The Effect of Chlorination Effluent Recycle on Toxicity and Organically Bound Chlorine in Softwood Kraft Pulp Bleaching

Factorial experiments were conducted to investigate the effect of chlorination filtrate recycle on toxicity and organically bound chlorine in the first two stages of softwood kraftpulp bleaching. The effects of the mode of chemical addition, the substitution of chlorine dioxide, and the kappa factor were also studied. A continuous laboratory-scale (6.0 o.d.g/min) chlorination apparatus was designed to rapidly produce suficient efluent for trout bioassays. An increase in recycle equivalent to 18 m3/adt caused the first-stage toxic emission factor to decrease by 314 TU.m3/adt. Thus, toxicity data from chlorination efluents that are produced in the laboratory without recycle are not representative of the industrial process. A regression equation was formulated for the organically bound chlorine (AOX) results, which did not depend on recycle.