82    CHARACTERIZATION AND BIOLOGICAL TREATMENT OF
BLEACH PLANT EFFLUENT
Cai-Fang Yin, Graduate Student
Thomas W. Joyce, Professor
H. M. Chang, Professor
Department of Wood and Paper Science
North Carolina State University
Raleigh, North Carolina 27695
INTRODUCTION
The pulp and paper industry uses a large quantity of chlorine to bleach pulp to achieve the required
brightness and cleanliness of pulp. The advantage of using chlorine is its high selectivity towards
lignin removal and its low cost.
In the conventional softwood chlorination (C) stage bleaching, approximately 63 kg active chlorine
is applied per ton of pulp. Of this, about 44 kg is converted to inorganic chloride and 3 kg to
organically bound chlorine (OC1); the remainder remains in the pulp. In the first alkaline extraction
(El) stage, 9 kg more of chlorine is converted to chloride and an additional 3 kg of OC1 is generated in
the effluent.1 Overall, about 10% of the active chlorine used in the chlorination stage will be discharged as OC1 in the bleach effluent. The actual value may vary depending upon wood species,
bleaching sequence, and the degree of closure in the bleach plant.
Approximately 70 kg organic material per ton of pulp is discharged in the bleach plant effluent
during softwood bleaching. Of this amount, 50 kg is lignin, 19 kg is carbohydrates, and 1 kg is
extractives.2 About 75% of the dissolved material (COD and color) is in the chlorination and first
extraction (C + E,) stages.
The dissolved, heavily modified and chlorinated compounds exert not only oxygen demand (BOD
and COD), but also cause the effluent color and toxicity. Traditionally, the purpose of the pulp mill
effluent treatment system is to reduce the oxygen-consuming material (BOD5) and the suspended
solids (SS). A conventional waste treatment using a primary clarifier and a secondary biological
treatment such as an aerated lagoon, activated sludge, or a fixed film reactor, has been proven to be
very effective in removing SS, BOD5, and in most cases, toxicity.3,4 The amount of BOD5 has been
reduced at many mills to a level where other pollutants are beginning to be of greater concern. These
concerns include COD, color, and OC1.
A major contributor to COD, color, and OC1 are high molecular weight chlorinated lignins called
chlorolignins. Although these high molecular weight chlorolignins do not cause BODs and acute
toxicity in the environment, the long-term effect of these chlorolignins on the receiving waters and the
ecosystem is essentially unknown. The behavior of these compounds during conventional biological
treatment is not well known either. Therefore, the purpose of this study was to investigate the
biological treatability of bleach plant effluent based on COD, color, and OC1 removal efficiency. The
ability of the wood-degrading fungus to remove these pollutants was also studied. The particular
strain selected was Phanerochaete chrysosporium, which has already demonstrated its ability to
degrade lignin and modified lignins.5
Effluents being treated were first characterized in terms of COD, color, and OC1. Distribution of
these pollutants in high molecular weight material (HMM) and low molecular weight material (LMM)
was determined by first separating them by ultrafiltration. Gel Permeation Chromatography (GPC)
was used to study the molecular size distribution of chlorolignins in the effluents.
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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