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In this study a combined anaerobic biological-nanofiltration method was studied as a means of internal purification of a thermomechanical pulp (TMP) plant effluent. A TMP plant clear filtrate was first subjected to a thermophilic anaerobic treatment at 55 or 70°C in an upflow anaerobic sludge blanket reactor and then nanofiltered in a flat-sheet laboratory-scale module. The anaerobic treatment removed 55% of the chemical oxygen demand at 70°C and 65% at 55°C. Sugars were removed both at 55 and 70°C while the low molar mass ligneous material was removed only at 55°C. By nanofiltration the remaining low molar mass ligneous material was removed by about 98-99% and the high molar mass ligneous material by 96-99%. Sugar was removed by 88-98% and chemical oxygen demand by 78431%. It was also shown that most of the pulp-brightness-decreasing substances had been removed. The permeate flux depended on the sample but was at its best [about 38 L/(m*.h) at 8 bar] for the first hour of filtration for the sample anaerobically treated at 55°C. The samples did not cause permanent fouling of the membrane. In this study it was shown that the combined anaerobic biological-nanofiltration method is a competitive internal purification method for TMP plant clear filtrate resulting in a very clean water, which could be reused in the water circula-