Page 955 - The Toxicology of Fishes

Case Study: Pulp and Paper Mill Impacts 935

The objective of this chapter is to present the history of pulp and paper effects assessment on fish as
a case study in fish toxicology. Overviews are provided of the pulp and paper process, historical impacts
of effluents, regulatory changes, industry improvements, and methods used for assessing effects. Current
gaps in knowledge and active areas of research and development are also discussed.

The Pulp and Paper Industry

Paper and paper products are essential to people living in modernized societies. As a result, the pulp
and paper industry plays a vital role in the global economy. Approximately 15% of the world’s paper
mills are in North America, and these mills produce 36% of its paper (Smook, 1994). Canada alone is
one of the largest producers and exporters of pulp and paper; it supplies 34% of the world’s wood pulp
exports and more than 50% of its newsprint, and the forest sector in general employs over 1 million
people (FPAC, 2002). To understand how PMEs affect fish, it is important to grasp the fundamentals of
pulp production. Each pulp mill is unique, employing different techniques to remove the fiber from trees,
including thermo-mechanical processes and chemical processes. Each mill has a unique combination of
equipment and processing options, tailored to meet the specific needs of the customers for their pulp
and the nature of the product (e.g., newsprint, cardboard boxes, tissue paper, photographic paper, writing
paper). Some mills have an integrated paper facility, while others produce only pulp. Furthermore, many
mills have multiple lines, including thermomechanical and chemical processes, and some mills alternate
processes, fiber sources, and bleaching sequences (e.g., chlorine, chlorine dioxide, hydrogen peroxide),
depending on the needs of a specific customer. In many cases, a paper mill may buy their pulp from
other sources and have no on-site pulping facilities.
The basic principle of pulping is to convert wood chips into fibrous raw material called pulp, which
is a suspension of wood cellulose fibers in solution. Wood consists of the fibrous carbohydrates cellulose
(45 to 50%) and hemicellulose (25% for softwoods), lignin (25 to 35% for softwoods), and compounds
easily extracted from the wood (2 to 8%), such as terpenes, resin and fatty acids, plant sterols, and
phenols (Biermann, 1996; LaFleur, 1996; NCASI, 1989; Smook, 1994). Cellulose is the backbone
component of wood fiber and is the most important component for pulp production. Hemicelluloses are
the filler of the cellulose fiber and are more soluble and labile than cellulose, resulting in greater
susceptibility to degradation in chemical pulping. Lignin is a high-molecular-weight polymer that binds
or cements the cellulose fibers together in a matrix (Kringstad and Lindström, 1984). The objective of
pulping is to separate and recover the cellulose fibers from the lignin and other wood constituents with
maximum fiber yield and minimum fiber degradation (LaFleur, 1996).
The most common pulping process is the kraft (or sulfate) process that was patented by C. F. Dahl
in 1854 as a modification of the soda process (Smook, 1994). In the kraft process, wood chips are
digested, or cooked, at high temperature (160 to 180°C) and pressure (800 kPa) with white liquor, which
is a mixture of hot caustic soda (NaOH) and sodium sulfide (Na S) (Biermann, 1996) (Figure 24.2). The
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lignin and wood extractives are solubilized in the cooking chemicals, leaving the less soluble cellulose
fibers as pulp (Kringstad and Lindström, 1984; McCubbin and Folke, 1992; McLeay and Associates,
1987). After digestion, the pulp is washed, screened, and cleaned in the brownstock washing area, and
the pulp fiber is separated from the residual weak black liquor. Weak black liquor is a complex mixture
containing waste lignin, cooking chemicals, and wood extractives (Biermann, 1996). Following brown-
stock washing, the pulp may be sent to a bleach plant, where residual lignin is removed and the pulp is
bleached to achieve a desired brightness. At some mills, pulp is sent through an oxygen delignification
stage to remove additional lignin prior to bleaching (Figure 24.2). This reduces the amount of lignin
entering the bleach plant and decreases the volume of bleaching chemicals required (NCASI, 1989).
In the bleaching process at bleached kraft pulp mills (BKPMs), pulp is treated in multiple, alternating
stages with various bleaching chemicals containing chlorine or oxygen followed by extractions with
alkali (Biermann, 1996). Due to environmental concerns, bleaching technologies have changed through-
out the years. Elemental chlorine was the dominant bleaching agent in the 1970s and 1980s and was
largely replaced by chlorine dioxide (elemental chlorine-free [ECF] bleaching) in the 1990s (Folke,
1996; Johnson et al., 2003). Complete elimination of bleaching using chlorine-based compounds (totally
chlorine free [TCF]) is also an emerging technology.