A kraft mill bleach plant effluent was treated in a pilot station with a common pretreatment stage and three parallel biological treatment plants. The possibility to membrane filter the E-stage effluent before biological treatment was provided. In the first pilot plant, three 0.7 m3 anaerobic fixed film reactors were followed by an activated sludge plant with two 0.7 m3 aerobic tanks in series. The second plant comprised a 10 m3 anaerobic hybrid reactor followed by a 2.5 m3 activated sludge plant. The third line was a 4.5 m3 activated sludge plant. All three plants were connected on-line to the bleach plant in a kraft mill. During the study, the bleaching conditions were changed from 75% substitution of chlorine for chlorine dioxide to 100% chlorine dioxide bleaching. In the comparisons, a total hydraulic retention time of 12 hours was used in all the three systems. The influent temperature was kept at 37-38°C. Biomass concentrations were 1.5-4 kg TSS/m3 in the aerobic parts. The concentration of fixed biomass was estimated to be 10-20 kg/m3 in the anaerobic reactors after the experiments.

The change in bleaching conditions decreased the discharge of AOX more than 50% and had a positive effect on the biological treatability of the bleach plant effluent. The transformation of organic and chloroorganic material increased, a more stable operation could be obtained and the quality of the sludge improved. Except for chlorate, there were no major differences in treatment efficiency between the three systems studied. Anaerobic pretreatment resulted in a somewhat higher AOX removal and a somewhat better tolerance to shock loads, especially with chlorine used in bleaching. Membrane filtration of the E-stage effluent gave a significant contribution to the overall treatment result with 75% chlorine dioxide (D) in the first bleaching stage, but contributed only marginally with 100% D.

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