## 3 Results
### Improvement in AD Performance though Bioaugmentation with the Syntrophic Enrichment Culture  
The ability of the syntrophic enrichment culture adapted to high VFA concentrations to improve the performance of AD of readily biodegradable waste (dog food) was tested at different ISRs (Table 2).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**VS Removal, \%**} & **Cumulative Methane** & **Average Methane** & **Maximum Methane** \\  & & **Production,** & **Production Rate, mL** & **Production Rate, mL** \\  & & **mL/g VS** & **CH/(g VS Day)** & **CH/(g VS Day)** \\ \hline Bioaugmentation Mixture 1 & & & & \\ (ISR = 50/50) & 34.6 \(\pm\) 1.7 & 178.5 \(\pm\) 8.7 & 5.19 \(\pm\) 0.11 & 24.5 \(\pm\) 0.4 \\ Control 1 & 12.0 \(\pm\) 0.6 & 28.6 \(\pm\) 1.3 & 1.50 \(\pm\) 0.04 & 8.19 \(\pm\) 0.16 \\ Bioaugmentation Mixture 2 & & & & \\ (ISR = 20/80) & 38.1 \(\pm\) 1.8 & 174.0 \(\pm\) 8.7 & 5.11 \(\pm\) 0.11 & 27.7 \(\pm\) 0.5 \\ Control 2 & & & & \\ (ISR = 20/80) & 5.2 \(\pm\) 0.3 & 4.25 \(\pm\) 0.22 & -* & 2.04 \(\pm\) 0.04 \\ \hline \hline \end{tabular}  
* Calculation of average methane production rate was not possible because methanogenesis ceased completely after 4–5 days of incubation.  
\end{table}
Table 2: The performance efficiency of bioaugmentation with enrichment culture to prevent AD failure at different ISRs.  
Figure 4: The phylogenetic tree of archaeal 16S rRNA gene sequence of the representative clones of each OTU and related type strains of the most represented archaeal groups of the enrichment culture. The scale bar represents 5% sequence divergence.  
The enrichment culture exhibited high activity both under-favorable (ISR = 50/50) and unfavorable (ISR = 20/80) conditions. Cumulative methane production in the bioaugmented mixtures after 30 days of incubation was maximum both for favorable and unfavorable ISR, reaching 178.5 \(\pm\) 8.7 and 174.0 \(\pm\) 8.7 mL/g VS, respectively. In the mixtures with ISR of 50/50 the AD process was stable, with the average methane production rate of 5.19 \(\pm\) 0.11 mL CH\({}_{4}\)/(g VS day). The average rate of methane production in the control samples did not exceed 1.50 \(\pm\) 0.04 mL CH\({}_{4}\)/(g VS day), i.e., it was 3.5 times lower than in the bioaugmented samples. In the mixtures with ISR of 20/80, a suppression of methanogenesis was observed on days 3-5. Subsequently, however, the AD process stabilized and attained the average methane production rate of 5.11 \(\pm\) 0.11 mmol CH\({}_{4}\)/(g VS day). In the control samples (ISR = 20/80), methane production stopped on days 3-5 of incubation (Table 2) due to the inhibition of methanogenic community caused by high VFA build-up (~8.0 g/L), represented mainly by butyrate (53.4 mM), acetate (37.8 mM) and ethanol (20.2 mM).  
The maximum concentration of total VFAs was as high as 7.6-7.9 g/L on days 2-3, the main components being butyrate (48.8-52.5 mM), acetate (35.7-38.9 mM) and ethanol (23.1-26.7 mM). In the mixtures supplemented with the enrichment culture, the maximum VFAs concentration was 1.5-2 times higher than in the control, which indicates better substrate decomposition by the microorganisms of the bioaugmented microbial community. This is apparently due to the lack of inhibition by metabolite products, which are rapidly consumed and do not accumulate to inhibitory concentrations. In both bioaugmentation and control mixtures with ISR of 50/50, VFAs were almost completely consumed by the end of incubation (12 mM propionate and 5.5 mM acetate remained). Hydrogen concentration in the headspace did not exceed 0.71 mmol/L for the control and 0.15 mmol/L for the bioaugmentation mixture. While in all control mixtures with ISR of 20/80, H\({}_{2}\) concentrations were high (from 15.86 mmol/L at the onset of incubation to 12.14 mmol/L by its end). Such high H\({}_{2}\) concentrations indicate a severe imbalance in the AD process. Thus, our findings showed that the syntrophic enrichment culture significantly improved AD performance.