Our society, despite once-upon-a-time dreams, still runs on paper—and to make paper, you first have to make pulp.
Wood is made up of fibers of cellulose bound together by a substance called lignin. The goal of pulp making is to get rid of the lignin and keep the fibers.
Once upon a time, all pulp was made using the kraft process, in which wood chips are placed with strong chemicals in huge heated vats known as digesters. The chemicals and heat dissolves the lignin without breaking the cellulose fibers. But kraft pulp making has been an environmental nightmare, producing pollutants that range from dioxins and furans to alcohols, chlorates, heavy metals and more.
A newer kind of pulp, bleached chemi-thermo-mechanical pulp (BCTMP), is more environmentally friendly. It requires only half as many trees as the kraft process to produce the same amount of pulp, and also produces fewer pollutants. In the BCTMP process wood chips are treated with mild chemicals (chemi-) and steam (thermo-), then ground between large steel disks (mechanical). The resulting pulp is screened, cleaned and filtered, then bleached in a hydrogen-peroxide solution, which can be partially recycled.
The process still takes a lot of fresh water, which flows counter-current through the mill from the drying end to the front end. Traditionally, fresh water from a stream or lake comes in and the contaminated water that comes out is treated before being discharged back into the environment.
But in 1992, a new pulp mill in Saskatchewan went one step further. By recycling the water it used, it became the world’s first successful zero-effluent pulp mill.
Effluent at the mill is screened, then treated in two flotation clarifiers, in which tiny bubbles lift the lightest particles in the water to the surface, while the heaviest particles settle to the bottom (thanks in part to chemicals that prompt particles to clump together).
The clarified effluent goes into three 30.5-metre-tall evaporators filled with more than 190 kilometres of heated stainless-steel tubes. The effluent is pumped to the top of the evaporators, and flows down the inside of the tubes as a thin film. As it does so, a portion of it evaporates. The falling film drags the vapor down to the bottom of the tubes, where it is drawn off into a compressor and sent to the outside of the tube bundle.
The steam first flows through the clean condensate region, where most of it condenses into distilled water. The vapor which remains, full of volatile organic compounds such as methanol and acetic acid, condenses in the foul condensate region.
Seventy percent of the clean condensate is sent back to the pulp mill for use as hot wash water at the mill’s back end. The rest goes to the distillate equalization pond. There it is combined with makeup water from the nearby lake (which is required at a rate of about 2.3 cubic metres of water per air-dried tonne of pulp). The foul condensate is sent through a steam stripper, which removes the volatile organic substances in the water (they boil at a lower temperature than water). The stripped condensate is combined with the clean condensate for reuse.
The effluent left after the pass through the evaporators is about 35 percent solids. It’s sent to two concentrators, which work pretty much the same as the evaporators. The effluent coming out of the final concentrator is a thick “liquor,” 67 percent solids, that can be incinerated along with the volatile organics stripped out of the foul condensate. The incineration takes place in the recovery boiler, and generates steam to operate the concentrators. Inorganic chemicals are recovered as smelt (a molten solid), which flows out into a conveyor belt of moving pans. The smelt solidifies into ingots, then is taken off and stored.
As a result, the mill uses only about a tenth as much water as a typical mill of its type, and has no effluent discharge. Of the 8,000 litres per minute of effluent sent to the system, 7,820 are recovered as high purity water for reuse in the pulping process.
And where is this paragon of pulp mills? Meadow Lake, of course.
Today the Meadow Lake pulp mill only seems to make the news because of financial woes. It would be a shame if that were to cause people to forget that it’s also a world-class engineering achievement—one right here in Saskatchewan.
