## 1 Introduction  
In the last decade, poultry meat consumption in the European Union increased from 20.6 kg per capita/year in 2010 to 23.5 kg per capita/year per person in 2021 [1, 2]. This increase is chiefly reflected in a growing number of producers as well as the increased amount of raw material processed and loads of post-slaughter waste and highly polluted sewage generated [3]. Due to the potential adverse impact on the natural environment, susceptibility to putrefraction, and sanitary risk, their handling is strictly regulated by law [4]. They can be a substrate featuring a vast potential for use in utilization biogas plants. This type of biomass is characterized by a high content of organic compounds susceptible to biodegradation, which directly affects the increase in the volume and improvement in the quality of the biogas produced [5]. Wastes generated in the meat processing plants are by-products posing a low sanitary risk. They are included in three categories of wastes, i.e., animal-derived by-products recognized as suitable for consumption, foodstuffs of animal origin that pose no threat to animal and human health, and withdrawn foodstuffs of animal origin [6]. The rules for dealing with this type of waste are stipulated in the provisions of Regulation (EC) No. 1069/2009 of the European Parliament and of the Council of 21 October 2009 as they laid down sanitary rules for animal by-products not intended for human consumption [7]. One of the acceptable and promoted means of neutralizing this type of waste is its transformation into biogas, fertilizers, and soil improvers [8]. The requirements for its hygienization before the anaerobic digestion process are to keep the material pre-disintegrated to a maximum particle diameter of 60 mm at 70 \({}^{\circ}\)C for 60 minutes, according to the provisions of the Regulation of the European Commission No. 142/2011 of 25 February 2011 on the implementation of Regulation (EC) No. 1069/2009 of the European Parliament and of the Council [9]. The thermal pre-treatment of post-slaughter waste is carried out mainly via hydrothermal depolymerization or thermochemical methods [10]. It allows for the effective hygienization of the processed medium as well as the destruction and release of complex organic molecules into the dissolved phase [11]. Hydrolysates are used in an easier and more accessible way by extracellular enzymes of anaerobic microorganisms, which leads to an improvement in anaerobic degradation efficiency [12]. Multiple methods of heat treatment of post-slaughter waste have been developed and tested so far. They differ in the source and method of heat supply, the range of temperatures used, the retention time during hydrothermal depolymerization, and the methods of chemical catalysis [13]. Waste heat from cogeneration systems was often used for the legally required thermal treatment of category 3 (C3) waste prior to the process of anaerobic digestion [14]. This method of biogas management allows for the production of electricity and significant amounts of low-temperature waste heat that is difficult to manage but can be used to heat post-slaughter waste (60 min, 70 \({}^{\circ}\)C). Currently, other competitive ways of exploiting biogas are increasingly deployed. In many cases, it is more reasonable from the economic and environmental points of view to enrich and refine raw biogas into biomethane, and then to inject it into gas networks or use it directly to power engines of agricultural machines or public transport vehicles [15]. Such alternative avenues of biogas exploitation eliminate waste heat generation in the biogas plant facility, which can be used for the thermal pre-treatment of C3 waste [16]. Therefore, there is a justified need to look for other competitive methods of heating organic substrates which can boost the final effects of methane fermentation. So far, little attention has been paid to the possibility of using ultrasounds (UD) in the combined process of ultrasonic disintegration and hydrothermal depolymerization.