Installed closed-loop wastewater recycling system.

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Curtis Lyons
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1 MNNESOTA TECHNCAL ASSSTANCE PROGRAM 000?/ Wastewater Recycling System Used in Paper Manufacturing Reduces Wastewater Discharge CASE STUDY Company Producthdustry Waste streaxdchemical Process Change Benefits Hennepin Paper company Little Falls, Minnesota Paper Mill Wastewater Manufacture colored construction-grade paper using once-through water system. nstalled closed-loop wastewater recycling system. Reduced wastewater discharge from approximately 2 million gallons per day to 45,000 gallons per day. Based on an average operating year, the following approximate annual releases have been eliminated: 562 million gallons of wastewater, 149,000 pounds of total suspended solids, and 57,000 pounds of biochemical oxygen demand. Hennepin Paper Company, a specialty paper mill located in Little Falls, Minnesota, manufactures colored construction-grade paper. The paper is made from a mixture of recycled paper, groundwood pulp and kraft paper. n July 19m, Hennepin Paper Company initiated a pollution prevention program with the goal of eliminating the discharge of wastewater into the Mississippi River. This would be accomplished by rerouting the wastewater back into the manufacturing process for reuse. Another goal of the program was to reduce usage of river water. At times a local power company s hydroelectric power plant, located immediately upstream from Hennepin Paper Company, would lower the river level to do maintenance work on the power plant. This would disrupt Hennepin Paper s intake of river water. By reusing wastewater, Hennepin Paper would eliminate its reliance on the river and be ensured an uninterrupted water supply. Page 1 (continued) th St- SE suit^ 807 Minnmmpoiir, Minnemota b C CEO CMinnrmotm only1 FAX re123 e27-47~~ Tho Mlnnrqtr Otlom of Wntr Mlmrgomont~ Mn TAP program im supported with a grant to thm Bohooi of Publlc Health. Olvlmion of EnviMnmrntal and Occupntlonal Health, at the Univerrity of Minnrrotm. Printed on recycled paper

2 Before initiating the pollution prevention program, all of the mill s process and cooling water was supplied by the Mississippi River. Freshwater from the river entered the mill, passed over a fine-mesh screen, entered a 2,700-gallon freshwater tank, and was then pumped to water-demand points throughout the mill. The resulting wastewater was processed through. a treatment system and discharged to the Mississippi River through two outfalls: one discharged process wastewater and the other discharged cooling wastewater. Averages of 607,000 gallons of process wastewater and 1,140,000 gallons of cooling wastewater were discharged each day. Process Change The first step in the company s pollution prevention program was to reroute the process wastewater and pump it back into the freshwater tank. An automated valve was placed in the incoming freshwater line to control the water level in the freshwater tank. When the volume of wastewater supplied to the freshwater tank was insufficient to meet the processing demand, the valve opened to bring the water level up to the required amount. Conversely, when the supply of wastewater exceeded the amount required for processing, the wastewater would overflow the freshwater tank and reenter the wastewater treatment system. By pumping increased amounts of treated wastewater into the freshwater tank, the mill s process and cooling water became nearly 100 percent recycled wastewater. To determine what type of pumping and piping system would be required, Hennepin Paper collected data on water volume input and output. Obtaining accurate flow volume measurements was difficult, particularly in assessing the amount of wastewater recycled daily through the system. Two, 900 gallons-perminute pumps and approximately 1,000 feet of PVC and stainless steel piping were installed to reroute and pump the wastewater back to the freshwater tank. To prevent treated wastewater from overflowing the freshwater tank and reentering the Wastewater treatment system when supply exceeded demand, a surge tank was incorporated into the system. The surge tank holds the treated wastewater in reserve until water demand increases (see attached diagram). Product and Equipment Problems L Several problems in manufacturing operations resulted from the reuse of wastewater. During the production of colored construction paper, the wastewater becomes colored from the dyes used to color the paper. One of the first problems was to determine how to modify production using colored wastewater instead of clear water. Originally, colored paper was manufactured on demand with no specific schedule for a particular color. With the use of colored wastewater, the manufacturing schedule was arranged 90 the color of the wastewater would not affect the final color quality of the paper. After experimenting with various formula combinations, the company found that a manufacturing schedule which started with lighter colors could progress to deeper and darker colors, culminating in black. After black, successively lighter shades could be made, such as grays, browns or blues, finally ending with white. This Page 2 (continued)

3 ~ of color schedule went through several changes before the required color qualities were produced. Another problem was an increased wear on pumps, and plugged pipes and showers caused by higher rates of suspended solids and biological contaminants in the wastewater. Cleaning, repairing and replacing equipment increased labor costs by 32 percent a month and material costs by 145 percent. To help alleviate the problem of biofouling and pipe plugging, a chlorine dioxide disinfectant was added to the wastewater. As an alternative to chlorine dioxide, hydrogen peroxide was also evaluated for its ability to control.biological growth, and was found to be effective in controlling some biological contaminants. Bacterial contamination also created problems with heat exchangers used to condense steam from the paper machine dryers. The recycled wastewater caused the cooling tubes in the heat exchangers to become fouled or plugged. This impeded heat transfer causing a loss of steam and decreased production rates. To eliminate this problem, a closed-loop cooling water system was installed to provide clean cooling water for the heat exchangers. River water now circulates through this system and is cooled with an induced draft cooling tower. Water from this syst+m is constantly drawn off to feed a crucial shower on the paper machine, which is also unable to operate properly using recycled wastewater., i Other areas of the papermaking process also require water of higher quality than the recycled wastewater. For these applications, city water must be used. Because riqer water and city water are added to the closed loop system, an excess appraximztely 40,1)00 gallons per day of wastewater is produced. This wastewater is subsequently treated and discharged to the river. Customer and Employee Acceptance Changing the paper color production schedule from manufactured on demand to manufactured on a specific weekly schedule created few problems with customers. Hennepin Paper explained their reasons for the process change to their customers who generally accepted the change and adjusted their expectations for delivery dates to adapt to the. mill s production schedule. As a result, Hennepin Paper lost no customers. 1 Hennepin Paper s employees were essential to the success of the wastewater recycling program. However, employee acceptance of the process change was slow. At the beginning of the process change, employees had to endure the frustrations caused by equipment problems and loss of product quality. The ability of the employees to devise a scheduling system that allowed for the use of colored wastewater was very important to the success of the system. Through experimentation and constant monitoring, employees learned how to produce a specific paper color while using wastewater that had previously been used to Page 3 (continued)

4 . A. produce a different color(s). Various combinations and amounts of coloring dyes were tried before the employees determined what worked best. The paper produced during the experimental stage often did not meet quality requirements. This rejected paper was not discarded, but instead was reused as stock at the beginning of the production process. Achieving Pollution Prevention Hennepin Paper Company s wastewater reuse system has eliminated over 95 percent of the wastewater discharged from the mill to the Mississippi River. Total suspended solids (TSS) and bioclhemical oxygen demand (BOD) discharged to the river have decreased 80 percent and 50 percent, respectively. One way to reduce or eliminate biofouling in equipment is to improve the quality of treated wastewater. Hennepin Paper s current wastewater treatment plant does not have a primary clarifier to control amounts of wood fiber entering the system. The chief constituent of the fiber is cellulose, which does not decompose in the wastewater treatment plant. By removing more of the fiber, it is hypothesized that the organic loading of the treatment system could be eliminated and more efficient biological treatment would result. This would increase the qudity of the wastewater reused in the manufacturing process. To achieve their ultimate pollution prevention goal of zero discharge, Hennepin Paper Company will be filtering out more of the wood fiber and is installing sand and carbon filters to improve the quality of the treated wastewater. This should allow the treated wastewater to be used in all operations throughout the mill, and result in a fully recycled system and zero discharge of wastewater to the Mississippi River. This publication was created with the assistance of Hennepin Paper Company s Environmental Maim Manager, Ronald Klinker. Copyright 1992 by MnTAP.. Material in this publication may be reprinted only with permission &om MnTAP.

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