## 3 Results
### Application of the Flocculant to Restore Methanogenesis in Destabilized Bioreactors  
In our previous experiments [16], it was shown that the maximum concentration of PAM (40 g/kg TS) can contribute to the maintenance of methanogenesis in experimental reactors with a high organic load. To confirm the possibility of applying enforced flocculation caused by the PAM flocculant to restore methanogenesis, a model of a destabilized reactor was developed. Digestion was carried out at a lower inoculum-to-substrate ratio (ISR, 30%/70% based on total VS in the digesting mixture), elevated concentration of easily degradable s-OFMSW (45%), lowered moisture content (~92%) and high VS concentration in the mixture (40 g/kg). On days 3-6 of the experiment, evidence of destabilization of AD due to substrate overloading was observed. During the first few days, significant amounts of hydrogen were accumulated (up to 17.2 \(\pm\) 4.7% of the gas phase volume). VFA, especially acetate, were produced and accumulated; acetate concentration increased 4.2-6.0 times over three days. The overall VFA concentration remained relatively low, 3.42 \(\pm\) 0.05 g/L. In all samples, pH decreased from 7.2 at the beginning of the experiment to 5.8 \(\pm\) 0.3 after 3-6 days. A combination of these factors resulted in complete suppression of methane production by day 6 of the experiment. On day 7, the PAM solution was added (the control samples were supplemented with the same volume of distilled water). The humidity of the mixture increased from 92.1 \(\pm\) 0.2% to 93.9 \(\pm\) 0.1%. The experimental samples were then subdivided into two groups so that digestion was carried out either without or with mixing (shaking at 100 rpm). Positive dynamics of methane accumulation and a decrease in the VFA concentration were observed only in the non-mixing samples. The samples treated with the PAM at 20 mg/g TS exhibited restoration of methanogenesis after acidification-caused destabilization after 4-5 days. While insignificant positive changes were observed in PAM-40 samples 8 days after the flocculant treatment, a significant shift occurred only after 45 days. Dynamics of methane accumulation in non-mixing samples and dynamics of VFA and hydrogen accumulation and consumption are shown in Figure 2a,b and Figure 3, respectively. The process characteristics are presented in Table 3.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**The Concentration of**}} & \multirow{2}{*}{**VFA Concentration, g/L**} & \multicolumn{2}{c}{**Ammonium Concentration,**} & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**Methane**} & **VS Decomposition**, \\  & & & & **mg/L** & & **VSinitial** & **\%** \\ \cline{2-7} \cline{6-7} \cl  
Figure 3: Dynamics of volatile fatty acids (VFA) accumulation and consumption (overall concentrations of acetate, propionate, and butyrate, g/L): Control (polyacrylamideamide (PAM)-0) without the flocculant; PAM-20, PAM-40, and PAM-60, the flocculant concentrations 20, 40, and 60 mg/g total solids (TS), respectively. Time of the flocculant addition is indicated by the arrow. Figure 2: Dynamics of methane accumulation (ml/g VS initial) (**a**) and hydrogen content (%) (**b**) in biogas in bioreactors destabilized due to accumulation of high volatile fatty acid (VFA) concentrations operated without mixing after addition of different polyacrylamide (PAM) concentrations for the restoration of methanogenesis: Control (PAM-0) without the flocculant; PAM-20, PAM-40, and PAM-60, the flocculant concentrations 20, 40, and 60 mg/g TS, respectively. The X-axis begins (0 days) on day 7 of the experiment when the flocculant was added. A correlation between the time of methanogenesis activation and the beginning of a decrease in the VFA concentration may be seen in Figures 2 and 3. In the PAM-20 mixtures, the VFA concentration peaked and began to decrease 5-7 days after the flocculant addition, which corresponded to the onset of methane accumulation in the gas phase.