** & **Resolution** & **Response Time** & **Odour Threshold (OT)** \\  & & & **(RT) t\({}_{00}\) (s) * & **(ppm)** \\ \hline Volatile organic & & & & \\ compounds (VOCs) & 0–100 ppm & 10 ppb (0.01 ppm) & 15 & based on main \\  & & & & components ** \\
10.6 eV & & & & \\ \hline
**Electrochemical Sensor** & **Range** & **Resolution** & **Response Time** & **Odour Threshold (OT)** \\  & & & **(RS) t\({}_{00}\) (s) * & **(ppm)** \\ \hline Ammonia (NH\({}_{3}\)) & 0–100 ppm & 1 ppm & 60 & 5.2 [14] \\ Hydrogen sulphide (H\({}_{2}\)S) & 0–100 ppm & 0.1 ppm & 35 & 0.0081 [45] \\ Methanethiol (CH\({}_{3}\)SH) & 0–10 ppm & 0.1 ppm & \textless{}35 & 0.00393 [45] \\ \hline \hline \end{tabular}  
* Response Time (t\({}_{00}\)): The time for a sensor to reach 90% of its final stable reading. Typically, an exposure of twice the t\({}_{00}\) time is required to get a stable reading. **[**IE = 8.82 eV; OT = 0.33 ppm [46])** is the dominant compound in the mixture of volatile organic compounds (VOCs) in the tested process gases in analyzed waste biogas plants (based on own, yet unpublished, chromatographic analysis); moreover based on [41] toluene can be used with a close relationship (R\({}^{2}\) = 0.95) in conversion from a GC-based VOC concentration to a PID-based VOC concentration; other compounds present in the highest amounts in process gases from the studied biogas plants processing municipal waste are styrene (IE = 8.43 eV; OT = 0.047 ppm [47]) and phenol (IE = 8.51 eV; OT = 0.048 ppm [48]).  
\end{table} 
Table 1: Sensor specifications of a gas detector MultiRae Pro and threshold level of odour identification for selected chemical compounds. \begin{table}
\begin{tabular}{c c} \hline \hline \multirow{2}{*}{**Mark of Measurement Point**} & **Biogas Plant** \\ \cline{2-3}  & **Biala Podlaska** & **Promnik** & **Wolka Rokicka** \\ \hline
1 & Waste storage plant (inside the hall) \\
2 & Mixed waste storage *** \\
3 & Selectively collected waste storage *** \\
4 & Mechanical treatment \\
5 & Fermentation preparation *** \\
6 & Oxygen stabilization *** \\
7 & Technological wastewater pumping station \\
8 & Biofilter *** & Digestate dewatering \\ \hline \end{tabular}  
*** Surface sources for which the gas sample was taken from under the static shield.  
\end{table} 
Table 2: Measurement points in particular waste treatment plants. Figure 1: Measurement points locations at biogas plant in Biala Podlaska. Figure 2: Measurement points locations at biogas plant in Promnik. OAV parameters were calculated by equation:  
\[\text{OAV}=\frac{\text{OC}_{\text{i}}}{\text{OT}_{\text{i}}} \tag{1}\]  
where:  
OC\({}_{\text{i}}\)--concentration of individual odorant (ppm),  
OT\({}_{\text{i}}\)--odour threshold of individual odorant (ppm).