###### Abstract  
Excessive accumulation of volatile fatty acids (VFA) is one of the major factors destabilizing methanogenic digestion of organic wastes in anaerobic bioreactors. Existing methods of stabilization of this process are mostly expensive and labor-intensive, often requiring removal of a considerable portion of acidified biomass from the bioreactor. We propose a method for methanogenesis restoration in such soured reactors by the addition of a cationic polyacrylamide flocculant (PAM) at 20 mg/g total solids. After flocculant addition, mixing should be minimized to prolong the existence of the floccules formed in the presence of the flocculant. While partial microbial degradation of the polyacrylamide flocculant was observed during the thermophilic anaerobic process, complete PAM mineralization did not occur. Significant inhibition of anaerobic processes, primarily in the activity of syntrophic propionate-oxidizing bacteria, was observed at PAM concentrations above 40 mg/g total solids. anerobic digestion; volatile fatty acids; polyacrylamide flocculant; acidification; methanogenesis; restoration Article  
Application of Polyacrylamide Floculant for Stabilization of Anaerobic Digestion under Conditions of Excessive Accumulation of Volatile Fatty Acids [PERSON]  
1 Federal Research Center \"Fundamentals of Biotechnology\" of the Russian Academy of Sciences, 119071 Moscow, Russia; [PERSON] [EMAIL_ADDRESS] (A.A.N. ); [EMAIL_ADDRESS] (A.A.E. ); [EMAIL_ADDRESS] (A.E.Z.) Federal Scientific Agropineering Center VIM, 109428 Moscow, Russia; [EMAIL_ADDRESS] (A.A.K. ); [EMAIL_ADDRESS] (A.A.K.) Department of Theoretical and Applied Mechanics, Russian University of Transport, 127994 Moscow, Russia, [EMAIL_ADDRESS]  
Correspondence:process. VFA produced by fermenting bacteria are then utilized by syntrophic bacteria and acetoclastic methanogens. The growth rates of both syntrophs and methanogens are lower than that of fermenting organisms. Therefore an excessive accumulation of VFA can suppress the activity of the whole microbial community and destabilize the anaerobic digestion [5,6]. Development of approaches to overcome AD destabilization caused by excessive VFA accumulation is an important task for researchers and technologists. Addition of alkaline reagents is a known approach used to restore methanogenesis in soured bioreactors [10]. Thus, doses of lime, sodium carbonate, sodium hydroxide, ammonium hydroxide, or gaseous ammonia may be added to neutralize the acids produced during fermentation. Application of lime is not desirable, since insoluble CaCO\({}_{3}\) is formed, which is deposited in the pipes and on the reactor walls [6]. While ammonia and its compounds may be used for pH stabilization, they have a toxic effect on microorganisms and result in an additional nitrogen load in the anaerobic bioreactors [11]. Moreover, the addition of one or two portions of alkaline agents is inefficient, since increased pH results in rapid restoration of activity of hydrolytic and acidogenic microorganisms, accompanied by a still greater increase in VFA concentration (up to 14-15 g/L). Methanogenic archaea, with their growth rate and activity lower than those of the hydrolytic and acidogenic microorganisms, cannot cope with converting VFA to biogas [12]. To restore methanogenesis in the soured digester treating the organic fraction of MSW (OFMSW), an alkali and an additional source of the methanogenic microbial community, such as anaerobically digested sewage sludge or soil from the anaerobic zone of a landfill site, were added simultaneously [12]. While this method proved efficient in methanogenesis restoration, it implies a significant dilution of soured digesting mass in the limited reactor volume and requires the utilization of excessive soured biomass, which has a sharp unpleasant odor [12]. Microorganisms thriving as various biofilms are known to be more resistant to unfavorable conditions and biocidal factors than free-living cells.