This presentation will deal about technologies for the reduction of COD from the bleach plant, mainly focused on the partial closure system, and also referred to the closed cycle of the bleach plant.
Based on laboratory evaluations of partial closure when applied to the ECF bleaching of conventional and oxygen delignified eucalyptus kraft pulps, the following conclusions can be made:
·about 50% of the bleach plant COD, BOD and color will be returned to the recovery system.
·less than 20% of Ba. Ca, Fe. Mg, and Mn and about 35% of the oxalate will be recirculated.
·only about 5% of the total chloride will be returned.
·the eucalyptus effluents are easy to treat with obtainable AOX and COD reductions of about 80% and 70% respectively.
Ultrafiltration and partial closure of alkaline filtrates in ECF bleaching can reduce final bleach plant effluent COD by as much as 60% and AOX by as much as 40%.
Ultrafiltration of ECF alkaline filtrates can be achieved with both high fluxes and little fouling using commercially available membranes.
Experience in full scale from a pulp mill shows that partial closure of the bleach plant is possible, while retaining the ECF concept.
Based on a reduction of the total emission from the mill a more effective biological treatment should be considered.
Results from pilot plant work shows that the ECM system is able to transform the bleach plant effluent to clean condensate, a methanol containing condensate, carbon dioxide exhaust gases and a mixed solution of sodium salt and heavy metals in a sludge.
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