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

The effect of shock loads of long chain fatty acids (LCFA) on the activity of granular methanogenic sludge was studied with capric acid as model 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 properties: the acetogenic and methanogenic population is killed virtually completely when the concentration exceeds 6.7 to 9.0 mol/m3 capric acid. The observed variation in the threshold level may be attributable to differences in mass transfer rate from liquid to granules in different experiments. At most, 0.2% of the acetotrophic methanogens survives, when the LCFA concentration in a methanogenic digester exceeds the lethal 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 number of survivors. Acetotrophic methanogens do not adapt to LCFA, neither after repeated exposure to toxic concentrations, nor after prolonged exposure to non-toxic concentrations.' A low LCFA (lipid) concentration in the influent promotes faster growth of acetogenic bacteria capable of degrading LCFA.