The severity of heavy metal inhibition on anaerobic digestion is dependent on the metal species and their dissolved concentration in the digester. The general sequence of inhibition on anaerobic digestion of municipal sewage sludge was found with Ni > Cu > Cd > Cr > Pb. Metal immobilization affinity in the sludge followed the reverse sequence. Due to sulfide production during digestion high quantities of heavy metals are precipitating as highly insoluble sulfide salts.

Nickel was immobilized to 94 % in the digester and indicated the most dramatic effect on anaerobic digestion. At a concentration of 250 to 300 g Ni m-3 toxicity occurred. Lower nickel concentrations resulted in reversible process inhibition. Copper up to 1000 g Cu m-3 caused reversible inhibition of acid producing, fermentative, and methanogenic bacteria. The time necessary for recovery of the process was dependent on the initial copper concentration in the digester. The organisms indicated capability of adaptation to copper. The copper uptake in the digester was 97 %. Cadmium inhibited digestion of sewage sludge up to approximately 50 % at 650 g Cd m-3. For long durations of acclimation a tendency toward recovery was observed. The cadmium uptake in the digested sludge was 99 %. Chromium and lead were uptaken at 99.9 % during digestion. Hence, the addition of these metals up to 1000 mg Cr/l and 600 mg Pb/l showed only little effect on anaerobic digestion.

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