## 1 Introduction  
The beginning of the development of biogas technologies, which continues to this day, dates back to the 1930s when the basic principles of the process were elaborated and the first digestion tanks appeared. After the Second World War, the biogas achievements of Germany that pioneered the \"biogas evolution\" spread throughout the world, and biogas became an object of global interest, particularly due to the need to search for energy from renewable sources [1]. Recent years have seen a steadily growing interest in innovative and environmentally-friendly technologies aimed at a reduction in the use of fossil fuels and the prevention of environmental pollution, and therefore further development of biogas technologies [2, 3]. Lignocellulosic biomass is the most abundant renewable feedstock in nature [4]. Lignocellulosic material, due to the origin of the raw material, can be divided into four groups: forest resources, biodegradable municipal waste, paper waste, and residue of energy crops [5]. Chicken manure, which is often one of the largest organic waste streams, is treated as lignocellulosic biomass due to its high straw content of over 60%. The production of chicken manure worldwide is estimated at 20,708 Mt per annum, which often raises environmental concerns while potentially providing a readily available feedstock for biogas production. In theory, using all of the chicken manure resources can generate 40 \(\times\) 10\({}^{14}\) kJ/year of energy [6]. The lignocellulosic complex comprises three polymers, i.e., crystalline cellulose, amorphous hemicellulose, and highly branched lignin. These three components form a strongly intertwined, stabilised compound which makes lignocellulosic biomass difficult to dissolve in water and resistant to direct hydrolysis with the formation of monosaccharides [7, 8, 9, 10]. Therefore, the use of lignocellulosic biomass for biofuel production requires a minimum of three stages: the first stage is the pretreatment aimed at increasing the release of cellulosic polymers from the cross-linked biomass structure, the second stage involves the hydrolysis of pretreated biomass into monosaccharides, and the third stage involves digestion or rehydration of monosaccharides to biorefinery products [7,11]. The pretreatment mentioned above is of key importance to ensure a high yield of monosaccharides from polysaccharides of the feedstock under treatment. Hydrolysis omitting the pretreatment stage achieves an efficiency of less than 20%, whereas applying the pretreatment process increases its efficiency to 90% [5]. Effective pretreatment should satisfy several criteria, e.g., ensure the separation of lignin from cellulose, increase the proportion of amorphous cellulose, increase substrate porosity, eliminate loss of sugars, reduce inhibitor formation, and minimise energy costs. In order to increase the efficiency of energy production from biomass, several physical, physicochemical, chemical, and biological treatments are carried out. An important issue is establishing technological conditions that enable an efficient, economically viable, and safe substrate preparation process [9,12-14]. This study compared two innovative biomass pretreatment methods preceding the anaerobic digestion process, i.e., ultrasonic pretreatment (UP) and pulsed electric field (PEF). The research described in the article was aimed at testing whether the analysed methods improve the efficiency of methane fermentation of lignocellulosic substrates. Although the application of UP for biomass conditioning is a relatively new solution, the literature reports indicate the impact of the compression and cavitation effects on delginification, the reduction in biomass particle size, and the reduction in the degree of biomass polymerisation [7,15,16]. UP has been successfully applied for the pretreatment of agricultural waste [7,15,17]. PEF is a non-thermal method which uses electrical pulses to destroy plant structures by creating pores in the cell membrane.