Ozone Decolorization of Pulp and Paper Mill
Secondary Effluents
CARL NEBEL, Manager
RONALD D. GOTTSCHLING, Engineer
H.J. O'NEILL, Engineer
Advanced Treatment Systems
Welsbach Ozone Systems Corporation
Philadelphia, Pennsylvania 19129
INTRODUCTION
With some qualifications, methods are available for the removal of biologically
oxidizable matter from pulp and paper mill effluents (1). Biological secondary treatment
appears to be effective in removing those materials which cause deoxygenation and slime
growth in receiving waters. Furthermore secondary treatment is also effective in removing
those materials from kraft mill effluents which adversely effect fish productivity.
A major remaining problem involves the removal of the biologically refractive fraction
of pulp and paper mill secondary effluents which is responsible for color. Wood species,
age, processing variables and the nature of the secondary treatment all effect the quantity of
color remaining.
Research and development work for color removal started over 30 years ago in the
United States. These investigations were prompted by complaints of colorization of small
streams by mills located on them rather than by governmental regulations. Today most
states have specific color regulations which must be met within a short time frame.
CAUSES OF COLOR
The causes of color are two-fold, inorganic and organic. In the pulp and paper industry
one is concerned predominately with colors of organic origin, i.e. materials having carbon
as their backbone.
Inorganic color forming minerals are often denoted as pigments and are only rarely
found in pulp and paper mill secondary effluents. Pigments generally are readily removed in
the effluents primary treatment stage by sedimentation. In some rare paper product lines
soluble pigments are employed as coloring agents. Examples will be presented where an
ozone induced flotation process can remove such species.
MECHANISM OF COLOR FORMATION AND ELIMINATION
The major source of color is organic in nature. Technically speaking, almost all organic
compounds are colored but fortunately most do not absorb light in the visible portion of the
spectrum. The majority of organic compounds absorb light in the ultraviolet region of the
spectrum. The mechanism of light absorption is accomplished by organic functional groups
known as chromophoric groups. These light absorbing groups are those which have excess
electrons such as the carbon-carbon double and triple bonds, six carbon membered
aromatic rings, nitro groups, sulfur and oxygen containing groups, and heterocyclic
compounds containing oxygen, nitrogen, or sulfur as a member of the cyclic ring. Those
compounds which absorb light in the visible spectrum have the above mentioned high
electron density functional groups arranged in a long chain. The length of the chain and the
nature of the electron-rich groups dictate the shade of the color. Cleavage of one of these
groups breaks the molecular chain and generally shifts the color from the visible portion of
the spectrum to the invisible portion of the spectrum which can be either the ultraviolet or
infrared region.
948