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 degrees C in an upflow anaerobic sludge blanket reactor and then nanofiltered in a flat-sheet laboratory-scale module. The anaerobic treatment re moved 55% of the chemical oxygen demand at 70 degrees C and 65% at 55 degrees C, Sugars were removed both at 55 and 70 degrees C while the low molar mass ligneous material was removed only at 55 degrees C, By nanofiltration the remaining low molar mass ligneous material was removed by about 98-99% and the high molar mass ligneous material by 95-99%. Sugar was removed by 88-98% and chemical oxygen demand by 78-81%, 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(2) . h) at 8 bar] for the first hour of filtration for the sample anaerobically treated at 55 degrees 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 circulation system of the plant or in the paper machine white-water system. However, further studies for optimization of the process are needed.