Inhibition of bioactivity of UASB biogranules by electroplating metals

Among the high-rate anaerobic wastewater treatment processes, the upflow anaerobic sludge blanket (UASB) reactor has attracted most commercial and research interests. In this study, the toxicity of five heavy metals commonly found in the electroplating effluent on the activity of anaerobic biogranules was investigated. Biogranules were sampled from UASB reactors treating four types of wastewaters containing, individually, acetate, propionate, starch and benzoate as the sole substrate. All reactors were operated at 37 degrees C for over six months at a loading rate of 10 g COD(-1)d(-1). The methanogenic activity of biogranules treating each type of wastewater decreased with the increase of metal concentration. The toxicity of each metal to the biogranules was measured by the reduction of methanogenic activity of biogranules in serum vials, as compared to the controls. Results show that, in general, acetate- and propionate-degrading biogranules exhibited higher resistance to electroplating metals than the starch- and benzoate-degrading ones. Acetate conversion is a one-step process conducted by acetotrophic methanogens alone. Propionate degradation, on the other hand, is a two-step process conducted by acetogens and methanogens; whereas, benzoate and starch degradations are multi-step processes conducted jointly by fermentative/acidogenic bacteria, acetogens and methanogens. Results of this study imply that heavy metals from electroplating effluent, in general, inhibit the bioactivities of fermentative/acidogenic bacteria more than those of methanogens and acetogens. For the latter two groups of bacteria, the toxicity of the heavy metals were in the following descending order: zinc > nickel > copper > cadmium > chromium.