## 3 Results
### Application of the Flocculant to Restore Methanogenesis in Destabilized Bioreactors
Similar changes were observed in the PAM-40 mixtures after 40 days of the experiment. VFA consumption resulted in a pH shift to more neutral values (Table 3). However, only in the PAM-20 samples, the pH reached the optimal values by the end of the experiment. In the PAM-60 mixtures and the control (without PAM), the total VFA concentration increased until the end of the experiment. While the average concentrations of propionate (22.4 and 23.0 mM) and butyrate (70.4 and 73.4 mM) in the control and the PAM-60 samples were similar, acetate concentration in PAM-60 exceeded the value for the control by 17.5 \(\pm\) 1.9 mM. Increased acetate concentration was probably due to a partial restoration of the activity of fermenting, homoacetogenic and syntrophic bacteria or partial degradation of PAM followed by acetate production. Hydrogen ratio in biogas reached a maximum on day 3 of incubation and then was gradually declining till the end of the experiment. Active hydrogen production attributes to active hydrolysis and fermentation in the samples. Coincidently, it reveals that the activity of hydrogenotrophic methanogens was not sufficient or was inhibited. The concentration of total ammonia nitrogen (TAN) produced during digestion did not exceed 1.1 g/L, with the highest values in the samples PAM-20 and PAM-40 (Table 3). Increasing TAN concentration indicated the hydrolysis of proteins and nucleic acids. Lower TAN concentrations in the PAM-60 samples resulted probably from suppression of the microbial community, including the decreased activity of proteolytic bacteria. Another possible reason may be a decreased rate of PAM decomposition under destabilized conditions or at concentrations above 40 g/kg TS. Reduced bioavailability of proteins after PAM addition also might account for lower TAN concentration [17]. The degree of TS degradation determined using the standard methods [16] was 20.1% for the control and 45.3, 38.5, and 36.5% for the mixtures PAM-20, PAM-40, and PAM-60, respectively. The degree of VS degradation varied from 35 to 60.4% depending on the amount of added PAM (Table 3). It should be noted that the products of decomposition of organic matter from the substrates were not converted to biogas completely. Thus, in the control mixtures VS degradation resulted in the formation of VFA, ethanol, and other fermentation products, while no further conversion of these compounds to methane occurred due to suppressed activity of syntrophic bacteria and methanogenic archaea. Methanogenesis was not restored in the mixing samples. In all samples hydrogen was accumulated to 35.9-40.3% 7 days after the PAM addition; by the end of the experiment, its concentration decreased to 12.6-25.5%. The lowest hydrogen concentration in the gas phase was observed for the PAM-20 samples. High hydrogen concentrations in the gas phase indicated destabilization of the AD process; it also suppressed the activity of syntrophic bacteria involved in VFA utilization [20,21]. Thus, VFA accumulated, and their concentration by the end of the experiment was as high as 9.54 \(\pm\) 0.15 g/L. It should be noted that VFA accumulation in mixing samples occurred slower than in non-mixing samples, and VFA concentrations did not reach the maxima observed for digestion without mixing (12.4-13.1 g/L). This was probably due to considerable inhibition of all anaerobic processes, including acidogenesis. The TAN content in all mixtures was 1039-1083 mg N-NH4/l, independent on the amount of PAM added. The degree of VS decomposition in the course of digestion was -40 to 45%. The floccules size and the duration of their existence were shown to be of critical importance for the restoration of methanogenesis.