## 4 Discussion  
### Removal of the Ether Extract, Phenols, Chlorides and Sulfates from Sewage in Municipal WWTPs  
In light of the risk of exceeding permissible levels of substances extractable with petroleum ether in technological sewage from a confectionery plant, [PERSON] et al. [11] explored ways to enhance the efficiency of the existing sewage pre-treatment systems. Proposed solutions included installing additional deflectors or partitions and implementing biological sewage treatment under aerobic conditions. Subsequent technological tests conducted by [PERSON] et al. [12] on sewage from the confectionery industry, using the activated sludge method in SBR batch reactors, demonstrated satisfactory efficiency in removing substances extractable with petroleum ether. These results confirmed the viability of biological processes for treating this type of sewage. Similarly, our studies have shown that the processes taking place in the biological part of the WWTP play a key role in ether extract reduction. However, the first stage of treatment processes, i.e., mechanical treatment, which resulted in a 28% reduction of the ether extract, is also important. Therefore, the installation of additional deflectors or partitions in order to increase the efficiency of removing this type of pollutant, as proposed by [PERSON] et al. [12], seems to be justified. At one municipal WWTP, [PERSON] [15] reported an average removal efficiency of 42.7% for fat and oil substances in sewage. The highest load of substances extractable with petroleum ether was removed from initial sewage sludge, while the removal of fats and oils through sand and screenings was minimal. It was observed that the degradation of fat substances occurs predominantly during sludge processing, facilitated by microbiological decomposition in the fermentation chamber. This process does not occur in the biological treatment line of the WWTP. This is because the biodegradation of fats and oils by activated  
Figure 9: Biomass vs. biogas production at the subject WWTP. sludge microorganisms requires more time than the hydraulic retention time of the sewage and activated sludge mixture in the biological treatment section. Instead, fats and oils are adsorbed onto activated sludge flocs and subsequently removed in the stream of excess sludge. The research by [PERSON] et al. [28] demonstrated that the presence of phenols and sulfides in sewage (along with chromium, nickel and copper) was a source of disturbances in the biological treatment processes in a WWTP operating activated sludge technology. The decrease in the efficiency of removing chemical oxygen demand, suspended solids and nitrogen was caused, among others, by the presence of phenols in raw sewage (January-March and October-December), which is associated with olive cultivation specific to the research region. In the period from May to September, i.e., when there were no phenols in the sewage, and other toxic elements were present, the treatment efficiency was good, thanks to the activated sludge acclimatization. In their study, [PERSON] and [PERSON] [29] proved that when phenols are the only source of carbon in sewage and their concentration does not exceed 30 mg/L, then activated sludge quickly decomposes them, thus shortening the time of phenols' toxic effect. However, when sewage contains other, more easily assimilable sources of carbon, even low concentrations of phenols are toxic to activated sludge, because activated sludge uses the most easily assimilable source of carbon. Undecomposed phenols can lead to a weakening of the activated sludge's activity and even cause it to die. Therefore, the authors highlight that attempts to adapt activated sludge to the decomposition of phenolic sewage in municipal sewage can be risky, especially in WWTPs not adapted to phenols decomposition. [PERSON] et al. [30] investigated the effectiveness of various technologies for phenols removal used in several municipal WWTPs.