## 1 Introduction
59 mg kg\({}^{-1}\) for oxytetracycline, while [PERSON] et al. [22], analyzing the same kind of samples, noted the concentration 90 mg kg\({}^{-1}\) for sulfdaiazine. Antimicrobial substances can affect methanogens and bacteria in various ways. Their direct effects may include the inhibition of DNA replication, RNA transcription or ATP generation. They can also retard cell division, protein translation or synthesis of the cell wall and nucleotides [23,24]. Antibiotics can also affect microorganisms indirectly by influencing the unbalance between microbial groups activities in four steps including hydrolysis, acidogenesis, acetogenesis and methanogenesis. For the carrying out the first three stages of methane fermentation, various groups of bacteria are responsible, which supply the substrates for biogas production to methanogens. Acetic bacteria convert the products like propionate to acetate, glucose to acetate, and ethanol to acetate, which can then be used by methanogenic microorganisms as a substrate. However, methanogens belonging to the _Archaea_ domain are necessary for the success of the AD process and the stable operation of bioreactors. Among the methanogens, due to their ability to convert the substrate, there are acetonitrile and hydrogenotrophic methanogens [25]. These microorganisms are responsible for catalyzing the final and most sensitive stage of the anaerobic process (methanogenesis). The acetonitrile methanogens play an extremely important role in the production of CH\({}_{4}\), since 70% of the methane produced as a product of the process comes from acetate [26]. Antibiotics in wastewater and cattle slurry used as substrates in the AD process can pose serious problems for anaerobic microorganisms. Drugs can reduce the activity of methanogenic microorganisms or alter the microbial populations, influencing pollutant removal and biogas production. The presence of antibiotics during anaerobic treatment may also disturb the balance of the process through the accumulation of metabolic intermediates such as volatile fatty acids (VFAs), which consequently may lead to a complete inhibition of the process [27]. The occurrence of drugs in AD-processed substrates has a direct influence on metabolic functions of both methanogens and bacteria active in the first three stages of the process, which are responsible for supplying methanogenic _Archaea_ with substrates for CH4 production [28]. Many authors have confirmed the negative impact of several antimicrobial substances on the efficiency of AD, with such manifestations as a decreased yield of biogas or its lower CH4 content [29; 30]. As evidenced in a study by [PERSON] et al. [25], the presence of different combinations of antibiotics in a substrate undergoing AD first and foremost inhibits the activity methanogens and acetogenic bacteria. This may be a consequence of changes in the structure of microorganisms in bioreactors due to their exposure to antibacterial compounds, which has a direct effect on biogas production [30]. To the best of our knowledge, no studies have been conducted so far that would analyze the effect of the most common antimicrobial substances used in veterinary and human medicine on the AD process, inclusive of the processing parameters and the microbiological aspect of AD and using a model of substrates of different origin. It is worth noting that the relevant scientific references [31; 32] do not provide data concerning the effect of metronidazoleade (characterized by the direct influence on methanogens) on the efficiency of biogas production. The purpose of this study has been to determine the effect of amoxicillin (AMO), oxytetracycline (OXY), sulfamethoxazole (SMX) and metronidazoleade (MET) on the efficiency of the AD process of sewage sludge (SS) and cattle slurry (CS). The effect of the drugs on the production of biogas and its content of CH4 was determined, while simultaneously analyzing concentrations of VFAs.