INFLUENCE OF BLEACHING CONDITIONS AND MEMBRANE FILTRATION ON PILOT-SCALE BIOLOGICAL TREATMENT OF KRAFT MILL BLEACH PLANT EFFLUENT

A kraft mill bleach plant effluent was treated in a pilot station with a common pretreatment stage and three parallel biological treatment p lants. 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 compr ised a 10 m3 anaerobic hybrid reactor followed by a 2.5 m3 activated s ludge plant. The third line was a 4.5 m3 activated sludge plant. All t hree plants were connected on-line to the bleach plant in a kraft mill . During the study, the bleaching conditions were changed from 75% sub stitution of chlorine for chlorine dioxide to 100% chlorine dioxide bl eaching. In the comparisons, a total hydraulic retention time of 12 ho urs was used in all the three systems. The influent temperature was ke pt at 37-38-degrees-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 c hange in bleaching conditions decreased the discharge of AOX more than 50% and had a positive effect on the biological treatability of the b leach 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 maj or differences in treatment efficiency between the three systems studi ed. Anaerobic pretreatment resulted in a somewhat higher AOX removal a nd a somewhat better tolerance to shock loads, especially with chlorin e used in bleaching. Membrane filtration of the E-stage effluent gave a significant contribution to the overall treatment result with 75% ch lorine dioxide (D) in the first bleaching stage, but contributed only marginally with 100% D.