We studied biogas fermentation from alcohol waste fluid to evaluate the anaerobic digestion process and the production of vitamin B-12 as a byproduct. Anaerobic digestion using acclimated methanogens was performed using the continuously stirred tank reactor (CSTR) and fixed-bed reactor packed with rock wool as carrier material at 55degreesC. We also studied the effects of metal ions added to the culture broth on methane and vitamin B-12, formation. Vitamin B-12 production was 2.92 mg/L in the broth of the fixed-bed reactor, twice that of the CSTR. The optimum concentrations of trace metal ions added to the culture liquid for methane and vitamin B-12 production were 1.0 and 8 mL/L for the CSTR and fixed-bed reactor, respectively. Furthermore, an effective method for extracting and purifying vitamin B-12 from digested fluid was developed.