# Determining Effective Threshold Range of Image Pixel Values for Municipal Waste-Contaminated Clay
## 4 Discussion  
Figure 9 shows binarized SEM images of clay polluted with CH3 COOH at a concentration of 0.6 mol/L for 12 image pixel values (T denotes the pixel value). The following three factors were used to validate the effective threshold range of 110-140.  
First, there is a connection between the effective threshold range of image pixel values and the apparent void ratio. The same SEM images of municipal waste-contaminated clay resulted in different threshold ranges of image pixel values, which led to different binary images. Since more image pixels were regarded as pores given a larger threshold range, there were more black pixels and a higher apparent void ratio. At the same time, when the pixel threshold value was 130, the apparent void ratio approximated the actual void ratio the most. Figure 9 shows this phenomenon.  
Second, pore obstruction varies depending on the seepage depth. The amounts of calcium carbonate and organic matter per kilogram of clay generated are shown in Table 3. It can be seen that the amounts of calcium carbonate and organic matter produced per kilogram of clay gradually decrease with the depth of seepage. This occurs because during waste-contaminant seepage, organic matter (organisms) and suspended debris obstruct the pores, and organic matter and its degradation substances chemically react with the clay to generate calcium carbonate and other deposits in the clay pores [46; 47]. Therefore, the smaller pores in the upper soil prevent the leachate from moving lower, resulting in less chance of clay contamination and a higher void ratio when municipal waste contaminants keep to greater depths. The results indicated that the threshold determination method was reasonable given that the curves corresponding to the various thresholds depicted in Figure 6 were consistent with this classification.  
Figure 8: Relationship between fractal dimension and the pixel value of clay particles at different concentrations.  
Third, from the perspective of macroscopic parameters, the suitability of the threshold values was confirmed. In this study, a parameter, namely, the compaction factor, which is equal to the ratio of the dry density to the apparent void ratio, was defined as the effective combination of the soil macroscopic properties (dry density) and microstructure (apparent void ratio). The larger the compaction factor is, the more compact the soil. The compaction factor values for municipal waste-contaminated soils at appropriate thresholds could be estimated using the apparent void ratio values corresponded to the thresholds for the seeping and soaking municipal waste-contaminated soils determined in Section 3.3.  
Figure 10 shows that the compaction factor decreased with increasing seepage depth, while the compactness of clay also decreased, which agrees with the finding that the void ratio of municipal waste-contaminated clay increased with pollutant seepage. The  
Figure 9: Binarization of SEM images of CH\({}_{3}\)COOH-contaminated clay with the concentration of 0.6 mol/L segmented by different pixel values.  
compaction factor increased with increasing CaCO\({}_{3}\) concentration and decreased with increasing CH\({}_{3}\)COOH concentration, which is consistent with the finding that CaCO\({}_{3}\) imposed a cementation effect on soil and that CH\({}_{3}\)COOH exerted a corrosion effect on soil. This suggests that the thresholds were appropriate.  
Therefore, it is reasonable to conclude that the pixel threshold value ranges from 110 to 140.