INTERNAL PURIFICATION OF THERMOMECHANICAL PULP CLEAR FILTRATE WITH A COMBINED BIOLOGICAL AND MEMBRANE FILTRATION METHOD - A PRELIMINARY-STUDY

In this study a combined anaerobic biological-nanofiltration method wa s studied as a means of internal purification of a thermomechanical pu lp (TMP) plant effluent. A TMP plant clear filtrate was first subjecte d to a thermophilic anaerobic treatment at 55 or 70 degrees C in an up flow 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, S ugars were removed both at 55 and 70 degrees C while the low molar mas s ligneous material was removed only at 55 degrees C, By nanofiltratio n 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 sh own that most of the pulp-brightness-decreasing substances had been re moved. The permeate flux depended on the sample but was at its best [a bout 38 L/(m(2) . h) at 8 bar] for the first hour of filtration for th e sample anaerobically treated at 55 degrees C. The samples did not ca use permanent fouling of the membrane. In this study it was shown that the combined anaerobic biological-nanofiltration method is a competit ive internal purification method for TMP plant clear filtrate resultin g in a very clean water, which could be reused in the water circulatio n system of the plant or in the paper machine white-water system. Howe ver, further studies for optimization of the process are needed.