Bleaching and the environment in the 21st century.

A decade ago, pulp bleaching was the hot research area. A major reason for that was the concern that by-products from chlorine-based bleaching processes were potentially toxic. Dioxin was a particular concern, and chlorine-containing chemicals (measured as AOX) were suspect generally. Alternative bleaching processes and sequences were developed, modified, and optimized.

Through the mid-1990s, U.S. pulp producers were unsure of how extreme revised government effluent restrictions would be. The U.S. Environmental Protection Agency (EPA) delayed its final ruling several times as it gathered information about best available technologies (BAT) and made adjustments to the proposed requirements. When the U.S. EPA finally published the Cluster Rule in 1998, U.S. pulp producers were well on their way to eliminating chlorine bleaching. (For the complete text of the Cluster Rule, visit http://www.epa.gov/waterscience/p ulppaper/cluster.html.)

Environmental groups such as Greenpeace have advocated totally chlorine free bleaching (TCF) and zero effluent processes. Several mills, mostly in Scandinavia and Western Europe, have opted to produce TCF pulp, while nearly all mills in North America use elemental chlorine free (ECF) processes. Both processes have effectively reduced or eliminated dioxins and other toxics to "nondetect" levels, as efforts continue to minimize discharges.

This progress was recently documented in the March 2002 "Review of Scientific Basis for AOX Effluent Standard in British Columbia," by John Carey, Eric Hall, and Nell McCubbin (available as a PDF at www.aoxpanel.ca). The review evaluated the scientific rationale to implement the zero AOX discharge requirement of a 1992 provincial regulation. Pulp mills would have had to comply with that requirement by the end of 2002. The panel concluded that further reductions in effluent AOX beyond those already achieved by British Columbia's bleached kraft mills would not produce demonstrable environmental benefit.

The panel did suggest setting discharge limits to those already being achieved by the mills. The average annual AOX discharge rate for the mills in 2000 was less than 0.5 kg/ton, compared to an average of 6.5 kg/ton in 1988. The panel also suggested that further benefits could be achieved by minimizing the discharge of black liquor.

MOVING TO OTHER CAUSES

Although the watchdog groups Reach for Unbleached (www.rfu.org) and West Coast Environmental Law Association (www.wcel.org) criticized the panel's findings, most environmental groups are devoting the bulk of their efforts to other causes, for now.

At the Institute of Paper Science and Technology (IPST) in Atlanta, Georgia, USA, and elsewhere, research on pulping and bleaching processes continues, though the emphasis is shifting more toward product development and energy efficiency. Ana Retzlaff, a PhD candidate at IPST supervised by Arthur J. Ragauskas, has been studying the effectiveness of chlorine dioxide and ozone in various sequences. The goal is to help mills save money by determining the comparative effectiveness of various sequences. An IPST-North Carolina State University study, sponsored by the U.S. Department of Energy through Agenda 2020, is aimed at developing new extended oxygen delignification systems (see http://home.ipst.edu/~aragausk/Projects/National_CompetativeGrants/bo dy_national_competative_grants.html).

"Oxygen is the lowest cost bleaching chemical available today," said consultant Wayne Bucher ([e-mail]). Using it effectively, however, usually requires some investment for washing and pressure vessels, he said. "As usual, there are trade-offs between capital and operating costs."

Bucher emphasizes that each mill is unique. "It is impossible to identify one sequence as the best for all pulp mills," he said. "However, a modern OD(Eop)D sequence will be very effective for many integrated mills.

"Modern, two-stage oxygen delignification systems are powerful and adaptable," Bucher continued. "They are environmentally sound, as the organic load is burned rather than sent to the sewer. Oxygen delignification removes lignin that historically was removed from the fiber by attacking it with chlorine and/or chlorine dioxide."

Within five years, more economical and sophisticated control systems will further improve bleaching processes. Enzymes also will gain wider use and will be better tailored to specific uses, noted Bucher.