## 2 Materials and Methods  
### Study Methodology  
The study covers two measurement series (1 and 2) at six plants (A, B, C, D, E, F). It was carried out at the turn of 2018 and 2019. The dates of the series are presented in Table 1. The measurements always took place in various seasons, always during the day. The research involves the determination of air temperature and relative humidity at each designated measurement point (receptors marked as a, b, c, d, e, f, g), simultaneously measuring wind speed and direction at points located outside buildings and plant processing buildings. Meteorological conditions were determined in the plume at a height of 1.5 m by means of a Weather Meter 4500 NV by Kestrel based on the European Standard VDI 3940 [37]. The results of air temperature and relative humidity presented in Table 5 are the arithmetic mean of five parallel measurements. Furthermore, in each receptor, odour intensity (i\({}_{\text{od}}\)) was determined by means of the method of sensory evaluation in a six-grade scale, in accordance with Table 2 [31], as well as the odour concentration (\({}_{\text{cod}}\)) expressed in odour units per cubic metre (ou/m\({}^{3}\)), analogically to European norm PN-EN 13725:2007 [38]. Olfactometric tests were performed in situ with the use of dynamic field olfactometric measurements. The device used in the study was the olfactometer Nasal Ranger(r) (St. Croix Sensory, Inc., Stillwater MN, USA). Odour concentrations in every measurement point were calculated based on two readings of the \(D/T\) parameter (dilution-to-threshold ratio), using the following formulas [2; 31; 35; 39; 40]:  
\[Z_{YES}=\left(D/T\right)_{YES}+1,\]  
where \(Z_{YES}\) means dilution ratio, at which the odour was perceptible (-); \((D/T)_{YES}\) means dilution ratio agreeing for the moment when the odour was perceptible for the first time (-),  
\[Z_{NO}=\left(D/T\right)_{NO}+1,\]  
where \(Z_{NO}\) means dilution ratio, at which the odour was imperceptible; \((D/T)_{NO}\) means dilution ratio agreeing for the moment when the odour was imperceptible just before the dilution \((D/T)_{YES}\),  
\[Z_{ITE}=\sqrt{Z_{YES}\cdot Z_{NO}},\]  
where \(Z_{ITE}\) means the assessment of the individual threshold, expressed as dilution ratio (-). Values of the odour concentrations were calculated based on a geometric mean of the set of all individual estimations (\(Z_{ITE}\)) for a given measurement point:  
\[c_{od}=\sqrt[n]{\sum\
olimits_{i=1}^{n}Z_{ITE,i}},\]  
where \(n\) means amount of all estimates. Chemical tests, including the determination of concentrations of volatile organic compounds (VOC) and ammonia (NH\({}_{3}\)), were performed with the MultiRae Pro gas detector (RAE Systems, Inc., San Jose, CA, USA). The sensor's characteristic is presented in Table 3. At each of the measurement \begin{table}
\begin{tabular}{c c c} \hline \hline
**Location of Biogas Plant** & **Date of 1 st Series** & **Date of 2 nd Series** \\ \hline Jarocin & 21 July 2018 & 27 February 2019 \\ Tychy & 27 September 2018 & 5 March 2019 \\ Promnik & 12 April 2019 & 15 May 2019 \\ Stalouva Wola & 30 August 2018 & 19 February 2019 \\ Wolka Rokicka & 28 August 2018 & 19 February 2019 \\ Biala Podlaska & 26 April 2019 & 15 May 2019 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Schedule of measurement series at the analysed biogas plants. \begin{table}
\begin{tabular}{c c} \hline \hline
**Scale of Odour Intensity, i\({}_{\text{od}}\)** & **Discernible Odour Intensity** \\ \hline
**0** & No odour \\ \hline
**1** & Odour almost imperceptible \\ \hline
**2** & Very weak odour \\ \hline
**3** & Weak odour \\ \hline
**4** & Strong odour \\ \hline
**5** & Very strong odour \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Scale of odour intensity according to the sensory evaluation method [31]. points, measurements of the analysed compounds were made in five repetitions. The ionisation energy of the sensor was 10.6 eV. The sensor detects only those compounds that are able to ionise [41; 42].