## 3 Results and Discussion
### Energy Balance  
The thermal pre-treatment of the H-LSPW was conducted in the IS-1K reactor with an active volume of 1.0 dm\({}^{3}\). Depending on the experimental variant, the assumed heating temperature was achieved by means of electrical heaters (CTP) or ultrasounds (UTP). It needs to be emphasized that H-LSPW was category 3 waste, which requires pre-disintegration to the maximal particle diameter of 60 mm and then 60-minute retention at a temperature of 70 \({}^{\circ}\)C prior to the AD process, as stipulated in the legal regulations in force [38]. Hence, the thermal pre-treatment is an indispensable technological procedure that cannot be eliminated [39]. Given the above, there emerges a justified need to search for a method of heat energy supply, in the case of which the process of anaerobic disintegration of this type of waste would be technologically, energetically, and environmentally viable. The energy balance performed proved substrate heating to 70 \({}^{\circ}\)C to be the most advisable variant of thermal pre-treatment, regardless of whether the heating proceeded via CTP or UTP (Table 5). Considering the volume of biogas produced, methane content, and unit energy value of this biogas fraction, reaching 0.00917 Wh/dm\({}^{3}\), the highest gross energetic efficiency was demonstrated in variants S4V3 (UTP 90 \({}^{\circ}\)C, OLR = 6.0 gCOD/dm\({}^{3}\)) and S3V3 (UTP 70 \({}^{\circ}\)C, OLR = 6.0 gCOD/dm\({}^{3}\)), i.e., 11.84 \(\pm\) 0.24 Wh and 11.41 \(\pm\) 0.27 Wh, respectively. In S50V3 (control, OLR = 6.0 gCOD/dm\({}^{3}\)), without the pre-treatment, the energy content of the produced CH\({}_{4}\) was significantly lower and reached 10.48 \(\pm\) 0.74 Wh (Table 5). Taking into account the energy inputs, the highest net energy efficiencies were determined in S1V3 (CTP 70 \({}^{\circ}\)C, OLR = 6.0 gCOD/dm\({}^{3}\)) and S3V3 (UTP 70 \({}^{\circ}\)C, OLR = 6.0 gCOD/dm\({}^{3}\)), and reached 5.92 \(\pm\) 0.43 Wh and 5.80 \(\pm\) 0.42, respectively (Table 5). The observed differences were not statistically significant. The net energy gain achieved in the remaining thermal pre-treatment variants was significantly lower, ranging from 2.00 \(\pm\) 0.13 Wh (S2V1) to 4.98 \(\pm\) 0.42 Wh (S2V3). The energetic efficiency obtained was found to be influenced to the greatest extent by the OLRs applied. This was mainly reflected in the lowest total CH\({}_{4}\) yield noted at OLR = 6.0 gCOD/dm\({}^{3}\) (V3). Due to the active volume of respirometers (500 cm\({}^{3}\)), the OLR was 3.0 gCOD in these variants. Depending on the experimental stage, this variant yielded from 795 \(\pm\) 48 cm\({}^{3}\)CH\({}_{4}\) (S1V3) to 906 \(\pm\) 24 cm\({}^{3}\)CH\({}_{4}\) (S4V3). In the variants with the initial OLR = 2.0 gCOD/dm\({}^{3}\), the total volume of CH\({}_{4}\) produced was lower (Table 5) and ranged from 304 \(\pm\) 13 cm\({}^{3}\)CH\({}_{4}\) (S1V1) to 343 \(\pm\) 7 cm\({}^{3}\)CH\({}_{4}\) (S4V1). Despite the control stage (S0) producing the lowest total CH\({}_{4}\) yields, ranging from 270 \(\pm\) 21 cm\({}^{3}\)CH\({}_{4}\) (S0V1) to 687 \(\pm\) 81 cm\({}^{3}\)CH\({}_{4}\) (SE0V3), it yielded the highest net energy, i.e., 10.48 \(\pm\) 0.74 (S0V3). This was due to the fact that H-LSPW was not pre-treated at this stage, hence the gross energy determined in CH\({}_{4}\) was the same as the net energy. It should be again emphasized that H-LSPW is a category 3-type waste requiring substrate pre-treatment prior to AD and increasing energy expenditure. Other researchers have also demonstrated that UD generates additional energy expenditures for the wastewater treatment system [40]. It was found, however, that an appropriate technology can be coupled with aerobic digesters to facilitate the stage of hydrolysis and boost the efficiency of complex wastewater treatment systems [41].