## 1 Introduction  
Recycling waste is becoming a relevant challenge, combined with an important aspect of the environment. The reuse of organic waste has been supported by the approval of Directive 2018/851/EC of the European Parliament, which regulates waste utilization management by supporting the principles of the circular economy and promoting the minimization of waste production [1, 2]. Anaerobic digestion (AD) is a popular method of stabilizing organic matter such as sewage sludge (SS) and cattle slurry (CS), enabling receive mainly biomethane and fertilizers [3, 4] which leads to the production of methane (CH\({}_{4}\)). Methanogens, mostly microorganisms belonging to the families of _Methanosetaceae_ and _Methanosarcinaceae_, directly participate in CH\({}_{4}\) synthesis [5, 6]. _Methanosarcinaceae_ rely on H\({}_{2}\), CO\({}_{2}\), or acetate to produce CH\({}_{4}\), and they easily adapt to changing environmental conditions [7]. In turn, methanogens of the family _Methanosetaceae_ are resistant to elevated concentrations of acetate [8]. However, the presence of antibiotics and their metabolites can compromise the efficiency of AD [9, 10]. Drugs and their transformation products can inhibit AD by altering the structure of microbial communities and decreasing CH\({}_{4}\) yields. The _mcr_A gene encoding methyl-coenzyme M reductase is specific to methanogens, and it can be used as a functional marker to evaluate the efficiency of AD. The efficiency of the analyzed process can also be indirectly determined based on methanogen counts, mostly microorganisms belonging to the families of _Methanosarcinaceae_ and _Methanosetaceae_[11, 12]. The overuse of antibiotics in human and veterinary medicine, as well as in livestock and crop farming, increases the concentrations of antibiotics and their metabolites in municipal sewage and CS that are discharged to the environment [13]. The stability of these substances in biomass and the environment poses a significant problem. Municipal sewage containing antibiotics is processed in wastewater treatment plants (WWTPs). Pharmaceuticals are accumulated in SS, which becomes a reservoir of antibiotics [14; 15]. In turn, excessive antibiotic use in veterinary medicine promotes the spread of antimicrobials in animal wastes such as CS. The medicated premixes are the main source of contamination with antibiotic residues. Antimicrobials are used both in the treatment and prevention of infectious diseases [16]. Substrates, such as SS and CS, which contain antibiotics and their metabolites, are processed by AD and often reused in agronomy [16; 17]. Although there are European legal limits for contamination of stabilized organic matter before its disposal in the environment (Directive 86/278/EEC), these do not include antibiotics. The presence of antimicrobial substances in SS and CS before stabilization is also not legally controlled. Importantly, due to the growth of the human population and the intensification of animal production, higher production of SS and CS is predicted [2]. Since the worldwide production of antibiotics is still rising [18], the accumulation of antimicrobials in these substrates may reduce the effectiveness of their stabilization. Research has demonstrated that raw wastewater often contains amoxicillin, ciprofloxacin, and metronidazolezole [19], whereas amoxicillin, metronidazolezole, and enrofloxacin have been detected in CS [20]. These antibiotics are widely used in human and veterinary medicine, but they are not always effectively absorbed in the intestines. As a result, 30-90% of the parent compound may be excreted from the body in the unmodified form [21] and transferred to the environment with SS and CS. The presence of antibiotics in these substrates may affect the efficiency of their anaerobic treatment. Worrying is that the scope and consequences of antibiotic pollution have not yet been fully elucidated, especially concerning substrates particularly liable to the accumulation of antimicrobial substances.