RECYCLING

A thorough process review for the best water and wastewater management practices can minimize effluent volume and its effects on receiving streams

BY ROBERT J. SYNK

Water Reuse, Reduction Projects Save Money at Recycling Mills

Careful management of process water usage and wastewater treatment practices in OCC recycling mills can reduce operating costs and help achieve compliance with environmental permit requirements. Although reducing water use is usually easier and less costly during the initial engineering of new mills, procedural modifications and design changes to existing operations can also save water and reduce effluent flows. Recent experience in North American linerboard mills has shown how a thorough engineering review of the process can yield opportunities to reduce, reuse, or recycle water to decrease the wastewater stream volume and concentration.

OCC recycling mills are facing ever-tightening limitations on water use and wastewater effluent from the federal and state governments and local authorities. In 1993, the U.S. Environmental Protection Agency (EPA) announced its proposed "Cluster Rule" regulations for reducing air and water releases of pollutants by pulp and paper mills
(40 CFR, Parts 63 and 430).

The regulations that affect secondary fiber non-deink mills have been reviewed and will remain unchanged, based on negotiations with the EPA. Mills targeted in Subpart 3 of Part 430, which is titled "Secondary Fiber Non-Deink Subcategory," are those recycled mills that produce bleached and unbleached tissue, linerboard, corrugated medium, and roofing felt but do not deink.

Existing sources in this category would have faced the proposed limitations on biological oxygen demand (BOD) and total suspended solids (TSS) shown in Table 1. For some existing mills, these limits would have required an order-of-magnitude reduction in wastewater effluents from current discharges.

EPA's New Source Performance Standards (NSPS) for builder's paper, roofing felt, and paperboard mills, which include linerboard and corrugated medium recycling mills, state, "No new source within this segment of this subpart shall discharge wastewater to any waters of the United States." Simply put, this means new OCC recycled linerboard and corrugated medium mills would have been required to achieve zero wastewater discharge.

The limitations shown in Table 2 would have applied to new mills producing other fiber products from non-deink secondary fiber, i.e., pulp and tissue. Compliance with these very stringent regulations by existing and new OCC recycling mills would have required a considerable effort from both mill designers and operators. Although these proposed regulations were not promulgated, the goal is clear: that the EPA may seek significant reductions in water use and wastewater effluents in the future.

WATER MANAGEMENT STRATEGY. How can a new or existing mill economically reduce the operating costs and environmental impacts associated with water use and wastewater treatment? The answer is to carefully manage the mill's process water system to reduce, reuse, or recycle as much process water as possible, decreasing the wastewater stream volume and concentration.

How is this accomplished? The first step is to establish a mill water consumption baseline balance equal to the mill's freshwater consumption rate if the mill were operated with zero wastewater discharge. The next step is to investigate each typical use of freshwater and determine how reduction, reuse, or recycling techniques can be best applied to all mill processes to develop a mill water balance that approaches the baseline water consumption value. Then a return-on-investment analysis will determine an implementation priority for mill process changes based on improving economics or achieving compliance.

BASELINE WATER CONSUMPTION. To establish the baseline value of the mills' water consumption, it is necessary to look at how water is discharged from the mill in ways other than as wastewater effluent. The paper machine dryer section discharges substantial amounts of water as vapor (about 20 gpm/100 machine dry tons produced/day). Atmospheric flash tanks, vacuum pump exhausts, and cooling towers also discharge water vapor into the air. The evaporation at each can be easily calculated.

Water also leaves the system with sludge or other rejects. Each dry ton of rejects at 35% solids takes 445 gallons of water with it to the boiler or landfill. This equals about 0.3 gpm for each daily dry ton of solids, or 0.1 gpm per daily wet ton at 35% solids.

The final product leaving the mill, either linerboard or medium, contains about 6% moisture. This results in the discharge of approximately 1.0 gpm of water for each 100 machine dry tons produced per day.

What is the total volume of water discharged from an OCC recycling mill through these effluent points? A 450-tpd, OCC-to-corrugating medium mill with zero effluent will have an allowable water intake, or water consumption baseline value, of about 130 gpm, which is about 420 gal/daily ton, if the OCC furnish is received with 10% moisture. Any additional water used must then be recycled from sources within the mill.

TYPICAL FRESHWATER USE REDUCTION. The freshwater consumers in the mill present the best opportunity for reducing water use and wastewater flow. Some of these include the following.

Washdown water. Washdown water can be from any source that is not too hot or contaminated. A possible source is clarified effluent. The washdown hoses should be on an independent header made of corrosion-resistant material. Training and operator awareness are the main contributors to significantly reducing washdown water use.

Cooling water. Cooling water can come from either freshwater or from water cycled through a cooling tower. In some cases, warm water that has been used previously for cooling may be suitable for some additional cooling applications, such as roll cooling and pump seal water.

Major users of freshwater for cooling in some mills are the liquid ring vacuum pumps. Water use may be reduced by cascading the cooling water through trains of vacuum pumps. Also, this is a good application for a cooling tower that allows the water to be cooled and recycled to the vacuum pumps. This water may be contaminated with machine felt hairs and fine fibers and should be filtered and kept separate from clean warm water.

To keep the amount of water used to a minimum, temperature controls can be installed at each use of cooling water to reduce the flow by heating the cooling water to the highest temperature that accomplishes the purpose. Some inexpensive, self-contained controls are available.

Clean warm water should be collected in one or more clean warm water tanks. The cost of this collection can be fairly expensive for small flows. Depending on the layout, small standpipes with pumps and level control may need to be strategically located to collect warm water and pump it to a warm water collection tank. The best approach to properly sizing the warm water tank to reduce spills is to make the best possible determination of the quantity of excess warm water that can accumulate during a partial shutdown or production upset.

If properly monitored by conductivity meters in strategic locations, the warm water should be suitable for most freshwater applications. Proper collection and distribution of cooling water can allow almost total reuse. This warm water should be used as much as practical for seal water or high-pressure showers, where cleanliness is essential but the higher temperature is not a problem.

Cooling towers on clean water become necessary when temperatures in the "used" cooling water are excessive and must be reduced before the water can be reused. An additional benefit of using a cooling tower is that the cooling tower evaporation will allow a significant increase in the mill freshwater intake without increasing the volume of wastewater discharged.

Seal water. Seal water for pumps and other equipment can be drastically reduced in many cases. Seals are of three basic types: packed stuffing boxes, mechanical seals, and dynamic seals.

Packed stuffing boxes for cool liquids typically require only 0.2 to 0.5 gpm for proper packing lubrication. As few as 40 to 60 drops/min out of the stuffing box will properly lubricate most packing. Some packing is adequately self-lubricating, but it may need water flushing to keep abrasives out of the stuffing box or may need cooling by a small water flow if the temperature of the pumped fluid is higher than 100F.

Types of mechanical seals are numerous. Most require some liquid for lubrication and cooling.

Dynamic seals theoretically do not require sealing water during pump operation. For stock applications, having a short flush as soon as the pump is shut down is a good idea. This keeps stock from filling the dynamic sealing vanes. The flush can be controlled by a timer to reduce the amount of water used. Dynamic seals are not practical if the pump suction pressure is high or if the discharge pressure varies widely.

For pumps or other rotating equipment, the manufacturer is probably the best source for information on reducing water use. Requests for a quotation for equipment requiring sealing should include a request for alternative seal pricing and discussion of sealing options, with an estimate of seal water requirements for each. Recent interest in controlling seal water has produced a number of different seal water flow controllers which, under certain circumstances, work well to reduce pump and agitator seal water use.

Cleaning showers. Freshwater use in cleaning showers can be reduced by taking one of two basic approaches. The first is to use filtered whitewater. The filter can be just a strainer to protect against upsets if shower nozzles are large enough or are self-cleaning. Some showers may require an extensive filtration system to allow satisfactory operation.

The second approach is to replace the existing shower with a freshwater shower that uses significantly less water. Equivalent cleaning can often be accomplished with much less water at a higher pressure. Also, single-nozzle, high-pressure needle showers can often replace multiple-nozzle oscillating showers, particularly on machines using newer, more open fabrics.

Equipment manufacturers and clothing suppliers should be able to describe the water quality needed and suggest a shower design that can give satisfactory service while possibly reducing freshwater use.

Lubrication showers. Lubrication showers for Uhle boxes can probably use filtered whitewater. Lubrication water for suction rolls should be a high-priority use for clean water.

Dilution of additives. Some additives can be successfully diluted with whitewater or filtered whitewater. However, caution is advised, since some additives can react unfavorably with dissolved or suspended solids in recycled water. Attempt this only with the agreement of the chemical supplier and only if possible variations in dilutent are carefully considered.

Recovery of spills. Installing spill sumps and chests that are connected to tank and chest overflows and drains allows spilled stock or whitewater to be re-entered directly into the process. Depending on the finished product and the process equipment available, some U-drain flows may be collected, clarified, and reused in the process or even reused without clarification. For example, some mills that produce linerboard and corrugating medium from an OCC furnish pump the U-drains directly into the OCC pulper. Most contaminants in the U-drains are successfully removed by the OCC screening and cleaning systems.

In new installations, larger tanks and chests can be installed to reduce the possibility of spills. Also, extensive use of a fan pump type of arrangement can simplify startup and greatly reduce spills. In an arrangement like this, a pump suction pipe is connected to a whitewater chest or tank, and the stock feed is tied into the suction pipe. Typically, several stock pumps will be connected to the same whitewater chest or tank. If the stock feed entrains significant air, a standpipe between the whitewater chest or tank and the pump may be required.

With proper design, this type of system can be started on water in reverse operating sequence, and then the stock feed can be started. This approach greatly reduces the probability of spills when compared with startup, shutdown, or operation of a system with many stock tanks.

WATER RECYCLING CONSIDERATIONS. As a mill increases recirculation of water and reduces the amount of effluent from the system, dissolved solids and chemicals will build to higher concentrations. This higher concentration adversely affects papermaking and normally increases corrosion. The allowable dissolved solids level will vary widely depending on the grades produced. As the recirculation of process water is increased, the corrosion resistance of piping, valves, tanks, and equipment needs to be re-evaluated.

Higher concentrations can also seriously affect chemical reactions, chemical type, and chemical quantity requirements. Additional equipment to condition recycled water, or adjustments in the process chemistry to compensate for residuals in the recirculation loops, or both may be required.

Existing mills with wastewater discharge permits can reduce flows and contamination levels in the effluent. Additional wastewater processing may
be required for compliance with tighter effluent limits. The discharge of a wastewater stream, even at a greatly reduced volume, can keep the increase in concentration of harmful chemicals and dissolved solids in the process water to a manageable minimum.

For new mills, which may be required to meet zero-effluent regulations, treatment of a wastewater stream by distillation prior to recycling may be required to reduce the harmful dissolved solids and chemicals buildup to a manageable level.

Lessons gained from experience

Recent environmental compliance projects at several North American OCC recycling mills produced the following recommendations for economically reducing environmental impacts by carefully managing process water use and wastewater treatment practices. These mills typically use 100% secondary fiber furnish and discharge effluent to a publicly owned treatment works. The consulting engineer led detailed discussions among mill operators, mill engineering, and equipment suppliers to identify these process changes for evaluation.

• Install a spill collection, storage, and reuse system to reduce fiber losses and stabilize flow rates to the wastewater system.
• Reroute piping to discharge U-drains and the spill collection system to the pulper rather than to the wastewater treatment system.
• Change process piping to discharge clear whitewater rather than contaminated sump water to the wastewater treatment system to reduce TSS (including fiber) to the wastewater system.
• Optimize the effluent clarifier efficiency by proportioning the unit operations and polymer additions to the wastewater flow rate.
• Change the paper machine retention aid program to improve starch retention and reduce mill effluent COD levels.
• Recycle settling basin sludge to the aerated stabilization basin to increase sludge age, which improves biological treatment and reduces effluent concentration.
• Recycle treated wastewater for starch makeup.
• Control and recycle press roll cooling water.
• Rework the arrangement of the paper machine shower nozzles to reduce water use.
• Recycle boiler and cooling tower blowdown water.

   

REFERENCES

Tom Bolick and John Yolton, "The Role of Engineering in Water Conservation," TAPPI Journal, Vol. 79, No. 12,
p. 125.

G. Bosar, Mechanical Seals: Guidelines for the Pulp and Paper Industry, TAPPI Press, Atlanta, 1993.

C.J. English, "Water Reduction Opportunities in Recycled Pulp Production," Paprican, CPPA Montreal, 1994,
Chapter 6.

Christine Foster and Dominic Rende, "How Recycling, Water Reuse Impact Chemistry," PIMA's Papermaker, January 1997, p. 48.

Goulds Pump Manual, 5th Edition, Goulds Pumps Inc., CPPA Montreal, 1994, 1988.

Carl W. Kohler, "Tighter Regulations Spur Design of Zero-Discharge Recycled Mills," PULP & PAPER, May 1994,
p. 89.

ROBERT J. SYNK, P.E., is director, business development, Sandwell Inc., Atlanta, Ga.