The effect of process variables on alkalization and removal of typical contaminating toxic metals from a simulated acid leachate by continuous mixed cultures of sulphate-reducing bacteria was studied. It was shown that the amount of metal removed and rise in pH both varied with the amount of sulphate reduction occurring, the residual sulphate concentration being the main determinant of final pH. Factorial experiments showed that sulphate reduction was enhanced by increasing the substrate concentration and inhibited by the initial sulphate concentration. The dilution rate did not exert a primary effect, but the existence of a significant interactive effect between the substrate and sulphate concentrations and the flow rate was indicative of a quantitative modification of the effect of the former two variables by the latter. The biomass concentration in the cultures was only affected by the substrate concentration indicating that the other variables acted by selection for or against sulphate-reducing components of the mixed culture. A response-surface analysis of the yield of sulphate reduction and alkalization against substrate concentration and dilution rate indicated that sulphate reduction (and alkalization) was sensitive to both of these variables where the substrate: sulphate stoichiometry was in the range 1: 1-3: 1. At lower sulphate concentrations complete reduction occurred at all levels while at higher sulphate concentrations washout occurred in all runs, which indicated that the key variable was the substrate:sulphate stoichiometry and its interaction with the dilution rate. Attention is drawn to the efficiency of the experimental designs employed for elucidating these factors.


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