BLEACHING ENZYMES WORLD PULP&PAPER 50 dioxide used in the conventional bleach sequence. At high dosages of xylanase, there can be a small yield loss. It is interesting to note that the maximum brightness is achieved at about the same xylanase dosage where the yield is equivalent to the conventionally treated pulp. Brightness gain was not observed at higher xylanase dosages but yield and COD release continue to be adversely impacted at higher dosages. There is also concern about the impact of the removal of xylans with regards to final paper properties. There are many theories and hypotheses as to what the true impact is by removing xylan, some of which are contradictory. This is outside the scope of this paper; however, multiple mill trials have shown that the action of enzymes prior to bleaching has had no significant impact on paper properties. A sample of beater curves, with and without enzymatic pre-treatment is presented in Figure 2. Time and time again, the results are essentially the same. When the enzyme is applied at recommended levels, no significant impact upon paper properties was reported. End user characteristics in graphics papers have also been studied, and no ill effects have been found from paper produced from enzymatically treated pulp. Another concern with the use of enzymes is the release of additional BOD/COD to the wastewater treatment plant. Enzymes do increase the release of lignin and xylan from brownstock (although the ratio is pH dependent). In mills where there is a prebleached washing stage between the brown High Density storage (the typical site of prebleaching enzyme addition), this organic loading is sent back to the recovery system and increases energy content of the black liquor. The net impact in these particular systems is a reduction in BOD/COD from the bleach plant to the wastewater treatment system. In principle, BOD is converted into usable Btus in the recovery cycle. One mill configured this way has seen a 14% reduction in effluent BOD where another mill has reduced COD loading to the wastewater treatment plant by about 10%. In cases where there is no prebleached washing, enzyme activity will cause an increase in BOD/COD loading; however, it should be pointed out that the increase is in the form of simple sugars and related organic acids. These materials are much easier to digest than the chlorinated phenolic compounds created in conventional bleaching. A study of the impact of filtrates from a xylanase treated pulp on biological oxygen uptake was conducted in the laboratory. The results indicated a 17% increase in respiration, an indicator of bioavailability, as is shown in Figure 3. In another study BOD and COD levels in the bleaching filtrates were compared against a baseline. Increasing xylanase dosage increases both BOD and COD loading; however, it takes a high dosage of xylanase to exceed the BOD loading equivalent to a conventionally bleached pulp. It is interesting to note that the increase in BOD loading was much more gradual than that of COD loading with an increase in xylanase dosage – a consequence of high lignin release relative to xylan release. The results of this testing are shown in Figure 4. maximum brightness is achieved at about the same xylanase dosage where the yield is equivalent to the conventionally treated pulp