How mill closure affects additives and paper quality.
Antoin Deul, senior research engineer, Advanced Recycle Technologies, Nalco Co., Naperville, Illinois, USA.
Norris Johnston, applications manager, water management technology, Hercules Pulp and Paper Division, Huntsville, Alabama.
Kasy King, president, Papermaking Process Consulting, LLC, Appleton, Wisconsin.
Len Olavessen, marketing manager, water treatment, Buckman Laboratories International, Memphis, Tennessee.
Steve Tremont, director, business operations, Ciba Specialty Chemicals, Water & Paper Treatment, Suffolk, Virginia.
George Totura, program manager--Nalco Co. Water, Paper Services Division, Nalco Co.
SOLUTIONS! What are the current trends in machine closure, and what is the impact on wet end chemistry?
JOHNSTON: Environmental issues and reducing high energy costs are key drivers that continue to propel this trend. For example, Hercules has recently been involved with several energy audits that often point to reuse/recycle as a significant opportunity to lower the overall mill energy costs per unit of production. With closure, both temperature and conductivity increase and pH can fluctuate significantly, upsetting the typical balance between wet end additives. Retention will clearly be impacted due to increased fines loading as well as increased levels of dissolved solids. This makes it more challenging to retain just about every wet end additive--from fillers to functional chemistry additives (i.e., size, wet strength). Fines build-up can also stabilize foam. Deposition can become problematic since pitch and scale build-up due to recirculation can plug shower nozzles, screens, and washer wires.
KING: In my travels to different mills, I have yet to see significant efforts towards closing up water systems. To date there has not been enough price pressure on the mills to make them devote resources to any type of water conservation projects. With the reduction in mill technical and supplier technical staffing/support, there have been few efforts to point out the possible benefits and savings to water conservation. Even where there is recognition of the need to conserve water, there are usually not enough technical resources available to initiate or implement this type of project.
This trend may change direction as energy costs go higher. The payback for water conservation lies in heat and energy savings as well as lower incoming and outgoing water treatment costs. When the savings are significant enough to offset the increased operating difficulties, water closure will become a higher priority. As has been the case for other technologies, Europe is farther along in water conservation than is North America. Europe is somewhat more advanced in project planning/staffing, and the environmental/conservation lobby there is a bit more vocal.
OLAVESSEN: Closure involves water re-use (using the drains from one system going directly to another system without any treatment), or reclaim water (water from one system that is treated to make it more suitable for use in the next system). In both re-use water and reclaim water strategies, we see microbiological problems that normally require more microbicide usage per ton than before. These added costs could be substantial, depending on product requirements.
We have to understand the additional contaminants in re-use water and how they affect the chemistry of the receiving application--such as additional suspended and dissolved solids going to the receiving application that might affect charged chemistry methods. Also, additional solids can cause deposits and fouling we did not see before relying on that re-use water. In reclaim water, recycled water must be treated to suit application needs. This reduces demand on wet end chemistry but adds chemical costs per ton. It may also involve capital expenditures requiring return on investment (ROI) calculations to determine if the cost is acceptable.
TREMONT: Trends in machine closure for both water reuse and air emissions are still dependent on current regulatory legislation and how well a mill is equipped to comply. The U.S. EPA fact sheet highlights that U.S. pulp and paper mills are a significant source of environmental pollutants, with 19 mills associated with dioxin-based fish advisories and approximately 245,000 metric tons of toxic air pollutants released annually.
To address the dioxin issue, the EPA's Cluster Rule--enacted in the 1990s--helped move the industry to accept and invest in elemental chlorine free (ECF) pulp bleaching by replacing elemental chlorine with chlorine dioxide. However, the Cluster Rule fell short on regulating volatile organic compounds (VOCs), which play a significant role in the formation of ozone, and adsorbable organic halides (AOX), which exhibit toxicity and may bioaccumulate in fish tissue.
Because of the VOC issue, the impact of specific wet end chemicals on air and water emissions must be considered in any machine closure strategy. Some specific wet end additives to consider for VOC monitoring include certain oil-in-water emulsion polymer retention and drainage aids and the volatile surfactant levels needed to produce on-site ASA wet end sizing emulsions. To help reduce VOCs, the use of "dry" polyacrylamide retention and drainage aids is a viable option.
TOTURA: The paper industry is increasingly aware of the responsibility to use resources wisely, including water. Paper mills should develop an understanding of how reducing total cost of operation (TCO) through water system closure must be precisely aligned with sheet properties established in the wet end. Different paper grades have widely varying critical properties, so the impact on a recycled linerboard manufacturer with a totally closed mill water system will be entirely different from a tissue producer.
SOLUTIONS! As closure increases, how does water quality affect performance chemicals and paper quality?
JOHNSTON: Variability introduced into the system will, in turn, generate variable effects on performance chemicals and therefore sheet quality--at least until a new equilibrium has been established. Whenever a water system is closed, the water chemistry changes, temperature increases, and dissolved oxygen decreases. Increased scaling and corrosion will result. Additionally, anaerobic bacteria thrive in these new conditions and can cause a number of problems. Sheet quality issues include strength reduction and odor complaints.
System closure demands a closely coordinated team effort using a combination of technical inputs--including mill and corporate technical resources, suppliers, and consultants. The inherent technological issues should be considered and well thought out before the closure process begins. Modeling is frequently used to understand the impact that seemingly small changes will have on the entire process. Changes to wet end chemicals are often needed. Over time, a properly closed mill will operate at a lower cost, add value to the company's products, and increase long-term profitability.
KING: This is a complicated question. Perhaps intricate is a better word, since closure takes many directions and has many possibilities. What goes in must come out, with an equilibrium established in the water phase for all soluble and insoluble materials. Once that equilibrium is reached, excess material must go out of the system either with the paper, the effluent, or in the form of deposits. As less water is used, these equilibrium concentrations are reached faster, leading to a need to understand the influences on performance chemicals and paper quality.
As water closure increases, the level of contaminants in the wet end also increases. This means that performance chemicals such as retention aids, sizing agents, strength additives, and defoamers/deaerators work harder and cost more for the same result. I have seen both positive and negative impacts on dyes and optical brighteners. Higher contamination levels usually lead to higher dye use, but sometimes the higher rate of recycle means more dyed product is recycled and lower levels of dye are realized. Deposit control chemicals, whether biological or chemical, usually have to work harder and thus cost more with higher recycle. I have seen a few cases where mills successfully took more of the water contaminants out with the paper so that deposit control cost actually went down with greater water reuse.
Most chemical additives today have some type of electrical charge impact on the wet end, either through their direct charge or by the way they influence other charges. Thus, conductivity and electrical charge are good methods to track wet end chemistry, water closure, and contaminant build up. Each machine will be able to develop correlations for conductivity and charge that relate to machine runnability. The correlations are used to adjust chemical levels and machine process parameters.
As water use is reduced, soluble and insoluble concentrations increase. This makes paper quality targets more difficult to reach. A few examples of paper quality loss might include the following:
* Brightness will go down as color bodies increase with higher water recycle
* Internal sizing is more difficult as contaminants interfere with sizing chemicals
* Strength and tensile properties are harder to reach since many contaminants are debonders and lower strength
* Bulk and caliper are more difficult to reach as recycled materials are compact and more dense than virgin materials.
Closing up the wet end can lead to significant difficulties. Papermakers and technical personnel must find the balance between operational savings in heat and water treatment costs and the increased costs of maintaining runnability and paper quality.
OLAVESSEN: With increased closure, paper quality can be affected by stubborn microbiological infestations that cause higher paper biological counts, holes, and machine slime accumulation that affects shower heads and slitter sprays. Managing this requires an analysis of all wastewater stream qualities over all operating ranges, and matching those qualities to application requirements and receiving application points. You also must understand the range of water quality and flows from each system--not just one or two "snapshot" water samples. After your analysis, you can sort through cost effective re-use applications and reclaim applications strategies to minimize reclaim treatment sizing.
TREMONT: On-line conductivity measurements have shown that increased water closure produces a proportional buildup of dissolved salts and organic BOD that can reduce the effectiveness of wet end process and functional chemicals applied for machine deposit control, retention, strength, sizing, and brightness. Brownstock linerboard machines have achieved zero effluent by controlling machine runnability and strength with novel wet end additives designed to perform under high conductivity conditions with tolerance to "anionic trash." Also, the negative impacts of water closure on unbleached board brightness and color are a relatively low priority in corrugated boxes.
In bleached papers, paper quality becomes more of an issue with the possibility of significant brightness reversion and sheet defects as contaminants build up with system closure. Microfiltration and reverse osmosis technology can control dissolved solids buildup and maintain brightness parameters.
TOTURA: The impact of process water recycling can best be described by its impact on "charge management." There is an ideal operating range of anionic process constituents, including the fiber, that must be counterbalanced by cationic polymer feed for optimal wet end operation. Recycling process water can bring fiber fines and fillers into play that change the previous balance, requiring adjustments in applied charges. This dynamic process is critical to final sheet quality. A recycling opportunity must consider the impact on this delicate charge balance. There will often be an increased chemical cost resulting from the need for more anionic character, but this is well-justified if it maintains sheet properties.
DEUL: Water resource management is complex due to strong interaction between pulp, water, contaminants, additives, and process equipment. Most systems can tolerate a certain degree of closure. However, the real battle begins when the level of contaminants--which increases with closure--negatively impacts productivity and product quality. The key to successful water resource management is to predict the change in water chemistry upon closure and its impact on operations, chemical programs, and paper quality, and then prevent potential problems. System closure should be implemented systematically, considering specific mill operating parameters and using appropriate process analysis tools.
The systematic approach to water resource management uses a three-level hierarchy: water conservation, water reuse and water recycle. The main steps are:
* A mill audit to benchmark and develop a picture of operating practices and sampling protocol to establish heat and mass balances for the water system
* Conceptual design of potential reuse and recycle scenarios
* Development of an aqueous based steady state simulation model with the following objectives:
* Simulate the effect of water reuse/recycle options on water chemistry (scaling tendency, contaminants concentration, temp and pH) throughout the system
* Examine the effects, size and location of potential point source treatment/kidney technology applications to treat water prior to reuse
* Risk and benefit analysis of proposed solutions, which may include lab studies to support simulation data and evaluate the impact of water chemistry on performance chemicals
* Implementation, monitoring and control.
SOLUTIONS! Is there any trend to recycle effluent water back to the paper or pulp mill? If so, what are the benefits and downsides?
JOHNSTON: Yes. Several mills have already implemented this practice or are actively pursuing modeling. Benefits are energy savings, environmental impacts, simpler operation, and reduced chemical costs. Beyond the obvious "conservation" benefit of reducing fresh water requirements, there is usually a considerable energy benefit. Wastewater is usually warm and returning warm water to the process not only saves water, it reduces the overall energy input required to reach a desired operating temperature (whether in the boiler feedwater makeup system, the paper mill, or the pulp mill).
The downside of effluent is either physical or chemical. Physical risk is related to deposition from increased suspended solids. Chemical risk pertains to changes in ion concentrations and their impact on process treatments, functional treatments, water corrosivity, and overall product quality.
Both mechanical and chemical treatment scenarios are available to address the threats posed by effluent recycle. Experienced technicians should assess the impact of effluent recycle on a mill water system. Each change that addresses a chemical or physical risk must be evaluated from a "big-picture" perspective so that surprises are avoided when these changes are implemented.
KING: A few studies in the 1970s showed the possibility of recycling clean effluent back to the mill intake pipe. Some mills were successful and some were not willing to take the risk. Successful mills typically have a high volume river flow or large lake to help dilute the effluent water. Also, mills with less variable effluent discharges find it easier to recycle. The variability of effluent water quality usually discourages mills from using recycled water for digester make down, shower water, consistency control, etc. As discussed earlier, the advantage of using mill effluent water back into the pulp or paper mill is the heat recovered. This produces energy savings or reduced water usage and treatment costs. The downsides are much the same as discussed in the other questions, including increased chemical and operating costs along with deposit and paper quality issues.
OLAVESSEN: As mills seek to close their operations, their efforts lead them to their final effluent water and how to bring that water back into the mill. Issues include microbiological issues, color issues, and additional contaminants not seen in the normal mill supply water that can influence scale formation, fouling and process chemistries. Mills must analyze and understand these issues. If millions of gallons of influent treatment can be reduced through smart water recycling strategies, mills can realize environmental benefits and possibly reduce costs per ton.
TREMONT: With the use of dissolved air flotation (DAF) clarification to remove solids, deinked pulps mills are able to recycle "gray water" back to the process to achieve a minimum level of system closure. Integrated paper and board mills routinely recycle excess white water to the pulp mill after clarification. Recycling final effluent water is an excellent way to close up a white water system, since the water quality of a well-run secondary effluent plant is significantly improved over an internal recycle loop. The downside is that once the mill runs by returning final effluent, the primary and secondary treatment facilities must be monitored as closely as any other critical internal water loop, since an effluent treatment plant upset would significantly affect the papermaking process.
TOTURA: We see a trend toward using treated wastewater in a number of mill processes, but it must be done with care. An obvious concern is reseeding a process stream with a certain category of biological activity called filamentous bacteria. Incoming fresh water carries a far lower loading of filamentous bacteria than would be the case with effluent from a mill's wastewater treatment plant. Although pathogenic microorganisms are eliminated in treated effluent, filamentous bacteria easily survive doses of disinfectant that adequately kill pathogens. Filamentous bacteria form particularly tenacious deposits--an obvious problem.
Also, unexpected problems can arise. One mill using its own treated wastewater as just a fraction of the mill water supply experienced serious problems with its membrane-based boiler pretreatment equipment. This was not apparent in any water analysis or other forecasting capability, but became a serious problem that eventually impacted production. It takes a complete system perspective--watching for these sorts of seemingly unrelated changes--to assure that a good concept does not impair performance and profitability.
WHAT YOU WILL LEARN:
* Current trends in machine closure.
* The impact of closure on wet end chemistry and paper quality.
* The pros and cons of recycling effluent water back to pulp and paper mills.
* "New technology in water treatment," by Alan Rooks, Solutions!, February 2004. To access this article, go to www.tappi.org and type the following Product Code in the search field: 04FEBSO31.
* "Water treatment: A Solutions! Roundtable," edited by Alan Rooks, Solutions!, February 2003. Product Code: 03FEBSO19.
* Water Supply and Treatment, edited by Jack G. Walters. ISBN: 0898524563. Available through TAPPI Press: Product Code: 0101R156. This in-depth study of water use in the pulp and paper industry can help planning and engineering personnel select equipment for a water treatment system. 1989. 87 pages, soft cover.
Member Price: $22.00
Non-Member Price: $32.00.
EDITED BY ALAN ROOKS, EDITORIAL DIRECTOR
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|Title Annotation:||WATER TREATMENT|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Feb 1, 2005|
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