A bleach plant effluent from softwood kraft pulping was treated in the laboratory with a combination of ultrafiltration and different biological methods. The E-stage effluent was firstly membrane filtered using membranes with a nominal cut-off of 8,000 Dalton. In the filtration, a concentration factor of 15 was used at 55°C. The treatment resulted in 89% AOX removal and 87% COD removal. Calculated in relation to the actual flows of E-stage and (C+D)-stage effluent at the mill, this corresponded to AOX and COD removals of 20% and 41% respectively. Before the biological treatment, the permeate was mixed with (C+D)-stage effluent and treated in three parallel biological systems, an aerated lagoon with and without solids recycle, an airlift system with a mixed fungal flora and an anaerobic filter. For the lagoon treatment, a hydraulic retention time of 7 days was used at biomass concentrations of 70, 350 and 480 mg TSS/l and 20-22°C. The fungal system was evaluated at retention times of 5.5 11 and 22 h, 770 mg TSS/l and 37°C. For the anaerobic filter, retention times of 1, 5 and 25 h at a temperature of 35°C were used.

Very promising results were obtained with the combination of physical and biological treatment. The aerated lagoon with solids recycle gave the best results with 66% of AOX, 72% of COD and 95% of the BOD being removed in the combined process. The anaerobic filter also gave good results, but in practice a small aerobic post-treatment would probably be necessary. The three systems were also evaluated for the removal of chlorinated phenolic compounds and acute toxicity according to the Microtox test. The results suggest that a combination of membrane filtration and anaerobic/aerobic or just aerobic treatment would be an attractive way to handle kraft mill bleach plant effluents.

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