MIXED SULFATE-REDUCING BACTERIAL CULTURES FOR BIOPRECIPITATION OF TOXIC METALS - FACTORIAL AND RESPONSE-SURFACE ANALYSIS OF THE EFFECTS OF DILUTION RATE, SULFATE AND SUBSTRATE CONCENTRATION

The effect of process variables on alkalization and removal of typical contaminating toxic metals from a simulated acid leachate by continuo us mixed cultures of sulphate-reducing bacteria was studied. It was sh own that the amount of metal removed and rise in ph both varied with t he amount of sulphate reduction occurring, the residual sulphate conce ntration being the main determinant of final ph. Factorial experiments showed that sulphate reduction was enhanced by increasing the substra te 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 c oncentrations and the flow rate was indicative of a quantitative modif ication of the effect of the former two variables by the latter. The b iomass concentration in the cultures was only affected by the substrat e concentration indicating that the other variables acted by selection for or against sulphate-reducing components of the mixed culture. A r esponse-surface analysis of the yield of sulphate reduction and alkali zation against substrate concentration and dilution rate indicated tha t 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 occurre d in all runs, which indicated that the key variable was the substrate :sulphate stoichiometry and its interaction with the dilution rate. At tention is drawn to the efficiency of the experimental designs employe d for elucidating these factors.