Electrically Induced Coagulation of
Lignin from Kraft Mill Wastewaters
HUGH J. CAMPBELL, JR., Graduate Student
FRANKLIN E. WOODARD, Assistant Professor
DAVID O. HERER, Research Assistant
Department of Civil Engineering
University of Maine
Orono, Maine
INTRODUCTION
General
The objectionable consequences of pollution originating with the manufacture
of wood pulp have led to a massive search for methods for treating this industry's
wastewaters. Despite this, no single method has been developed for treating all of the
various types of water pollutants arising from the pulping of wood.
The major pollutants found in pulp mill wastewaters are materials that exert
oxygen demand and that cause color in the receiving waters. Oxygen demand results
from the biodegradation of carbohydrate components present in these wastewaters
(1). The color causing materials are carmelized sugars and lignin degradation
products. Lignin degradation products may also be a significant source of
biochemical oxygen demand (BOD) However, these materials are not normally
measurable by the standard five-day BOD test, since non-acclimated seed organisms
are usually used for the test (2,3). Biological treatment systems have been utilized to
remove BOD measurable by the five-day BOD test. Chemical treatment has been used
effectively for the removal of color. No treatment has been developed, however,
which satisfactorily removes both types of pollutants.
Source of Lignin
The function of pulp digestion is to separate cellulose from the other
constituents of wood chips. Cellulose is the only part of the wood normally used in
the making of paper. The lignin and other components are considered as wastes and
are disposed of in one of several ways. For example, the kraft pulping process
employs a chemical recovery furnace which burns most of the lignin and waste
carbohydrates. However, a substantial quantity of lignin remains attached to the
cellulose fibers after digestion, and is removed by further processing in a bleaching
plant. The wastewaters from this operation are not subjected to the chemical
recovery furnace. Thus, the bleaching plants are the primary sources of color causing
pollutants in kraft pulp mills.
The feasibility of removing lignin polymers from wastewaters by coagulation
has been demonstrated. Woodard and Etzel (1) used electrophoretic techniques to
separate the sulfonated lignin in pulp mill wastewaters into two fractions, termed
Fraction A and Fraction B. Both fractions were shown to originate from the same
source, i.e., lignin. However, Fraction A is composed of larger polymers and thus is
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