Aims: The aim of this investigation was to develop an empirical model for the autotrophic biodegradation of thiocyanate using an activated sludge reactor. Methods and Results: The methods used for this purpose included the use of a laboratory scale activated sludge reactor unit using thiocyante feed concentrations from 200 to 550 mg l )1 . Reactor ef¯uent concentrations of <1 mg l )1 thiocyanate were consistently achieved for the entire duration of the investigation at a hydraulic retention time of 8 h, solids (biomass) retention of 18 h and biomass (dry weight) concentrations ranging from 2 to 4 g l )1 . A biomass speci®c degradation rate factor was used to relate thiocyanate degradation in the reactor to the prevailing biomass and thiocyanate feed concentrations. A maximum biomass speci®c degradation rate of 16 mg )1 g )1 h )1 (mg thiocyanate consumed per gram biomass per hour) was achieved at a thiocyanate feed concentration of 550 mg l )1 . The overall yield coef®cient was found to be 0á086 (biomass dry weight produced per mass of thiocyanate consumed). Conclusions: Using the results generated by this investigation, an empirical model was developed, based on thiocyanate feed concentration and reactor biomass concentration, to calculate the required absolute hydraulic retention time at which a single-stage continuously stirred tank activated sludge reactor could be operated in order to achieve an ef¯uent concentration of <1 mg l )1 . The use of an empirical model rather than a mechanistic-based kinetic model was proposed due to the low prevailing thiocyanate concentrations in the reactor. Signi®cance and Impact of the Study: These results represent the ®rst empirical model, based on a comprehensive data set, that could be used for the design of thiocyanate-degrading activated sludge systems.

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Agitated bacterial tank bioleaching reactors are currently sparged with air to satisfy both oxygen and CO(2) requirements of microbial cells. Under high-sulphide loading conditions, as is the case with high-grade metal sulphide concentrates, the microbial and chemical demand for oxygen is significantly increased during the bioleaching process. Sparging with enriched oxygen gas may offer an alternative process option to increased agitation and sparged aeration, to overcome the mass transfer difficulties at elevated temperatures where thermophilic Archaea, rather than Bacteria, are used. In the case of air sparging, the DO (dissolved oxygen) concentration in tank reactors could not be increased to a point where it would become inhibitory due to the limited oxygen content of air (20.9% O(2)). The use of enriched oxygen in such reactors at large scale does, however, pose its own set of process risks. The first aim of this investigation was, therefore, to determine the effects of various DO concentrations, in both the limiting and inhibitory ranges, on the microbial activity of Sulfolobus sp. U40813, a typical thermophilic mineral-leaching archaeon. Secondly, the effect of CO(2) concentration on the rate of ferrous iron oxidation was investigated. Both the oxygen and CO(2) kinetics were examined in controlled batch cultures at 78 degrees C, using ferrous sulphate and potassium tetrathionate as energy sources. The optimal DO concentration for iron oxidation was found to be between 1.5 and 4.1 mg.l(-1). The use of elevated DO concentrations (above 4.1 mg.l(-1)) inhibited the ferrous oxidation rates. The optimal gas CO(2) concentration for ferrous iron oxidation was found to be in the range 7-17% (v/v). The iron oxidation rates were, however, severely limited at CO(2) concentrations less than 7%, indicating that the CO(2) supply was limiting in this range and inhibited the microbial growth rate.