The effects of cadmium on an anaerobic acetate enrichment system were
determined in both batch (serum bottles) and continuous (chemostat) st
udies. Unlike total soluble or total added cadmium, free cadmium conce
ntrations showed a good correlation to toxicity in batch systems. Free
cadmium concentrations (measured using a combination of dialysis and
ion exchange) of 0.1 mg/l or less caused no more than 10% inhibition o
f methanogenesis and concentration in excess of 0.12 mg/l caused compl
ete failure in serum bottles. Chemostats operated at a 15-d SRT and an
organic loading rate of 500 mg-COD/l-d as acetic acid were spiked wit
h slug doses of 50, 75 and 100 mg/l of cadmium. Ninety percent recover
y of the chemostats was observed when the free cadmium was 0.1 mg/l in
the Cd-inhibited cultures in the chemostats. Based on these studies,
a free Cd level of 0.1 mg/l appears to be a ''safe limit'' for acetate
-utilizing methanogens.