Use of Chlorine Dioxide for the Bleaching of Pulp: A Re-Evaluation of Ecological Risks Based on Scientific Progress Since 1993, 1998 Environmental Conference Proceedings

A science advisory panel reviewed studies published since 1993 to assess the ecological risks associated with final effluents from pulp mills using 100% ClO2 in bleaching processes (often referred to as elemental chlorine free, or ECF mills). The panel addressed two major questions: (i) What are the risks to aquatic organisms and ecosystems due to the release of final effluent from ECF pulp mills, including all contributing factors in the process stream? and (ii) What are the specific risks, if any, contributed by the use of ClO2 as opposed to other mill processes? All available data indicate that chlorinated compounds in effluents from ECF bleaching have a much lower degree of chlorine substitution than those produced during bleaching with Cl2 , and are therefore generally less persistent and bioaccumulative. A screening level, or - where merited - probabilistic risk assessment of chlorinated compounds in ECF mill final indicated a negligible risk to aquatic organisms. With regard to the broader, first question, it is becoming increasingly clear that there are non-chlorinated substances, which are unmodified or slightly modified wood extractives released primarily during pulping, that appear to induce biochemical responses and reproductive effects in some fish species in the receiving environments for some pulp mills. These substances are produced by a variety of pulp extraction and bleaching technologies, and also occur naturally; e.g., retene and other resin acids, b-sitosterol and other plant steroid, and/or various polar, lignin-derived substances. A quantitative risk assessment of these compounds is not yet possible, owing to the con current lack of knowledge of concentrations in final effluent and the receiving environment, toxicological dose-response relationships, and ecological relevance. In general, significant environmental gains have come from 100% ClO2 substitution and the widespread adoption of efficient secondary treatment.