Pulp & Paper

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Bagasse Paper Mill

February 21, 2008

Productora de Papeles (PROPAL), based in Cali, Colombia, is a group of two paper mills. Originally International Paper operations, they are now owned by the Carvajal family group. Both mills produce paper using sugar cane bagasse as furnish. There are many sugar cane mills in the area, so a year-round supply of raw material is assured.

Commissioned in 1990, today their #2 Mill produces 330 metric tons of paper per day. This paper is used for copier paper, although books and notebooks are also produced with it. Propal has recently purchased a Model VP-24 screw press which will dewater the primary clarifier underflow at this mill.

The project was driven by two factors: environmental and economic. From the environmental standpoint, using the press will eliminate a landfill operation. This landfill can cause run-off and groundwater contamination, besides presenting an ever-growing environmental situation.

Economically the press has a quick payback because the press cake will be dry enough to be combined with their boiler fuel. The press cake has a BTU value of approximately 8,500 BTU per pound of dry matter (compared to 10,000 for coal). Propal's B&W boilers burn coal. By blending the press cake with this coal, a reduction in coal consumption will result. The use of such biomass fuel has been receiving much attention at paper mills because of increasing energy costs.

The load to be dewatered is 40 metric tons per day (MTPD) of dry solids. It was felt that a Model VP-16 could likely have handled the load. The VP-24 was selected on a conservative basis, anticipating future operating conditions. It is also possible that it may be used to remediate the material in their landfill.

It is noteworthy that the mill already burns 70 MTPD dry solids of chaff and pith that is separated ahead of the digester. This material is pumped to an Andritz belt press where it is dewatered to 40% solids prior to burning.

Propal made use of the Vincent rental fleet. A Model CP-4 press was air freighted to Cali and used for a variety of testing. With this testing it was determined that no pre-thickening or polymer will be required, and press cake with a solids content of 50% can be produced without undue strain or problems. Rent was waived for the test period, and a Vincent engineer assisted at no charge.

The clarifier is an Eimco, 150' in diameter, handling 9000 gpm. Sand is separated ahead of the clarifier in order to reduce torque loading, and the flow is neutralized. The reject material, mostly medulla or pith, settles to a thick 5% underflow. It is because of this relatively high underflow consistency that no pre-thickening is required.

The original equipment at the WWTP (wastewater treatment plant) consisted of a small Eimco rotary drum screen, followed by a pair of Andritz belt presses. The rotary drum screen is only, approximately, 3' diameter by 5' long. This increases the underflow consistency from 5% to 8%. The belt presses, now worn and in need of overhaul, are about 3 meters wide. The cake produced by the belt presses normally has only 15% solids.

The new installation will place the screw press in parallel with the belt presses. That way it will be possible to retain the belt presses, at least for the time being, as back-up. It will also permit the rebuilding of these belt presses. In the future the belt presses may be used to for pond remediation, dewatering sludge from the mill's treatment lagoons.

The Vincent press was selected over competition because we were able to clearly demonstrate the performance being guaranteed. Because it was an export contract, strong financial assurances were offered in the form of two Letters of Credit. The first L/C, issued by Bank of America to Propal, assures Propal that their down payment money will be refunded should Vincent fail to ship them the press as promised. The second L/C will assure Propal that Bank of America will refund the full purchase price of the press should Propal elect to return the machine. Customer satisfaction is guaranteed.

Issue 197

Boiler Ash

October 16, 1997                                                                                                                                                                                                       ISSUE #68

Recently our sales rep in North Western Ontario, Process Flow Systems, took us to visit the one of the largest paper mills in North America. There are kraft, newsprint, and recycle mills all on the same property.

We were asked to demonstrate our small CP-4 with the sludge that comes from a scrubber on their hog fuel boiler. The water and particulate coming from the spray chamber were a filthy jet of boiler ash.

We did not expect the test to work. As expected, the material just oozed through the press with no dewatering. However it looked like there was potential because some polymer had been added. With the polymer, clear water could be seen forming in the nearby puddles and streams.

By chance nearby there was a pile of paper mill waste fiber. This was added to the ash and water solution, and, bingo! It was like magic. With the fiber as a press aid, the press started producing cake that measured 60% solids.

The application was found to work well with 10% by volume reject fiber. By weight the percentage would have been much lower, as is normal with the use of a press aid.

On-site testing is planned with a larger scale rental press.

Boiler Fuel

January 11, 1995
Rev. Oct 1997

We frequently are asked about the fuel value of waste materials from pulp and paper mills. This question arises because, when screen rejects and knots & shives are pressed, the dry press cake that is formed can be burned instead of landfilled. The combustion can be either in a coal fired boiler or in a bark burner. Tipping fees are an important part of the analysis. A high rate, like $30/ton, would make press cake disposal more important.

The heat value of the press cake is a function of the percent moisture as well as the ash and clay content. In general, the solids will have 2,900 BTU/# "as fired". Even when you take into account that there can be up to 60% water in the press cake, there is still some economic advantage to burning the sludge, in terms of energy contribution. That is, if you start with two pounds of press cake at 50% moisture, there will be one pound of solids burned, contributing 2,900 BTU, but there will also be one pound of water to evaporate, requiring 1,200 BTU. Thus there would be a net contribution of 850 BTU per pound of press cake.

Comparable figures are 10,000 BTU/# for coal and 5,000 BTU/# for wood. The fuel value of primary sludge, 4,000 BTU/#, is good. Coal is an ideal fuel for burning along with pressed screen rejects because it burns hot and has low moisture.

Most commonly the coal fired boiler will use one of two types of grates for burning press cake (rejects, sludge): Rotograte and Vibratory. With the Rotograte there is a limit of 400º F inlet air temperature, so the amount of sludge that can be dried is limited. Blending 10% press cake with coal would be recommended.

With the Vibratory grate the inlet air temperature can go up to 600º to 650º F because the grate is water cooled. This permits mixing the press cake with the fuel up to a higher percentage. General guidelines for the mixture are 30% press cake with 70% bark or 40% press cake with 60% coal.

It should be noted that, with the low inlet air condition, the system counts on radiant heat to evaporate the water in the press cake. This cools down the furnace, causing combustion problems. Also, running up the ratio of press cake to coal leads to poor air distribution across the grate, which also leads to uneven drying and burning.

It should be remembered that if a ton of press cake is burned, there will still be 400 pounds to haul to landfill: that is 1,000 pounds of water evaporated, 600 pounds burned up, and 400 pounds of ash.

Issue 20

Clarifier Sludge

August 28, 1998                                                                                                                                                                                                    ISSUE #82

In 1997 Putney Paper in Putney, Vermont purchased a Vincent VP-16 screw press. Its function was to further dewater sludge from the primary clarifier in their wastewater treatment plant. The sludge was pre-thickened with a belt press ahead of the screw press.

The mill both recycled OCC (Old Corrugated Container) and did deinking of other waste paper. The OCC operation put fiber in the clarifier, which improved the subsequent dewatering. Deinking, on the other hand, results in a higher proportion of clay and other materials that are hard to dewater in a screw press.

Testing with a rental machine and initial operation with the VP-16 gave excellent results. Press cake solids of 55% were obtained. This addressed problems associated with landfill hauling and fees.

A year later the press would not work. Capacity had fallen off and dewatering was minimal. Examination revealed that the sludge no longer had the fiber content that had existed previously. This had occured as a result of improved fiber recovery. Nothing could be done with the mud-consistency material coming from the belt press. The mill stopped using the screw press.

Trials elsewhere prompted Vincent to make this proposal to mill management: if they would substitute the existing 1800 rpm drive motor with a 900 rpm model, Vincent would supply the belts and sheaves to cut the screw speed in half. This combination cut the speed of the press from the normal 13 rpm down to about 3 rpm.

The results were a dramatic improvement in dewatering capability. The 35% solids cake from the belt press was further dewatered to a respectable 45% solids in the Vincent press.

Unfortunately capacity of the screw press fell off with the speed reduction. At times this was a problem for the mill, so the 1800 rpm motor was re-installed. With this change, press cake solids fell to 40%, but capacity was up. Later, a frequency inverter was installed so that the press could be run at speeds from 3 to 7 rpm. The speed was set according to the nature of the sludge being pressed at any given time.

Clarifier Underflow - A

October 11, 1999

An excellent installation was made in 1999 at a Weyerhaeuser paper mill. The mill installed a Model VP-16 screw press, along with a pair of sidehill screens. This equipment is used to dewater the underflow from their clarifier. Photos of the installation are available.

The clarifier is located at the waste water treatment plant. The flow into the clarifier is the combined flow of all of the waste streams in the mill. It includes the knots, shives, and screen rejects from all sources. Since the mill is a virgin fiber Kraft mill, there are negligible contaminants such as the ash (clay), dirt and ink that typify a recycle paper mill. We did note the presence lime, which comes from the lime kiln and cooking liquor preparation areas.

This waste stream dewatered and pressed in very good fashion. The cake, with over 40% solids, is added to the bark pile and sold as fuel to a local co-gen (electricity generating) plant.

The two Vincent sidehill screens, each 3' wide, were installed over the inlet to the press. These face each other so that any splash flow falls into the press. We have nearly identical installations at Bowater and Kimberly-Clark.

At Weyerhaeuser the clarifier underflow pumped to the sidehills was anticipated to be 300 gpm at 1.5% solids. The flow is thickened to an estimated 5% to 8% by the time it falls into the screw press. The press capacity was measured in excess of 40 TPD,DS (tons per day of air dry solids), which is somewhat greater than the expected load.

The original inquiry came from a direct mailing of the Tappi Journal article co-authored by Vincent. Two problems came together that made possible the justification required for the project. There was an existing belt press that was severely deteriorated and in need of major maintenance. Furthermore, the cake from the belt press was too wet to sell along with bark. This cake had to be landfilled, and the mill's landfill was nearing its maximum capacity.

Issue 98

Contract Wastewater Treatment

December 4, 2001                                                                                                                                                                                                   ISSUE #123

It was at a Tappi Environmental show that Vincent met representatives of Integrated Technical Services of Baton Rouge, Louisiana. This firm specializes in handling waste disposal on a contract basis. Pulp and paper is but one of many industries served by ITS.

Working with Vincent, ITS secured a three year contract to handle wastewater disposal at a Georgia-Pacific mill. The mill, in Crossett, Arkansas, was once the largest mill in the United States. It is rated at 1,600 tons per day, producing softwood Kraft pulp.

The combined wastewater flows of the mill are pumped a few miles off-site to an in-the-ground clarifier. The flow to the clarifier averages fourteen million gallons per day. It includes the undercooked chips, shives, screen rejects, and wash water from all sources.

Underflow from this clarifier is pumped to sidehill screens mounted above Vincent Model VP-16 screw presses. The arrangement has a pair of 3' sidehills over the inlet of each of two presses. These face each other so that any splash flow falls into the press.

The flow to the sidehill screens averages 1% to 2% solids. Even under adverse conditions this is concentrated sufficiently by the screens for the screw presses to operate reliably. Only under very difficult conditions is any polymer used in the treatment.

Press capacity is extremely high. This is due to the effectiveness of the sidehill screens and the high freeness of the Kraft fiber coming from the mill. Throughputs in excess of 70 TPD of air-dry solids per press have been measured. This is an outdoors installation. The only building is a trailer used by an operator. Currently the cake from the presses is landfilled, although there are plans to use the cake as boiler fuel. 


July 13, 1999                                                                                                                                                                                                         ISSUE #96

Deinking Mills are a major specialty among paper recycling mills. The principal raw material categories for these mills are newsprint, magazines, telephone directories, and mixed office waste. There are many sub-specialties. For example, some mills will recycle only newsprint, while others will mix magazines with newsprint. This adds fiber strength and brightness, but it requires the addition of equipment to screen ash from the furnish. (Ash is mostly the kaolin clay coating used in glossy magazine paper.) MOW (Mixed Office Waste) mills use the least expensive material; however, it requires special equipment for removing a broad range of contaminants such as Styrofoam and stickies (contact cement).

The common feature of deinking mills is that they must separate printers' ink from the fiber. This is made difficult because the objective of ink formulators is to produce a product that stays bound to the paper fibers. Obviously there are many types of ink: water based, latex, those that work on ground wood paper, those that work for glossy coated paper, etc. One of the worst is laser printer ink, because it is heat bound to the fibers.

Once the ink and ash have been separated from the fibers, these contaminants must be screened from the flow. Traditional technology is to use a machine similar to a belt press for that purpose. The performance of these deinking machines is principally measured by (a) the brightness of the good fiber that is accepted, and (b) the amount of good fiber going in the reject steam.

Last year a Fiber Filter machine was tested for deinking capability. This was done in the laboratory of Thermo Black Clawson, the premier manufacturer of recycle paper mill machinery. The results were extremely encouraging. Compared to the conventional equipment, the Fiber Filter scored higher in brightness and, simultaneously, higher in fiber recovery. This means that more ink went through the fabric sleeve while at the same time that less good fiber was getting through the same sleeve. Ash separation was also excellent.

Feed consistencies ranging from 0.25% to 2.0% were tested. Accept consistencies were in the range of 7.5% to 14% solids, with flow rates of 150 to 250 gpm in the Model FF-12.

Kneader Prethickener

September 19, 1997

In 1997 Vincent was awarded a contract for a Model VP-16 screw press that was used in an important pulp & paper application. The order was received from Thermo Black Clawson, the leading American supplier of paper mill machinery. The press is for Papelera Aragua, their client in Cagua, Venezuela.

The application was at a deinking mill that recycled ONP (old news print). Once hydrapulped, the newspaper must be scrubbed to remove ink from the cellulose fibers. This is achieved in a Thermo Black Clawson machine known as a Kneader. In the Kneader the fibers are rubbed against one another in order to separate the ink.


The Kneader requires a furnish with 25% to 30% solids. This is pre-thickened from a flow with 2% concentration by the Vincent screw press. The installation processes 30 MTPD (metric tons of air dry solids per day). The freeness varies considerably, from 150 to 500 csf (Canadian standard freeness).

The Vincent interrupted screw negates the need for a variable frequency drive. This press design uses interruptions in the screw flights with stationary resistor teeth inserted in the gaps. The resulting agitation allows the press to operate consistetly, without co-rotation, over wide ranges of both throughput and feed consistency. The use of an air cylinder actuated discharge cone allows production of press cake of steady solids content.

This job is important to Vincent because it is a step into yet another pulp & paper application: thickening pulp for both wet storage and baling purposes. Frequently there is a need for storage between pulping operations and the paper making machine. Water must be removed, without damaging the fibers, to reduce the storage bulk to a manageable volume. We have one small press operating in this capacity at a Georgia-Pacific mill, so we have confidence our larger presses can do the job.

Issue 67

Knots & Shives - B

October 4, 2001

Vincent has supplied screw presses to several Kraft mills for use on knots and shives. These rejects are material that is not made into paper pulp. The knots come from knotters following the digester, while shives are bundles of fiber, high in lignin, that are separated by pressure screens.

This waste is screened. Shaker screens are used to separate fiber and chemical-rich liquor from the knots, and commonly dewatering screens separate wash water from the shives. Depending on the quality of paper being produced, these rejects may be redirected to the digesters. More commonly, after screening the waste is landfilled. At other mills the shives are left in the water going to the wastewater treatment plant.

The advantage of adding a screw press to this process is that the knots and shives can be made into a press cake. The cake moisture can be held in a range of 35% to 50% by adjusting the air pressure on the discharge cone. Mills use the dry cake as supplemental fuel for their boilers. Other mills have found that run-off problems are avoided at the knot pile, the knots do not drip while being hauled to the landfill, and the press cake adds stability to a landfill.

Advantageous chemical and fiber recovery results where hot stock screening is used. With hot screening the rejects from the unwashed stock, at 190° F, are the material furnished to the Vincent screw press. The press liquor, containing 15% chemicals and some usable fiber, is returned to the chemical and fiber recovery flow. Chemical recovery (mostly caustic) can be increased by 10%.

Typically the primary knotters account for about one quarter of the load. The shives usually arrive in a flow of 1% to 2% consistency. Sidehill screens have been used successfully in installations that required pre-thickening. Mills where shives were sluiced with wastewater have benefited from reduced loading in the wastewater treatment plant.

The most commanding performance feature of the Vincent screw press is the ability, without operator attention, to handle a wide variety of conditions. Flow rates can be varied from "off", to surge, and back to normal. Tests were run with only mill water going into the press, followed by thin and thick flows. The press rarely wants to purge. There is a tendency to overload and jam at very low flows, so reinforced profile bar screens and heavy duty drives and frames are used for knots and shives.

One characteristic that catches everyone's eye is that knots are disintegrated into small bundles of fiber in the Vincent press. This results in an excellent boiler fuel and opens the possibility of additional fiber recovery. This will depend on the paper quality produced at the mill.

Issue 55 - B

Knots & Shives

November 30, 1994
Rev.Aug 1997

In 1994 Vincent presses were used for on-site testing at two paper mills, a James River plant and Buckeye Florida in Perry, FL. The tests at James River were arranged by the Atlanta office of Sandwell Inc., a consulting firm, while the tests at Buckeye were initiated by Agri-Products, a landscaping materials firm. Since then both mills have bought VP-16 presses.

Both plants produce paper pulp, which starts with wood chips that are digested. The mass discharged from the digesters contains pulp fiber, spent chemicals, and waste materials. The liquid is very black in color, hence its name, black liquor. This liquid gives pulp mills their characteristic odor. Conventional practice is to screen out the usable pulp fiber. The screen rejects are waste materials which, along with some black liquor, are sent to landfill.

These screen rejects consist of knots, shives, as well as some good pulp fiber. The shives are little bundles of cellulose fiber that are still bound by lignin that failed to get dissolved in the cooking process. At James River the knots were fairly evident, about the size of quarters, while at Buckeye they rarely appeared. This was a function of the species, as well as age and condition, of the trees being harvested.

The amount of good fiber present in the waste stream varied considerably; at Buckeye it was low, around 25%, while at James River, on the softwood side, it was running unusually high, probably around 80%.

Moisture content of the material coming into the presses from the vibratory screens varied widely. Readings ranged from 76% to 91% moisture. Successful operation is also achieved with some flows in the 96% to 99% moisture range.

Excellent results are evident pressing the knots and shives at both plants. Black liquor, with its chemicals, is a financially important recovery at James River. At this hardwood mill, the rejects are unwashed, coming directly from a continuous digester. At Buckeye the chemicals are less concentrated because the rejects come from the brown stock wash system.

Solids content of the press cake at both mills averages 40% to 50%. Some material can be taken up to 55%. The James River material can be pressed so that it is suitable for a boiler fuel.

Buckeye is entertaining a proposal involving mixing the pressed material with cypress mulch for landscaping application. Another potential use at James River is as a filler in the production of tar paper. In any case landfill problems of contamination and availability are avoided.

The most commanding performance feature of the press is the ability, without operator attention, to handle a wide variety of conditions. Flow rates can be varied from "off" to surge to normal. Tests were run with only plant water going into the press, followed by thin and thick flows. The press rarely wants to purge. There is a tendency to overload and jam on dry material, so reinforced profile bar screens and the heaviest available drives are used for knots and shives.

One characteristic that catches everyone's eye is that knots are disintegrated into small pieces in the Vincent press. This opens the possibility of additional fiber recovery.

In anticipation of future pollution regulations, presses for black liquor applications are offered with vapor barrier construction.

Issue 18

Non-Pressable Sludge

October 4, 1994
Rev. Jul 1997

Very frequently we are asked if some material can be dewatered in a Vincent press.  There is a very simple fist test that indicates if a screw press will work: First, put a small mount of material in the palm of your hand.  Next, close your fingers gently around the mass of material.  Work the material with your palm and fingers so that something is squeezed out between your fingers.  If a liquid comes out between your fingers and if, in the end, there is some solid material left in your palm, then a screw press might succeed.

For example, if you ran this test with mashed potatoes in your fist, you would see that it cannot be pressed.  On the other hand, if you shredded some paper, mixed it with water, and worked that material in the palm of your hand, you would see that paper pulp can be pressed successfully.

Digested organic material is the most important non-pressable sludge.  This includes sludge from sewage treatment, slaughterhouse, and cooked food plants. At these treatment facilities, the fine cloud in the wastewater is agglomerated with polymer flocculent.  In a DAF (Dissolved Air Flotation) system this sludge floats to the surface and is skimmed over the edge of a tank.  In other systems it is allowed to decant to the bottom of a clarifier tank, from which it is pumped out as underflow.

When sludge is skimmed off it will have 80% to 95% moisture; clarifier underflow is even wetter.  It is very expensive to dispose of because it can represent a huge tonnage going to landfill.  Generally, belt presses can be used to filter out some more of the water. But the end result cake is still likely to have more than 80% moisture.

Other sludges that will not press are finely ground inorganic materials.  These include settled materials such as pond dregs, tank bottoms, and clarifier silt.  Use the fist test if you are in doubt.

Because of the big potential payoff, we have run sludge tests using variety of press aids.  We also tried heating sludge to 200°F before pressing it.  Another effort involved adding bleach to break down the polymer.  Not one of these efforts has come close to working.

We have seen conflicting results with the use of polymer. We had a case where water was readily squeezed from a belt press cake with a bare hand; yet, the Vincent screw press could remove nothing.  In fact, after passing the sludge through our screw press, the fist test was no longer successful. At the same time there are several paper mill installations where the Vincent screw press works only when polymer is used on the waste stream.  The fist test is not a fail-proof determinant.

Issue 16


Paper Mill Prethickening Options

December 12, 2000

In some pulp & paper screw press applications there is no need to prethicken the flow to the press. This is the case both with rejects from pressure screens and knots from shaker tables. Similarly, most clarifier underflow has 2% to 3% solids consistency, which, especially in virgin fiber mills, is adequate for dewatering in a screw press.

Even in these easy applications it is recommended that a simple sidehill screen be installed over the inlet to the screw press. This assures proper press operation even under upset or shut-down conditions where the flow can become very dilute. (In dilute flow conditions the high flow of water through the press screen washes the solids through the screen so that the capture rate becomes unacceptable.)

Where a paper mill wastewater flow is being coagulated and/or flocculated, there is an aversion to using a sidehill screen. This is because the surface of the screen will become coated with the polymerized sludge, blinding the screen.

In these installations there are a number of prethickening machines that can be used. The simplest is a rotary drum screen. The use of continuous sprays on the outside of the drum keep the screening surface clear. FKC is a competitor who makes frequent use of these devices. Where the mill specifies a rotary drum screen ahead of the press, Vincent has included equipment designed and manufactured by IPEC Industries in British Columbia.

Many times the mill will specify that a gravity table (or table thickener) be supplied with the press. These are a specialty of Andritz, a very strong competitor that offers the Dupps screw press. A gravity table is in principle the upper deck of a belt press.

Probably the most effective prethickening machine is a belt press. Cake solids from a belt press will range from a low of 20% where there is a high content of secondary sludge, to a high of 35% or 40% solids where there is no biological content. For the last five years Vincent presses have been offered in conjunction with Phoenix belt presses.

In one mill Vincent screw presses were supplied along with gravity tables whose cake was fed to belt presses before being fed to the Vincent presses. The final cake is burned.

Issue 113

Paper Recycle Mills

April 23, 1996

Paper recycling is not new: Vincent presses have been installed at two mills that are over fifty years old. At the same time there has been a large construction boom of paper recycling mills in the 1990's. The mills are scattered across the country, generally being located close to population centers. There are seventeen in Los Angeles alone.

Each of these mills specializes in recycling a specific kind of waste paper. The most common is OCC (old corrugated container). Most OCC mills avoid waxed corrugated or corrugated from the far east (because of its weaker fiber make-up). Other mills handle these sub-grades.

The other recycle mills are called deinking plants. They specialize in categories such as ONP (old newspaper), telephone directories, MOW (mixed office waste), magazines and MWP (mixed waste paper).

Each of these categories of waste paper presents special challenges and requires specific technology. Much of the chemistry has to do with ink. Ink manufacturers continuously improve their ink for better adherence to paper fibers, while the recyclers are focused on separating the ink from the fiber. Equally important to the paper recycler is the technology to separate the ink not only from the pulp fibers but also from the stream of usable fiber. The waste rejects frequently end up in a Vincent screw press.

Recycling high quality, glossy magazines presents special challenges. These magazines are generally made of one third kraft fiber for strength, one third ground wood fiber for economy, and one third clay coating for printing quality. The high proportion of clay in the waste stream can make it all but impossible for a Vincent press to operate correctly.

MOW is one of the least desirable forms of waste paper to recycle. The wide variety of contaminants, especially plastics, cause problems for the paper maker. Laser printer inks affect quality because they are thermally (opposed to chemically) bound to the paper fibers.

The common characteristic of recycle paper mills is that they generate a considerable load that most frequently goes to landfill. For example a 300 TPD OCC recycle mill will generate 10%, or 30 TPD (solids) of reject material. This waste is rejected by screens that sort out the cellulose fibers that can be used to produce acceptable pulp. Reject streams containing 1% to 2% solids can be fed directly into a Vincent press to reduce the solids to press cake.

Reducing the solids to press cake reduces the tonnage that must be hauled to landfill. Many landfills require at least 30% solids before they will accept loads of waste. An additional bonus is that pressing eliminates liquid drainage on the highway when the material is being hauled.

Some paper mills press the reject material in order to dewater it to the point where it becomes a suitable boiler fuel.

Issue 42

Paper Cup Trim Recycling

Pressing News
August 3, 2015

An interesting niche exists in the secondary fiber market. It involves firms which recycle cut-out scrap from producers of water resistant paper containers.

There are many firms which produce pails for KFC® chicken, paper cups for Coca Cola®, gable top containers for Tropicana® orange juice, boxes for Ben & Jerry's® ice cream, and similar containers. These items are made of high quality bleached paper which is coated on both sides with printable polyethylene. The products are made from shapes which are die cut from large rolls of paper. This paper is produced by well known firms such as International Paper and Georgia-Pacific.

The trim resulting from the production of the required cut-outs can be recycled to recover the long fiber pulp found between the layers of polyethylene. The pulp is sold as market pulp. Because of the high quality of the fiber, the pulp is blended with other fibers to improve the overall strength of specialty papers.

The recycling begins with loading the trim into a hydrapulper. Water is blended with the trim and a thick slurry is produced. The pulper can have a screened bottom which holds back the plastic film, but which allows the fiber slurry to pass through.

As the trim is pulped, the fiber is rubbed free of the plastic film. A dilute flow of this fiber is drained from the pulper. At one mill is it prethickened with a sidehill screen and then dewatered with a Vincent screw press. The press cake is filled into bulk bags for shipment to the end users.

Typically the pulp is run through a dryer to produce thick sheets of market pulp.

Another application for a Vincent screw press, at these same facilities, is to dewater the rejected polyethylene film. This plastic is sold to plastic recyclers who process it into plastic pellets or proprietary plastic products.





Issue #275

Pressing Cake from a Belt Press

May 30, 2002

Energy costs have caused several mills to look at adding a screw press to their wastewater treatment plant. This is done so that the sludge from the facility can be used as boiler fuel. Typically the screw press is fed the cake produced by an existing belt press.

Belt presses and screw presses have fundamental similarities. Both operate on a continuous basis (not batch) to separate liquid from solids. However, each machine design has different strengths and weaknesses.

Because a belt press works with a filter made of fabric, it achieves much finer filtration than is possible with a screw press. The metal screen (perforated metal, profile bar, or drilled plate) in a screw press results in far more suspended particles in the press liquor than is characteristic of a belt press. The screening area of a belt press is much greater than a screw press, so it has greater hydraulic capacity.

The other side of the coin is that a screw press can squeeze a lot tighter than a belt press. The all-metal construction of a screw press allows higher, more concentrated, pressure and higher torque. The result is that a screw press can make press cake of a lower moisture content than is possible with a belt press.

Several paper mills have taken advantage of these characteristics by adding a screw press in series with a belt press. The belt press filters out the suspended solids, and the screw press squeezes out additional water.

Other mills have been motivated to add a screw press because of trouble maintaining acceptable consistency in the cake from their belt press. With surges in secondary (biological) sludge flow, the cake can become too moist. Instances of landslides at the landfill are not unheard of. Placing a screw press after the belt press will increase the final press cake solids from a range of 20% to 35% up to 25% to 50%, depending on the amount of secondary treatment sludge that is present.

A benefit to adding a screw press is that the belt press can now be operated with minimum belt pressure and no nip. A belt press will run almost maintenance free under such conditions.

The Vincent interrupted flight design has a particular advantage in this application. It can handle a wide range of input consistencies and flow rates without adjustment, all while continuing to produce a consistently dry cake.

Issue 58 - D

Pulp & Paper Installations


JULY, 2010


1994 Smurfit-Stone Container, Wabash, Indiana (now Paprworks Industries)
Two VP-16's doing screen rejects at a recycle boxboard mill. Tom Manley,
513-746-6493, was the plant engineer; we co-authored a TAPPI paper. We
arrived at this, our first paper mill, in June 1994.

1995 Domtar (formerly Georgia-Pacific), Port Edwards, Wisconsin
They bought a CP-6 to thicken screen rejects which are stored and later added to
special runs of bond paper. [Mill closed.]

1995 Coastal Paper, Mississippi
This is a paper maker that buys market pulp and converts it into specialty papers.
The tiny CP-4 works perfectly for their screen rejects.

1995 Bowater (formerly Halla Paper), Mokpo, Korea
This is a deinking mill. The furnish is 95% old newspapers and 5% groundwood a
fiber. They have four CP-12's, which are beautiful machines. The presses dewater 3
screen rejects and some primary clarifier sludge, all coming from belt presses.
The cake, at 55% solids, goes to a fluidized sand bed boiler.

1996 Georgia-Pacific (formerly Fort James), Pennington, Alabama
The VP-16 for dewatering knots and shives at this large virgin fiber mill is
working well. Pat Braud, 205-459-1737, is the cognizant engineer.

1996 CANFOR (Formerly FIBRECO), Taylor, British Columbia
This CTMP mill has purchased a VP-16 for pressing primary sludge with some
fraction of secondary sludge blended in. The sludge comes from a belt press.
They produce and sell market pulp made primarily from wood chips

1996 Evergreen Pulp (Formerly Samoa Pacific Cellulose) , Samoa, California
The last virgin fiber mill in California. They have a pair of VP-10's plus a rental
CP-10. One is used to dewater bleached rejects from just ahead of a paper
machine. Another is for shives. It enables them to haul the rejects to landfill
without environmental problems. [Mill closed.]
1996 Celotex, Quincy, Illinois
They had a CP-10 rented medium term and have since commissioned a VP-
16. They are a recycle mill that makes the outer paper layer for plasterboard.
The press is used to dewater screen rejects after the hydrapulper. They solve
a landfilling cost and problem by dewatering the rejects. [Mill shut in 2002.]

1996 Liberty Paper, Becker, Minnesota
This is an OCC recycle mill pressing tertiary screen rejects plus DAF sludge.
Following successful tests, they purchased a pair of VP-16's. Steve McPherson,
Production Manager, 763-261-6120.

1996 Durango Paper (formerly Gilman), St. Marys, Georgia
This firm has a pair of VP-12 presses, all-stainless construction (including the
frames), along with sidehill pre-thickening screens. Quaternary rejects are pressed
prior to burning. [Mill shut in 2002.]

1996 Georgia-Pacific (formerly Fort James), Clatskanie, Oregon
Two VP-30 presses were intended to take rain water out of sawdust. With 400 hp
drives, these are 35' long and weigh 80,000 pounds each. The units are now idle.

1996 APC Paper Company, Claremont, New Hampshire
This firm has a VP-10 to dewater OCC waste sludge prior to landfill.
Environmental problems associated with an unstable landfill led to the testing that
led to the purchase. They pre-thicken the flow to the press with a sidehill screen.

1996 Millar Western, Meadow Lake, Saskatchewan
A Model VP-16 dewatering screw press is used to increase solids in cake from a
belt press. This zero effluent mill burns the cake in a hog fuel boiler. The primary
sludge solids are increased from 30% or less to 40% or more with the screw press.

1996 Simpson Paper, Gilman, Vermont (Dirigo Paper)
This mill uses market pulp to make high-grade specialty paper. Reject material
from a large clarifier is fed to Somat presses. The sludge from the Somat's, at 10%
to 15% consistency, is fed to a Vincent Model CP-6 press. The Vincent takes it up
to 35% or 40% solids. [Mill closed in 2007.]

1997 Ponderay Newsprint (AbitibiBowater) Usk, Washington
This newsprint mill is 80% virgin fiber, 20% recycle. They are pressing a wide
range of screen rejects from both operations. Following a long rental period, they
bought a VP-16 in T-316 stainless. Their order included sidehill screens for use
ahead of the press.

1997 Buckeye (formerly Merfin), Delta, British Columbia
This mill makes sanitary napkins. A rental CP-6 worked well in competition
with a FAN press. They purchased a CP-10 for more capacity. They are pressing
reject pulp, which is collected in a Krofta clarifier.

1997 Louisiana-Pacific, Chetland, British Colombia
This mill acquired a CP-10 for sludge dewatering shortly before the mill was
1997 Buckeye (formerly Merfin Europe Ltd.), County Cork, Ireland
This sister plant has purchased a VP-10 for dewatering reject pulp from a system
that converts market pulp into paper product on a dry basis (Air Laid).

1997 Kadant Black Clawson, Mason, Ohio
A VP-16 press for use in a secondary fiber mill in Venezuela. The press,
positioned head of a deinking kneader, is used to thicken 35 TPD of stock to the
range of 30 to 35% solids. The user is Papelera Aragua.

1997 Putney Paper, Putney, Vermont
This firm purchased a VP-16 for dewatering clarifier underflow. An improved
fiber recovery system seriously reduced the capacity of the press. [Mill closed.]

1997 Buckeye Florida, Perry, Florida
After on-site testing, this 1,200 TPD softwood Kraft mill purchased a VP-16
for dewatering knots and shives. This relieves landfill load and groundwater
contamination, while creating a saleable by-product. A sidehill screen thickens
the flow ahead of the press.

1998 St. Anne-Nackawic, Nackawic, New Brunswick
This mill rented a VP-16 for two summers before purchasing the machine. It is
used to dewater reject fiber that was otherwise being taken to landfill in a dripping
wet condition. After pressing the cake burns extremely well in their hog fuel

1998 Kerwin Paper, Appleton, Wisconsin
This recycle paper mill produces colored construction paper. The VP-10 press
dewaters rejects that arise when a color change is made. The press cake is re-used
in black construction paper.

1998 Terrace Bay Pulp, Terrace Bay, Ontario
(Formerly Neenah Paper, Kimberly-Clark)
This large virgin fiber mill ran tests using a rental CP-10, dewatering primary
sludge that was pumped directly from the clarifier. They ordered a pair of VP-
16's and sidehill screens, and they are very pleased with these. The press cake
is burned in their boiler. In 2001 a third VP-16 was ordered to handle a mill
expansion, and all the presses were relocated from the WWTP to the boiler house.

1999 ALBERTA RESEARCH COUNCIL, Edmonton, Alberta
A Model CP-10 press is used for laboratory and pilot testing at the Alberta
Research Council.

1999 International Paper, Oswego, New York
This mill had a CP-6 rented for half a year. While we were switching up to a
CP-10, they tried a press by Press Technology, a Springfield, Ohio firm. That
press failed due to wear which was related to high screw rpm. [The mill is shut.]

1999 Crown Vantage, Milford, New Jersey
This mill produces food grade wrapping paper from market pulp. Rated at 200
tons per day, the mill was sending 4 to 8 tons of clarifier underflow sludge to
landfill daily. The purchase of a CP-10 screw press was justified by reduced
landfill charges. The screw press increases the solids of cake from a Komline-
Sanderson belt press from 20-27% to in excess of 50%.

1999 Corrugated Services, Forney, Texas
This recycle mill uses a small KP-6 for light dewatering of solids screened from
the wastewater flow. "The best capital investment this company ever made."

1999 Weyerhaeuser Paper, New Bern, North Carolina
This Kraft mill replaced a worn belt press with a Model VP-16 screw press and a
pair of Vincent sidehills. Final press cake solids were improved from the 30's to
45%; the cake is now sold as a fuel to a cogen power plant. No polymer is used in
the clarifier.

1999 Nevamar (formerly International Paper), Hampton, South Carolina
This mill is using a Model CP-12 screw press to dewater fiber and phenolic dust.
The dust comes from the spray chamber dust collector. The mill specializes in the
manufacture of hard board used in decorative door facings, counter tops and screw
conveyor hanger bearings.

1999 Celulosa Arauco (Alto Parana), Misiones, Argentina
This Chilean paper company procured a Vincent Model CP-12 screw press for
dewatering rejects from a brown stock tertiary screening stage. The installation is
at a softwood Kraft mill in Argentina. The objective is to recover the chemicals in
the black liquor filtrate.

1999 FiberMark, Brattleboro, Vermont
This specialty mill produces coated or embossed pressboard, using a furnish of
30% virgin and 70% recycled market pulp. They have a Vincent Model CP-12
screw press that is used to further dewater primary clarifier sludge that is first
prethickened to 22% solids with a vacuum coil filter. The Vincent press increases
the consistency to the 45% range, significantly reducing hauling and landfill

1999 Minas Basin Pulp, Hantsport, Nova Scotia
This mill has a CP-12 to dewater OCC waste sludge and tertiary/quaternary rejects
prior to hauling them to landfill. Press cake is in the range of 50% solids, and
alternatives such as burning are being considered. Pre-thickening is done with a
sidehill screen.

2000 Georgia-Pacific, Crossett, Arkansas
Integrated Technical Services of Baton Rouge, LA bought a pair of VP-16's, each
with a pair of sidehills for prethickening, for dewatering the clarifier flow at this
1,600 TPD hardwood and softwood mill. Later, LARCO purchased another VP-
16 for use under a similar contract at the mill.

2001 Gulf States Paper, Demopolis, Alabama
This mill used a VP-16 from our rental fleet while a new machine was built to
their specifications. The load is estimated at 41 TPD, dry solids.

2001 Rock-Tenn, Sheldon Springs, Vermont
A small CP-6 is used to dewater 7 TPD of rejects. They feed from the Kadant
Black Clawson Ultra Sorter at 25% solids. The press cake, at 45% solids, is sent
to landfill.

2001 Caraustar Mill Group (formerly Smurfit-Stone), Lafayette, Indiana
A rental CP-10 was used dewatering rejects from screens at this recycle paper
mill. Mark Lindstrom, 765-423-5631, is the cognizant engineer.

2001 Durango-McKinley Paper, Prewitt, New Mexico.
Furnish for this 550 TPD linerboard mill arrives by train and trailer. Four waste
streams, with varying flow rates, including one that is biological, are fed to an
Andritz belt press. The 35% solids cake from the belt press is further dewatered
to 37% to 45% solids (depending on the biological component) with a Model VP-
16 turning 5 rpm. This rental press was used until another press at the mill could
be rebuilt. Ed Tom, 505-876-2171, supervises operations. Rented another unit in

2002 Georgia-Pacific, Cedar Springs, Georgia
All wastewater solids from this 2,300 tons per day mill are dewatered in a pair
of VP-16 presses. Pairs of sidehill screens are mounted over the inlet of each
of screw press. The WWTP is operated and managed by Integrated Technical
Services of Baton Rouge, Louisiana.

2002 Rock-Tenn, Aurora, Illinois
A rental CP-10 was purchased for dewatering screen rejects in this book binders
board mill. John Goll, Assistant General Manager, 630-898-4231.

2002 Noss AB, Sweden
This paper industry equipment supplier has purchased a Model CP-4 laboratory
press for use in pulp thickening trials.

2002 Ohio Pulp Mills, Cincinnati, Ohio
This mill recovers secondary fiber from gable-top cartons, ice cream cartons, and
meat pads. A great deal of plastic film is present in the rejects. A custom Model
KP-10 press, cut short for low headroom, dewaters their waste flows ahead of the

2002 Buckeye, Mt. Holly, North Carolina
This conversion mill has air laid paper machines making diaper products. Excess
pulp from the paper machines and floor drains feeds to a DAF or sidehill, then is
pumped to a CP-10. The Vincent press replaced a plate and frame press. Contact
is Tim Kistemaker, 704-822-6400.

2002 Linpac Paper, Cowpens, South Carolina
Rejects at this 450 TPD OCC mill include unusually high quantities of plastic.
A flow of 200 gpm is fed directly to a Model CP-12 screw press, without
prethickening. Bill Johnson, 864-463-9090, is the Mill Engineer.

2003 P. H. Glatfelter Co., Spring Grove, Pennsylvania
A simple, robust Model CP-10 screw press was purchased in 2003. An enlarged
headbox was included to handle surges. Trials were run with a rental CP-6 prior
to the purchase. Since the feed flow can reach 100 gpm, a 3/32" perforated screen
was selected. Reclaimed liquor is returned to process.

2003 Flakeboard Company, St. Stephen, New Brunswick
A small CP-4 press was purchased to recover valuable oil from waste in this board
manufacturing facility.

2003 AV Cell, Atholville, New Brunswick
This mill has a Model VP-16 press on extended rental. (A new 316 stainless press
was purchased in 2006.)

2003 Inland Paper, Orange, Texas
A VP-16 was purchased by Bruce Brothers Dredging under a contract to dewater
WWTP sludge at this virgin fiber mill. Clarifier underflow is prethickened with
sidehill screens.

2003 US Gypsum, Gypsum, Ohio
This mill makes liner for gypsum wallboard. They use a Model VP-16 press for
sludge dewatering. An IPEC rotary drum screen is used to prethicken the flow to
the press.

2004 Blue Ridge Paper, Canton, North Carolina
This virgin fiber mill purchased CP-10 presses for dewatering screen rejects in
both their softwood and hardwood mills. The reclaimed liquor is returned to the
system. The press cake is burned.

2004 Sindihan Paper (UEM), Egypt
A Model CP-10 press was sold for dewatering clarifier sludge at this Egyptian
paper mill. In 2006 they inquired about purchasing another identical press for a
new project.

2004 Georgia-Pacific, Green Bay, Wisconsin
The KP-16 press at this 1000 TPD recycle mill is in an unusual application.
Floating wastes, mostly Styrofoam and woodchips, are skimmed at about 200 gpm
from the WWTP clarifiers. The press dewaters this waste stream. This is our first
installation of a Model KP-16 press in the pulp and paper industry.

2005 International Paper, Pensacola, Florida
Two screw presses, a VP-16 and a KP-16 were used in series and parallel to
thicken stock for a specialty paper.

2005 Eurocan, Kitimat, British Columbia
A Model VP-22 is used by Jose's Excavating, under contract with the mill,
to dewater primary clarifier sludge. The sludge is loaded to the press after
draining in a pond. The press cake is landfilled. [Mill closed in 2009.]

2005 Rayonier, Fernandina Beach, Florida
A Model VP-22 was purchased for dewatering knots. The press was built in
all-316 stainless, including the base frame, because of adverse environmental
conditions. The press cake is burned in a mill boiler.

2005 Evergreen Packaging (formerly International Paper), Pine Bluff, Arkansas
This mill is renting two Model VP-16 presses for dewatering a combination of
clarifier sludge and knots/rejects. Vincent Sidehill screens are used to prethicken
the flow to the presses. The cake is burned.

2006 Elite Kraft, Thailand
A Model CP-12 press is used to dewater sludge that is prethickened with a tray
belt prethickener.

2006 AV Cell, Atholville, New Brunswick
This mill purchased a Model VP-16 press built in 316 stainless construction. This
followed extended use of a similar model in 304 stainless.

2006 Inland Paper, Orange, Texas
A KP-16 was rented by Bruce Brothers Dredging under a contract to dewater
WWTP sludge at this virgin fiber mill. Clarifier underflow, at very low
consistency is pumped directly into the press. The cake produced is quite suitable
for landfill stacking.

2006 Evergreen Packaging (formerly International Paper) Pine Bluff, Arkansas
This mill rented (since purchased) a Model KP-16 press for dewatering small
reject fibers after bleaching. A feed of 200+ gpm at 0.5 - 1.3% consistency
results in press cake at 38% solids. The load is about 7 TPD dry solids. This is
a very successful application for a Series KP press. The cake is sold to a roofing

2007 Elite Kraft, Thailand
A Model KP-16S press has been ordered to dewater small pieces of plastic film,
Styrofoam and similar Combisorter or Serraplast rejects.

2007 First Quality Tissue, Lock Haven, Pennsylvania
A pair of CP-12 presses, each with paired sidehill screens for pre-thickening, is
used to dewater clarifier underflow.

2007 Pactiv Corporation, Plattsburgh, NY
A Model CP-6 press was purchased for de-watering tertiary rejects.

2007 PROPAL (Productora de Papeles), Cali, Colombia
This bagasse paper mill purchased a Model VP-24 for dewatering clarifier
underflow. No polymer was required. The cake was to be blended with coal
for use as boiler fuel. This job was a failure; however, it presents an interesting

2007 Sappi Fine Papers, Cloquet, MN
A Model VP-24 has been purchased for dewatering knots. The load is
approximately 50 TPD of dry solids. The liquor will be recycled and the crushed
knots returned to the digester.

2007 Flambeau River Papers, Park Falls, Wisconsin
A Model CP-10 has been used on a rental basis for an extended period. With a
3600 rpm motor, this press achieves outstanding capacity.

2007 Inland Paper, Orange, Texas
A KP-16 was rented long term by Bruce Brothers Dredging under a contract
to dewater WWTP sludge at this virgin fiber mill. Clarifier underflow is
prethickened with sidehill screens.

2008 Rayonier, Jesup, GA
A model KP-16, owned by Absorption Corp, is in service at the mill. The
dewatered fiber is used in the production of litter for domestic pets.

2008 UPPC (United Pulp & Paper Company), Philippines
A Model CP-12 is giving excellent results dewatering cake from belt presses at the
wastewater treatment plant.

2008 Blue Heron Paper, Oregon City, OR
This recycle mill has rented a Model KP-10 press for an extended period. The
waste includes a considerable amount of Styrofoam and plastic.

2008 Hartford City Paper, Hartford, Indiana
Based on a project carried out by AMEC consulting engineers, this mill has
purchased a Model KP-10 press for dewatering WWTP sludge. The flow comes
from a Poseidon clarifier and an Andritz sidehill screen. The sidehill, with 0.005"
slots, is PLC controlled.

2008 SAICA, Venizel, France
This large recycle mill uses a Model KP-16 to dewater sludge ahead of landfill.
The press was selected after trials with German equipment.

2008 Georgia-Pacific, Crossett, Arkansas
Larco, a contractor operating the WWTP, bought a Model KP-16 for assisting in
dewatering the clarifier flow at this 1,600 TPD combined hardwood and softwood

2008 Fiber Solutions, Pine Bluff, AR
This contractor has rented a Model KP-16 for over a year. It is being used to
recover fiber from whitewater. The press cake is sold for the manufacture of
shingles and other by-products.

2008 Georgia-Pacific, Palatka, FL
This mill rented a Model KP-16 press was used on knots and shives. The
installation was temporary while other mill machinery was being rebuilt.

2008 Sonoco, Menasha, WI
This secondary fiber mill installed a Model KP-16 to de-water sludge and screen
rejects. It replaces a screw press associated with excessive maintenance expense.

2008 NGC (National Gypsum Company), Pryor, OK
This plant uses a Model KP-16 press to dewater screen rejects and clarifier sludge.

2008 SAICA, Zaragoza, Spain

2008 Alabama Rivera Pulp, Perdue Hill, AL
A ModelVP-16 screw press is used to dewater knots at this pine mill. The press
cake is conveyed to the chip yard.

2009 Norampac, Niagara Falls, NY
This mill has rented a Model KP-16 de-watering sludge.

2009 University of British Colombia, British Colombia, Canada
The University has purchased a Model CP-4 press for laboratory and pilot
operations at the Pulp & Paper Centre.

2009 PCA (Packaging Corporation of America), Filer City, MI
This mill has installed a pair of Model VP-16 presses. An initial objective was to
reduce landfill expense. Permitting was sought to allow the cake to be sold for a
biofuel project. Additional savings were realized by the elimination of polymer
usage. A biological waste stream is to be blended into the feed to the press.

2009 Hutamaki, Sacramento, CA
This secondary fiber mill is using a Vincent Fiber Filter to pre-thicken a dilute
flow of extractor pressate ahead of a Model KP-6 screw press. This has resolved
problems with a minor waste stream and provides a saleable by-product which is
shipped to a plastics recycler.

2009 Tama Paperboard (Caraustar Industries), Tama, IA
This mill is using a rental Model KP-10 to de-water sludge and Poseidon rejects.

2009 Kimberly-Clark, New Milford, CT
This mill purchased a Fiber Filter FF-6 to separate plastic flakes and beards and
crumbs from a clarifier overflow.

2009 Kimberly-Clark, Beech Island, SC
This mill is using a rental Fiber Filter FF-6 to clean up short fibers and stickies
from a stream. The sludge from the Fiber Filter is then de-watered in a Model CP-
4 screw press.

2009 Lydall Filtration/Separation, Rochester, NH
This mill is using a rental KP-10 for dewatering sludge. This unit is replacing a
PT&M press.

2009 Glatfelter, Spring Grove, PA
This mill has a specially modified CP-4 which is used to de-water grit produced
when they slake their lime.

2009 PCA (Packaging Corporation of America), Counce, TN
A rental KP-16 has seen extended service dewatering screen rejects at a recycle

2009 Weidmann Electrical Technology, St. Johnsbury, VT
This mill purchased a CP-12 press for dewatering sludge. The mill produces paper
insulation for electrical transformers.

2010 Bataan 2020, Philippines
A Model CP-12 is giving excellent results dewatering cake from belt presses at the
wastewater treatment plant. The flow is pre-thickened with a sidehill screen.


Pressing Dilute Flows

November 30, 2005                                                                                                                                                                                              ISSUE #167

Occasionally questions arise about feeding a screw press with extremely dilute flows. We have one good set of data that involves this condition. They come from an OCC (old corrugated container) recycle mill, Liberty Paper, in Becker, MN.

They fed a flow of 260 gpm into a Vincent Model VP-16 press. This flow was mill effluent with a solids consistency of only 0.7% (7,000 ppm). This represents a feed of 11 tons per day of dry solids going into the screw press. The result was that 4 TPD,DS were captured and came out as press cake with 45% to 50% solids. The press liquor (effluent) from the press had about 4,500 ppm of solids. Thus the capture rate of the press was about 35%.

This operating condition occurred only during mill shut-downs because normally the feed to the press was in the range of 2% to 3%. (The city objected to the 4,500 ppm discharge periods, so at the time Liberty had to add sidehill screens ahead of the screw press, just to cover down periods.)

Thus it is seen that the press does not become inoperable due to extremely low consistency feed. Even with straight water going into the press, water will not purge from the solids discharge end of the press. On the other hand, the capture rate does go down significantly.

The Smurfit mill in Wabash, Indiana tested this to the limit in 1994. They ran the press normally for a while and then replaced feed flow to the press with a firewater hose. The press was run this way in order to confirm that a plug of fiber at the solids discharge would hold, preventing any water from coming out the solids discharge end of the press. All of the fire water came out through the screen. Normal press operation resumed automatically when the normal flow was re-admitted to the press.

(The other extreme of this same test was to have the press in normal operation and then switch the flow into press to cake from the press. That is, the press was fed only cake with 50% solids. This material passed through the press without the press tripping out on overload or damaging itself. Negligible press liquor came through the screens when operating in this manner.) 

Prethickening with a Sidehill Screen

July 22, 2004                                                                                                                                                                                                      ISSUE #151

Many flows require prethickening before they can be dewatered in a screw press.  If the feed to a screw press is too dilute, two problems are likely.  The obvious problem is that the press may not have sufficient open screen area to pass all the water.  The press can become limited by its hydraulic capacity rather than the solids discharge capacity.  This can be avoided with either a larger screw press or some form of prethickening.  The prethickening option almost always costs less.

A more subtle problem associated with dilute flow in a screw press is that the solids capture rate will drop.  The reduced capture of solids occurs because the velocity of water passing through the screen increases to where the solids particles are swept through the screen.  This is apparent in manure dewatering applications:  the solids capture rate with scraped manure barns is 50%, while it drops to only 25% at flush barn installations.

In general, the least expensive prethickening device is a sidehill screen.  (These are also known as parabolic, gravity, sloped, and static screens.)  Sidehills are inexpensive, and they have the advantage of having no moving parts.  However, sidehills get a bad rap.

Engineers and operators will frequently voice objections to sidehills because they are not consistent.  It is common for them to flood when the flow goes up or when the screen surface becomes dirty and blinded.  This changes the consistency of the discharge sludge, which may cause problems in downstream equipment.

If the downstream equipment is a screw press of the continuous flighting design, yes, there are apt to be problems.  A reduction in feed consistency to a screw press of this design can cause the press to produce wet cake, or even purge, if its speed is not reduced.

In contrast, the Vincent screw press, because of its interrupted flighting design, works very well with sidehill prethickeners.  The gaps in flighting between each of the compression stages of the screw must fill with solids of some consistency before material will move toward the discharge of the press.  Consequently, even if the feed consistency decreases due to flooding of the sidehill, the Vincent screw press will continue to produce discharge cake with a steady solids content.  No adjustment or change in speed is required.            

This advantage of the interrupted screw design has been demonstrated most vividly in the pulp and paper industry.  A significant number of installations, in both virgin fiber and recycle mills, have now been operating for enough years to offer convincing evidence.

Pulp & Paper I

August 30, 1994

It was in 1994 that Sales Representative Gary Gann came to us with a chance to test in a paper mill. His customer, a major recycle paper mill, had a severe problem because their primary and secondary sewage treatment plants were no longer able to carry the load. Would we be able to filter the waste stream?

Despite four decades of pressing, Vincent had never had equipment in a pulp mill. Sensing a unique opportunity, we responded by dispatching two small presses for trials. By the time these presses made it back to Tampa they had been modified many times. It took that to meet the challenge.

Engineering Management at the plant hoped that a screw press would be able to remove solids from their waste stream. They wanted to avoid adding another belt press because of the high maintenance expense and need for an operator. It turned out that three other screw press manufacturers had tried their equipment at the plant, without success.

We learned from their experience. Blinding on ash and clay was avoided by going to a profile bar (heavy wedgewire) screen. A purging problem was solved by improving the arrangement of the auto- adjusting discharge cone. Press plugging was avoided with a combination of the interrupted screw flight design along with a unique by-pass and dilution system. It took several modifications to our standard citrus screw press to make it work.

Today the plant is avoiding pollution problems. They used one of our larger rental press until a pair of all-stainless VP-16's could be delivered. In fact, they were able to shut down one of their two waste digesters.

The application was challenging because of the very wide range of conditions that exist. The plant produces boxboard by recycling pre-consumer waste paper, so screen rejects from a variety of stages reach the press. When metal is detected, thick stock is diverted to the press. Periodically their clarifier backwashes fine sludge to the press. Other times the inflow becomes extremely dilute because flushing is going on in the plant. Our one press is handling this full range of materials!

5/97 Update: Today Pulp & Paper is more than half our business!

Issue 13

Pulp & Paper II

March 21, 1996
Rev. March 1998

It was in June of 1994 that Vincent was invited to their first on-site testing in a paper mill. This has led to a major change in the Company: today more than half the presses being built are going to the pulp and paper industry.

Paper mills fall into three main categories:

    • The virgin fiber mills where digesters cook wood chips to make paper pulp
    • Recycle mills where waste paper is pulped by agitating it in a water bath
    • Mills that buy market pulp from other mills and convert it into their own specialty papers.

Vincent has screw presses operating in all three of these industry segments. In most cases our press is being used to dewater waste streams.

The waste streams generally have consistencies in the range of 1% to 12% solids. They are dewatered so that the solids become a bulky material that can be readily handled. At the same time the pressate liquid frequently becomes clear enough that it is acceptable for re-use in the mill.

The waste streams that can be dewatered in a Vincent press consist of various forms of screen rejects. In a virgin fiber mill rejects come both before and after bleaching. Ahead of bleaching we deal with brown stock rejects: knots from the knotters, and shives (along with a fair percentage of good fiber) from the tertiary or quatenary pressure screens.

In a recycle mill a variety of screenings are used to separate unusable fiber along with dirt, ink, plastic, etc. All of the waste streams will contain some measure of good fiber.

The press cake will typically be in the range of 40% to 50% solids. The lower solids content is adequate for easy handling and transport, while the higher solids material might be used for boiler fuel. (In the boiler fuel application the press cake is combined with 80% to 90% coal.) Frequently the press cake is landfilled. However occasionally the press cake is sold to mills capable of using the material in the production of lower grade pulp products. Other uses include filler for tar paper and shingles as well as landscaping mulch.

This is where we find the market.

The most common inquiry from the pulp and paper industry involves pressing clarifier sludge at the wastewater treatment plant. We have found that our press will usually work well on this material. This is especially true if there is little or no biological (secondary treatment) sludge present. Our competitors in this market are the immense continuous screw presses offered by Andritz and FKC.

With our press design we have found that the best performance is achieved if the clarifier sludge is prethickened with a gravity table or a belt press. To address this opportunity we have established a relationship with Phoenix Process Equipment Company, a major supplier of belt presses to the pulp and paper industry.

Future issues of Pressing News will detail our applications in both virgin fiber and recycle mills.

Issue 40


January 23, 1996
[Rev. September 2002]

Largely because of action by environmental groups, the cost and availability of timber in the northwest has become a problem for paper mills. Paper makers now make paper that contains fiber extracted from lumber mill sawdust. Because lumber mills normally burn their sawdust, it represents an economic source of raw material for a pulp mill.

In 1996 this situation led to pressing trials using sawdust. The tests were conducted in Tampa with engineers from Fort James trying out three different Vincent screw presses.

The thin, high speed saw blades used at the lumber mills are cooled with water. This process increases the moisture content of the sawdust slightly from the normal 52% found in green lumber. Rainfall adds even greater moisture content. The samples tested in Tampa had 65% moisture (35% solids).

The normal press configuration was found to produce press cake with 42% solids. The horsepower required was in the high range for Vincent presses. Since the Fort James specification called for 45% to 48% solids, a special screw design was used in subsequent testing.

It is noteworthy that the pressing action is only removing free water. We are not breaking open the cells of the wood. A competitive press requiring 3,500 hp could achieve solids closer to 60%. However this broke down the cellulose fibers, reducing the quality of the paper being produced.

Therefore, the final contract called for two VP-30 presses, each with 300 horsepower drives. These subsequently were modified to 400 horsepower. The bulk density of the sawdust usually is low, about 22 pounds per cubic foot. This is compressed dramatically, to the point where water is expelled, by changing the screw shaft diameter just ahead of the first resistor tooth. The pair of presses can handle approximately 100,000 pounds per hour of sawdust.

The presses were supplied with an unusual feature: The resistor teeth were drilled for the possible addition of steam directly into the sawdust as it is being pressed. This technique was used many years ago in both fish meal and citrus peel to achieve lower cake moisture. To date it has not been used at the paper mill.

Unfortunately, the need at the Fort James Clatskanie mill is unique. They have an overloaded sawdust digester. The arrangement does not allow for the addition of sufficient steam when the sawdust is wet. Thus the capacity of the digester drops from 300 TPD in the summer to 240 TPD in the winter rainy season.

Another problem is that James River in Clatskanie has a limited amount of white liquor generating capacity. The excess water in the sawdust requires extra white liquor to overcome a dilution effect.

On the other hand, improvements in the material handling system appear to have resulted in increased digester capacity. Simply being able to feed more tonnage of the damp sawdust into the digester may have been all that was required.

The installation of the VP-30 screw presses never quite succeeded, primarily due to stick-slip vibration that occurs. This vibration occurs when tight pressing is attempted. A variety of mechanical failures have resulted from the vibration, in spite of numerous modifications.

Issue 38

Shower Water Filtration

October 25, 2001                                                                                                                                                                                                 ISSUE #122

Tests were recently conducted using a Vincent Fiber Filter on shower water at Atlantic Packaging in Scarboro, Ontario. The Fiber Filter was selected because of its small footprint and attractive capital cost compared to conventional filters.

This recycle paper mill has a combination OCC (Old Corrugated Container) and tissue operations. The most important use of shower water is on the felt of paper machines. Other applications are on trommel and rotary drum screens.

The primary source of shower water comes from disc strainers used at the end of the stock prep process. The disc strainers raise the paper pulp from 1.2% to 12% consistency. There are three effluents: cloudy, clear, and super clear. The cloudy effluent is pumped back to the start of the process at the hydrapulpers. The clear effluent is re-used half way back in the stock prep process, with the excess going to sewer.

The disc filter itself uses some of the super clear; the rest is filtered prior to use as shower water. The existing super clear water filters are Sweco machines that feature a vertical axis with a spinning drum screen. The drums have fabric panels backed up with coarse mesh steel screen. The drums are about 4' in diameter.

These Sweco's are very old and have served their purpose; maintenance expense has become excessive. Thus our Fiber Filter is being evaluated as a potential replacement.

The very low solids consistency of the super clear water resulted in high capacity in the Fiber Filter even with very fine fabric sleeves. With a feed consistency of 70 ppm, using a 31 micron sleeve, we were able to produce filtrate with 20 to 25 ppm. This was done with the Model FF-12 being fed about 200 gpm and producing about 1 gpm of fiber-sludge discharge.

A goal of the project is to eliminate the need for addition of city water make-up to the shower water tank.

There is a rule-of-thumb used in selecting filters used on flows ahead of spray nozzles: The filter basket should have openings one tenth the size of the openings of the shower nozzles. This is achieved with the 20 and 31 microns sleeves available for use with the Fiber Filter machines.

Spray Chamber Dust

February 27, 2001

Everyone is familiar with Formica counter tops.  This material is made from ground wood dust that is combined with phenolic resin.  The material is heated and pressed to produce a rigid, homogeneous, void-free material.  In addition to counter tops, the material is used for wall paneling (Masonite board) and furniture.  Vincent Corporation buys rods and tubes of this material, called Ryertex, to manufacture certain bushings and screw conveyor hanger bearings.

The International Paper plant in Hampton, South Carolina produces Micarta sheets and rods of this material.  It is known for its electrical insulating properties.  One of their engineers happened upon the Vincent web site, www.vincentcorp.com, and thought of using a screw press to dewater their waste stream.  A phone call was made and, within a very few months, a Model CP-12 screw press was built and installed.

Sawing and sanding operations are part of the manufacturing process, which creates a lot of dust.  Dust-laden exhaust air is drawn through the chamber, and water is sprayed into the air.  The dust and water fall to the bottom of the chamber, forming a sludge.  There was a need to separate the free water because the waste is sent to landfill.

To dewater the sludge from the spray chamber it is elevated by a drag flight conveyor to the CP-12.  This flow is about 1,000 pph at 16% consistency.  The press cake produced by the press is a dry material with only 32% moisture, and the press liquor is clear enough for reuse within the plant.

Initially the CP-12 did not perform very well.  Then the screw was slowed by switching to a 1200 rpm motor.  With this change the cake became much drier.

An unexpected problem was encountered at the installation.  The powdery press cake entered between the screw shaft and the cone bushing.  The heat was sufficient to melt resin that is present, and the cone started seizing to the screw shaft.  The condition was caught before serious damage occurred.

To address the situation an automatic lube system was designed and supplied.  It uses compressed air to pump lubricating grease to the cone bushing.  A timer sets the frequency and duration of the pump cycle.  This system is now being offered as standard in a variety of Vincent applications.

Issue 115


Stock Thickening

April 20, 2004

The best Pulp & Paper application for Vincent presses has been to squeeze as much water as possible from reject fiber. Typically we are working with screen rejects, knots & shives, and clarifier underflow. If the waste solids are to be landfilled, they are squeezed to only 35% to 40% solids, minimizing abrasive wear within the press. If the waste is to be used as boiler fuel, then the discharge pressure of the same press is increased and cake with 50% solids is achieved.

A different application involves thickening a flow of fiber and water to a solids consistency of only 30% to 35%. This need exists where good fiber must be thickened (a) for temporary storage, (b) ahead a deinking kneader, or (c) between stages of a counterflow wash or bleaching operation. In these applications it is important not to squeeze too hard as fiber damage can result, and the cake produced will no longer be sufficiently fluid.

On three occasions we have sold Series VP presses for stock thickening, with good results.

Earlier this year excellent results were achieved using a Series KP press. This series of machines was introduced in 1996 for "soft squeeze" applications. The initial market, dewatering cannery and fresh-cut produce waste, required a light, low-torque machine. This changed almost immediately as we entered the farm market for producing cow bedding from manure. This application required the same economical press, but with three times the horsepower.

Until last year the market for Series KP presses was limited to 16" screws. However sales were made to corn canneries that needed machines that could handle up to 100 tons per hour of corn husk and cob. That led to the introduction of 24" and 30" Series KP machines. Converting this waste into silage by-product was once again a high torque application.

It was recognized that the Series KP press technology had evolved to where it could be suitable for stock thickening in paper mills. Tests were run, demonstrating that the press was easy to set to hold 33% to 36% output solids. Typical of operation with the interrupted flight screw design used by Vincent, this output consistency held over a wide range of feed consistencies. Capacity of the 20-hp KP-16 was 30 tons per day, air dry solids, so a Model KP-30 press was recommended for the 100 TPD load at the mill.

This testing was performed at the Linpac mill in Cowpens, South Carolina, where deinking capacity is limited by insufficient prethickening capacity ahead of the kneader. The testing demonstrated that a Series KP screw press could be placed in parallel with the existing prethickening press in order to relieve the bottleneck.

Issue 152

TAPPI - Pressing Knots & Shives

Tappi Journal, Vol. 82, No. 2, February 1999

The project described in this article all started with an ad in The Tappi Journal. The ad was brought to the attention of Joe Lukasik, an engineer at the Atlanta office of Sandwell Inc. At the time Sandwell was working on a fiber recovery project at the James River Naheola Mill in Pennington, Alabama. Through Sandwell, arrangements were made for on- site testing. The success of the trials ultimately led to the four installations described in this article.

An unusual project with many advantages has recently come on stream at the Gilman Paper Mill in St. Marys, Georgia. The installation features a pair of screw presses that are used to dewater quaternary screen rejects. This technology parallels a similar project at Fort James Naheola Mill, with interesting differences.

The Gilman mill, rated at 1,200 tons, has been in service since 1941. It has both hardwood and softwood kraft operations employing thirteen batch digesters. About half of the tonnage goes to bleached and unbleached kraft multiwall specialty products, with the other half going to bleached board, coated and uncoated.

Similarly, the Naheola Mill, rated at 1,100 tons, has both hardwood and softwood kraft operations. About half of the pulp produced in converted on-site to tissue and towels, while the rest is used for packaging board.

At both mills the screw press project was part of a new fiber preparation system that features Thermo Black Clawson pressure screens. At Gilman, the principal benefit of the new system is improved stock quality to the #1 paper machine, while at the same time converting waste materials into useful boiler fuel. In contrast, at Fort James the principal benefit is increased chemical and fiber recovery, while at the same time reducing loading at the wastewater treatment plant.

At Gilman prior to the installation of the new pressure screens and screw presses, rejects from secondary screens were refined and pumped to the blow tank. This was undesirable because it resulted in poor stock quality to one of the paper machines.

In contrast at Fort James, prior to the installation, insufficient quaternary screening was available. Rejects from vibratory (secondary) knotters were accumulated at ground level prior to hauling to landfill, and rejects (mostly shives) from the quaternary pressure screen were diverted to the wastewater treatment plant. This system had poor yield and excessive loss of digester chemicals.

With the new systems, both mills use Vincent screw presses to dewater combined flows of knots, shives, and other rejects.

At Gilman the new system uses a Vincent sidehill screen to thicken the 160 gpm 2.3% consistency flow ahead of each of the screw presses. The screen tailings are funnelled directly into the 12" presses. At Fort James, in contrast, the flow of quaternary rejects and knots from the knotters flow directly to a 16" press without benefit of a sidehill screen. Surge flows of up to 400 gpm go to the press.

Both mills use a hot stock screening system. The rejects from the unwashed stock, at 190º F, are what go to the screw press. The press liquor, containing chemicals and some usable fiber, is returned either to the rejects tank supplying the pine side secondary screen or to the chemical and fiber recovery flow.

The horizontal screw press has two screening sections: at the inlet hopper and over the compression stage. An initial knot dewatering application supplied by Vincent Corporation to Fort James Naheola mill had profile bar screens (baskets) with nominal 0.020" (one half millimeter) slots. To optimize chemical and fiber recovery these were subsequently changed to 3/32" perforated screens. Based on this experience, the Gilman presses had press screens with 3/32" perforations from the start.

It was found at Fort James that in normal operation the inlet hopper screen of the press allows free liquor to drop out, which consists of up to 40% of the total pressate flow. In addition an unexpected source of liquor recovery was found to arise from pressing the knots. Testing showed that 20% by weight of knotter rejects is converted into press liquor in a Vincent screw press. It is estimated that this liquid flow amounts to 10% of the total chemical recovery, as well as some of the fiber recovery.

The screw presses were designed specifically for the pulp and paper industry. All contact parts are made of stainless steel (316 at Gilman; 304 at Fort James), and weld applied hardfacing is used on the wear areas of the screw and discharge cone. Heavy duty drive and screw flighting allow the press to operate under conditions of a "hard cook" when material similar to ground wood enters the press. Vibration was minimized in the Gilman presses by specifying that the drive motor be mounted in line with the gearbox. Since this eliminates V-belts from the drive train, a variable frequency drive (VFD) is available. Gasketing compatible with H2S was employed, and all bronze materials (nuts and bushings) were eliminated.

Gasketed covers were supplied with provision for modification for vacuum recovery of vapors. This is in anticipation of future requirements for collection of Total Reduced Sulphur (TRS) emissions.

In passing through the press, the knots are broken into small bundles of fiber. This occurs because the press design is based on an interrupted screw flight with stationary resistor teeth. The agitation and shear caused by these members break down the knots. Some fiber recovery improvement is achieved as a result of this action. It is difficult to spot the difference between the knots and the shives in the press cake.

The press cake moisture can be controlled by adjusting the air pressure actuating the discharge cone (also called a stopper or plug). With this devise press cake moisture in the range of 45% to 55% solids is maintained. In mills where the cake is landfilled, lower cone pressures and higher cake moistures are typical.

At Gilman the press cake is made of elements that previously were refined and recirculated, while at James River they were sent either to landfill or wastewater treatment. With the new installations the rejects are used as boiler fuel at both mills.

Similar installations exist at two other paper mills. At these the press cake is being sold as landscaping mulch and as raw material for an asphalt shingle manufacturing operation. Vincent has supplied screw presses for use on shives to Louisiana-Pacific, Samoa, California and for knots and shives to Buckeye Cellulose, Perry, Florida. The motivating factors at these mills were to facilitate off-site hauling, to avoid landfill problems, and to extend landfill life.

The success of these installations has generated interest in pressing knots and shives. The projects are relatively small and simple. Incremental improvements in pulp making and abatement of environmental pressures are the benefits.

Ross is Chief Engineer, Gilman Paper, 1000 Osborne St., St. Marys, Georgia 31558; and Johnston is Professional Engineer, Vincent Corp., 2810 East 5th Avenue, Tampa, FL 33605.


The flow of hot black liquor being squeezed from the quaternary rejects is seen dramatically when the screw press covers are removed.


One of a pair of Vincent Model VP-12 presses. Tailings from sidehill screens on the floor above drop through the vertical chute into the press.


Press cake drops to a concrete bunker at ground level prior to being transported to the hog fuel boilers.


This press cake typically has 50% solids and has excellent combustion characteristics.

TAPPI - Pulp & Paper Waste Dewatering

Tappi Journal, Vol.78, No. 12, December 1995.

Prepared by Thomas H. Manley, Plant Engineer, Boxboard Mill Division, Jefferson Smurfit Corporation, Wabash, Indiana; and Robert B. Johnston, P.E., Vincent Corporation, Tampa, Florida.

Screw presses installed at the Jefferson Smurfit boxboard mill in Wabash, IN have significantly decreased the load on the wastewater treatment facility and facilitated the capture and disposal of fines in the primary clarifier sludge.

These benefits were achieved by reversing the position in which screw presses are normally used. In the typical installation, the screw press goes at the end of the cycle, receiving the sludge from clarifiers and/or DAF systems. At Jefferson Smurfit the presses were instead placed to receive reject material flows ahead of the clarifiers.

Higher than anticipated reject rates from the mill's cleaning systems had increased the load on the existing wastewater treatment facilities. Conditions reached a level where, during upsets, unacceptable discharges could occur. Resolution of this problem was necessary to ensure continued compliance with the mill's NPDES permit.

Screw presses offered key advantages. They operate continuously through wide swings in flow rate and solids concentration; they operate unattended; and they require minimal maintenance.


Established as a recycle mill in 1892, today this plant specializes in producing high quality boxboard. Typical end uses include breakfast cereal boxes.

The nominal mill capacity is 365 TPD. Basic machinery includes six Hydrapulpers and two paper machines: a 96" ten cylinder (400 fpm) Multiply and a 120" eight unit (500 fpm) Ultraformer. Both machine coated and uncoated combination boxboard is produced.

Wastewater Sources

There are a great many point sources of wastewater in the plant. Important ones include pulper detrashing screens, pressure screen rejects, unclaimed cooling water, and tank overflows. Rejects from the pulp cleaning system are the focus of this paper. These rejects are pumped directly to a screw press. They include forward cleaner rejects as well as fine and coarse screen rejects.

Large trash from the Hydrapulpers is removed continuously by a continuous scavenger system. This bulky material is moved by conveyor to a dump hopper.

Wastewater Treatment

Primary wastewater treatment is performed in an Infilco clarifier.

The secondary treatment plant is an activated sludge system. It is physically located on an adjoining property. Originally it was operated by the City of Wabash, treating both mill and municipal wastewater. It consists of three rectangular aeration basins, two rectangular digesters and three final settling tanks. The water is discharged into the Wabash River in accordance with an NPDES permit.

Polymer is added to sludge that is pumped from the Digesters. This sludge is then dewatered on a belt press. The belt press requires an operator on each shift, and it is generally regarded as a high maintenance machine.

To minimize the tonnage or cubic feet going to landfill, 40% or higher solids is desirable in the press cake. The belt press used at the secondary treatment plant can achieve only 30% (approximately). Although screw press material, due to its characteristics, can be spread with solids up to 50%, the belt press cake material cannot be spread at consistencies above 30% solids. This is due to the operation of the feeders on the trucks that are used to landspread the press cake.

There are various practical and theoretical means of disposing of sludge from the belt press. One of the most economic is land application: farmers accept the material without charge because of its benefits to the soil, and the farm acreage in the immediate area of the plant currently supports these operations.

Placing the press cake in a landfill was very economic in the past. However with the decline in landfill sites in the immediate area, plus regulations applied to landfill operations, this disposal option has lost favor.

Additional potential future disposal means are under review. The sale to other business operations is especially attractive. Potential buyers include paper recyclers capable of using the fiber that is rejected at Wabash because of stringent product specifications. Also, it is recognized that material that is dewatered to approximately 50% moisture might be used as a boiler fuel by blending with coal. A final option under review involves coal mine reclamation activities.

Press Application

Most waste water streams from the mill are combined ahead of wastewater treatment. The combined flow is pumped across a bank of inclined screens to remove long fiber prior to entering the primary treatment. The long fiber is returned to the mill for re-processing.

Reject streams from forward cleaners and pressure screens do not pass over the sidehills. Instead they are fed directly into a pair of screw presses. Filtrate water from the screw presses flows to the primary clarifier. Excess clarified water then overflows to the secondary treatment plant.

The screw presses generate 7 to 21 dry tons per day of press cake at up to 50% moisture.

It is important to note that the wastewater treatment facility does not have to handle this tonnage of solids. By capturing the solids with a screw press, a significant reduction of load on the wastewater treatment plant is achieved.

Capture of clarifier sludge in a screw press is difficult. There is a tendency for the fines (clay or ash) to blind the screens of the press, which results in drastically reduced press throughput capacity.

The operation results in screw press filtrate water with 500 to 1000 ppm solids. This range of solids is within an acceptable range for treatment and capture in the secondary treatment plant.

Selection of a Screw Press

Six presses by four different manufacturers were tested on-site with varying results. In the end a design manufactured by Vincent Corporation was selected. The design is a modified version of their standard citrus press, a machine used in converting orange peel into cattle feed. The modifications were required because, while wet fiber dewaters much more readily than citrus peel, it is much less compressible once the free water is removed.

During the trial operation, efforts were made to develop a set of specifications for the screw presses. This effort began with a focus on normal technical details such as gpm capacity, horsepower requirements, press cake moisture and screw diameter. This proved unsatisfactory because of the very wide range of flow rates and solids concentrations that were encountered. The varying nature of the inbound flow (easy to press fiber as compared to difficult to press sludge) made the specifications difficult to write.

In the end, the unique purchase specifications were as follows: The primary performance criteria for satisfactory operation of each press are (1) it must not plug or jam and (2) it must not pass large quantities of unpressed liquid into the flow of press cake. The press must operate like a pump: reliably, unattended, and with very infrequent maintenance.

The presses that were purchased have many unique features. For example, it was found that the use of wedgewire screens, as opposed to perforated metal, not only increased physical strength but also reduced the concentration of suspended particles in the press filtrate. Wedgewire appeared to be more self-cleaning than perforated metal.

Accommodating the absolute peak flow under conditions of maximum blinding would have required an excessively large screw press. Rather than purchasing such a large machine, provisions were made to allow the incoming flow to overflow the inlet hopper during the unusual peaks. This overflow is directed back into the treatment system. It is estimated that this overflow provision is used less than 5% of the time.

A pneumatically adjustable cone at the press discharge allows the press to operate satisfactorily over a wide range of flow rates and solids concentrations. If the inbound solids are low, the cone pinches off the discharge to prevent liquid from purging into the press cake discharge. The design of this cone mechanism negated the need for a variable speed drive on the screw press, which represented a significant capital savings.

The presses also feature an interrupted screw flight design, as opposed to a continuous screw. Because the screw is discontinuous, fixed resistor teeth can be mounted to the press frame, protruding into the flow of material inside the screen. This design reduces co-rotation, the condition where material rotates with the screw and nothing either enters or leaves the press. The stirring action by the teeth allows for a shorter machine that requires less horsepower to operate.

To assist in un-blinding the filter screen, presses were acquired that have a wiper-brush mounted on the screw auger. This clears blinding material from the screen surface. The feature assists operation during periods of high sludge content.

Maintenance requirements also guided the press selection process. Presses were purchased with all contact parts made of T-304 stainless steel, which specification will minimize maintenance requirements over many years. Similarly, presses in a horizontal configuration were selected because of the ease of disassembly in the event of screw, screen, drive, or cone maintenance. Finally, the presses selected make use of standard OEM gear boxes, bearings, seals, etc., which further reduces maintenance expense over the long run.


The principal result of the installation of the screw presses has been to relieve solids loading on the wastewater treatment facilities. A recent expansion of the mill cleaning system had resulted in serious overloading of reject material going to the wastewater plant. With the addition of the screw presses this condition has been resolved.

One side benefit of removing such large quantities of solids ahead of the treatment plant has been a reduction in the amount of sludge to be belt pressed. The sludge from this source has been reduced from 1,000 to 600 dry tons per month.

Results from when the wastewater treatment plant was at times overloaded to conditions following the installation of the first screw press have been compared. The analysis shows that suspended solids were reduced from an average of 75 mg/l (or ppm) to 25 mg/l.



During an extended period of trials at Jefferson Smurfit, numerous problems were encountered. These included:

  1. Overload and Trip-Out. This was apt to occur when pressing too tight. Typically it was a consequence of feeding thick stock to the press during upset conditions. Solutions included providing for dilution water at times of high amperage draw and oversizing the press.
  2. Purge of Liquid in the Press Cake. Some presses had difficulty when insufficient fiber was present to form a press cake. Pre-thickening can relieve this problem. The design of the press discharge cone is very important.
  3. Screen Blinding. The screens of most presses tend to blind on clarifier sludge. It appears that platelets of clay in the sludge bridge the openings in the screen and prevent the flow of liquid through the openings. This problem is even more pronounced if biological (secondary treatment) sludge is present. At times the addition of fiber waste will act as a press aid and wipe the screen clear. Also, pre- thickening with a belt press can help. Alternatively, the use of a low rpm continuous screw press will address this problem.
  4. Excessive Solids in Press Filtrate. With certain screen configurations, the parts per million of suspended solids in the press filtrate were found to be ten times greater than the acceptable range. The use of wedgewire screens resulted in the best performance.
  5. Interrupted Operations. Some presses required operator attention before satisfactory operation was achieved on a re-start. This occurred under conditions such as a period of no incoming flow followed by resumption of mill operations. Dilute flows during flushing and wash-down can also require operation attention to the screw press. The machines at the Wabash boxboard mill were selected to be able to handle these swings without adjustment.
  6. Screw Wear. Excessive abrasive wear was noted on press components such as the screw flights and discharge cone. This is addressed with the addition of hardfacing in the wear areas.

Venezuelan Paper Mills

January 21, 2001

In December Vincent conducted a screw press workshop and presented a paper at a Pulp and Paper conference in Maracay, Venezuela. The trip offered a chance to study the pulp and paper industry typical of many small countries.

There are nine paper mills in Venezuela, one of which is shut down and another operating in bankruptcy. All of the nine have secondary fiber (recycle) operations. Four mills have virgin fiber operations: one Kraft long fiber (pine) mill; one CTMP (Chemical Thermo Mechanical Pulp) mill; one ground wood, and one that uses sugar cane bagasse as a furnish. Almost all of the mills have US parent firms. There is only one small mill that is still locally owned. Because of parent firm involvement, the paper making technology is relatively advanced. The engineers we met, all Venezuelan, were at a par with the personnel found in North American mills.

At a typical mill we saw coarse hydrapulper rejects going, quite wet, from a trommel (rotary drum) screen to landfill. No baler or screw press was used for further dewatering. The same mill had a pair of belt presses dewatering flocculated (polymer added) sludge and clarifier underflow. The cake from these was quite dry, 35% solids, due to a high ash content. This cake also was sent to landfill.

In North America we would have been enthused with the possibility of using a screw press for further dewatering in order to prevent drainage on the highway or to produce boiler fuel. However, in Venezuela the environmental regulations are fairly lax, and fuel is very cheap, so there is little opportunity of selling a press.

One mill did use a screw press on their sludge. This was acquired with an aim to producing fuel for a boiler. However the project was abandoned because of the high capital cost of a fluid bed combustor along with the low price of fuel oil.

The country is in poor financial condition and relatively unstable.

Issue 114

Virgin Fiber Mills

June 4, 1996

Paper mills that process chipped logs and sawmill waste into paper are located in the woodlands of the northern and southeastern United States.

We have found several applications where Vincent screw presses can be used advantageously. These involve dewatering waste material such as screen rejects, shives and knots.

Screen rejects are bits of material that are not suitable for making paper. They can come from pressure (filter) screens both ahead of and following a bleaching operation. At times they are in thin flows containing 1% solids, while at other times they may have been thickened to approximately 10% solids. Because the solids are large enough to be caught in the screen of our press and the water is loose (that is, not bound) excellent pressing results are achieved.

Shives dewater beautifully in our press. Shives are tiny bundles of cellulose fibers and lignin that are not suitable for producing paper. The shives are screened from the acceptable pulp and become a waste stream. Dewatering in a press removes them from the waste stream going to the wastewater treatment plant. The press liquor from the press can contain both fiber and chemicals that are worth recycling. At the same time, the press cake will be useful as boiler fuel; landscaping mulch; and filler for asphalt shingles and tar paper.

Another waste product at a virgin fiber mill is knots. These are the remnants of branches that are seen in lumber products. Knots absorb a great deal of digester chemicals, but they do not become useful pulp fiber. They are screened from the flow of cooked wood chips with vibrating shaker screens called knotters. Knots are either burned or sent to landfill. Pressing knots in a Vincent press results in 20% by weight being separated as a thick black liquor. This enables valuable chemical recovery and also makes the knots into a better fuel. Pressing knots also addresses an environmental run-off situation which can occur if high pH (12 or more) drainage occurs from the knot pile.

Issue 44

Wax Coated Box Recycling

July 30, 1998

Recently Thermo Black Clawson of Middletown, Ohio has started promoting a new, patented recycling process. The system, trade named Xtrax, is used to recycle wax-coated corrugated containers. Inland Paperboard and Packaging Inc. is a partner with Thermo Black Clawson in the development.

Wax-coated corrugated containers have largely replaced wooden crates for transporting produce and fruit. The tonnage of these cartons produced per year is quite large: an estimated 1,500,000 tons. We were surprised to learn that, by weight, these cartons are 30% wax.

Paper machines at normal OCC (Old Corrugated Container) recycling mills can tolerate up to about 3% wax. Because of the problems caused to the paper machine clothing by excess wax, the wax-coated boxes are segregated. Up until now the great majority of them have been incinerated or landfilled.

The new Thermo Black Clawson process features the addition of steam to the hydrapulper. The heat melts the wax. Then a conterflow series of TBC Reverse Screens are used to wash the wax from the flow. In the end the furnish has less than 1% wax.

In April of 1998 Vincent Corporation was honored to be invited to a Trial Demonstration held at the Middletown laboratory of TBC. Representatives from ten paper companies attended along with engineers from Poseidon (clarifiers), BetzDearborn (flocculents), Brown & Root (consultants), and CITGO (waxes).

Vincent screw presses were used in two phases of the process. One CP-4 was used for primary, first wash dewatering. Testing was done at 165º, 185º and 150º. The wax content in all press cake samples was on the high side, 4-1/2%; we feel that this can be improved by using higher cone pressures.

A second CP-4 was used on the DAF sludge coming from a unique Poseidon clarifier. Most clarifiers are very large in diameter and of relatively short height. The Poseidon machine was shaped more like a grain silo, with up-side-down umbrellas on the inside. Flocculated wastewater is pumped into the side, at the bottom. The clarified water makes a U-turn into the umbrellas and drains from the bottom. The sludge floats to the top and over a weir. Excellent results were achieved pressing this sludge by greatly reducing the rpm of our press.

Issue 81