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Theoretical and experimental studies have been undertaken to assess the-potential value of sulphide precipitation for removing heavy metals from sewage. The behaviour of cadmium, chromium, copper, lead, nickel, zinc and mercury has been examined. Thermodynamic calculations predict that metal sulphides are likely to precipitate in an upward flow anaerobic sludge blanket (UASB) reactor, and that the equilibrium solubilities will be very low. The experimental investigation was carried out on a pilot-scale UASB reactor treating settled sewage of mainly domestic origin, with and without metal "spikes". The results show that the effectiveness of anaerobic treatment for removing the metals depends markedly on their physico-chemical state. Metals in particulate form appear to pass through the reactor with no change in concentration, unless they are only loosely bound to the sewage solids. However, very high removal efficiencies can be attained for dissolved and weakly-bound metals.

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