Activated sludge respirometry to assess solar detoxification of a metal finishing effluent.

Inhibition of the respiration of activated sludge has been tested as a convenient method to estimate toxicity of aqueous solutions containing copper and cyanide, such as metal finishing effluents; according to this method, an EC50 of 0.5 mg/l was determined for CN(-) and 3.0 mg/l for copper. Solar detoxification of cyanide-containing solutions was studied using TiO2, but this process was unfavourable because of the inhibitory role that plays the copper ions present in real effluents on the oxidation of cyanide. On the other hand, the oxidative effect of hydrogen peroxide was greatly enhanced by Cu2+ and solar irradiation, as complete elimination of free and complexed cyanide could be accomplished, together with precipitation of copper, in experiments carried out at pilot plant scale with real metal finishing effluents. Under these conditions, total detoxification was achieved according to respirometric measurements although some remaining toxicity was determined by more sensitive Vibrio fischeri luminescent assay.