BACTERICIDAL EFFECT OF LONG-CHAIN FATTY-ACIDS IN ANAEROBIC-DIGESTION

The effect of shock loads of long chain fatty acids (LCFA) on the acti vity of granular methanogenic sludge was studied with capric acid as m odel substrate. The results show that inhibition is primarily related to the LCFA concentration; the LCFA:biomass ratio is less important. A lethal threshold LCFA concentration can be distinguished in reactors with identical physical and chemical conditions and inoculum propertie s: the acetogenic and methanogenic population is killed virtually comp letely when the concentration exceeds 6.7 to 9.0 mol/m3 capric acid. T he observed variation in the threshold level may be attributable to di fferences in mass transfer rate from liquid to granules in different e xperiments. At most, 0.2% of the acetotrophic methanogens survives, wh en the LCFA concentration in a methanogenic digester exceeds the letha l threshold level. Mass balance analysis shows that obligate hydrogen producing acetogenic bacteria and hydrogenotrophic methanogens recover first, followed by acetotrophic methanogens. Recovery of acetotrophic methanogenesis can be described by exponential growth of a small numb er of survivors. Acetotrophic methanogens do not adapt to LCFA, neithe r after repeated exposure to toxic concentrations, nor after prolonged exposure to non-toxic concentrations.' A low LCFA (lipid) concentrati on in the influent promotes faster growth of acetogenic bacteria capab le of degrading LCFA.