Pulp Mills

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Contents

Introduction

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Pulp mills are large industrial facilities that convert timber, wood chips or other wood products into wood pulp that is then used to produce paper, cardboard or other products. The manufacturing process uses a substantial amount of water and can produce large quantities of wastewater. Historically, these facilities used chlorine to bleach the wood pulp to produce white paper. Use of chlorine in the pulping process causes the formation of toxic chemicals such as dioxins and furans in the wastewater. Depending on the degree of wastewater treatment the pulp mill uses, the wastewater effluent may also contain high levels of suspended solids, biological oxygen demand, and high or low pH values. The characteristics of pulp mill effluents have the potential to be hazardous to both humans and marine life.

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The pulp mill industry is one of the largest producers of water pollution in the world. One of Surfrider Foundation’s first and largest victories was a successful Clean Water Act lawsuit filed in 1989 and settled in 1991 against pulp mills in Humboldt County, California that were operated by Simpson Paper Company and Louisiana-Pacific Corporation. These pulp mills used chlorine in the “Kraft” bleaching process to turn brown wood pulp into white paper. In so doing, the mills discharged approximately 40 million gallons per day of untreated wastewater into the near-shore surf zone. This wastewater contained excessive concentrations of Biological Oxygen Demand and suspended solids, had a pH value outside of the permit limits, and contained dioxins and furans, extremely toxic chemicals created by the use of chlorine in the bleaching process. Surfers in the area of the discharge complained of skin rashes and nausea. The mills also produced hazardous, irritating air emissions.

The settlement won by Surfrider Foundation and the U.S. Environmental Protection Agency (at the time, one of the largest penalties levied by the EPA under the Clean Water Act, and the largest in the Western U.S.) required the pulp mills to construct wastewater treatment facilities to eliminate toxic discharges to the ocean and extend their ocean outfall pipes. The mills also agreed to evaluate the elimination of the use of chlorine in the pulp bleaching process through substitution of chlorine dioxide and/or hydrogen peroxide in an Elemental Chlorine Free (ECF) or Totally Chlorine Free (TCF) process. The Louisiana-Pacific (now owned by Evergreen Pulp, Inc.) mill became the first TCF bleached Kraft pulp mill in the U.S. in 1993.

In an interesting case of history (sort of) repeating itself, the Evergreen pulp mill, in a settlement reached in March 2007, agreed to comply with the requirements of the Clean Air Act and to install new pollution-control devices on their smelt dissolver tank and their lime kiln, by September 2008, to significantly reduce overall air emissions. This pulp mill is now shut down, and the local community is involved in a process to determine a proper use for the site. Here's another article discussing the site re-purposing. Of course, there will need to be some site cleanup prior to any re-use of the site.

In 2007, Surfrider Foundation was involved with pulp mill issues in Oregon, Tasmania, Chile and other locations.

Pulp Mill Processes

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Pulp mills convert wood, recycled paper or other products containing cellulose into pulp, which is then used to make paper and related products. In developing countries, about 60% of cellulose fibers originate from non-wood raw materials such as sugar cane fibers, cereal straw, bamboo, reeds, esparto grass, jute, flax, and sisal.

The main steps in pulp and paper manufacturing are raw material preparation, such as wood debarking and chip making; pulp manufacturing; pulp bleaching; paper manufacturing; and fiber recycling. Pulp mills and paper mills may exist separately or as integrated operations. Manufactured pulp is used as a source of cellulose for fiber manufacture and for conversion into paper or cardboard.

The manufacture of pulp for paper and cardboard can use both chemical and mechanical methods. Chemical methods generally consist of cooking (digesting) the raw materials, using the Kraft (sulfate) and sulfite processes. Wood chips are cooked with caustic soda (sodium hydroxide) to produce brownstock, which is then washed with water to remove cooking (black) liquor for the recovery of chemicals and energy. Pulp can also be also manufactured from recycled paper and cardboard.

Mechanical pulp can be used without bleaching to make printing papers for applications in which low brightness is acceptable—primarily, newsprint. However, for most printing needs, the pulp has to be bleached. For mechanical pulps, most of the original lignin in the raw pulp is retained but is bleached with peroxides and hydrosulfites. In the case of chemical pulps (Kraft and sulfite), the objective of bleaching is to remove the small fraction of the lignin remaining after cooking. Oxygen, hydrogen peroxide, ozone, peracetic acid, sodium hypochlorite, chlorine dioxide, chlorine, and other chemicals are used to transform lignin into an alkali-soluble form. An alkali, such as sodium hydroxide, is necessary in the bleaching process to extract the alkali-soluble form of lignin. Pulp is washed with water in the bleaching process.

In modern mills, oxygen is normally used in the first stage of bleaching. The trend is to avoid the use of any kind of chlorine chemicals and employ “total chlorine-free” (TCF) bleaching. TCF processes allow the bleaching effluents to be fed to the recovery boiler for steam generation; the steam is then used to generate electricity, thereby reducing the amount of pollutants discharged. Elemental chlorine-free (ECF) processes, which use chlorine dioxide, are required for bleaching certain grades of pulp.

Increasingly, the use of elemental chlorine for bleaching is being phased out. Only ECF or TCF processes are acceptable, and, from an environmental perspective, TCF processes are preferred. Two schematic diagrams are presented below, showing the pulp mill manufacturing process and typical water and air emissions from these types of facilities.


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Pulp Mill Wastewater and Air Emissions

Because of the use of chlorine or chlorine dioxide in the pulping and bleaching processes, the soluble organic substances removed from the pulp in bleaching stages, as well as the substances removed in the subsequent alkaline stages, are chlorinated. Some of these chlorinated organic substances are toxic; they include dioxins, chlorinated phenols, and many other chemicals. In addition, the wastewater may contain non-chlorinated organics, nutrients (phosphorous and nitrogen-containing compounds) and metals such as manganese. Wastewaters are also high in biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids.

Air emissions often contain highly malodorous compounds such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. Other typical air emissions include particulate matter, sulfur oxides, nitrogen oxides, and volatile organic compounds (VOCs). Steam- and electricity-generating units associated with the pulp mill using coal or fuel oil typically emit fly ash, sulfur oxides, and nitrogen oxides.

Pollution Prevention and Control

As mentioned above, often the most significant environmental issues associated with pulp mills are the discharge of chlorine-based organic compounds (from bleaching) and other toxic organics. In processes that don’t use chlorine, the “black liquor” from the pulping process can be recycled for re-use and energy generation. Some mills are approaching 100% recovery. Industry developments demonstrate that total chlorine-free bleaching is feasible for many pulp and paper products. Pulp mill pollution prevention programs focus on reducing wastewater discharges and on minimizing air emissions. The key production and control practices to minimize emissions and waste include:

  • Use of dry debarking processes
  • Prevention and control of black liquor spills
  • Use of total chlorine-free (TCF) processes, or at a minimum, use of elemental chlorine-free (ECF) bleaching systems.
  • Reducing the use of hazardous bleaching chemicals by extended cooking and oxygen delignification.
  • Pushing for zero-effluent discharge where feasible. Reducing any non-recyclable wastewater discharges to the extent feasible. * Incineration of liquid effluents from the pulping and bleaching processes.
  • Reducing the odor from sulfur compound emissions by collection and incineration and by using modern, low-odor recovery boilers fired at over 75% concentration of black liquor.
  • Dewatering and properly managing sludges.
  • Where wood is used as a raw material to the process, encourage plantation of trees to ensure sustainability of forests.


Implementation of cleaner production processes and pollution prevention measures can yield both economic and environmental benefits.

Treatment Technologies

Air

Sulfur oxide emissions are scrubbed with slightly alkaline solutions. Electrostatic precipitators are used to control the release of particulate matter into the atmosphere.

Wastewater

Wastewater treatment typically includes (a) neutralization, screening, sedimentation, and floatation/hydrocycloning to remove suspended solids and (b) biological/secondary treatment to reduce the organic content in wastewater and destroy toxic organics. Chemical precipitation is also used to remove certain substances such as metals. Fibers collected in primary treatment should be recovered and recycled. A mechanical clarifier or a settling pond is used in primary treatment. Flocculation to assist in the removal of suspended solids is also sometimes necessary. Biological treatment systems, such as activated sludge, aerated lagoons, and anaerobic fermentation, can reduce BOD by over 99% and achieve a COD reduction of 50% to 90%. Tertiary treatment may be performed to further reduce toxicity, suspended solids, and color.

References


This article is part of a series on Clean Water which looks at various threats to the water quality of our oceans, and the negative impacts polluted waters can have on the environment and human health.

For information about laws, policies, programs and conditions impacting water quality in a specific state, please visit Surfrider's State of the Beach report to find the State Report for that state, and click on the "Water Quality" indicator link.