## 4 Conclusions  
Exposure of SS and CS to the antimicrobials chosen for this study resulted in differences in the composition of microbiota engaged in the AD process. It was observed that antimicrobial substances produced different effects on this process. The presence of drugs in substrates undergoing AD may manifest itself through the accumulation of particular VFAs in bioreactors, diminished amounts of generated biogas or lower CH\({}_{4}\) content of biogas. Drugs and products of their transformations present in substrates can also have an adverse influence on changes in the composition of microbiota that are characteristic of a given substrate, which was observed in this study. The microorganisms present in  
Figure 4: Heatmap with the logged concentrations (number of copies in g\({}_{\text{D}}\)\({}^{-1}\)) of _MST_, _MSC_ and _mcrA_ genes in process and control bioreactors with sewage sludge (SS) and cattle slurry (CS). (SSC) control samples of sewage sludge, (CSC) control samples of cattle slurry.  
digestate samples differed in their sensitivity to the particular antimicrobial substances, and the prevalence of one of the two analyzed families from the domain _Archaea_ in SS or CS (_Methanosarcinaceae_ and _Methanos eataceae_, respectively) could have resulted in the different CH\({}_{4}\) production potential determined for the two tested substrates.  
The above results confirm the assumption that the presence of medicinal substances in SS and CS may hinder the degradation of these substrates in the AD process. The research also confirm the claim that antimicrobial substances have a substantial influence on the efficiency of AD, in a manner dependent on the concentration of these drugs in substrate and on the type of substrate. Despite exposure of SS and CS digestate samples to the same concentration of a given antimicrobials, changes in the number of copies of genes that are characteristic of _Archaea_ as well as changes in AD parameters followed different trends.  
The results obtained in the course of this study are not sufficient to clearly identify the degree to which the tested drugs affect the selected part of the methanogenic community of microorganisms in SS or CS substrates. It is essential to pursue this research further, including such aspects as an assessment of abundance and activity of other groups of microorganisms engaged in this process, as well as analyses of changes in the number of the _mcrA_ gene transcript. Studies based exclusively on the presence and abundance of certain groups of methanogens are insufficient to monitor their activity in substrates undergoing AD. Compared to an analysis of the number of the _mcrA_ gene copies, expression of the gene might be a more accurate instrument for determination of the activity of methanogens, as it provides a better insight into the dynamics of microbial metabolism during an AD process.  
The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/1/369/s1, Table S1: Oligonucleotide primers and PCR reaction profile, and Figure S1: Average values of methane production in bioreactors during anaerobic digestion of substrates such as (A) sewage sludge (SS) with MET, AMO, OXY, SMX supplementation, and control reactor (SSC), and (B) cattle slurry (CS) with MET, AMO, OXY, SMX supplementation, and control reactor (CSC).  
Conceptualization, M.H., E.K., I.K. and M.C.; methodology, M.H.; formal analysis, M.H., I.K. and M.C.; investigation, I.K. and M.C.; resources, M.H.; data curation, I.K. and M.C.; writing--original draft preparation, I.K. and M.C.; writing--review and editing, M.H. and E.K.; visualization, M.C.; supervision, M.H. and E.K.; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the NATIONAL SCIENCE CENTER, grant number 2016/23/B/NZ9/03669.  
Data Availability Statement: Data sharing not applicable.  
The authors declare no conflict of interest.