Page 956 - The Toxicology of Fishes

936 The Toxicology of Fishes

Pulp Digesters Oxygen Delignification

Bleach Plant
Wood Chips

White
Liquor Screens
Brownstock Washers Post-Oxygen Washers
Weak Black Finishing &
Liquor Shipping
Chemical Recovery

Slaking
Evaporation
Strong
Green
Liquor Black
Combustion Liquor
FIGURE 24.2 Kraft pulp and paper process schematic showing digestion of chips to produce pulp, bleaching of the pulp,
and the chemical recovery cycle.

To recover the cooking chemicals used in pulp digestion, weak black liquor is sent through the chemical
recovery phase of the mill operations (Figure 24.2). Modern chemical recovery systems are designed to
recover 96 to 99.5% of the spent cooking chemicals (McCubbin and Folke, 1992). This phase of
operations transforms weak black liquor to strong black liquor through a series of evaporators and
combusts it in a recovery furnace to form inorganic smelt. The smelt is dissolved in water to form green
liquor, which is causticized with lime to regenerate white cooking liquor (McLeay and Associates, 1987;
Smook, 1994).
In addition to the production of pulp, a mill produces large volumes of waste effluent. A typical BKPM
(700 tonne/day capacity) can discharge between 90 and 130 million liters per day of effluent into surface
waters (lakes, rivers, estuaries, oceans) (Walden, 1976). In Canada, effluent concentrations in receiving
waters range between 0.2% (100 m from the outfall on the Abitibi River, Ontario) and 30% (100 m from
the outfall on Riviere du Nord, Quebec) (Environment Canada, 2003). Effluent concentrations vary from
mill to mill, depending on the amount of water used per tonne of pulp, the size of the receiving
environment, and fluctuations in receiver flow on a seasonal and annual basis. Significant efforts have
been made by the pulping industry to reduce freshwater use and, hence, the discharge of effluent. In
Canada, pulp and paper mills have reduced their water consumption per tonne of output by 30% since
1990 (FPAC, 2002). Many mills now recycle process streams within the mill (e.g., using condensates
from the chemical recovery process for brownstock washing) and use countercurrent washing techniques
to reduce water use (LaFleur and Barton, 2003). One Canadian mill has achieved zero discharge and
does not release effluent. The final effluent discharged from a BKPM is a complex chemical mixture
consisting of several effluents produced at different stages within the process. Effluent produced in the
debarking of trees, brownstock washing wastewaters, bleach plant filtrates, black liquor and chemical
spills, and condensed vapors that are produced during weak black liquor evaporation (i.e., condensates)
are examples of process effluents that can be discharged into the main chemical sewer (LaFleur, 1996;
Smook, 1994). Each process effluent has a unique chemical composition and can interact with other
effluents, resulting in a complex final mixture. In addition, the quality of effluent will vary with the
species of tree used as the fiber source, whether the trees are debarked by wet or dry processes, and
whether the fiber source is pulp wood, wood chips, or recycled fibers. As a result, the effluent quality
is not static, and a generalization among mills is seldom possible.