Treatment efficiency of eucalypt kraft pulp bleaching effluents: influence of dissolved organic matter.

Application: Knowing how effluent composition affects treatment efficiency can help mills manage their treatment systems in ways that avoid treatment efficiency loss after mill process changes.

Elemental chlorine free (ECF) and totally chlorine free (TCF) bleaching effluents are believed to be equally treatable in efficiently operated treatment plants. However, contradictory results of the comparative effects of ECF and TCF effluents are found in the literature. Biological treatment efficiency depends on the metabolic capacity of the biological sludge, which, inturn, depends on adaptation of the microorganisms present in the sludge to the effluent being treated.

In this study the treatability of laboratory generated eucalypt kraft pulp ECF ((D(EO)DD) and TCF (Q(OP)(QZ)(PO)) bleaching effluents was compared in a bench scale activated sludge system. Untreated TCF effluent presented 3.75 times as much BOD and 2.4 times as much COD as ECF effluent.

Over a six-week treatment period, BOD removal was similar for both effluents (>97%) whereas removal of COD was greater in TCF (77%-85%) than in ECF (47%-68%) effluent. We attributed that largely to the removal of high molecular mass organic matter, which represented a significant fraction of TCF effluent but not ECF effluent. Microtox toxicity was consistently eliminated from both effluents, whereas chronic toxicity to algae was reduced but not removed during treatment. Effluent toxicity was not correlated with BOD, COD, or AOX levels. TCF effluent contained more readily removable high molecular mass organic, matter than ECF effluent. Removal efficiency was not simply a function of organic matter molecular mass, but also of the nature of the dissolved organic matter. The ECF effluent fraction of intermediate molecular mass was the most highly chlorinated and presented the lowest removal efficiency.

Mounteer and Silva are with the Department of Microbiology, and Colodette is with the Department of Forest Engineering at Federal University of Vicosa, Minas Gerais, Brazil. Contact Mounteer by email at [e-mail]