Section 6. PAPER WASTES
REMOVAL OF COLOR FROM PULP AND PAPER MILL EFFLUENTS
BY BIOLOGICAL PROCESSES
Alton G. Campbell, Jr., Research Assistant
Thomas W. Joyce, Associate Professor
Department of Wood and Paper Science
North Carolina State University
Raleigh, North Carolina 27650
INTRODUCTION
The discharge of highly colored effluents into surface waters is a potentially serious
problem for the pulp and paper industry. In the near future, the quantity of color discharged by the industry may be regulated by some state or federal agencies. An efficient
and economical color removal process is needed by the pulp and paper industry.
The current chemical and physical processes best suited for color removal include such
methods as chemical precipitation, ozonation, carbon adsorption, ion-exchange, resin
adsorption, ultrafiltration and reverse osmosis [ 1 ]. Even though some of these methods
are excellent color removal processes, all of them have certain basic problems, such as
excessive cost per unit volume of effluent treated or unreliability. Several processes, such
as reverse osmosis and ultrafiltration, require further process development. Inherent problems associated with each process will probably deter their use as a color removal technology within the pulp and paper industry.
Biological color removal may hold considerable promise in decolorization technology.
The cost of biological effluent treatment is usually about one order of magnitude less
expensive than either chemical or advanced wastewater treatment. Conventional biological
process technology for waste treatment in the pulp and paper industry has proven to be
cost effective and reliable.
COLOR SOURCES AND REMOVAL
The high color content of bleached kraft paper mill effluent (BKME) originates predominantly from the bleach plant. This waste stream is also notable for its toxic and mutagenic characteristics [2,3]. Due to its high chloride content and the resulting corrosion
problems if chloride is recycled, waste streams from the conventional bleach plant cannot
be burned in the kraft recovery system and recycled in the mill, and, therefore, must be
treated and discharged. The primary source of color in bleach plants is the degradation
products of lignin that are formed during the bleaching sequence. During bleaching, the
lignin polymer is oxidized, chlorinated, and degraded into products soluble in water and
alkali. The chomophoric units in the bleach plant effluent which contribute to the majority
of the color are quinones, quinone-methides and alkenes conjugated with aromatic rings
[4,5]. A successful biological treatment system that would reduce color must be capable
of enzymatically oxidizing the lignin fragments or of chemically modifying the conjugated
chromophoric groups. Lignin and related compounds, which include color bodies, have been
found to be very resistant to bacterial degradation.
The biological treatment systems currently used in the pulp and paper industry are
successful in reducing BOD. However, within the time frame of a biological treatment
system, bacteria cannot significantly reduce the COD or color content of BKME. Bacteria
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