The effects of chlorine dioxide substitution for elemental chlorine in the chlorination stage, aimed at the minimization of chlorinated organics and subsequent oxygen reinforcement in the proceeding extraction stage on the characteristics of bleaching effluents have been examined. The characteristics of the effluents were assessed in terms of molecular weight distribution by gel filtration, so that alternatives for the treatment of these effluents can be proposed. The analysis of chlorination and extraction stage effluents with different chlorine dioxide substitution levels and varying available chlorine charges by using a Sephadex G-50 column have indicated that indeed a marked change in the molecular weight distribution of chlorinated organic compounds have occurred. The molecular size of chlorinated organic compounds in the chlorination stage has increased with the chlorine dioxide substitution, while that in the extraction stage has decreased when compared with the molecular size of these compounds in original bleaching effluents with only elemental chlorine used in the chlorination stage and followed by extraction stage. Molecular size of the chlorinated organic compounds in the chlorination stage effluents was not seriously affected by the chlorine dioxide substitution level; while it was increased greatly by the increased total chlorine dosage. In the extraction stage effluents, chlorine dioxide substitution level has not appeared to be an important parameter influencing the molecular size distribution.
Combined effluents from a bleached Kraft pulp mill using annual plants were treated in a lab-scale activated sludge (AS) system. The effects of operating solids retention time (SRT) and concentration of additional carbon source on AOX removal were investigated. Higher AOX removals (30%) were observed with long SRTs, 99% of which was pure metabolization. As SRT was decreased AOX removal efficiency also decreased (4%) and principal AOX removal mechanism changed to adsorption onto wasted biomass. The AOX removal efficiency was lower (10%) on a 20% decrease in the supplemental carbon source, with no apparent effect on the removal mechanism.
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