The product of the biological sulphate reduction is sulphide. High concentrations of molecular H 2 S (g) can be inhibitory for microbial activity, especially at a reactor pH of 6 to 7. This paper focuses on the effect of high sulphide concentrations on the sulphate reduction rates. The results of three investigations operating a continuous reactor, a column reactor and batchtest reactors have shown that increased sulphide concentrations have resulted in improved biological sulphate reduction. In all instances the reactor pH was kept at 7.5 to 8.5. It was shown that when the sulphide concentration was 700 mg/ℓ in a continuously operated reactor, the sulphate reduction rate was 12 gSO 4 /ℓ•d. When operating batch-test reactors the results showed that when the sulphide concentration increased, to 1 400 mg/ℓ, the volumetric and specific sulphate reduction rates correspondingly increased to 4.9 gSO 4 /ℓ•d and 1.5 gSO 4 /gVSS, respectively. Thirdly, operating a tall column reactor using H 2 and CO 2 as the energy source, showed that when the initial sulphide concentration of the feed water was 0, 100 and 268 mg/ℓ, the average biological sulphate removals were 650, 1 275 and 1 475 mg/ℓ, respectively. These obtained results indicated that the addition of sulphide to the feed water to the reactor had a positive effect on sulphate removal. Improved sulphate removal results in increased alkalinity production and in an increased reactor pH, which in turn is favourable for a decrease in the redox potential, when a dominant redox couple, like sulphate: sulphide, is present in a reactor.

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