3.2. Odour Concentration (cod) and Intensity (iod)
, Figure 3
, Figure 4
, Figure 5
, Figure 6
and Figure 7
show the results of odour concentration (cod
) and intensity (iod
) 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.
At individual measurement points, cod varies from 2 ou/m3 to 448 ou/m3 depending on the kind of technological procedures. The highest values of cod is observed during digestate dewatering at A biogas plant (448 ou/m3) and waste storage at the biogas plant at E biogas plant (106 ou/m3), while the lowest ones are identified from the surface of the open biological filter in Wólka Rokicka (E) and Tychy (B) (respectively 2 ou/m3 and 3 ou/m3) and in the oxygen stabilisation of mixed waste sieve fraction (15–80 mm) plant in Biała Podlaska (F) (3 ou/m3). It should be also noted that, at many odour sources, the measured values of cod is between 4 and 8 ou/m3.
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 cod
is accompanied by a higher iod
. 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 (Figure 2
, Figure 3
, Figure 4
, Figure 5
, Figure 6
and Figure 7
). It should be noted that the results of iod
are less varied between individual measurement series than cod
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
3.4. Impact of Air Temperature and Humidity and Technological Factors on Odour Concentration and Intensity: Analyses
At four of the examined biogas plants (A, B, E, F) for two research series, bigger cod
(at designated points) is accompanied by higher air temperatures (irrespective of differences between the temperatures in a range from 3 °C to more than 18 °C). The same relationships are also reported in other papers [34
]. In the case of B biogas plant, despite small differences between mean temperatures of individual measurement series (at a level less than 3 °C), such dependencies are observed at almost all measuring points. In turn, at the biogas plant in Biała Podlaska in spite of a slight variation in cod
values (from 3 ou/m3
to 11 ou/m3
), the dependencies between cod
at measurement sites and average air temperature are evidently the same. It is the air temperature that seems to have a major impact among environmental drivers on odour emission. This is particularly evident in relation to cod
at A biogas plant. During the second measurement series, the average air temperature is lower by about 61% compared to the air temperature in the first series, and in individual points, lower cod
values are also noted—lower by 33% (in b and g points) to 84% (in point a) as is graphically illustrated in Figure 2
. This trend is not observed only at one point: point f (oxygen stabilisation). At other biogas plants, the tendency is similar, and the lower temperature is accompanied by lower cod
values (compared in the same odour sources)—lower by 17% to 90%. This is illustrated graphically in Figure 3
, Figure 4
, Figure 5
, Figure 6
and Figure 7
. The dependencies between cod
and average air temperature are not so evident in Stalowa Wola. The variability of cod
between the series of tests is probably determined more by the technological factors.
This tendency is less pronounced with respect to iod due to the aforementioned methodology that was used. At two examined biogas plants (B and F), dependencies which mirrored the observed cod. A temperature relationship is also observed for iod and air temperature; a higher iod is found at higher air temperatures. At other biogas plants (A, D and E), iod is less dependent on atmospheric and ambient conditions because it was characterised by lower variability between the measurement series.
At A biogas plant, the difference between average air temperatures during the test series is too small (0.3 °C) to assess its effect on odour nuisance. It makes the comparison impossible.
At two biogas plants (A and F), a similar impact is observed of mean air humidity on cod
compared to the effect presented in the literature [35
]—a higher cod
at lower air humidity. However, this relationship is observed in the case of differences in air humidity at a minimum level of about 20%. It should be noted that differences in average relative air humidity at the level of 10% to 14% (as measured respectively at C and E biogas plants) do not have a significant effect on cod
. At two biogas plants, the differences between average air humidity during separate research series are too small to assess their effect on odour nuisance. The air temperature should be indicated as the main environmental factor visible primarily in relation to cod
. The strength of the correlation relationship between the odour concentration and air temperature was determined by means of Spearman’s rank correlation coefficient. The choice of the applied coefficient results from the type of features, number of observations and shape of the relation. The correlation coefficient rho
for all measurements is rho
= 0.306 and the significance level α = 0.022. The results obtained for the source d (digestate dewatering plant) were not used for calculations. The implementation of dewatering process varies significantly between plants and is largely influenced by the technological solutions used there.
However, the influence of wind cannot be ignored. The greatest wind influence was observed in the case of F biogas plant for e source (oxygen stabilisation in the open air): in the first series v1
= 1.5 m/s, cod
= 22 ou/m3
; in the second series v2
= 2.3 m/s, cod
= 11 ou/m3
. The obtained relation (an increase in wind speed results in a decrease in odour concentration) is consistent with the results of tests carried out by Sakawi et al. [44
]. This impact should be taken into account during further testing.
The obtained results of air temperature and humidity impact on odour emissions are in accordance with results presented by other authors; but it should be emphasised that the scope of the research reported in the literature has not allowed for unequivocal, forecastable and obvious assessment of the discussed dependencies. Thus, this paper has contributed significantly to the subject. Knowledge of the impact of various factors (meteorological, ambient and technical) on the odour emissions and nuisance of waste treatment plants will permit to apply corrective actions to minimise them.
The conducted research also shows the impact of technological processes, operating regimentation and changes in waste properties. The variability of these factors can be observed on the basis of the results of VOC and NH3
concentrations, changing between separate series. In most cases, the increase in VOC or NH3
concentration is accompanied by an increase in cod
(in five out of six installations—B, C, D, E and F—at all measuring points (E and F biogas plants)) or in most of them (Table 5
). It demonstrates the effect of technological factors on the odour impact of biogas facilities. The influence of technological factors takes place in both the mechanical and biological parts of the installations. Especially at B and C biogas plants (in Tychy and Promnik), the impact of technological factors on the odour effect is significant (higher cod
at higher VOC or NH3
concentration), because in these cases the research was accompanied by small differences in air temperature and humidity between individual measurement series, so their influence needs to be omitted.
The increase in VOC or NH3 concentration is not always accompanied by an increase in cod. In these cases, a dominant influence of atmospheric factors is observed (according to the previously presented relationships). It is particularly evident at A biogas plant (in Jarocin), especially at the measuring points in the mechanical part of the installation. In turn, the biological part of this plant (point f: oxygen stabilisation) shows the prevailing impact of technological factors over atmospheric agents in relation to odour emissions.
The highest VOC concentrations are recorded in the plants of waste storage (7740 ppb), pre-treatment (6040 ppb and 5710 ppb) and digestate dewatering (6410 ppb) at B and C biogas plant (points a, b, c, d according to Table 6
). Among the places where the highest NH3
concentrations are observed for digestate dewatering plant (32 ppm, 22 ppm and 20 ppm), installation of oxygen stabilisation (20 ppm) and open biofilter surface (13 ppm and 14 ppm) should be mentioned (points d, f, g at A, B, D, F biogas plants according to Table 6
). Further research should indicate the dominance of some element in the technological line in this respect. Analysing the presented results, it should be stated that at different measuring points (in the technological line) different odorants (VOC or NH3
) have the main impact on the level and changes of odour concentration. At points a, b, d, f (waste storage, pre-treatment, digestate dewatering and oxygen stabilisation), more VOC concentration changes accompanying cod
changes are observed and marked in grey in Table 6
. In other elements of the installations, the influence of NH3
or both odorants dominate. Further research should indicate which odorant is responsible for higher odour emission.