organic halides (AOX). The less than stellar performance of the first generations of bleaching enzyme technologies has left many in the industry understandably wary of attempting to adopt this technology once again. Nonetheless, mills are increasingly venturing back into this arena with the new technology, and the results are promising. Several have commercialized this technology and are using it on a full-time basis. The rest of this article will address some of the typical concerns mills have when they are considering this technology. The main concerns include yield loss, adverse impacts to the wastewater treatment plants, and final pulp properties. It makes sense to think that if the enzymes selectively act upon xylans, the potential exists to solubilise material and thereby cause a loss of yield through the bleach plant. All bleaching reduces yield in an absolute sense but we must look deeper into the relative change in yield when an enzymatically treated pulp is compared to one bleached with conventional methods. One has to remember that bleaching, by nature, results in a yield loss. Chlorine dioxide, while selective, still consumes not only the surface xylan but some of the cellulose as well. One must look at the selectivity of enzymes versus the selectivity of chlorine dioxide in total. Figure 1 presents the results of a meticulous lab study in which the post-bleaching yields of a pulp after pre-treatment with different dosages of a xylanase are compared against those of a conventionally bleached pulp. The xylanase pre-treated pulps are bleached with 80% of the chlorine WORLD PULP&PAPER 49 This required extensive acidification at the end of the brownstock washers to achieve the required 6.5-7.5 pH range. The acidification of this pulp proved to be problematic as residual lignin, not washed out in the system, would reprecipitate causing more issues than could be overcome with the enzymes. There were also significant safety issues as hydrogen sulfide gas could be released during the acidification process. Some mills had limited success, but for the most part, the concept of bleaching enzymes was abandoned. For the mills that could implement the process, there were issues with substantial yield loss through the bleach plant. The issue was with the quality of the enzymes used as many of the commercially available products contained an unintended yet significant amount of cellulases which caused breakdown of the cellulose along with the hemicelluloses. In the past few years, the technology of enzyme discovery, isolation and production has changed to the point where the new generation of materials warrant a closer look. Through careful manipulation of the biology producing the enzymes, not only have we been able to produce enzymes that perform in a wider pH regime, but we also have been able to remove residual cellulases and their undesired side effects in the bleaching process. This has provided the industry with another opportunity to not only reduce bleaching costs through the reduction of expensive chlorine dioxide, but also to produce a less toxic effluent through the reduction of chloro-organic compounds, some of which are measured as adsorbable Figure 2. Impact of enzymes upon standard beater curves (PFI revolutions)