The effect of heavy metal ions on the performance of a two-phase thermophilic anaerobic digester

A two-phase, thermophilic anaerobic reactor was operated with a starch-based feed and the effect of adding, individually, four metals (copper, nickel, zinc and lead) was examined. In each case, four concentrations (0.5, 1, 2 and 3 mM) were used and the addition was made continuously over a period of 30 hours. Viewing the process overall, the addition of the metals had a marked effect on gas production but there was a rapid recovery when the metal addition was stopped. Zinc and lead had the greatest effect. The presence of metal ions affected both the COD removal efficiency and volatile fatty acid (VFA) production in the acidogenic reactor. Nickel and zinc tended to stimulate the production of propionate and butyrate whilst copper and lead tended to suppress VFA formation. When binding series were examined, zinc gave an anomalous behaviour for which no explanation can be given.