Toxic effects of cadmium on methanogenic systems

The effects of cadmium on an anaerobic acetate enrichment system were determined in both batch (serum bottles) and continuous (chemostat) studies. Unlike total soluble or total added cadmium, free cadmium concentrations 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 of methanogenesis and concentration in excess of 0.12 mg/l caused complete 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 with slug doses of 50, 75 and 100 mg/l of cadmium. Ninety percent recovery 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.