## 3 Results and Discussion  
### Effects of Ultrasonic Pretreatment on Solubilization of Organic Matter  
The experimental results showed a significant increase in the concentration of the soluble organic compounds in variants V1-V4. In V1, where the M-BGS biomass was not pretreated with ultrasonics, the soluble COD concentration was on the level of 119 \(\pm\) 11 mgO\({}_{2}\)/L, while the soluble TOC was 89 \(\pm\) 6 mg/L (Table 4). A dynamic transfer of the COD and TOC to the dissolved phases in V2 and V3 was found, which, respectively, reached 344 \(\pm\) 39 mgO\({}_{2}\)/L and 302 \(\pm\) 27 mg/L in V2, and in V3, respectively, was 617 \(\pm\) 43 mgO\({}_{2}\)/L and 476 \(\pm\) 52 mg/L (Table 4). The COD solubilization degrees were 25 \(\pm\) 2.1% (V2) and 55.3 \(\pm\) 3.9% (V3), respectively. For TOC, this indicator was 35.0 \(\pm\) 2.4% (V2) and 63.6 \(\pm\) 7.0% (V3). In the next variant of the experiment, the COD and TOC solubilizationswere still at a high level. In V4, the COD and TOC concentrations were, respectively, 719 \(\pm\) 49 mgO\({}_{2}\)/L and 533 \(\pm\) 42 mg/L, while the solubilization degrees were 66.7 \(\pm\) 3.0% and 73.0 \(\pm\) 7.9% (Table 4). A higher ultrasonic energy input connected with a longer sonication time of 200 s did not enhance the solubilization of the organic matter. In V5, no significant increase in the concentrations of the organic compounds in the dissolved phase was found. The amounts of soluble COD and TOC were 771 \(\pm\) 83 mgO\({}_{2}\)/L (solubilization degree 72.4 \(\pm\) 7.3%) and 560 \(\pm\) 42 mg/L (solubilization degree 77.5 \(\pm\) 8.5%), respectively (Table 4). Ultrasonication is a very popular and effective mechanical pretreatment method used for treating different kinds of biomass prior to AD [44; 45; 46; 47; 48]. This method improves the biodegradability of biomass by disrupting its structure, thus changing its physical, chemical, and biological properties [45; 49]. The primary purpose of the ultrasonic technique is to damage the structure of the cell walls, and then release the available intracellular matter for subsequent biodegradation in AD [50; 51]. The main mechanism of action in ultrasound is cavitation, which is generated by the formation, growth, and violent collapse of bubbles created under excessively large negative pressure in the rarefaction regions [52; 53]. The degree of biomass disintegration is evaluated by the changes in the physical properties (e.g., the particle size of biomass, turbidity, and settleability), chemical composition (increase in soluble organic matter concentration, and in protein, polysaccharide, ammonium nitrogen, and nitrate nitrogen concentrations in the dissolved phase), and biological characteristics (e.g., specific oxygen uptake rate) [49]. The solubilization of organic matter is the main parameter used to evaluate the efficiency of biomass disintegration [45; 54; 55; 56]. However, the obtained results depend on the biomass characteristics and the parameters of the ultrasonic device, such as the power supply, frequency, and sonication time [46; 57]. In this study, in most of the variants, the soluble COD and TOC concentrations increased by more than six times (more than 600%) compared to the variant without UP. The extension of the sonication time to 200 s consequently increased the centration of the soluble COD and TOC. Similarly, the ultrasonic treatment of the excess sludge at 0.5 W/mL for 30 min increased the soluble COD concentration by 690% [58]. [PERSON] et al. [31] studied the ultrasonic disintegration of _Senedesmus_ sp. and _Pinnularia_ sp. microalgae. They found that sonication for 200 s with a 400 W, 24 kHz ultrasonic device increased the soluble COD and TOC, respectively, from 64 \(\pm\) 7 mg O\({}_{2}\)/L to 512 \(\pm\) 31 mg O\({}_{2}\)/L and from 47 \(\pm\) 6 mg/L to 441 \(\pm\) 35 mg/L for _Senedesmus_ sp. and from 59 \(\pm\) 4 mg O\({}_{2}\)/L to 360 \(\pm\) 12 mg O\({}_{2}\)/L and from 50 \(\pm\) 2 mg/L to 263 \(\pm\) 18 mg/L for _Pinnularia_ sp. Lower effects were observed by [PERSON] et al.