New technology in water treatment.
GLEN BOWEN, applications project manager Hercules Pulp and Paper Division, Buffalo, New York, USA.
JAMES GRAHAM, business manager-papermaking technologies water treatment, Buckman Laboratories, Memphis, Tennessee.
NORRIS JOHNSTON, applications team manager, Hercules Pulp and Paper Division, Huntsville, Alabama.
GEORGE TOTURA, program manager--water/paper services division, Nalco Co., Naperville, Illinois.
STEVE TREMONT, marketing director, Ciba Specialty Chemicals, Water and Paper Treatment, Suffolk, Virginia.
SOLUTIONS! What are the "basic" process control systems paper mills should be using in their water treatment facilities and how can they optimize these control systems with upgrades or new technology?
GRAHAM: Some of the basic process control systems paper mills should be using include.
* flow-proportioned chemical feed,
* trimming the feed based on turbidity of finished water,
* using temperature to determine dosage of chemistry programs,
* using streaming current detector (SCD) and oxidation reduction potential (ORP) readings to adjust chemical dosages to target levels, and
* fabricating a pilot clarifier and making adjustments based on the effluent of this system to mimic the actual system.
Mills should use data collection and trending to recognize the needs for adjustment and the degree of adjustment required, and to monitor trace residuals in the neat product to adjust feed levels. The following new technologies may help:
* PLC-based control systems to interpret several variables at once, rather than just one,
* algorithms to model the system dynamics, and
* systems that hunt for the optimized dosage, based on predetermined measurements.
JOHNSTON: Process control in water treatment is typically confined to chemical dosing. Basic control is achieved by aligning feedrate of specialty chemicals with process flow according to a prescribed dosage, usually expressed in parts per million (ppm). A more refined control scheme might further trim chemical dose based upon certain process conditions, such as temperature, pH, or dissolved solids.
Maximum control is achieved when the dosing of a particular chemical is based on results. If, for example, the chemical is added to achieve a particular turbidity value, then a continuous turbidimeter might feed back to the dosing system to alter feed based upon turbidity. Other control systems might be aligned with analyzers such as conductivity, pH, silica, phosphate, molybdate, or corrosion rate. The degree of sophistication the mill desires will dictate what "level" of process control should be applied. Often the decision weighs existing process variability, desired variability, and cost to achieve that level of desired variability.
TOTURA: Achieving precise boiler chemistry control is a very important objective for many mills. This is best accomplished with an automated feed system that incorporates the ability to adjust to process changes and verifies that the treatment is actually fed into the system. Such a system should be able to incorporate a boiler blowdown control loop to manage this variable as well. With energy costs such a big component of a mill's powerhouse budget, automated feed system control strategies are coming into more widespread use.
TREMONT: A 100% fully automated water treatment system is not yet available to control all the mechanical and chemical variables for pulp and paper mill water treatment applications. Water treatment systems rely heavily on living micro organisms. It is important to correlate the effects of cost-effective biological treatment with daily testing. The best practices available combine trained people with a degree of process control automation.
SOLUTIONS! Some mills in urban settings or with very strict effluent limits have installed or are considering combination anaerobic-aerobic water treatment systems. What is the potential for other paper mills to use this type of treatment systems?
BOWEN: Treatment of paper mill waste streams using microorganisms to reduce biological oxygen demand (BOD) is fast becoming a "best practice." Discharge permits, municipal charges for processing industrial wastes, community demands, and good environmental stewardship all point to increased sophistication in the waste treatment area. The development of highly efficient activated sludge plants has reduced the required "real estate" so that even the most landlocked facility might be eligible for installation of such a system.
Mills located in urban settings will typically have a high percent of recycled fiber, resulting in a "high strength" waste stream, sometimes greater than 2000 ppm of BO[D.sub.5]. To address strict effluent limitations or reduce municipal processing costs, mills can use a combination of anaerobic and aerobic treatment systems. The anaerobic pre-treatment can remove more than 80% of the BO[D.sub.5] load and is efficient at treating high strength effluent of this kind (that is, BO[D.sub.5] greater than 2000 mg/L). Most of the remaining BO[D.sub.5] will be removed or converted to suspended solids in the aerobic polishing step.
Control of odors and elimination of toxic gases in process equipment and waste treatment systems has spawned the development of new and exciting chemical technology that is being evaluated throughout the industry. The application of these specialty chemicals has the potential to reduce time needed to purge vessels prior to entry and the more environmentally significant capability to scavenge such odorous gases as hydrogen sulfide and mercaptans. As a result of these applications, the safety of the work place is improved and odors are reduced from the processes and waste treatment areas.
TREMONT: Anaerobic mills treat only a small water flow with a very large BOD. The big payback with an anaerobic water treatment system is energy costs from methane gas generated to fully power a boiler. Aerobic systems require a consistently warm temperature--which is why northern mills are considering combination aerobic-anaerobic systems for higher efficiency, especially during the colder winter months.
SOLUTIONS! What do you see as the most promising technology trends in water treatment, both chemical and mechanical?
GRAHAM: There are several promising, new technology trends in water treatment, such as the following:
* the use of enzyme technology to treat difficult problems such as BOD and color,
* refinements in reverse osmosis and dissolved gas removal, resulting in broader applications, and
* improvements in the types and accuracy of monitoring tools, which will mean a larger role for these tools in the future.
JOHNSTON: Regarding mechanical trends, computerized feed and control systems for specialty chemicals--with and without analyzers for feedback control--offer considerable economic advantages in those processes where constant operator attention is no longer feasible. The development of compact and efficient unit operations such as influent clarifiers, demineralizers, membrane separation equipment, and BOD reduction facilities all provide "more for less" to the paper industry.
The chemical arena requires continued development of specialty chemical formulations to address the dynamics of water treatment. The odor control chemistry previously mentioned is an example of technology development to meet an industry need. Development of more cost-effective coagulants and flocculants continues at a rapid pace. The development of oxidizing and non-oxidizing biocides for fresh and process water treatment needs is a focus for many specialty chemical suppliers.
TOTURA: Membrane-based water treatment is coming into more widespread use, just at the time when we need it. The waste created by classic ion exchange processes is getting to be a problem as mills tighten their water systems or are required to minimize their impact on the environment. Yet membrane systems are a considerable investment and, like most processes, they either benefit from or require chemical additives for optimal service.
Another trend we are seeing is the increased awareness of how important water quality is to the operation of the paper machine. Mills using surface supplies often experience water quality excursions that the clarifier just cannot handle when the river "turns over" seasonally. We are studying how this impacts the papermakers. If we can take extra steps up front in the system to maintain water quality, then we can have a big payoff at the machine. Any time you take a larger perspective rather than only attending to individual processes you can make a big difference in how the mill performs.
TREMONT: There is a trend toward less chemical and more mechanical water treatment with applications such as thin film and chlorine resistant cellulose acetate reverse osmosis technology to handle up to 500,000 gal/day. Novel on-line control systems are now available to measure phosphorous and BOD levels to optimize biological nutrient levels. Current on-line turbidity measurements for "feedback" control require daily/hourly operator sensor cleaning and calibration while future demands require "feed forward" control systems based on mass flow and density. The most promising trend in water treatment is the application of proven microparticle and micropolymer retention and drainage technology to water treatment to improve water clarification and sludge dewatering efficiency.
IN THIS ARTICLE, YOU WILL LEARN:
* The basic process control systems mills should use and how they can be upgraded.
* Whether mills should consider combination anaerobic-aerobic water treatment systems.
* The most promising technology trends in water treatment, both chemical and mechanical.
* "Water Treatment: A Solutions! Roundtable," edited by Alan Rooks and Jan Bottiglieri, Solutions! magazine, February 2003, p. 19 (available online at www.tappi.org).
* Go to www.tappi.org and type "water treatment" in the search box.
RELATED ARTICLE: SOLVAY OPERATES INNOVATIVE TREATMENT PLANT
Solvay Paperboard, Solvay, New York, USA recently installed a combination anaerobic-aerobic wastewater recycling facility that reduces both the amount of fresh water required for making paperboard (from recovered paper) and its wastewater discharge into the Onondaga County sewer system. The facility uses recycled wastewater from two paper machines to feed a third, reducing that machine's fresh-water requirement by 500,000+ gallons/day. A byproduct, methane gas, will replace coal-driven manufacturing processes. The US$ 14 million facility was funded, in part, by a US$ 250,000 grant from the New York State Energy Research and Development Authority. The 3-step process requires no lagoons:
1) Wastewater is collected from PMs 1 and 2 in a reactor vessel where organic contaminants (measured as biological oxygen demand over a 5-day period, or BO[D.sub.5]) are converted to carbon dioxide and methane gas by anaerobic bacteria.
2) The wastewater is sent to a "moving bed reactor," a tank partially filled with aerobic bacteria living in plastic media continuously mixed by air bubbles, removing 94% of the remaining BO[D.sub.5].
3) Remaining solids are forced to the surface by a dissolved air flotation clarifier. Solids are scraped away, leaving a clarified final effluent, which then becomes influent for PM3. The solids will eventually be returned to the mill as feedstock.
RELATED ARTICLE: WHEN SHOULD TREATMENT FACILITIES GET UPGRADES?
Water treatment at pulp and paper mills offers less and less room for error. That means facilities must be kept up to date--but at what cost? "The stakes are very high for managing paper mill water treatment systems these days," said George Totura of Nalco Chemical Co. "The absolute requirement for 100% system reliability has not changed; the paper machine always needs steam to put finished product onto the reel. But now there is also the absolute requirement for 100% environmental compliance. Any control strategy must accept these premises as fact."
Totura noted that the total cost of operating the processes must also be considered. "Mills are not just competing with other corporations. They vie within their own companies to be the lowest cost producer. Deploying new technology can only be done when it enhances mill competitiveness."
What happens when mills get to the point where the existing equipment is aging and costing more to operate than new alternatives? At some point, with the business proposition fully defined, mills will have to consider water system upgrades just to maintain production, said Totura. "The key point in this is that mills will have to project how a system change impacts the total mill, not just the one-unit operation. A lot of thought and many process simulations are required to study any major technology prior to actual deployment," he concluded.
EDITED BY ALAN ROOKS, EDITORIAL DIRECTOR
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|Title Annotation:||Water Treatment|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Feb 1, 2004|
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