Increasing the Biotreatability of ECF Bleaching Effluents by Ultrafiltration and Partial Closure of Alkaline Filtrates, 1999 Environmental Conference Proceedings

Solvie Herstad Svärd, Pia Jour, Patrick S. Bryant, Jiri Basta, Pierre Aggarwal--Eka Chemicals’ concept of ECF partial closure is based on keeping the alkaline and acidic filtrates from the bleach plant separate, reducing the alkaline effluent volume with ultrafiltration, and purging chlorides and potassium from the recovery boiler ESP dust. The alkaline filtrates are recycled to the brownstock fiberline and the acidic filtrates are treated in a biological waste treatment system.

Mill pilot scale ultrafiltration trials on ECF alkaline bleach filtrate have been completed in an operating kraft softwood mill using oxygen delignification. The alkaline filtrate used was the combined EPO and EP filtrates in the bleaching sequence D(EOP)D(EP)D. Along with the traditional performance measurements of COD, AOX, color and chloride, the alkaline feed to the ultrafiltration unit, the concentrate and the permeate were characterized for both inorganic and organic components. The alkaline feed, the permeate and the acidic filtrates, separate and in combination, were tested for their biological treatability and the environmental parameters were evaluated both prior to and after biological treatment.

The results showed that the treatability efficiency of the mixed effluent COD was increased from 45% to 60% when the alkaline feed was exposed to ultrafiltration. The unfiltered alkaline effluent had a treatability efficiency of about 40% for COD , while the ultrafiltration unit permeate had a COD reduction efficiency of more than 70%. The increased reduction was due to the reduction of the high molecular weight compounds in the permeate compared to the alkaline feed.

Studies were also performed comparing a two stage activated sludge process with a low loaded aerated lagoon system. In large scale the activated sludge process normally gives better efficiencies, among other things due to better sludge separation and higher sludge concentration. These studies showed that the efficiencies of the activated sludge process and the aerated lagoon were comparable, although possible effects of poorer sludge separation in the case of the aerated lagoon are hard to predict in laboratory scale studies.