THE TREATMENT OF A HIGH-STRENGTH PULP AND PAPER-MILL EFFLUENT FOR WASTE-WATER REUSE - II) BIOLOGICAL SULFATE 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 whi ch 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 operate d over a 3 month period. A simultaneous high-rate conversion of sulpha te to sulphide and COD to methane and CO2 was observed in the anaerobi c reactor. Sulphide was then oxidised by sulphide oxidising bacteria i n 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.