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

Abstract

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 complete-mixed 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, 1230 mg/l and 205 mg/l (as SO4) in the various stages respectively. The chemical cost with the integrated process amounts to R2.94/m(3), versus R12.44/m(3) when all the sulphate is removed using the biological stage only. Similarly, the cost for treating magnesium sulphate-rich mine water amounts to R1.92/m(3) for the integrated process, versus R3.11/m(3) for biological treatment only.