Section 3.    PAPER WASTES
INVESTIGATION OF THE POTENTIAL OF
AN ENZYMATIC TREATMENT OF PULP AND PAPER MILL EFFLUENT
TO FACILITATE DECOLORIZATION BY LIME PRECIPITATION
Janet C. Campbell, Research Assistant
Thomas W. Joyce, Associate Professor
Department of Wood and Paper Science
North Carolina State University
Raleigh, North Carolina 27650
INTRODUCTION
Color has been designated as a "nonconventional" water pollutant, one of three categories established by the Clean Water Act of 1977. Color, along with other nonconventional pollutants such as
ammonia, chemical oxygen demand (COD), and certain industry-specific substances, is so classified
since it is not included in either the conventional or toxic pollutant categories. Government regulations can limit discharge of any of these nonconventional pollutants where warranted.
As a water pollutant, color is of concern to the pulp and paper industry, the nation's third largest
industrial consumer of water [1]. An estimated two trillion gallons of wastewater is discharged annually by the pulp and paper industry [2], much of which is highly colored. This color is mainly attributable to lignin degradation products formed during various pulping and bleaching operations and has
some similarity to natural organic, aquatic color [3]. Investigations into the identity of chromophoric
structures have revealed the important ones to be quinones, quinonemethides, and vinyl groups conjugated with an aromatic ring [4]. Examples of possible chromophoric structures are presented in
Figure 1. These structures can be incorporated into larger, more complex molecular species. Those
present in bleachery spent liquors are generally thought to be acidic chlorine-substituted, oxidized
lignin fragments of relatively low weight and low aromatic content [4]. Although no uniform national
standard now exists for limiting color discharge, each mill is considered on a case-by-case basis for
regulation. There is presently no economically practical color removal process.
The color loadings of the various waste streams from pulping and papermaking differ from each
other dramatically (Figure 2 and Table I). The color from the bleach plant can be up to 80-90% of the
total from pulping and bleaching combined [5]. Of the bleachery color, as much as 90% is from the
alkaline extraction stage, meaning that up to 70% of the total mill color load is contributed by a
source that is only about one-fifth of the total volume.
Although the pulp and paper industry has been generally quite successful in removing conventional pollutants such as BOD5 and total suspended solids (TSS) from its waste streams, color reduction has proven to be a more difficult task in both technical and economic terms [5]. The current practice of primary clarification of wastewater followed by biological treatment, commonly aerated
lagoons or activated sludge units, can elmininate conventional pollutants at an efficiency up to 95%.
In contrast, these treatment systems are capable of color reductions of only 0-30% [5]. This resistance
of color bodies to biological treatment is largely due to the inability of the microbial population,
primarily bacteria, to metabolize lignin-derived chromophores. The ultimate solution to the color
problem most likely lies in various process modifications in pulping and bleaching technology, yet
some external effluent treatment will probably be necessary in many situations to reach the required
reductions [5].
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