## 2 Materials and Methods
### Research Data Handling and Statistical Analysis  
Research data for this study include the results of measurements of four polluting substances: ether extract (EE), phenols, chlorides and sulfates. Concentrations of these pollutants in sewage at three treatment stages (i.e., raw sewage, mechanically treated sewage and totally treated sewage) were measured in the accredited laboratory of the WWTP. Sewage samples for testing were collected by the WWTP operator over the period of one calendar year. Over this time, at each of the three treatment stages, both ether extract and phenols were tested 62 times, while both chlorides and sulfates were tested nearly 70 times. Standardized methods were employed to analyse sewage samples. Phenols content was measured using PN-ISO 6439:1994, involving distillation and UV-VIS spectrophotometry. Ether extract (EE) was determined via Soxhlet extraction under PN-EN 14331:2004. Chloride levels were assessed using silver nitrate titration (PN-ISO 9297:1994), while sulfate concentrations were analysed gravimetrically per PN-ISO 9280:2002.  
Initial statistical data analysis includes calculations of the basic descriptive statistics for polluting substance concentrations in sewage at the subsequent treatment stages: minimum (_Min_), average (_Avg_), maximum (_Max_), standard deviation (_STD_), coefficient of variation (_CV_), kurtosis (_Kurt_) and skewness (_Sk_). The interpretation of variability of the substance concentrations expressed by _CV_ coefficient was made based on the following rules: less than 0.25--low variability, between 0.25 and 0.45--average variability, between 0.45 and 1.00--strong variability, more than 1.00--very strong variability.  
The analysis referred to the maximum permissible values of pollutant concentrations in sewage specified by the obligatory Polish Regulation [26]: ether extract (i.e., substances extractable with petroleum ether)--50 mg/L, phenols--0.1 mg/L, chlorides--1000 mg/L, sulfates--500 mg/L.  
Based on the concentrations of polluting substances in raw sewage and in mechanically treated sewage, the average percentage reduction (\(\eta_{Avg}\)), along with the standard deviation of percentage reduction (\(\eta_{STD}\)) achieved as a result of mechanical treatment was counted. Similarly, based on the concentrations of polluting substances in raw sewage and in totally treated sewage, the percentage reduction (\(\eta_{Avg}\)), along with the standard deviation of percentage reduction (\(\eta_{STD}\)) achieved as a result of the overall mechanical-biological treatment, was determined. For this purpose, Equation (1) was used:  
\[\eta=[(C_{r}-C_{t})/C_{r})]\times 100\% \tag{1}\]  
where \(\eta\) is percentage reduction of polluting substance (%), \(C_{r}\) is polluting substance concentration in raw sewage (mg/L), and \(C_{t}\) is polluting substance concentration in totally treated sewage (mg/L).  
\begin{table}
\begin{tabular}{l} Coagulant tested for suspended particles removal: \\ \(\bullet\) polyaluminium chloride \\ Coagulant tested for struvite reduction: \\ \(\bullet\) Polystabil KWS \\ Parameters tested for biogas production: \\ \(\bullet\) biomass of sewage sludge \\ \(\bullet\) the volume of biogas \\ \end{tabular}
\end{table}
Table 1: _Cont._