## 4 Discussion
### Removal of the Ether Extract, Phenols, Chlorides and Sulfates from Sewage in Municipal WWTPs
The obtained efficiency reached high values, ranging from 88.95% (conventional activated sludge processes) to 99.97% (a combination of anaerobic/ anoxic/oxic processes, continuous microfiltration processes, ozone oxidation and chlorination processes). High efficiency of phenols removal was evidenced by the fact that seventeen different phenols were identified in the sewage influents, while only five phenols were found in the sewage effluents (including two regulated and three unregulated phenols). In the WWTP that was the subject of our research, the biological part of which is based on activated sludge technology, a similarly high level of phenols reduction was achieved, exceeding 91%. In their research work, [PERSON] et al. [31] draw attention to the fact that phenols concentration in phenolic sewage classified as low, medium and high is a key factor in selecting the appropriate treatment method in industrial sewage treatment systems. In the case of low phenols concentrations, biodegradation, chemical, electrochemical and photocatalytic oxidation, solid phase extraction, ozonation, reverse osmosis/nanofiltration and wet air oxidation are recommended for phenols removal. In the case of high phenols concentrations, liquid-liquid extraction, pervaporation, membrane-based solvent extraction, adsorption and distillation are recommended. [PERSON] et al. [32] observed an adverse effect of chlorides on biological removal of nutrients from sewage. While the removal of ammonia nitrogen was not disturbed by the increased chlorides concentration, the removal of phosphate phosphorus at higher chlorides concentrations was definitely less effective. At chlorides concentrations above 1000 mg/L, the concentration of phosphate phosphorus in the effluent was even higher than in the influent. Moreover, it was noted that the efficiency of removal of organic substances was significantly lower in a context of higher chlorides concentrations. The research by [PERSON] et al. [32] confirms the sensitivity of nitrifying activated sludge to high chlorides concentrations. Removal of chlorides from water and sewage is a difficult process, as a result of their high solubility and non-biodegradability. Chlorides removal methods include chemical precipitation, adsorption, oxidation and membrane separation [33]. As [PERSON] et al. [34] point out, domestic ion-exchange water softeners have a significant impact on the chlorides load in municipal WWTPs. Since chlorides removal technologies in WWTPs are limited and very expensive [34; 35], achieving sufficient chlorides reduction in treated sewage discharges is a real problem. A potential solution to this problem is to switch from domestic water softening to a centralized water softening system in water treatment plants. It turns out that centralized water softening methods using reverse osmosis technology or lime-softening technologies are more cost-effective solutions than home-based water softening with end-of-pipe chloride treatment; these methods are also characterized by environmental efficiency. Microbiological sulfate reduction is one of the main biological processes of great environmental and biogeochemical importance [36]. As indicated by [PERSON] [37], the use of sulfate-reducing bacteria in WWTPs brings advantages, including minimal sludge production, pathogens reduction, heavy metals removal and pre-treatment of anaerobic fermentation. The effectiveness of the sewage treatment system based on sulfate-reducing bacteria is also confirmed by [PERSON] et al. [38]. Not only does the use of this system make it possible to achieve a high level of sulfate reduction (in the case of [PERSON] et al. [38], it was 84%), but its integration with a sulfide fuel cell, which is an alternative to the energy-intensive oxygen sewage treatment process, also enables the generation of electricity. For better understanding the mechanisms of these biological processes, sulfate-reducing bacteria in anaerobic reactors of the active sludge technology municipal WWTPs were studied by [PERSON] et al.