## 1 Introduction
However, there have only been few reports on the AD of microalgal-bacterial granular sludge (M-BGS). Due to the different characteristics of M-BGS, there is a need to assess the suitability and technological effectiveness of the anaerobic treatment devices that treat sewage sludge for M-BGS digestion. Certainly, the structure, size, and biochemical characteristics of M-BGS can determine the performance of AD. One of the important factors affecting AD and the efficiency of biogas and methane production is the amount of easy available organic substances dissolved in the feedstock. Efficient hydrolysis and the rapid transfer of organic matter from the solid phase to the dissolved phase affect the rates of both acidogenesis and methanogenesis. When dispersed activated sludge or microalgae biomass is a feedstock in AD, the hydrolysis process takes place efficiently, consequently enhancing the anaerobic processes. In turn, the M-BGS has a greater hydrophobicity, density, compact granule structure, and EPS content, and there is a significant share of filamentous bacteria in the granule structure, which significantly reduce its susceptibility to anaerobic biodegradation and the rate of digestion. Thus, there is a need for studying the effective pretreatment of M-BGS prior to anaerobic digestion. Pretreatment solutions before anaerobic digestion are dynamically developing technologies. They cause the destruction of the substrate structure, including the fragmentation of flocs, damage to microbial cells, and the release of organic matter and extracellular polymers into the dissolved phase [37]. Various pretreatment methods are used, including mechanical, high-pressure, cavitation, biological, chemical, such as acidification, alkalization, oxidation, ozonation, and thermal methods, such as heat treatment and freeze-thaw. Combined, hybrid pre-treatment technologies are also used. The tested pretreatment methods in most cases produce satisfactory technological results related to the efficiency of the fermentation process. A significant increase in the biogas production efficiency, methane, and/or hydrogen content is obtained, and a higher degree of mineralization and decomposition of organic matter is observed [38]. So far, little attention has been paid to the possibility of using ultrasound (UD) in the combined process of ultrasonic disintegration and hydrothermal depolymerization. In systems exposed to UD, stresses appear that can cause numerous modifications in the structures of molecules. They can change the charge of the cell surface, the permeability of the cell membrane, and also promote cell rupture, disintegration, and fragmentation [38]. The phenomenon of cavitation is the formation of pulsating vacuum bubbles in a liquid, sometimes filled with saturated water vapor or gas dissolved in this liquid [39]. Vesicles oscillating around the cell membrane cause it to vibrate, which leads to stream movements in the biomass structure. As a result of these processes, the cell is exposed to shear stresses that can cause its damage, disintegration, and the transfer of organic matter to the dissolved phase [40]. The aim of this study was to determine the effect of an ultrasonic pretreatment (UP) of M-BGS on enhancing AD. In the study, the solubilization of organic matter was monitored, as well as the yields of the methane and biogas production. An energy gain assessment of the AD of the M-BGS for different sonication conditions was also performed.