Anaerobic treatment of sulfate - rich (COD/SO4 ratio 1.4-2.1) clarified Whitewater from a thermomechanical pulping (TMP) process was studied in three laboratory-scale upflow anaerobic sludge blanket (UASB) reactors at 55°C and in batch digesters at 55° and 65°C. Different seed materials were used in the UASB reactors.

The highest COD removal efficiency (effluent sulfide stripped) achieved was approximately 65 % in the UASB reactors. About 55 % COD removal efficiency was obtained at a loading rate of about 41 kgCODm−3d−1 in the UASB reactor seeded with thermophilic sludge cultivated with volatile fatty acids (VFAs). The total sulfide present in the liquid phase after anaerobic treatment accounted for approximately 65-78 % and 15-61 % of the removed COD in the batch digesters and the UASB reactors, respectively. Sulfate reduction was almost complete in the batch digesters, whereas about 24-64 % of sulfate was reduced in the UASB reactors. Acetate utilization for sulfate reduction was apparent in the batch digesters, whereas that could not be demonstrated in the UASB reactors. Sulfate reduction in the UASB reactors was obviously substrate limited.

In conclusion, thermophilic anaerobic treatment is an alternative for the treatment of warm sulfate rich TMP process water.

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