Water Use Reduction and Reuse

Water is used by all pulp, paper, and recycling mills in significant quantity. There are a number of processes which use water and generate waste water, but the primary uses are pulp washing following digestion and bleaching, and deinking and washing in recycling mills. Since most water used by a pulp mill is heated prior to use, reducing water usage can very significantly reduce energy usage. Energy costs and savings are often the most significant aspects of water usage reduction programs.

Brownstock Washing

Pulp washing is usually done in either of four washing systems, vacuum drum, pressure, diffusion, or Chemi. A brief description of each is provided below. Washing is done to remove cooking chemicals and to remove lignin breakdown byproducts as a prerequisite to pulp drying, bleaching, and paper making.

• Vacuum Drum Washing

Vacuum drum washers usually consist of three to six counter-flow stages. Each stage consists of a rotating screen drum which has a partial vacuum applied to the interior. The drum sits in a tank where pulp is diluted with wash water. The vacuum draws a pulp mat against the surface and wash water through the mat. The drum rotation advances the washed pulp mat to the next dilution tank. Wash water discharged from this wash stage is sent to the previous washing stage. At least six Georgia mills use vacuum drum washers.

• Pressure Washing

Pressure washing is similar to vacuum drum, but differs by spraying water under pressure through the pulp mat as the drum rotates. At least one mill in Georgia uses pressure washing.

• Diffusion Washing

Diffusion washing is a counterflow process that takes place in one or more stages. Pulp flow is upward and is carried on a perforated plate; water flow is downward through a series of baffles. This method offers a high degree of cleaning with low water use. At least three mills use diffusion washers in Georgia.

• Chemi or Belt Washing

A Chemi or belt washer is perhaps the simplest washing system design and offers reduced water usage with excellent cleaning. Belt washing is a counter flow process where pulp enters the washer area on a wire belt. Washing takes place under a series of showers. Clean water enters on the opposite end from the pulp and is sprayed vertically through the pulp. The used wash water is then collected and reapplied to the dirtier pulp by the next washing head countercurrent to the direction that the pulp moves. This process is continued through at least seven stages until the wash water is saturated with liquor after washing immediately incoming pulp. The wash water is then sent to the recovery process. Two mills in Georgia are known to use Chemi washers. Reduced dilution of the liquor recovered from washing will result in reduced energy usage in the recovery process.

The effluent from brownstock washing contains black liquor that must be recovered in addition to varying amounts of water. Diffusion washing and belt washing can reduce the amount of water used per ton of pulp in brownstock washing by 50% or more according to published data. However, the mills in Georgia do not show order of magnitude reductions when using the newer washing technologies. Comparison between mills is difficult due to product mixes of virgin and recycled pulp; mills that have replaced vacuum drum washing with diffusion or belt washing report that improved cleanliness or water reductions were achieved with the new technology, but exact figures were not available. Improved cleanliness is important in reducing bleaching load.

Costs of Pulp Washing With a Chemi Washer

Although several mills in Georgia have upgraded pulp washing, specific information on cost and water usage was not available. In the EPA's document, Pollution Prevention Technologies for the Bleached Kraft Segment of the U.S. Pulp and Paper Industry, the following estimate was provided.

Due to the long payback period, replacement of vacuum drum washers with a Chemi washer is more feasible in a new installation or when a new washer is required to replace a vacuum drum washer that has reached the end of its service life. The installation cost does include all support facilities such as a building to house the unit and piping. The actual replacement cost would be less if support facilities were already existing. Also, the operating cost reduction does not include savings in the bleaching stage. Improved washing will result in less lignin in the pulp thus reducing bleaching requirements.

Bleach Mill Effluent Reduction

Washing of brownstock prior to bleaching allows the recovery of wash effluent in the recovery boiler. Once the pulp has gone through a chlorination stage, recovery is not practical due to corrosiveness. Improved washing and increased delignification prior to bleaching decreases bleaching load and reduces the load placed on wastewater treatment. Bleach mill effluent reductions can then best be achieved prior to any chlorination stage. Improved delignification can allow bleached pulp mills to eliminate certain bleaching and extraction stages while achieving the same level of brightness. Elimination of a bleaching stage can reduce water usage by 2,500 gallons per ton of pulp production.

• Bleach Mill Effluent Recycling

Several mills and corporate organizations are exploring reuse or are reusing bleach mill effluent in previous bleaching stages. Bleach mill effluent accounts for approximately 50% of a bleached Kraft mill's water usage with effluent streams from each stage of bleaching. Recycling water to previous stages could reduce water usage by 60% depending upon the bleaching configuration. Wastewater from bleaching and pulping will contain methanol, other organics, and chlorinated compounds. Reuse in uncontrolled areas could increase air emissions. Typically, a mill will recycle white water from the paper machine back through the various bleaching stages in a counter-current process.

Other Water Use Reduction Opportunities

• Process Pump Seals

Seal water used in various pumps has been eliminated by replacing with mechanical face and lip seals in several facilities around the state. Seal water flows are typically not very high, but are continuous and can add up to large volumes over time. Replacing water seals with mechanical seals usually increases energy consumption due to friction. Seal water can also be collected and reused for other processes but may contain contaminants from the chemical passing through the pump.

Seals are also a source of air emissions, water pollution, and raw material loss due to leakage of the material being pumped through the seal. Chemical pump seals are being investigated by some mills for replacement with sealless and magnetically coupled pumps. An example of a sealless pump is a diastolic tube pump. Magnetically coupled pumps transmit power from the motor to the pump with a magnetic field. Traditionally, both designs are better suited for low torque applications. In either case, sealless pumps are not new technology and should work well in the pulp industry.

Union Camp in Savannah has replaced water lubricated seals with mechanical seals. According to an article published in the November 1995 issue of "Pulp and Paper," replacement of braided packing seals with a self-lubricating compound has reduced water usage and equipment failure. The compound used is a "colloidal mixture of lubricants amalgamated with Aragraphe fibers" manufactured by Tom-Pac of Montreal, Canada. The sealing compound also allows leaks to be repaired by injecting more compound into the seal. Replacement of water lubricated seals also reduces the potential of bearing and gearbox contamination with water, and seal water contamination with oil or other materials. Union Camp points out that 95% of the seal replacement projects have been successful. The benefits of this waterless sealing compound to Union Camp have been:

• reduced water usage
• reduced equipment damage
• reduced water contamination
• reduced maintenance time

Vacuum pumps are used to supply vacuum to vacuum drum pulp washers and other mill equipment. Vacuum pumps are typically liquid ring pumps which use water for sealing and cooling. Water usage is continuous and can be high depending upon the size of the pump. Liquid ring vacuum pumps represent the common technology used in hospitals and other facilities needing large vacuum volumes. In resent years, oil-free and oil-flooded rotary screw pumps have been used in some cases to replace liquid ring pumps to reduce water usage in hospitals. This technology may be useful in the paper industry to replace liquid ring vacuum pumps. Water discharged from liquid ring pumps should be suitable for reuse in any number of mill applications, but may contain some process contaminants.

• White Water Recycling

Paper machines can generate significant amounts of wastewater. One mill generated 3,000 gallons per ton of paper. White water is typically clean, but may contain some contaminants such as fiber, lignin compounds, and various organics. Some of Georgia's pulp mills are recycling paper machine water back to bleaching or pulping operations. Where bleaching does not exist, white water from the paper machine can be reused or partially reused directly in pulp washing or in the wet end of the paper machine. One Georgia mill recycling corrugated material reuses all white water after clarification. The sludge that settles during clarification is also reintroduced into the process. The company, Sweetwater Paper discharges approximately 810 gallons per ton of pulp produced.