## 4 Discussion
### Biogas Production and Coagulant Assessment to Enhance Treatment Processes  
As [PERSON] et al. [44] indicated, based on data from 2019, in the European Union, the largest biogas production was recorded in Germany (7547.5 ktoe) and the smallest in Malta (1.6 ktoe). At that time, Poland took seventh place, with a total biogas production of 298.5 ktoe. However, in Germany, biogas produced from sewage sludge in 2019 accounted for about 6.5% of the total biogas production, whereas in Poland it was 40%, and in Malta 62.5%. According to [PERSON] et al. [45], municipal WWTPs currently have a large energy potential and will continue to have such potential in the future. The authors estimate the prospects for the production of biogas from sewage sludge at a level of 408.87 million m\({}^{3}\) per year, with the figure recorded in 2021 being 153.74 million m\({}^{3}\). The data summarized by [PERSON] et al. [45] show that in Poland, the amount of biogas produced from sewage sludge is in second place; only agricultural biogas production is ahead of sewage sludge biogas production. Landfill biogas, municipal biogas and biogas from food industry waste is also produced, but in smaller quantities than biogas from sewage sludge. Biogas from anaerobic digestion in sewage treatment is converted into synthetic natural gas through catalytic methanation, achieving a gas mixture with CH\({}_{4}\geq\) 95%, H\({}_{2}\leq\) 5%, and CO\({}_{2}\leq\) 2.5%. Optimal conditions, including 4-5 bar pressure and reactor temperatures (400 \({}^{\circ}\)C and 300 \({}^{\circ}\)C), ensure efficient methane production for gas grid injection [46]. Our data highlighted energy usage and production over several years, showcasing a notable trend in increasing energy efficiency and coverage. For instance, between Year A and Year I (Table A1), there was a clear improvement in energy self-sufficiency, with the percentage coverage rising from 20% (Year A) to 90% (Year G). This indicates a successful strategy in enhancing the energy production capabilities of the WWTP, likely through investments in technology and processes. Moreover, maintaining a high level of energy share (up to 94%) suggested a well-balanced energy management approach, optimizing both usage and production. Biogas enhances the sewage treatment process by producing renewable energy from anaerobic digestion. Between 2014 and 2023 [47], there was a reduction in Biochemical Oxygen Demand (BOD\({}_{5}\)) and Chemical Oxygen Demand (COD), with reductions exceeding 97%. Advanced technologies and research have optimized methane production, contributing to efficient energy generation in a circular economy. From Year A to Year I (Table A2), the average monthly amount of composted sewage sludge generally decreased, starting at 834.5 mg/month in Year A and dropping to 721.6 mg/month in Year I. This reduction suggested improvements in the efficiency of biogas production processes or alternative treatment methods. The amount of discharged sludge also declined during the same period, from 279.35 thus. dry matter/month in Year A to 267.32 thus. dry matter/month in Year I (Table A2). This indicates an overall improvement in sludge management and treatment, potentially reducing the environmental impact. Studying phenols and biogas is important because phenolic compounds, found in nature and organic wastes, can be utilized in biogas production. The study of [PERSON] and [PERSON] [48] found that phenolic compounds inhibit biogas production at concentrations of 120 to 594 mg of compound/g Volatile Suspended Solids (VSS), with a maximum biodegradation of 63.85 \(\pm\) 2.73%. Understanding these effects can help optimize biogas production and manage the impact of phenolics in sewage treatment, as seen with inhibition concentrations between 800 and 1600 mg/L organic carbon. The increase in polyaluminium chloride (coagulant) consumption in Year I (Table A3) suggests that more coagulant was needed to achieve the desired sewage treatment outcomes.