## 3 Results and Discussion
### Odour Concentration (\(c_{\mathrm{od}}\)) and Intensity (\(i_{\mathrm{od}}\))  
Figures 2-7 show the results of odour concentration (\(c_{\mathrm{od}}\)) and intensity (\(i_{\mathrm{od}}\)) at individual odour sources (a-g) at the analysed biogas plants (for two measurement series: 1 and 2) in combination with the average temperature (T) and average relative humidity of air (RH). Air temperature and humidity values indicated in the figures are the arithmetic mean of the values measured at individual measuring points during one measurement series.  
Figure 2: Dependencies between average air temperature and relative humidity and \(c_{\mathrm{od}}\) and \(i_{\mathrm{od}}\) in two measurement series at Jarocin biogas plant (21 July 2018 and 27 February 2019), (**a**) T, RH and \(c_{\mathrm{od}}\); (**b**) T, RH and \(i_{\mathrm{od}}\).  
Figure 3: Dependencies between average air temperature and relative humidity and \(\rm{c_{od}}\) and \(\rm{i_{od}}\) in two measurement series at Tychy biogas plant (27 September 2018 and 5 March 2019), (a) T, RH and \(\rm{c_{od}}\); (b) T, RH and \(\rm{i_{od}}\).  
Figure 4: Dependencies between average air temperature and relative humidity and \(c_{\rm od}\) and \(i_{\rm od}\) in two measurement series at Promnik biogas plant (12 April 2019 and 14 May 2019), (**a**) T, RH and \(c_{\rm od}\); (**b**) T, RH and \(i_{\rm od}\).  
Figure 5: Dependencies between average air temperature and relative humidity and \(\rm{c_{od}}\) and \(\rm{i_{od}}\) in two measurement series at Stalowa Wola biogas plant (30 August 2018 and 20 February 2019), (**a**) T, RH and \(\rm{c_{od}}\); (**b**) T, RH and \(\rm{i_{od}}\).  
Figure 6: \(Cont\).  
Figure 6: Dependencies between average air temperature and relative humidity and \(\rm{c_{od}}\) and \(\rm{i_{od}}\) in two measurement series at Wölka Rokicka biogas plant (28 August 2018 and 19 February 2019), (**a**) T, RH and \(\rm{c_{od}}\); (**b**) T, RH and \(\rm{i_{od}}\).  
Figure 7: Dependencies between average air temperature and relative humidity and \(\rm{c_{od}}\) and \(\rm{i_{od}}\) in two measurement series at Biala Podlaska biogas plant (26 April 2019 and 14 May 2019), (**a**) T, RH and \(\rm{c_{od}}\); (**b**) T, RH and \(\rm{i_{od}}\).  
At individual measurement points, \(c_{\text{od}}\) varies from 2 ou/m\({}^{3}\) to 448 ou/m\({}^{3}\) depending on the kind of technological procedures. The highest values of \(c_{\text{od}}\) is observed during digestate dewatering at A biogas plant (448 ou/m\({}^{3}\)) and waste storage at the biogas plant at E biogas plant (106 ou/m\({}^{3}\)), while the lowest ones are identified from the surface of the open biological filter in Wolka Rokicka (E) and Tychy (B) (respectively 2 ou/m\({}^{3}\) and 3 ou/m\({}^{3}\)) and in the oxygen stabilisation of mixed waste sieve fraction (15-80 mm) plant in Biala Podlaska (F) (3 ou/m\({}^{3}\)). It should be also noted that, at many odour sources, the measured values of \(c_{\text{od}}\) is between 4 and 8 ou/m\({}^{3}\).  
Analysing the differences between the results obtained at the same odour source, but in two different measurement series, it should be stated that in most cases, a higher \(c_{\text{od}}\) is accompanied by a higher \(i_{\text{od}}\). It is visible at all analysed biogas plants (A) at odour sources a, c, f, g; (B) at odour sources c,d; (C) at odour sources a, c, d; (D) at odour sources a, e, f, (E) at odour sources: c, f, g; and (F) at odour sources b, c, e, f (Figures 2-7). It should be noted that the results of \(i_{\text{od}}\) are less varied between individual measurement series than \(c_{\text{od}}\) results what is related to the methodology of determining the odour intensity (using the scale from 0 to 5, in steps of 1; Table 2).