Water treatment: a solutions roundtable: in this exclusive roundtable, industry experts examine ways to use less and obtain more from the water stream at a mill.

Water, water everywhere, and not a drop to ... waste. Water reuse is becoming more important at pulp and paper mills dealing with water limitations. Changes in water quality and availability during drought conditions and limitations driven by environmental regulations also affect mills.

What technologies and "best practices" offer the best options for mills that are striving to reduce--sometimes dramatically--their fresh water usage? Mills must first make water use a priority according to Kasy King of Papermaking Process Consulting LLC, Appleton, Wisconsin, USA. "In my experience, 15% to 25% of water use can be eliminated by practicing good water use habits. Shut off running hoses, fix leaking pump seals, optimize saveall operation, set chest level alarms properly, and educate operating personnel on the cost of water."

"Next, eliminate fresh water use on key table and felt showers," King continued. "This is possible with clean process water from the clear saveall leg. Then use excess saveall cloudy water for consistency control on the paper machine and in the pulp mill. Finally, bring back primary and secondary discharged effluent for use for digesters, pulp make-down, and broke processing."

Mills should monitor and adjust process chemistry through all these steps as the soluble organic and inorganic materials concentrate. "Current available technologies of coagulants, high molecular weight polymers, and microparticulates allow these components to be retained and carried out with the sheet without quality losses," King said.

Mills should use a three-step, systematic approach to water resource management according to Scott Metcalfe, marketing manager, paper services, ONDEO Nalco Co., Naperville, Illinois. The approach includes basic conservation, water reuse (reclaiming water from one source and using it unchanged as makeup for another process), and water recycling (after treatment).

Metcalfe noted that the potential for increased use of water resources increases as a mill progresses up the "pyramid," as Fig. 1 shows. The size of the capital investment increases also. For minimum financial risk with maximum water savings, mills should consider this approach:

1. Mill audit to develop a detailed picture of current water use and mill operation practices

2. Aqueous process simulation of fiber, contaminant, and heat balance of mill operation

3. Design and simulation of new reuse/recycle scenarios

4. Cost, risk, and benefit analysis of reuse/recycle scenarios.

[FIGURE 1 OMITTED]

Several factors drive decisions about water recovery, including financial benefits, the requirements of the end-use stream, and the difficulty of cleaning the source stream, according to Jim Graham, business manager, papermaking technologies for Buckman Laboratories, Memphis, Tennessee. "Another factor that drives the decision for reuse is energy," he added. "A waste stream containing a high amount of heat may be recovered seasonally with the benefits of the recovered Btus driving the decision rather than the other factors," Graham noted.

Reuse becomes especially complex at higher levels (70%-80%) of closure when the accumulation of dissolved and colloidal solids leads to increased deposition, foam, corrosion, and microbiological activity, according to Steve Tremont, global marketing director for Ciba Specialty Chemicals, Suffolk, Virginia. He suggested that dissolved air flotation (DAF) clarification technology can act as a "kidney" for the mill to reduce dissolved and colloidal solids, including metals, pitch, stickies, and the associated chemical oxygen demand and biological oxygen demand accumulations.

"Effective DAF treatment of highly contaminated mechanical and secondary fiber water sources usually requires the use of bentonite clays with polyacrylamide flocculants or phenolic resins with polyethylene oxide polymers to remove the interfering colloidal contaminants," Tremont said. "Bentonite is especially effective in these water clarification applications and on paper machines and savealls. These inorganic microparticles act like a sponge to adsorb 20 times their weight in contaminants independent of the high ionic charge demands that limit conventional dual polymer and silica systems."

Norris Johnston, application team manager/water management technology for the Hercules Pulp and Paper Division, Jacksonville, Florida, recommends an accurate water mass balance as the first step to any water conservation program. "This approach often highlights 'low hanging fruit' that might reduce water use with simple, inexpensive operating procedure changes or minor process modifications," he said.

During an initial mass balance, a mill should determine the "normal" temperature of its various water streams since temperature will increase with system closure. "This data will be invaluable when considering reuse at various process points," Johnston said. "Mill management and process engineering with the water consultant should also establish 'minimum quality' standards acceptable for each process area, including temperature, total suspended solids (TSS), and total dissolved solids (TDS). These data become the basis for further water conservation opportunities."

The technology a mill chooses must be system specific, and water chemistry is a particular area for concern. This includes calcium, silica, or chlorides. It will dictate what combination of mechanical, chemical, or both treatments a mill will need. "When temperature is too high, the use of open-recirculating cooling towers rather than the more common 'once-through' cooling will conserve water. Where TSS requires reduction, liquid-solids separation techniques are very effective either with or without chemical coagulants. Where TDS is an issue, precipitation softening and ion exchange or the emerging membrane separation techniques might be justifiable. As with suspended solids reduction, dissolved solids reduction requires tailoring to the particular process need," Johnston noted.

When water reuse projects involve multiple streams, a computer model for predictive purposes can help the mill evaluate its options. "These predictive capabilities must include water flows, temperature, and quality as a result of reuse scenarios," said Johnston. Once the model predicts flow, temperature, and purity at each process critical point, mills can make better decisions about which treatments will be most effective.

TOMORROW'S TECHNOLOGIES

Cost, performance, and environmental reasons all drive water chemistry choices. These considerations have sparked a range of technological developments that can help mills reduce environmental impact and improve operator safety. Improved feed control and monitoring provided as a stand-alone PC system or connected to the distributed control system of a mill offer further benefits. "These automated systems can respond to process changes, system chemistry, and dosing pump variations to ensure economic and effective chemical treatment under most operating circumstances," Johnston noted.

"When you consider the true cost of water as a vital resource used throughout the mill, there is an estimated US$ 10 to US$ 20/ton of production associated with treating the cycle, depending on the grade produced," said Ciba's Tremont. Mills seeking lower production costs should create sound action plans for white water reuse to control dissolved and colloidal solids buildup efficiently in the white water circuits around the paper machine and saveall systems, he noted. "To control the accumulation of contaminants, these action plans may include investment in advanced microparticle retention, drainage, and formation systems including online chemical feed and retention monitoring technology to retain colloidal solids efficiently in the paper or board being produced."

King pointed to activated carbon as a promising technology. "It is expensive, but it will remove almost all organics and color bodies." It also offers reprocessing and reuse advantages, and the capital installment costs are low, he said.

Although environmental impact is a primary driver for improved water management, safety is also a perennial concern. "Alternatives to chlorine used for microbiological control will continue to be available since this a very difficult chemical to handle in a mill. The elimination of products containing volatile organic compounds (VOCs) and nonprocess elements (NPEs) in favor of environmentally friendly versions is also becoming common," said Buckman's Graham. "The improvements in feed and monitoring technology allow precise control of a concentrated product where an overfeed or underfeed situation could potentially negate any economic benefit gained by using such a program."

"A renewed emphasis on recovery boiler leak indication is likely as a result of the tragic explosions last year," added ONDEO Nalco's Metcalfe. "Higher performance polymer internal treatments that improve boiler operation and cleanliness will come to the market in 2003."

REVERSE OSMOSIS 

According to Johnston, reliable membrane and ultrafiltration technologies may significantly reduce environmental impact and improve process safety in water treatment. "The use of reverse osmosis (RO), for example, can greatly reduce or eliminate the need for ion exchange regenerant in the production of boiler feedwater makeup. Suppliers of 'emergency water' have long employed membrane technology to 'pretreat' water supplied to otherwise overloaded ion exchange systems. This provides increased water production with less waste. The approach has kept some mills operating where drought might have otherwise resulted ill a water shortage sufficient to shut down the powerhouse boilers."

RO has historically been a difficult-to-justify water treatment strategy, mainly due to high costs and uncertain returns. The technology has improved as membrane costs decrease and membrane integrity increases. King suggests that RO has particular application at mills whose existing technologies cannot cope with increasing soluble chemical concentrations. "These situations require an 'artificial kidney' to purge the system of contamination, and RO may work. Materials not going out with the product but purged by the kidney must still be discharged safely to the environment at a reasonable cost."

The financial investment is not the only concern. "In mills with a high potential for impacting boiler cycles with the implementation of a reverse osmosis system, energy management will drive the decision," suggested Graham. "In mills where there is a bottleneck in an existing high purity make-up system, improving the continuity of the operation of the make-up system may drive the decision. In mills where there is a need to reduce regenerant usage and waste, environmental drivers come into play."

According to Tremont, RO compares favorable to traditional ion exchange systems in water treatment. "Industrial thin film (TF) and chlorine resistant cellulose acetate (CTA) membranes available today can handle between 1000 and 500,000 gallons/day. Because of maintenance concerns with membrane plugging and bio-fouling, RO applications in pulp and paper mills can be cost-restricted to prefiltered or DAF clarified process water streams with low total solids, slightly acidic pH, and low hardness," he concluded.

IN THIS ARTICLE YOU WILL LEARN:

* "Best practices" that offer the best options for mills striving to reduce fresh, water use.

* Tips an water resource management.

* Water treatment technologies the offer the most promise for pulp and paper mills, including reverse Osmosis.

ADDITIONAL RESOURCES:

* For information about TAPPI committees and divisions that focus on water management, visit www.tappi.org