EFFECT OF HEAVY-METALS ON VOLATILE FATTY-ACID DEGRADATION IN ANAEROBIC-DIGESTION

The effects of chromium, cadmium, lead, copper, zinc and nickel on volatile fatty acid (VFA) degradation in anaerobic digestion were studied by using serum bottle assays. Acetic acid acclimated seed sludge (AASS) and mixed acids acclimated seed sludge (MASS) were used. The relative toxicity of heavy metals to degradation of acetic acid (HAc), propionic acid (HPr), and n-butyric acid (n-HBu) was Cd > Cu > Cr > Zn > Pb > Ni, Cd > Cu > is approximately equal to Zn is approximately equal to Cr > Pb > Ni, and Cd > Cu > Cr > Zn > Pb > Ni, respectively. Cd and Cu were the most and Pb and Ni were the least toxic heavy metals to VFA-degrading organisms. To some heavy metals, VFA-degrading acetogens were more sensitive than HAc-utilizing methanogens. The sensitivity of the VFA degradation to the metallic inhibition was HPr > HAc is approximately equal to HBu for Cr, HAc > HPr is approximately equal to HBu for Cd and Pb, HPr > HAc > HBu for Zn, HAc is approximately equal to HPr is approximately equal to HBu for Cu, and HAc > HPr > HBu for Ni. Mixtures of the heavy metals caused synergistic inhibition on HAc degradation. The inhibition patterns for MASS of Pb and Zn dosages were kinetically uncompetitive and competitive, respectively.