## 3 Results and Discussion
Here, the annual load of pollutants entering the wastewater treatment plant in 2020 (Table 5) was 191,902 kg BOD/year, which was 217,595 kg BOD lower than in 2019 (409,498 kg BOD/year). This difference was due to the closure of the border crossing due to COVID-19 and the dramatic reduction in the number of people entering this city, which is the first border town. Remarkably smaller differences between BOD and COD concentrations in 2019 and 2020 were observed in towns B, D, and E. Therefore, in town B, the average BOD of sewage was lower in 2020 than the corresponding value recorded in 2019 by 42 g O\({}_{2}\)/m\({}^{3}\), and that of COD by 117 g O\({}_{2}\)/m\({}^{3}\). In town D, the differences were 52 and 78 g O\({}_{2}\)/m\({}^{3}\), respectively. In the case of the largest town, E, the differences in pollutant concentrations were 73 and 45 g O\({}_{2}\)/m\({}^{3}\), respectively. Despite the fact that the recorded differences between the pollutant concentrations in towns B, D, and E were smaller than those in towns A and C, when taking into account the annual loads of organic pollutants expressed by the BOD indicator in towns B, D, and E, the differences between 2019 and 2020 were significant and amounted to 19,000 kg O\({}_{2}\)/year, 158,000 kg O\({}_{2}\)/year, and 616,000 kg O\({}_{2}\)/year, respectively. The COD values determined for towns B and D were 110,000 kg O\({}_{2}\)/year and 253,000 kg O\({}_{2}\)/year, respectively, whereas the load of COD pollutants received at the wastewater treatment plant in town E in 2020 was 144,000 kg O\({}_{2}\)/year lower than in 2019, which was primarily due to the higher average daily flows of influents. Similar trends were noted for total suspended solids (Figure 3). In towns A and C, their concentration determined in sewage in 2020 was about 45% of the values recorded in 2019. In towns D and E, these differences were minimal. In town B, the concentration of suspended solids accounted for 86% of the value recorded in 2019. In towns A, B and C, the total nitrogen concentration determined in 2020 was from 81 to 86% of the 2019 value (Figure 4). In town D, it was similar, as it was 1 g/m\({}^{3}\) higher than in 2019 (92 and 93 g/m\({}^{3}\), respectively), whereas in town E it accounted for 90% of the 2019 value. The pandemic period and the resulting lockdowns of catering facilities had the least effect on phosphorus concentration in raw sewage (Figure 5). In towns D and E, its values were the same in both years. In town A, in 2020, the concentration of phosphorus was 1 g/m\({}^{3}\) higher than in 2019 (10 and 11 g/m\({}^{3}\), respectively). In town B, its concentration recorded in 2020 accounted for 90% of the 2019 value, whereas in town C it accounted for 78%. Figure 3: Average suspended solids concentration of raw wastewater in 2019–2021. Based on literature data, it is possible to estimate the volume of food waste discharged to the sewerage system in 2019. This was primarily waste from catering facilities, because, in practice, FWDs are not used in individual households in Poland. According to the literature, one gram of dry food waste generates 1.21 g of COD, 0.58 g of BOD, 0.36 g of total suspended solids, 0.025 g of total nitrogen, and 0.013 g of total phosphorus [12]. Calculations made based on the BOD values show that about 227,600 kg of dry matter of food waste was discharged into the sewerage system throughout the year in the smallest town (town A). In town C, it was as much as approximately 374,150 kg of food waste. In the largest town studied, the annual amount of food waste discharged into the sewerage system exceeded 1 million kg (Table 6). A system for the collection of food waste from restaurants and other facilities for mass catering, and the transfer of this waste to facilities for the anaerobic disposal of solid waste, could be a viable means for the production of methane-rich biogas. According to [PERSON] et al. [33], waste from Polish catering facilities contains an average of 87.82 \(\pm\) 4.05% VS.