Experiments on finding the role of sulfate reducing bacteria in an anaerobic fluidized bed reactor were conducted with the substrates of a refined soybean protein and amino acids. Soybean protein consists of 60% of the soluble part and 40% of the suspended particle part. The soluble part of the protein was rapidly decomposed into organic acids in the reactor with the hydraulic retention time of 25 min. The organic acids were converted into acetic acid with reduction of sulfate, which meant the role of sulfate reducing bacteria was to convert certain low molecular organic acids into acetic acid. If the sulfate concentration in the influent is kept at the optimum ratio to the concentration of the protein, acetic acid occupies 44 to 76% of total organic acids in the effluent. A mixed substrate of 14 amino acids was also used for the influent of the reactor and showed high decomposition of the amino acids ranging from 23 to 95% decomposition for each amino acid. The reduction of sulfate and production of acetic acid were also observed. When single amino acid of 10 kinds was used for the substrate, decomposition and sulfate reduction were poor for most of the experiments. The stickland reactions of combinations of glycine-alanine, glycine-valine, glycine-leucine and arginine-leucine were investigated in the reactor. Compared to the experiments of single amino acid substrates of 10 kinds, the decomposition of all amio acids were more poor in the reactor.
