The treatment of a high strength pulp and paper mill effluent for wastewater re-use - II) Biological sulphate removal from effluent with a low COD/sulphate ratio

A two-stage anaerobic/aerobic biological process was investigated for its ability to remove sulphate from a pulp and paper mill effluent which had a low COD/sulphate ratio of 3.3. A bench-scale, continuous flow pilot-plant comprising an upward flow anaerobic sludge blanket (UASB) reactor and a fixed film sulphide oxidising reactor (SOR) was operated over a 3 month period. A simultaneous high-rate conversion of sulphate to sulphide and COD to methane and CO2 was observed in the anaerobic reactor. Sulphide was then oxidised by sulphide oxidising bacteria in the SOR to elemental sulphur. No significant inhibition was observed and the GOD and sulphate removal rates were 66% and 73% respectively for a hydraulic retention time in the UASB of 6 hours and 0.5 hours in the SOR. A granular sludge was maintained in the UASB which was found to be both stable and active at the COD/sulphate ratio investigated.