REMOVAL OF COLOR FROM KRAFT PULP MILL EFFLUENT
K. Larry Roberts, Associate Professor
Civil Engineering Department
Tennessee Technological University
Cookeville, Tennessee 38505
John Schoolfield, Environmental Engineer
U.S. EPA
Atlanta, Georgia 30365
INTRODUCTION
Pulp and paper mill wastewater is intensely brown colored by lignin, which is a byproduct of the
pulping process. The brown color of pulp mill waste is not affected by secondary biological treatmenl
used to remove most of the other pollutants from pulp and paper mill wastewater. When it is discharged into the environment, lignin can, under the right conditions, cause severe damage.
Despite years of research by the pulp and paper industry, there is still no method of color removal
which is economical and effective. This paper contains the results of an experimental verification
and optimization of a color removal method which has been developed over a period of several years.
It is based on the use of a class of cationic surfactants called quaternary ammonium compounds
(quals) to destabilize lignin molecules and remove them from solution.
BACKGROUND
Schoolfield [1] has reviewed the characteristics and effects on the environment of pulp and paper
mill waste. In summary, the pollution potential of the industry is high. For every ton of bleached
pulp produced, a Kraft mill produces 40,000 to 60,000 gallons of wastewater which contains a wide
variety of pollutants [2]. The industry has, however, invested considerable resources in pollution
control and has eliminated or mitigated most of its pollution problems [3J. In spite of years of research, the industry has been unable to develop a process which effectively and economically removes
color from its wastewater.
Although the pulp and paper industry has evaluated many unusual [4,5,6,7], even exotic, methods
of color removal, it has never evaluated the use of quats to remove color. Several years of research
have shown that it may be possible to use quats to remove color from pulp mill waste effectively and
economically.
QUATS
A quat molecule is surface active because of its unusual configuration. A quat molecule ionizes
to give a positively charged organic ion and a halogen anion [8]. One end of the organic cation is
hydrophobic (e.g., water-hating, water repellent) because it consists of a hydrocarbon chain which
has no attraction for water. The other end of the organic cation is hydrophilic (water-loving) because
it is charged and has a high attraction for the polar water molecule [9].
Because of its dual nature, the organic cation is attracted to surfaces, or interfaces [9]. For example, at an interface between water and air, the water-hating end of the cation can extend into the
air, which is nonpolar, and the water-loving end of the cation can extend into the water. Thus, at
an interface, a surface active ion is at its minimum energy configuration [8].
When surface active molecules gather at an interface, the energy of the interface is lowered. This
happens because the attraction of the surface active molecules for each other is less than the attraction
of water molecules for each other (9).
305