Biological and Advanced Treatment of Sulfate Pulp Bleaching Effluents

Abstract

Spent bleaching effluents (from chlorination (C) and extraction (E) stages) of a sulfate pulp mill were subjected to bench-scale biological and advanced treatment. Although > 90 % of the influent BOD5 could be removed in an activated sludge process, the effluent still contained high amounts of resistant substances. The maximum COD removal was about 50 %; the removal rates achieved in the parameters TOC, DOC, AOX, SAK (254 nm) were even lower. The biological treatment led to an increase in color (436 nm) up to 40 %. The biologically pretreated effluent was further treated by ozone or ozone/irradiation. The DOC, COD, color (436 nm), SAK (254 nm) and AOX removal rates amounted to 61 %, 81 %, 98 %, 92 % and 92 %, respectively. These methods led simultaneously to an increase in biological biodegradability as reflected by an increase in BOD5. A comparison of the results obtained for raw and biologically pretreated wastewaters showed that biodegradable substances should first be removed from the wastewater since otherwise the effectiveness of these methods decreased. The coagulation/flocculation of biologically pretreated effluent showed that FeCl3 was the most effective coagulant and that removal rates > 90 % could be achieved. The treatment with various powder activated carbons showed that a dosage of 10 g/l was required to achieve elimination rates > 90 % in the parameters DOC, COD, color (436 nm) and SAK (254 nm). Adsorption isotherms were developed for every activated carbon and adsorption constants were calculated.