30    REMOVAL OF CHLORINATED ORGANICS FROM KRAFT
SOFTWOOD BLEACH PLANT EFFLUENT BY
SEQUENTIAL BIOLOGICAL TREATMENT
USING WHITE-ROT FUNGUS
AND AN ANAEROBIC REACTOR
D.Y. Prasad, Visiting Scholar
Thomas W. Joyce, Professor
Dept. of Wood and Paper Science
NC State University
Raleigh, North Carolina 27695-8005
INTRODUCTION
Conventional pulping of softwood for a bleachable grade of pulp is generally stopped at a kappa
number of 28-32. A further reduction of the kappa number during pulping leads to the degradation of
the pulp, resulting in a lower yield and poor strength properties. Pulp produced by the kraft process is
bleached in a multi-stage sequence to obtain the desired brightness and cleanliness. Different bleaching agents are used for this purpose. Molecular chlorine is the most reactive bleach agent to the lignin
remaining in pulp and degrades and dissolves the lignin while producing chlorinated organic compounds of low and high molecular weights.1,2 These chlorinated organics are solubilized in dilute
alkali and extracted from the pulp in the subsequent extraction stage. The chlorinated organics are
major contributors to effluent color, BOD, and COD, mutagenicity, and carcinogenicity3 6 and acute
toxicity.7
The pollution load and amount of chlorinated organic compounds produced in the chlorination and
extraction stages are mainly dependent upon the incoming kappa number and the amount of chlorine
applied to the pulp. The major source for organic pollutants is the El stage effluent in the conventional bleaching sequence, CEDED, which contains about 12% of the total bleach plant effluent, but
accounts for about 95% of the color, 70% of the COD, and 50% of the BOD in the whole bleach
plant.8
Much research has been conducted to reduce the emission of chlorinated organics from bleach plant
effluents. Some of these include (a) modified cooking,9, ,0 (b) modified bleaching,""14 (c) recovery of
spent bleach liquors15,16 and (d) external treatment of spent bleach liquors.
Incinerating bleach plant effluent along with black liquor in the recovery system is considered an
uneconomical proposition due to the presence of the corrosive chloride ion and the high steam
demand for bleach plant effluent concentration in the multiple effect evaporators.
Numerous physico-chemical methods have been used to remove chlorinated organic compounds
from bleach plant effluent. The treatments include precipitation with lime,17 alum and metal ions,18,19
polymeric adsorbents,20,21 oxidation with ozone21 membrane techniques,22 soil media23 and UV-
irradiation.24 The major problems encountered with these methods are the commercial feasibility and
economic viability; i.e., they cost too much.25
As an alternative to physico-chemical treatments, biological (aerobic) treatment is known to be
effective in reducing the BOD and toxicity of kraft mill effluents.26,27. It is also a widely used
technique to remove BOD and chlorinated organics from bleached kraft mill effluent.28"30 However,
the removal of chlorinated organics in an aerobic environment is limited to rather low amounts.31"33
While the pollution load of the effluents with respect to BOD and COD can be treated to acceptable
limits with available technologies, the treatment of chlorinated organics continues to defy an economically viable solution. Thus, appropriate technology for treating chlorinated organics from kraft
softwood bleach plant effluent is of paramount importance to reduce the pollution load from the
paper industry.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
299