Treatment of acid and sulphate-rich effluents in an integrated biological/chemical process

A novel chemical/biological process is described in which sulphate and sulphide are removed simultaneously during biological treatment. Partial sulphate removal is achieved during chemical pre-treatment. In the biological stage sulphate is reduced to sulphide in a complete-mixed reactor through addition of sucrose or ethanol as a carbon and energy source. Sulphide is oxidised by allowing oxygen to enter the system in a controlled way. The experimental investigation of the process showed that sulphate and sulphide could be removed simultaneously due to co-existence of sulphate-reducing bacteria and sulphur oxidising bacteria. The volumetric sulphate reduction rate in a complete-mixed reactor, with sucrose as an organic carbon and energy source, amounts to 12.4 g SO4/(ℓ.d). The rate of biological sulphate removal was found to be directly related to the square root of sulphate, COD and VSS concentrations respectively, and inversely proportional to sulphide concentration. The practical value of simultaneous sulphate and sulphide removal is that only one stage is required for removal of both sulphate and sulphide; a conventional completemixed reactor can be used; and sulphate can be removed in a consistent way to below 200 mg/ℓ (as SO4) due to the stability of the process. By combining the biological stage with CaCO3- neutralisation and/or lime pre-treatment, the chemical cost can be reduced. Sulphate, associated with the over-saturated fraction after treatment with CaCO3 or lime, can be removed through gypsum crystallisation. In the integrated sulphate removal process (CaCO3- neutralisation, lime treatment and biological stages), sulphate can be removed from 9 200 mg/ℓ (typical sulphate concentration of coal discard leachate) to 2410 mg/ℓ, 1 230 mg/ℓ and 205 mg/ℓ (as SO4) in the various stages respectively. The chemical cost with the integrated process amounts to R2.94/m3, versus R12.44/m3 when all the sulphate is removed using the biological stage only. Similarly, the cost for treating magnesium sulphaterich mine water amounts to R1.92/m3 for the integrated process, versus R3.11/m3 for biological treatment only. WaterSA Vol.30 (2) 2004: 183-18

Improved sulphate removal rates at increased sulphide concentration in the sulphidogenic bioreactor

The product of the biological sulphate reduction is sulphide. High concentrations of molecular H2S (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 batch-test 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 gSO4/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 gSO4/ℓ·d and 1.5 gSO4/gVSS, respectively. Thirdly, operating a tall column reactor using H2 and CO2 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. Water SA Vol.31 (3) 2005: pp.351-35