## 3 Results and Discussion  
### The Effect of Antimicrobials on Biogas Yield  
The methane fermentation of SS led to a statistically significant decrease in the efficiency of anaerobic processing after the substrate had been added AMO, OXY and MET. Compared to the control, MET most strongly inhibited the AD process, which resulted in a nearly six-fold decline in the biogas yield and a decrease in its content of CH4 by over 50%. The concentration of seven out of nine analyzed VFAs was significantly higher in a sample of the digestate from SS exposed to MET than in the control (Table 2; Supplementary Materials Figure S1). The highest concentration was recorded for acetic acid. The SS digestate sample exposed to the presence of AMO was shown to have accumulated a considerable amount of purity acid. Another study [38] analyzing the effect of antimicrobial substances on sewage sludge AD proved that MET, AMO, OXY and SMX in low doses (8, 16, 8, and 8 mg kg\({}^{-1}\), respectively) did not have a significant effect on the efficiency of this process. The research results clearly indicate that the efficiency of the AD processing of sewage sludge is significantly decreased in the presence of high concentrations of antimicrobial substances. Drugs which enter WWTPs in high quantities, having been, for example, excreted by people [39], can induce such consequences as a lower efficiency or complete inhibition of SS methane fermentation. Antimicrobial substances are also present in by-products from animal farms, most probably due to their administration in treatment of animals. The presence of antibiotics can lower the rate or retard the conversion of such substrates by microorganisms engaged in the AD process [40]. The conducted research showed statistically significant decreased biogas production in the presence of all the four microbial substances when CS was used as a substrate. The most severe inhibition was due to the exposure to amoxicillin (AMO) and oxytetracycline (OXY). The presence of these microbial substances in CS resulted in a nearly four-fold and over two-fold decrease in biogas production due to the presence of AMO and OXY, respectively (Table 2; Supplementary Materials Figure S1). Moreover, the introduction of AMO to CS caused a statistically significant decrease in the biogas content of CH4, which declined by nearly 10%. Additionally, exposure to AMO caused significant accumulation of seven out of nine VFAs, indicating the inhibition of AD, and this was unobserved in any other bioreactor (Table 2). Volatile fatty acids are the major products of protein and carbohydrate synthesis in the AD process [41; 42], and their accumulation can implicate the inhibition of the activity of acetogenic microorganisms [43; 44]. The current results are convergent with the ones reported by [PERSON] et al. [45], who noted that higher AMO concentrations in sewage from a piggery submitted to AD, particularly when combined with the presence of other antibiotics, caused a decrease in the rate of CH4 production. Other researchers [46] noticed that AMO and OXY only slightly decreased the efficiency of the AD processing of swine slurry, although the concentrations of antimicrobial substances used in the cited experiment were several-fold lower than in the study reported in this paper (60-120 and 125-250 mg L\({}^{-1}\), respectively). Thus, our results demonstrate that higher concentrations of an antibiotic in a substrate of animal origin can substantially alter the efficiency of anaerobic processing, which is an extremely important finding in lightof the currently widespread application of antimicrobial compounds in both human and veterinary medicine. Concentrations of pharmaceuticals in a substrate in fermentation tanks have a direct influence on the efficiency of CH4 production. As demonstrated by [PERSON] et al. [47], yield of biogas is negatively correlated with an increase in the concentration of an antibiotic in a substrate undergoing AD processing.