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Filter system makes re-use of paper mills' white water possible
featurearticle Paper mills require large quantities of water, but with rising freshwater costs many are looking to recycle their wastewater. Traditional liquid~solid separation systems, however, often struggle to provide the fine filtration needed to turn a mill's white water, with its high suspended solids content, into re-usable water. Now, the Petax TMfiltration system from Kadant AES, USA, claims to be able to achieve just that. David McGowan explains
Filter System Makes Re-use of Paper Mills' White Water Possible anufactures of paper products consume large quantities of freshwater, with the highest volume users having intake requirements in excess of 100 m 3 per metric tonne. Costs associated with using such volumes can be high, with mills paying to purchase, heat and chemically condition the water. After use, wastewater must be treated in a local municipal system or on site at a mill-owned treatment plant. Today many mills are seeking to reduce operating costs, and water conservation is receiving careful consideration. The cost of freshwater is steadily increasing and its unlimited use is no longer a luxury available to most papermakers. Mills are looking to recycle water for reasons that are typically a combination of environmental and economics. Unlike freshwater, recycled white water contains fibre and can cause unique solids/liquid separation challenges. Most solids/liquid separation systems have difficulty operating when the requirements are to produce high quality water, to remove fine particles, to operate continuously and remove high quantities of fibre. To achieve water conservation objectives, many mills operate with inadequate filtration, or high levels of filter maintenance. Furthermore, many mills have abandoned water conservation efforts because of operational issues with filtration equipment. Even devices such as clarifiers lack the ability to consistently produce clear water suitable for many critical paper mill services. Inadequate filtration anywhere in a paper mill can create problems that include, but are by no means limited to, plugged paper machine shower nozzles, plugged heat exchangers, plugged vacuum pump seals and short mechanical seal life.
M
~REEN
B e n e f i t s of Fine F i l t r a t i o n Most mills would not consider using white water on critical paper machine applications such as the high pressure forming and high press felt cleaning showers. For this application, the water must be filtered to 20 micron and contain less than 20 part per million (ppm) of suspended solids. With inadequate filtration, small particles in the white water can become lodged within the paper machine press felt decreasing its porosity and life, resulting in an increase in machine downtime and production costs. Fine filtration offers many positive economical benefits, especially for mills that are recycling a significant portion of water. Areas of cost reduction include: • reduced waste treatment costs; • energy savings from heating water; • recovery of solids that can be returned to stock preparation; • reduction in surcharges that are calculated based upon solids sent to the sewer system; • quicker grade changes; and • reduced chemical costs by replacing other technology such as a dissolved floatation clarifier. Furthermore, cleaner white water with fewer solids can result in fewer plugged shower nozzles, cleaner forming fabrics and better paper machine operation. The Petax fine filtration system is the first commercially available unit that removes 1-20 micron particles from white water with up to 2000 ppm suspended solids, and can produce water of a quality that is acceptable for use in critical services, where only freshwater was previously used. Petax removes large quantities of solids without chemical flocculants, without filter aids, precoat, sweetener stock or a vacuum drop-leg. Its filtrate clarity typically ranges from 0-20 ppm for clarified white water applications that contain suspended solids of between 100 to 200 ppm.
Petax Filter System
Figure 1: The fluid flow through a Petax disk. 18
December 2002
The Petax filter contains a series of four to 16 disks mounted on a rotating hollow shaft (Figure 1). As the fully submerged filter disks rotate at'a speed that varies between 5-25 rpm, vessel pressure forces fluid through the engineered filter screens. Filtrate flows though the disks
www.filtsep.com
featurearticle Figure 2: Petax's two chamber take-off cleaning de~
This self-adjusting capability allows the system to maintain consistent filtration performance, even when process flows vary widely in volume and solids loading, as demonstrated in Figure 3. As a result, the filtrate quality and volume remain constant.
Filtration A l t e r n a t i v e s
I'ARi
into the central hollow shaft, and out of the vessel at atmospheric pressure. Depending on the process volume and other application considerations, 70-85% of the flow entering the unit leaves as filtrate, while 15-30% of the flow (including fines, fibres, fillers and other solids) leaves as concentrated liquid that is 3-6 times thicker than the inlet consistency. Solids are retained both on the disk's outer surface and within the fabric itself. To remove these solids, the system uses a threestage continuous cleaning system (Figure 2). The first cleaning stage uses the leading edge of a stationary take-off device to remove caked-on solids from the rotating disk. A positive displacement pump then draws solids from the device into a common header and out of the filter system. In the second cleaning stage, the outer fabric is backwashed to remove solids from below the surface of the filter mesh. A backflush/cleaning slot is located behind the leading edge of each take-off device. A second positive displacement pump pulls clean filtrate from inside the disk, through the filter screen, and out of the vessel. A single oscillating needle jet shower then removes any remaining solids from the disk's surface. The shower movement is coordinated with the rotation speed of the disk to ensure a complete screen cleaning with each pass of the shower. Although the continuous cleaning stages work to keep the fabric open, application dependent periodic cleaning and caustic washings are used to maximize system performance and screen life. The maintenance cleanings do not require operator involvement, other than pushing a button to initiate the cycle. The Petax system needs be taken off-line for between 15-30 minutes during the maintenance operation.
When evaluating equipment for water re-use projects, engineers have had few alternatives. Common choices mm include gravity strainers, tubular pressure filters, dissolved air floatation (DAF) clarifiers or reverse osmosis membrane filters. Gravity strainers are typically used to remove particles in the 75-150 micron range and with maximum inlet contaminant loadings of 1000 ppm. The average particle size of debris in a mill's white water system is now smaller than in the past because of the increased use of recycled fibres. In contrast, Petax can remove particles >20 micron in diameter, so its removal efficiency is significantly higher than that of a gravity strainer. Multiple barrel pressure filters tend to use wedge wire filter elements and are good for removing small quantities of fibre. However, they do not usually contain screens finer than 150 micron and cannot accommodate fluids that have a suspended solids content of more than 200 ppm. Thus, they are unsuitable for use in critical applications, where the removal of fine particles is essential. Membrane filters offer exceptional removal efficiency, i.e >1 micron, but they remain expensive to install and maintain. They typically require frequent membrane cleaning and installation of prefiltration equipment. In many instances their use is cost prohibitive. DAF clarifiers are commonly employed to help decrease freshwater use, but a Petax system exhibits several advantages over this technology. For flow rates up to 4000 litres/min, Petax typically occupies approximately a third of the floor space. While the Petax and DAF power requirements are virtually equal, the DAF chemical costs can be much higher than those spent on consumable spare parts for the Petax system. This cost differential is shown in Figure 4. Additionally, DAF clarifiers can experience upset sending unwanted debris with the clear
Flatbox Seal Water LINERBOARD & BAG PAPER
Changing P r o c e s s Conditions Petax automatically adjusts in response to changes in process conditions, such as solids loading and flow volume. Flow to the unit can be from a pump or by gravity from a standpipe. For systems using a feed pump, an increase or decrease in flow volume or solids will create a corresponding increase or decrease in operating pressure. The vessel pressure is monitored by a pressure transmitter. As pressure changes are detected, the control system automatically changes the disk rotational speed and the extraction pump flow rate to maintain a constant vessel pressure. Thus, the Petax system automatically responds to continuous changes in process conditions and continuously rejects variable amounts of solids from the filter vessel.
Filtration+Separation
300
2~0
200
150
-
F ' ~ M tnk~t P P ' M Ou~e~
100 50 0
Figure 3: Typical quality of the filtrate generated by the Petax filtration system. December 2002
19
featurearticle $80,000
$6O,OOO U
i Petax Parts
$40,000 $20,000
• DAF Chemicals
$0 225
450
675
900
Flow Rate (GPM) Figure 4: A cost comparison between Petax and DAF systems,
Figure 5: StoraEnso's 16-disk Petax unit enables it to recycle its white water. water into the process. Barrier filters, such as a gravity strainer or pressure filters are often installed on the clarifier clear discharge to capture debris during upsets. The barrier screens, however, can be ineffective at removing the fines, and so do not produce the same quality of water as a single-stage Petax system. Furthermore, many grades of paper c a n n o t tolerate chemicals in the water system, meaning DAF clarifiers would not longer be an option. Thus, while a Petax system has a higher installed cost compared to a DAF clarifier, its significnatly lower operating costs means it has a rapid payback period.
Case S t u d y A: S C A Graphics, A u s t r i a SCA Graphic's mill in Laakirchen, Switzerland, manufactures label paper on a 6.35 m paper machine. In 1997 the mill rebuilt the wet end and installed a new Valmet Gap Former. It has eight high-pressure showers compared with one high-pressure and three low-pressure showers that were in use prior to the rebuild. A demand for 1000 litres/min of additional clean water, with a m a x i m u m total suspended solids concentration of 20 ppm was created. The mill-owned wastewater treatment plant was operating at 101% of capacity, but additional freshwater use was not an option. A disk saveall reclaimed fibre and produced clarified white water with a suspended solids content of 87 ppm, but it was not suitable for use as the gap former high pressure shower water. The mill explored various filtration options, some of which utilized chemical flocculants. The Petax filtration system was
20
December 2002
found to be the only acceptable choice. The mill rented a trial filter, and subsequently purchased a system following positive test results. A 12-disk unit was installed in 1997, and the 87 ppm average feed is now typically clarified to 10 ppm. The unit has been running well since start-up and there have been no reported incidences of plugged nozzles.
Case S t u d y B: S t o r a E n s o , G e r m a n y Germany is one of the most environmentally conscious countries in the world and the StoraEnso mill at Uterson was already using white water on a n u m b e r of the paper machine showers when they initiated a project to further reduce water consumption. At this location in N o r t h e r n Germany, StoraEnso manufactures fine coated paper of a basis weight ranging between 170-400 g/m 2 on a 3.8 m wide paper machine. The mill's white water is pumped to a large settling clarifier, and the clear overflow is fed to a clarified white water tank. To achieve the water reduction objectives, a 16-disk Petax system was purchased (Figure 5). The clarifier's clear water typically contains 200-2000 ppm of suspended solids, but after passing through the Petax system, the suspended solids content of the white water is reduced to between 10-25 ppm. The treated white water is now used in all of the paper machine showers, including the critical high-pressure felt cleaning showers (nozzle diameters as small as 0.6 mm). At the start of the water reduction project the mill was treating 5680 m3/day of water in the waste treatment plant. After installing the Petax unit, the total daily volume to the waste treatment plant decreased to 5110 m3/day, representing a 10% decrease in volume. The total amount of water used in the mill was reduced from 9 m 3 to 7 m 3 per metric tonne of paper. The mill monitored manufacturing parameters and concluded there were no changes in paper quality and machine retention. There were slight, but insignificant increases in headbox conductivity, headbox COD, chlorine content and hardness. Changes were noted in biological activity and an increase in the total biocide use was necessary. No changes were detected in felt life and no plugged paper machine shower nozzles were noted. To summarize, the mill was able to satisfy the projects water reduction requirements and achieve the target payback time of less than one year without affecting the machine operations or paper quality. After installation of the Petax system, StoraEnso has become one of the most closed fine paper mills in the world.
Conclusions The Petax system is the first to combine the ability to remove fine particles and handle t~igh influent solids in the same unit. The system is currently allowing papermakers to economically re-use white water in applications where in the past it was considered too high risk to the production operations. 0
Contact: David R McGowan, Kadant AES, Queensbury, NY 12804, USA. Tel:+1 518 793 8801; Fax:+1 518 793 9392; E-maih Dave_McGowan@ KadantAES.com; Website: www.kadantaes.com
www.flltsep.com
Filter System Makes Re-use of Paper Mills' White Water Possible anufactures of paper products consume large quantities of freshwater, with the highest volume users having intake requirements in excess of 100 m 3 per metric tonne. Costs associated with using such volumes can be high, with mills paying to purchase, heat and chemically condition the water. After use, wastewater must be treated in a local municipal system or on site at a mill-owned treatment plant. Today many mills are seeking to reduce operating costs, and water conservation is receiving careful consideration. The cost of freshwater is steadily increasing and its unlimited use is no longer a luxury available to most papermakers. Mills are looking to recycle water for reasons that are typically a combination of environmental and economics. Unlike freshwater, recycled white water contains fibre and can cause unique solids/liquid separation challenges. Most solids/liquid separation systems have difficulty operating when the requirements are to produce high quality water, to remove fine particles, to operate continuously and remove high quantities of fibre. To achieve water conservation objectives, many mills operate with inadequate filtration, or high levels of filter maintenance. Furthermore, many mills have abandoned water conservation efforts because of operational issues with filtration equipment. Even devices such as clarifiers lack the ability to consistently produce clear water suitable for many critical paper mill services. Inadequate filtration anywhere in a paper mill can create problems that include, but are by no means limited to, plugged paper machine shower nozzles, plugged heat exchangers, plugged vacuum pump seals and short mechanical seal life.
M
~REEN
B e n e f i t s of Fine F i l t r a t i o n Most mills would not consider using white water on critical paper machine applications such as the high pressure forming and high press felt cleaning showers. For this application, the water must be filtered to 20 micron and contain less than 20 part per million (ppm) of suspended solids. With inadequate filtration, small particles in the white water can become lodged within the paper machine press felt decreasing its porosity and life, resulting in an increase in machine downtime and production costs. Fine filtration offers many positive economical benefits, especially for mills that are recycling a significant portion of water. Areas of cost reduction include: • reduced waste treatment costs; • energy savings from heating water; • recovery of solids that can be returned to stock preparation; • reduction in surcharges that are calculated based upon solids sent to the sewer system; • quicker grade changes; and • reduced chemical costs by replacing other technology such as a dissolved floatation clarifier. Furthermore, cleaner white water with fewer solids can result in fewer plugged shower nozzles, cleaner forming fabrics and better paper machine operation. The Petax fine filtration system is the first commercially available unit that removes 1-20 micron particles from white water with up to 2000 ppm suspended solids, and can produce water of a quality that is acceptable for use in critical services, where only freshwater was previously used. Petax removes large quantities of solids without chemical flocculants, without filter aids, precoat, sweetener stock or a vacuum drop-leg. Its filtrate clarity typically ranges from 0-20 ppm for clarified white water applications that contain suspended solids of between 100 to 200 ppm.
Petax Filter System
Figure 1: The fluid flow through a Petax disk. 18
December 2002
The Petax filter contains a series of four to 16 disks mounted on a rotating hollow shaft (Figure 1). As the fully submerged filter disks rotate at'a speed that varies between 5-25 rpm, vessel pressure forces fluid through the engineered filter screens. Filtrate flows though the disks
www.filtsep.com
featurearticle Figure 2: Petax's two chamber take-off cleaning de~
This self-adjusting capability allows the system to maintain consistent filtration performance, even when process flows vary widely in volume and solids loading, as demonstrated in Figure 3. As a result, the filtrate quality and volume remain constant.
Filtration A l t e r n a t i v e s
I'ARi
into the central hollow shaft, and out of the vessel at atmospheric pressure. Depending on the process volume and other application considerations, 70-85% of the flow entering the unit leaves as filtrate, while 15-30% of the flow (including fines, fibres, fillers and other solids) leaves as concentrated liquid that is 3-6 times thicker than the inlet consistency. Solids are retained both on the disk's outer surface and within the fabric itself. To remove these solids, the system uses a threestage continuous cleaning system (Figure 2). The first cleaning stage uses the leading edge of a stationary take-off device to remove caked-on solids from the rotating disk. A positive displacement pump then draws solids from the device into a common header and out of the filter system. In the second cleaning stage, the outer fabric is backwashed to remove solids from below the surface of the filter mesh. A backflush/cleaning slot is located behind the leading edge of each take-off device. A second positive displacement pump pulls clean filtrate from inside the disk, through the filter screen, and out of the vessel. A single oscillating needle jet shower then removes any remaining solids from the disk's surface. The shower movement is coordinated with the rotation speed of the disk to ensure a complete screen cleaning with each pass of the shower. Although the continuous cleaning stages work to keep the fabric open, application dependent periodic cleaning and caustic washings are used to maximize system performance and screen life. The maintenance cleanings do not require operator involvement, other than pushing a button to initiate the cycle. The Petax system needs be taken off-line for between 15-30 minutes during the maintenance operation.
When evaluating equipment for water re-use projects, engineers have had few alternatives. Common choices mm include gravity strainers, tubular pressure filters, dissolved air floatation (DAF) clarifiers or reverse osmosis membrane filters. Gravity strainers are typically used to remove particles in the 75-150 micron range and with maximum inlet contaminant loadings of 1000 ppm. The average particle size of debris in a mill's white water system is now smaller than in the past because of the increased use of recycled fibres. In contrast, Petax can remove particles >20 micron in diameter, so its removal efficiency is significantly higher than that of a gravity strainer. Multiple barrel pressure filters tend to use wedge wire filter elements and are good for removing small quantities of fibre. However, they do not usually contain screens finer than 150 micron and cannot accommodate fluids that have a suspended solids content of more than 200 ppm. Thus, they are unsuitable for use in critical applications, where the removal of fine particles is essential. Membrane filters offer exceptional removal efficiency, i.e >1 micron, but they remain expensive to install and maintain. They typically require frequent membrane cleaning and installation of prefiltration equipment. In many instances their use is cost prohibitive. DAF clarifiers are commonly employed to help decrease freshwater use, but a Petax system exhibits several advantages over this technology. For flow rates up to 4000 litres/min, Petax typically occupies approximately a third of the floor space. While the Petax and DAF power requirements are virtually equal, the DAF chemical costs can be much higher than those spent on consumable spare parts for the Petax system. This cost differential is shown in Figure 4. Additionally, DAF clarifiers can experience upset sending unwanted debris with the clear
Flatbox Seal Water LINERBOARD & BAG PAPER
Changing P r o c e s s Conditions Petax automatically adjusts in response to changes in process conditions, such as solids loading and flow volume. Flow to the unit can be from a pump or by gravity from a standpipe. For systems using a feed pump, an increase or decrease in flow volume or solids will create a corresponding increase or decrease in operating pressure. The vessel pressure is monitored by a pressure transmitter. As pressure changes are detected, the control system automatically changes the disk rotational speed and the extraction pump flow rate to maintain a constant vessel pressure. Thus, the Petax system automatically responds to continuous changes in process conditions and continuously rejects variable amounts of solids from the filter vessel.
Filtration+Separation
300
2~0
200
150
-
F ' ~ M tnk~t P P ' M Ou~e~
100 50 0
Figure 3: Typical quality of the filtrate generated by the Petax filtration system. December 2002
19
featurearticle $80,000
$6O,OOO U
i Petax Parts
$40,000 $20,000
• DAF Chemicals
$0 225
450
675
900
Flow Rate (GPM) Figure 4: A cost comparison between Petax and DAF systems,
Figure 5: StoraEnso's 16-disk Petax unit enables it to recycle its white water. water into the process. Barrier filters, such as a gravity strainer or pressure filters are often installed on the clarifier clear discharge to capture debris during upsets. The barrier screens, however, can be ineffective at removing the fines, and so do not produce the same quality of water as a single-stage Petax system. Furthermore, many grades of paper c a n n o t tolerate chemicals in the water system, meaning DAF clarifiers would not longer be an option. Thus, while a Petax system has a higher installed cost compared to a DAF clarifier, its significnatly lower operating costs means it has a rapid payback period.
Case S t u d y A: S C A Graphics, A u s t r i a SCA Graphic's mill in Laakirchen, Switzerland, manufactures label paper on a 6.35 m paper machine. In 1997 the mill rebuilt the wet end and installed a new Valmet Gap Former. It has eight high-pressure showers compared with one high-pressure and three low-pressure showers that were in use prior to the rebuild. A demand for 1000 litres/min of additional clean water, with a m a x i m u m total suspended solids concentration of 20 ppm was created. The mill-owned wastewater treatment plant was operating at 101% of capacity, but additional freshwater use was not an option. A disk saveall reclaimed fibre and produced clarified white water with a suspended solids content of 87 ppm, but it was not suitable for use as the gap former high pressure shower water. The mill explored various filtration options, some of which utilized chemical flocculants. The Petax filtration system was
20
December 2002
found to be the only acceptable choice. The mill rented a trial filter, and subsequently purchased a system following positive test results. A 12-disk unit was installed in 1997, and the 87 ppm average feed is now typically clarified to 10 ppm. The unit has been running well since start-up and there have been no reported incidences of plugged nozzles.
Case S t u d y B: S t o r a E n s o , G e r m a n y Germany is one of the most environmentally conscious countries in the world and the StoraEnso mill at Uterson was already using white water on a n u m b e r of the paper machine showers when they initiated a project to further reduce water consumption. At this location in N o r t h e r n Germany, StoraEnso manufactures fine coated paper of a basis weight ranging between 170-400 g/m 2 on a 3.8 m wide paper machine. The mill's white water is pumped to a large settling clarifier, and the clear overflow is fed to a clarified white water tank. To achieve the water reduction objectives, a 16-disk Petax system was purchased (Figure 5). The clarifier's clear water typically contains 200-2000 ppm of suspended solids, but after passing through the Petax system, the suspended solids content of the white water is reduced to between 10-25 ppm. The treated white water is now used in all of the paper machine showers, including the critical high-pressure felt cleaning showers (nozzle diameters as small as 0.6 mm). At the start of the water reduction project the mill was treating 5680 m3/day of water in the waste treatment plant. After installing the Petax unit, the total daily volume to the waste treatment plant decreased to 5110 m3/day, representing a 10% decrease in volume. The total amount of water used in the mill was reduced from 9 m 3 to 7 m 3 per metric tonne of paper. The mill monitored manufacturing parameters and concluded there were no changes in paper quality and machine retention. There were slight, but insignificant increases in headbox conductivity, headbox COD, chlorine content and hardness. Changes were noted in biological activity and an increase in the total biocide use was necessary. No changes were detected in felt life and no plugged paper machine shower nozzles were noted. To summarize, the mill was able to satisfy the projects water reduction requirements and achieve the target payback time of less than one year without affecting the machine operations or paper quality. After installation of the Petax system, StoraEnso has become one of the most closed fine paper mills in the world.
Conclusions The Petax system is the first to combine the ability to remove fine particles and handle t~igh influent solids in the same unit. The system is currently allowing papermakers to economically re-use white water in applications where in the past it was considered too high risk to the production operations. 0
Contact: David R McGowan, Kadant AES, Queensbury, NY 12804, USA. Tel:+1 518 793 8801; Fax:+1 518 793 9392; E-maih Dave_McGowan@ KadantAES.com; Website: www.kadantaes.com
www.flltsep.com