These results which suggest a difference in tolerance levels between archaea and bacteria are supported by the observation that the adverse effect of ERY and ST on MPA was more obvious when using ethanol as a substrate than when acetate was used. This is considered to be because the ethanol can be metabolized to acetate or hydrogen and carbon dioxide by SRB, and then hydrogen- producing acetogenic bacteria (HAB) or MPA can utilize them to cause methanogenesis. Thus, the slow down in the rate of ethanol metabolism was due to inhibition of the HAB by the SRB. On the other hand, CLX and TC did not exhibit an adverse effect on bacteria according to Fig. 2a and b. In this case, ethanol metabolism and methanogenesis appeared not to be inhibited by the antimicrobials. The reaction equations are as follows: