The $250 million Millar Western Meadow Lake Mill is located on a 247‐acre site about 200 miles northwest of Saskatoon, Saskatchewan. It uses mechanical action supplemented by mild chemicals to turn aspen wood chips into bleached chemi‐thermomechanical pulp (BCTMP), about 240 000 MT/Y.2 More efficient than the kraft process, this approach uses half the trees to make the same amount of pulp, producing almost one ton of pulp for each ton of wood on a water‐free basis. The Millar Western BCTMP process also eliminates chlorine compounds and odorous sulfur‐based impregnation chemicals. This environmentally friendly mill uses hydrogen peroxide to increase the brightness of the pulp, making it suitable for printing and writing grades of paper as well as for tissue and paper towels.
The plant is the first pulp mill in the world to operate a successful zero liquid discharge system. Effluent from the thermomechanical pulping process is concentrated from 2% solids to 35% solids by three falling film vapor compression evaporators, followed by two steam‐driven concentrators which further concentrate the effluent to about 70% solids. Of the 1760 gal/min of effluent sent to the system, 1720 is recovered as high‐purity water for reuse in the pulping process. Solids are burned in the boiler; the smelt is cast into ingots and stored on site for future chemical recovery.
In early1990s, when Millar Western Pulp (Meadow Lake) Ltd. announced plans to build a mill in northern Saskatchewan, the community was concerned about the pollution it would generate, especially effluent discharged to the Beaver River. Though a biological treatment system planned at the mill would have made the effluent cleaner than river water, Millar Western decided to go one step further and eliminate all effluent discharge from the pulp mill. The zero effluent system at Meadow Lake is the first of its kind in the world. The evaporator system, the key equipment in the water recovery process, was designed and supplied by Resources Conservation Company (Fosberg 1992). All effluent coming out of the mill is treated in the water recovery plant. As a result, the mill only needs about 300 gpm of makeup water to replace water lost to the atmosphere by evaporation. The same type of pulp mill without a water recovery plant would need about 2500 gpm of raw makeup water. The effluent treatment system started up in January 1992, when the mill went on line.
Millar Western’s Meadow Lake BCTMP mill is an example of successful closure of the water cycle in a mechanical pulp mill. An earlier attempt in Canada to close a kraft mill by recycling bleach plant effluents through the kraft chemical recovery process had failed on account of the buildup of corrosive materials (Smook 1992). Thus, it is useful to study in detail the advanced system in place at Meadow Lake.
The effluent produced by the BCTMP process at Meadow Lake is discharged at a rate of almost 1800 gpm. It has a temperature of 150 °F, a pH of about 8 and contains about 20 000 ppm dissolved solids. Figure 7.5 shows a more detailed view of the water recovery portion of the system, consisting of five stages: clarification, evaporation, concentration, stripping, and incineration.
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