## 3 Results  
### Concentrations of Pollution in sewage at the Subsequent Treatment Stages  
Among the four tested polluting substances, phenols were in the lowest concentrations (_Avg_ = 0.022 mg/L in raw sewage, _Avg_ = 0.002 mg/L in totally treated sewage). The greatest concentrations in raw sewage were found in case of the ether extract (the average was nearly 170 mg/L), while in totally treated sewage, it was noted for chlorides (the average was nearly 139 mg/L) (Table 2). Based on the calculated values of \(CV\) (Table 2), it can be stated that the ether extract was characterized by the lowest variability of concentrations in sewage (low variability; \(CV\) = 0.06 \(\div\) 0.10), while the greatest variability was attributed to the phenols (even strong variability). In turn, the concentrations of sulfates and chlorides were mainly characterized by average variability. Calculations of kurtosis (_Kurt_) and skewness (_Sk_) for all substances (Table 2) additionally allowed determining the shape and symmetry of the substances concentrations. The following rules were applied: greater kurtosis--more variables are accumulated around the average; skewness greater than zero--a prevalence of variables below the average; skewness less than zero--a prevalence of variables above the average. For example, based on the calculated skewness (Table 2), it can be stated that in most cases, concentrations of the polluting substances under consideration were below the average concentrations (skewness greater than zero). The graphs in Figure 2 show that the subsequent treatment stages did not cause a gradual reduction in the concentration of all tested substances. A visible reduction in the concentration through the subsequent treatment stages was observed in the case of the ether extract (Figure 2a) and phenols (Figure 2b). Considering the average concentrations of chlorides (Figure 2c) and sulfates (Figure 2d), these remained at a relatively even level at all three treatment stages. These findings were further developed into the results presented in Table 3. For the calculations of the average percentage reduction (\(\eta_{J,Avg}\)) of polluting substances, Equation (1) was used. Mechanical treatment caused a reduction of the ether extract and phenols concentrations averaging, respectively, 28% and 24%. In turn, the total mechanical-biological treatment provided a substantial reduction of over 91% in both of these substances. It was calculated that the standard deviation of the ether extract and phenols percentage reduction (\(\eta_{STD}\)) for the total treatment processes was smaller than for mechanical treatment (Table 3). For chlorides and sulfates, not only was there no gradual reduction in concentration as a result of mechanical treatment processes; in a significant part of the tested samples, the noted concentrations of chlorides and sulfates were greater in mechanically treated sewage than in raw sewage. This could have occurred as a result,for example, of the release of substances from solid phase of sewage into the liquid phase of sewage. Because of this, mechanical treatment for chlorides and sulfates disposal was evaluated as not applicable (Table 3). When it comes to the overall mechanical-biological treatment processes, these yielded low reduction levels (\(\eta_{Avg}\) for chlorides was 13% and for sulfates was 22%), with over 11% standard deviation of the percentage reduction (\(\eta_{STD}\)).