## 3 Results and Discussion
### Concentrations of Organic Compounds in the Dissolved Plased
(2020) [34] also proved UD to be an effective technique for post-slaughter wastewater pre-treatment. High-frequency UD may aid conventional treatment methods by promoting the initial oxidation of resistant organic pollutants. The study tested the influence of frequencies reaching 578, 800, and 1142 kHz, and a power density ranging from160 W/dm\({}^{3}\) to 1200 W/dm\({}^{3}\). At low power densities, the high frequency (1142 kHz) was more efficient in COD removal than the low one. In contrast, the low-frequency sonication (578 kHz) resulted in a higher COD load removed at high power densities. The highest COD removal percentage, i.e., 49%, and thus the highest increase in COD concentration in the dissolved phase, was achieved at the frequency of  
Figure 1: Effect of thermal pre-treatment method on the percentage increase in the concentration of organic compounds. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & & \multicolumn{6}{c}{**Indicator**} \\ \cline{3-8}  & & \multicolumn{2}{c}{**COD**} & \multicolumn{2}{c}{**TOC**} & \multicolumn{2}{c}{**BOD\({}_{5}\)**} \\ \cline{3-8}  & & **Concentration** & **Increase** & **Concentration** & **[mg/dm\({}^{3}\)]** & **Increase** & **Concentration** & **Increase** \\  & & **[mgO\({}_{2}\)/dm\({}^{3}\)]** & **[\%]** & **[mg/dm\({}^{3}\)]** & **[\%]** & **[mgO\({}_{2}\)/dm\({}^{3}\)]** & **[\%]** \\ \hline
0 & Raw wastewater & 29,400 \(\pm\) 1950 & - & 11,580 \  
578 kHz and a power density of 480 W/dm\({}^{3}\)[34]. In turn, [PERSON] et al. (2010) [35] used a low-frequency UD, i.e., 20 kHz, as a stage of pre-treatment of wastewater from the meat processing industry. The specific energy input ranged from 0 to 50 MJ/kgTS. The study proved the low-frequency UD to be highly effective in increasing the extent of dissolution of organic compounds. The concentration of TOC in the dissolved phase of non-pre-treated wastewater was 557 mg/dm\({}^{3}\). The maximal TOC concentration, reaching 654 mg/dm\({}^{3}\), was achieved for the UD energy input of 120 MJ/kgTS. Higher UD energies ensured partial mineralization of organic matter preceding the solubilization of the solid phase and TOC concentration decrease in the dissolved phase [35]. In another study, UD with the frequency of 20 kHz and power density of 500 W was used for the pre-treatment of olive mill wastewater (OMW), being one of the most complex types of industrial wastewater. The UD was applied for raw and tenfold-diluted OMW. This study demonstrated a small increase in the COD concentration in the dissolved phase of raw OMW (40,510 mg/dm\({}^{3}\)), reaching ca. 4-5% after 2 minutes of sonication (ca. 42,535 mg/dm\({}^{3}\)). This value remained unchanged in the sonicated sample within 10 min since UD. In the case of the diluted OMW, after 10 minutes of sonication, the concentration of soluble COD increased by ca. 23% compared to the raw OMW, approximating 49,827 mg/dm\({}^{3}\). In addition, the cited study analyzed the effect of UD lasting 0 to 60 min; however, it failed to demonstrate any successive increase in the COD concentration in the dissolved phase after 10 minutes of sonication [35].