Closing the loop at Stora Enso Uetersen: at one German mill, a new filtration system reduced a fine paper machine's freshwater consumption by nearly one third.

Stora Enso's mill in Uetersen, Germany is one of the most tightly closed fine paper mills in the world. Its PM 2 consumes just 4.2 liters of fresh water for each kilogram of paper produced--possibly the lowest consumption rate of any fine paper machine.

An aggressive program of white water reuse is critical to this closure program. More than 750 cubic meters ([m.sup.3]) per day of processed white water has replaced fresh water on every four-drinier and press section shower on the mill's PM 2, reducing fresh water intake by about one third. Even clog-sensitive, high-pressure cleaning showers have been using this filtered white water without incident for nearly 3 years.

One of the key parts of the mill's white water strategy is the PETAX[TM] filtration technology. "Previously, fresh water was the only shower water option. Other filters caused plugged nozzles and slime build-up on the showers," said Roll Bobell, production manager at the Uetersen mill.

The PETAX pressurized disc filter system was the only filtration technology that met all the application requirements, according to Bobell. Since its installation in 1999, the system has delivered clean filtrate to showers throughout PM 2. There have been no negative effects on machine runnability, fabric life, or paper quality, and the system has accomplished these goals with minimal downtime. The system's operating costs have been more than offset by reduced fresh water, energy, and wastewater treatment costs. "Based on our estimates, the PETAX filter system paid for itself in less than 1 year," says Klaus Behrendt, the mill's technical manager.

WHITEWATER LEADER

The 100-year-old Uetersen mill became a whitewater reuse leader for both environmental and economic reasons. Its 3.8-m wide PM 2 produces 160,000 metric tons/year of coated high quality catalog, magazine, and packaging paper in basis weights ranging from 170 to 400 grams per square meter (g/[m.sup.3]). The machine makes frequent grade changes and uses a wide variety of furnishes, from virgin fiber to sludge filler. Because of frequent process changes, white water solids can vary from 50 to 2000 ppm.

The mill had to make white water from PM 2's sedimentation tank suitable for use with high-pressure (HP) showers equipped with fine orifice nozzles as small as 0.6 mm. To accomplish this, it needed a new filter system to further clean the clarifier clear water. The selected equipment had to provide an average flow rate of 100 [m.sup.3]/h, with surges above 200 [m.sup.2]/h. In addition, the system had to remove fines, provide a constant volume of filtrate, and operate without upsets, filter screen clogging, or frequent downtime.

EVALUATING THE ALTERNATIVES

The mill researched and evaluated five filtration technologies. Management determined that conventional disk filters would not be compatible with existing mill equipment and would cope poorly with frequent grade changes and the high level of white water sediments. The mill also decided against using dissolved air flotation (DAF) clarifiers or backwashing pressure filters equipped with metallic filter elements.

In 1998, the mill approved a PETAX trial under the Stora ECO program. Trial results demonstrated a level of chemical-free fine filtration not achievable with other systems. The system handled wide fluctuations in both flow and solids without upsets or loss of filtration effectiveness. The system processed fluids with up to 2000 ppm of solids and produced filtrate with an average of only 8 ppm solids. Operators observed no plugging or biological problems in the HP showers during the trial. Based on the trial, Stora Enso installed a PETAX system on the Uetersen PM 2 in June 1999.

CENTRAL ROTATING SHAFT 

The Uetersen PETAX system uses 16 discs and can filter up to 240 [m.sup.3]/h flow. The discs are mounted on a central rotating shaft and submerged in fine fluid to be filtered. Pressure (typically below 0.3 bar) forces fluid through the filter medium. Channels on the disc guide filtrate towards the hollow central shaft and then out of the vessel (Fig. 1).

[FIGURE 1 OMITTED]

Filter screens trap fine particles entrained in the fluid. The system uses three cleaning stages to remove solids on and below the filter surface, preventing excess build-up that would otherwise reduce the system's flow rate and filtering effectiveness (Fig. 2).

[FIGURE 2 OMITTED]

The PETAX system automatically adjusts to changes in flow rate, solids loading, and fabric condition. When the inlet debris level rises, the system automatically detects an increase in system operating pressure and responds by raising both the disc rotation speed and the reject pump flows. The ability to automatically respond to fluctuating flow and loading conditions make the system particularly well suited to the Stora Enso/Uetersen application.

PM 2 stayed on line in full production mode during the installation. Because the system is compact, the mill mounted fine unit on the 10undation of an unused chemical tank After installation of the control system and piping, the PETAX system was started up following a routine 8-hour machine shutdown. "Startup went very smoothly, and optimization of the system took only 4 days," stated Thomas Schroetter, the mill's project engineer.

EXCEEDING EXPECTATIONS

Filtrate was averaging 5 to 10 ppm the day after start-up. The initial flow rate was 140 [m.sup.3]/h, increasing to 190 [m.sup.3/h by October 1999.

The system has met the mill's water capacity requirements. It filters 5000 [m.sup.3] of clarified white water per day at peak times. Despite filter inlet fluctuations in solids and flow rate, discharge filtrate quality has remained consistently high. Over a 17-month period, filtrate solids averaged less than 20 ppm. The 20-micron filter medium has kept filtrate solids well below the level required for HP shower operation.

Tests with other filtration systems resulted in plugged showers that caused streaking and contamination that damaged and shortened fabric life. By contrast, the mill has experienced none of these, problems with its PETAX system. Machine and press fabrics are lasting as long as they did when they were showered with fresh water.

Mill water system closure caused a slight increase in conductivity, headbox COD, process water chlorine, and hardness. However, these changes have not had a negative impact on paper quality. The only notable change has been an increased demand for biocide due to the mill's reduced fresh water intake.

As anticipated, system maintenance requirements have been minimal. Wear parts needing replacement include the filters screens and take-off device doctor blades. Filter screens are replaced after 6-9 months, and the doctor blades about once per year.

Since the system's introduction, PM 2's freshwater intake requirements have dropped by more than 30% and the mill has minimized water treatment costs. In addition, the process returns over 70% of white water solids, which are reused on PM 2. "With PETAX, our mill has saved more than Euro 562,000 (US$ 500,000) per yea1 in energy, water, fiber, and filler costs," said Ralf Lemitz, the mill's quality control manager.

MORE TRIALS

Other mills in North America and Europe have commissioned trials similar to the ones conducted by Stora Enso. Two of those trials have led to additional installations in Europe. In other trials, PETAX demonstrated 80%-98% removal efficiency in mills that produce tissue, newsprint, corrugated medium and linerboard.

David McGowan is product manager-filtration For Kadant AES, Queensbury, New York, USA. E-mail him at: dave_mcgowan@ aestf.com. Field data for this article was provided by Joachim Geldmacher, Petax Papier Ingenieur Technik GmbH, Numbrescht, Germany.