## 2 Materials and Methods
### Anaerobic Digestion Process  
The AD of SS and CS was conducted in 2 L bioreactors operating under semi-continuous, dynamic conditions. The initial substrate to inoculum ratio was 0.05 (VS basis). The substrates (CS or SS) fed to the process bioreactors (PB) were supplemented with a combination of antimicrobials. Control substrates without antibiotics were fed to the control bioreactors (CB). The experiments were conducted in two replications. Antibiotics were selected based on the results of our previous studies [19; 20] investigating the effects of individual doses of different antimicrobials on AD. The cited studies demonstrated that in SS supplemented with only one antibiotic, amoxicillin (AMO), ciprofloxacin (CIP), and metronidazoleze (MET) exerted the greatest effect on CH\({}_{4}\) yields and the structure of microbial communities, whereas, in bioreactors filled with CS, the most potent antimicrobials were AMO, enrofloxacin (ENR) and MET. Therefore, in the present study, PB containing SS were supplemented with a mixture of AMO, CIP, and MET, and those containing CS with a mixture of AMO, ENR, and MET in gradually increasing doses (Table 2). The study was divided into experimental series with different antibiotic concentrations: six series in bioreactors containing SS and seven series in bioreactors containing CS. Antibiotic doses were increased after the digester's hydraulic volume had doubled. Each experimental series lasted 45 and 59 days on average, and the entire experiment lasted 268 and 417 days for SS and CS, respectively. The bioreactors were operated at an organic loading rate of 2.8 g vs. L-d-1. Hydraulic retention time (HRT) was 22 and 28 days for SS and CS, respectively. The digesters were equipped with a mechanical stirrer and a feeding and discharge system, and they were connected to an automatic methane potential test system (AMPTS II) (Bioprocess Control, Lund, Sweden) which measured the amount of the produced CH\({}_{4}\). Gas was normalized for standard pressure and temperature (1.01325 bar and 273.2 K). Methane quality was \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **TS \({}^{a}\)**g\({}^{-1}\)**g\({}^{-1}\)**g** (mg) & **VS \({}^{c}\)**g\({}^{-1}\)**g** (mg) & **pH** & **TP \({}^{d}\)**g\({}_{TS}\)\({}^{-1}\)**g** (mg) & **TN \({}^{f}\)**g\({}_{TS}\)\({}^{-1}\)**g** (mg) \\ \hline SS 8 & 52.0 \(\pm\) 11.37 & 39.92 \(\pm\) 9.59 & 6.35 \(\pm\) 0.3 & 2.02 \(\pm\) 0.48 & 4.32 \(\pm\) 1.61 \\ \hline CS \({}^{h}\) & 107.46 \(\pm\) 29.01 & 84.23 \(\pm\) 22.17 & 7.75 \(\pm\) 0.42 & 0.95 \(\pm\) 0.26 & 4.10 \(\pm\) 1.64 \\ \hline Inoculum & 38.8 \(\pm\) 5.2 & 25.2 \(\pm\) 3.8 & 8.1 \(\pm\) 0.5 & 0.9 \(\pm\) 0.4 & 5.5 \(\pm\) 1.9 \\ \hline \hline \end{tabular}  
* TS—total solids; \({}^{b}\) g\({}^{-1}\)—the value of parameter per one gram of digestate samples; \({}^{c}\) VS—volatile solids; \({}^{d}\) TP—total phosphorus; \({}^{e}\) g\({}_{TS}\)\({}^{-1}\)—the value of parameter per one gram of TS; \({}^{f}\) TN—total nitrogen, \({}^{g}\) SS—sewage sludge, \({}^{h}\) CS—attle slurry.  
\end{table} 
Table 1: Characteristics of the substrates and the inoculum used in the AD process. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{6}{c}{**Antibiotic Concentrations (\(\mu\)g mL\({}^{-1}\))**} \\ \hline
**Substrate** & **Series** & **Dose ID** & **AMO** & **CIP** & **MET** & **ENR** \\ \hline \multirow{6}{*}{**SS**} & 1 & D1 & 1 & 0.25 & 0.25 & \\  & 2 & D2 & 2 & 0.5 & 0.5 & \\  & 3 & D3 & 4 & 1 & 1 & X \\  & 4 & D4 & 8 & 4 & 4 & X \\  & 5 & D5 & 16 & 8 & 8 & \\  & 6 & D6 & 36 & 16 & 16 & \\ \hline \multirow{6}{*}{**CS**} & 1 & D1 & 1 & & 0.25 & 0.25 \\  & 2 & D2 & 2 & & 0.5 & 0.5 \\  & 3 & D3 & 2.5 & & 0.75 & 0.75 \\ \cline{1-1}  & 4 & D4 & 5 & X & 1.5 & 1.5 \\ \cline{1-1}  & 5 & D5 & 10 & & 3 & 3 \\ \cline{1-1}  & 6 & D6 & 16 & & 4 & 4 \\ \cline{1-1}  & 7 & D7 & 32 & & 8 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concentrations of antimicrobials (Dose ID) added to SS and CS in successive experimental series.