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Capture Rate

June 28, 2001                                                                                                                                                                                                     ISSUE #M17

Engineers designing manure handling systems frequently ask about capture rate. They need to know the percent of the solids in the manure that will be captured in a manure separator. 

The captured solids are those that come out in the press cake. The solids not captured are those that flow in the liquid stream draining from the screen of the separator. The capture rate is calculated by dividing the solids in the press cake by the total solids pumped to the screw press.

Late last year capture rate tests were run at the University of Tennessee Dairy Experiment Station in Lewisburg, Tennessee. These tests confirmed a very significant difference in capture rate between flush and scrape barn systems.

The data show that when manure with 7% solids, typical of a scrape barn, are pumped to a screw press, the capture rate runs about 50%. In contrast manure with 2% solids, typical of a flush barn, had a capture rate of only 25%. (These figures are from a graph prepared by Dr. Robert Burns.)

The problem with a very dilute inbound flow can be explained as follows: The freeness tends to be high and a very large amount of water rushes through the screen. It goes through the screen so fast that what fiber is present tends to wash through the perforations or slots of the screen. Thus the capture rate is low with very dilute flows.

With thicker flows the longer particles tangle together, acting as a press aid. This forms a mat that entraps the finer particles. Thus the capture rate is higher with pre-thickened feed to a screw press.

It is noteworthy that solids in manure can be classified as either suspended solids or dissolved solids. The dissolved solids are like sugar in water: they cannot be captured with a mechanical filter. Since most of the manure flow to a screw press leaves as liquid, most of the dissolved solids pass through the press with this liquid. This places a definite limit on capture rate.



Cushman Farm Digester

October 1, 2001                                                                                                                                                                                                      ISSUE #M19

The Cushman Dairy in Franklin, Connecticut is a show case farm. The electrical energy used at this farm comes from a manure digester system developed by CST Industries (better known as Harvestore - Slurrystore). The original installation was made in 1998. 

The farm has 700 head, utilizing coarse sawdust as bedding. A scraped barn system is used to collect the manure, which is pumped to the digester.

The digester is an insulated tank made of the traditional Harvestore glass porcelainized steel panels. Manure is pumped in at the two-thirds level. Agitation is achieved with a centrifugal pump that takes its suction off the bottom of the tank. Digested manure sludge floats over a weir at the top and goes to a Vincent screw press for dewatering.

The biogas collected from the top of the digester flows to a positive displacement blower and then to a Cummins internal combustion engine. The engine is coupled to an induction generator that fills the farm's electrical needs. Radiator cooling water goes through a tubular heat exchanger that heats water used to maintain the digester temperature.

Digested manure flows to a Vincent Model KP-10 manure separator that was installed in 1998. Typically this produces 20 gpm of press liquor which flows to the manure pond. Press cake production normally runs 600 pph, with the press operating about ten hours per day.



Dairy Manure

April 15, 1997
Rev. 2008

The then-new KP screw press was first shown at the 1996 World Dairy Exhibition in Madison, Wisconsin. Rather than a sales effort, the objective of out booth was to determine the marketability of the product and to find suitable channels of distribution.

We knew that dairy farmers, especially large-scale operations, had been processing their manure through screw presses. With 68,000 attendees, the show gave us ample opportunity to find out why.

Keeping bedding and manure out of the treatment pond extends lagoon life. This reduces the frequency necessary to dredge the lagoon. This cost deferral is a financial benefit.

Environmental regulators are also a driving force. By keeping the fibrous solids out of the lagoon, odor generation is significantly reduced. Also, by pressing the manure ahead of the waste lagoon, a farmer can extend the capacity of existing lagoons. This avoids a number of permitting difficulties.

Farmers in northern climates must store their manure until the spring thaw. Pressing it to remove the solids can greatly facilitate this operation.

Pressing the manure separates it into two flows: dirty liquid and damp fibrous solids. Weather permitting, the liquid can be drained to the waste lagoon, or it can be pumped for irrigation purposes. (Full strength manure is too thick to pump very far, and it plugs irrigation spray nozzles and cakes the soil).

The manure solids from the screw press are composted for approximately ten days. This produces a rich fertilizer that is excellent for landspreading or tilling into a field. Some dairy farmers are selling this compost in bulk to nurseries and municipalities, while others are bagging the material for retail sale to home gardeners.

One increasingly common use of the compost is as bedding for dairy barns. The farmers we interviewed were using materials such as sand, sawdust, and rice hulls, which go either on the concrete floor or over a foam pad. (In fact, there were at least four exhibitors showing rubber or plastic "cow mattresses".)

University studies have shown that pressed manure has just the right moisture content (70%) for composting. This raises the manure temperature sufficiently to kill bacteria, thus making the compost safe for use as bedding. However, most of the farmers we talked to were reluctant to use the material for fear of infection (principally mastitis) spreading to the herd. (Since this original writing, it has become common to bed with manure solids straight from the screw press, without composting.)

Three different "manure separating" devices were exhibited at the 1996 show: sidehill screens; drag flight conveyors with slotted bottom troughs; squeeze rolls; and a screw press. Besides being large and mechanically complicated, the drag flight conveyors had limited, if any, squeezing action, so the dewatering that they achieved was marginal. The squeeze rolls dewatered a little better, but they were maintenance prone and costly.

The screw press being displayed was made by FAN of Germany. The principal complaint we heard were that it is expensive to maintain. Maintenance concerns centered on the availability and cost of spare parts such as the gearbox, screen and screw.

These areas have been addressed in our KP series of presses. The screw of the press is supported at both ends, rather than using a cantilevered support. This prevents the screw from hitting the screen once the machine loosens up. Also, the gearbox is separated from the inlet hopper by a drop-out gap; this saves the gearbox upon failure of the shaft seal in the inlet hopper. Our use of standard NEMA (American) motors also reduces maintenance costs.

A marketing problem was evident in that Vincent's industrial sales representatives do not call on dairy farms. To address this Vincent signed an agreement with A.O. Smith Harvestore. This firm, a leader in farm feed storage systems, will work with Vincent to develop models suitable for not only dairy, but also swine and cattle. This development contract is expected to lead to a distribution agreement. (This relationship failed to develop, and Vincent now sells through dairy equipment dealers.)


The KP-10 press on test at the University of Wisconsin will handle 50 gpm of manure and bedding, with only milking parlor water being added. The press achieves a separation of 43 gpm of liquid. The inbound solids split evenly, with 50% in the press liquid and 50% in the press cake. (This split is low because of the dissolved solids in the manure; a screw press cannot capture dissolved solids.) Press cake moisture content measures 72%. They process all the manure and bedding from 500-confined head in three and a quarter hours each day.

Issue 59

Fairgrove Farms Digester

December 6, 1999
Rev Jan 2001

Almost twenty years ago John Pueschel, Dave Pueschel, and Larry Kelly, owners of Fairgrove Farms in Sturgis, Michigan, installed a system to generate electricity from cow manure.

Manure from the dairy barns and milking parlor is pumped to a digester. This "plug flow" digester was designed by Perennial Energy of West Plains, Missouri. The digester is in the form of a horizontal concrete tank, like a long swimming pool. It is covered so that the bio-gas from the digestion process is collected. This gas flows, without the aid of a booster blower, to a Caterpillar internal combustion engine. The engine drives an induction generator. The generator in turn produces about $3,200 worth of electricity per month.

An induction generator was selected although it is a little less efficient that other types of generators. Its advantage is it produces electricity that is automatically in phase with the power grid that supplies the farm. The result is that surplus electricity can be pumped into the grid (sold to the public utility) without the need for expensive switchgear.

Selecting a Caterpillar engine that would run on the low 600 BTU (mostly methane) biogas, without a need for filtration or pressure boosting, was a key element in minimizing the capital investment.

In 1999 Fairgrove Farms purchased a Vincent Model KP-10 Manure Separator. This machine is used to dewater the sludge from the digester. It is manufactured in Tampa, Florida.

This separator is an all-stainless screw press with only one moving part, the auger. This screw features three stages of compression with weld applied hardsurfacing in the high abrasion surfaces. A screw recently completed over 4,000 hours of service before refurbishment was required.

The screen of the press is made of perforated metal screen with 3/32" openings.

The sludge is pumped to the screw press at the rate of about 40 gpm. The press liquor goes to the wastewater pond, while the press cake (at 70% moisture) is sold as bedding to nearby dairy farms. This generates an additional $3,000 in monthly revenue.

Issue 100

Fighting Cocks

June 29, 2005

We have a customer who, with an extended farm south of Guadalajara, Mexico, purchased a KP-6 screw press. They refer to it as "the recovery machine". It is used to separate undigested food solids from manure at the pig farm. Originally, this press failed to adequately dewater these solids. Both 0.030" perforated and 0.020" wedgewire screens were tried. It was only with the addition of a small 18" static screen, to pre-thicken the flow, that successful operation was achieved. The press cake is used as a feed supplement in the cattle grow-out operation.

One Sunday, after I ran out of things to tell him about screw presses, the owner told me of his own avocation. He spent twenty years playing and breeding fighting cocks. In his career he regularly worked in Argentina, Brazil, Chile, Peru, the United States (mostly in Arizona and Louisiana), and other countries.

For breeding, he imported birds from the Far East and Spain. The birds were characterized by their strength, how high they jump, and the skill with which they aim their spur. The trick is to breed the best of these qualities.

The fighting spurs are different in each country. Made of forged steel, some are longer or shorter, serrated or smooth, flat or pointed. In all cases the points are needle sharp, and, in the case of a flat blade, it is sharp enough to shave with. The spurs are made from spring steel, and some Swiss alloys are popular.

Mounting the blades is a science. Generally, the first step is to wrap two 3/8" wide bands, on the leg, above and below the bird's spur. Dr. Scholl's elastic bandage, cut into narrow strips, is preferred.

The bird's spur is a bony protrusion on the back side of the foot. It has a nail, but this is trimmed off. The saddle, made of leather or plastic, is wrapped around the foot, at the spur. There is a hole in the saddle that fits around the spur. Some saddles have aluminum inserts in these holes. The saddle is tied in place with a yard of surgical suture.

The saddle has a flat platform on which the metal spur is mounted. This is tied in place with the thread. The final assembly must be extremely rigid on the bird's leg.

In Mexico, only the left foot is armed. Just as most humans are right handed, most birds are left-footed. Right-footed birds do poorly in contest.

In the States, both feet are armed. The long, pointed spurs are bent inward at a slight angle. Because of this, spurs manufactured in the States are carefully etched with LH and RH markings. The SPCA has put great pressure on the sport in the States. The customer gave up the business three years ago. The participants were bringing in more weapons than before. Also, narcotraficantes have gravitated to the sport, and the police tend to arrest everyone when they conduct a raid, not just the drug people. Also, the hobby was too much of a distraction from his core businesses. Today the customer has reduced his flock to 1,500 birds. He belongs only to a local club that meets for a match every two weeks.

Issue 162

Gearbox Protection

June 26, 2000                                                                                                                                                                                                           ISSUE #M8

Vincent has never had a gearbox failure in a Series KP screw press. This record is attributable to two design features of these machines.

A critical observer will note that the construction used to mount the gearbox on the press is relatively expensive. The gearbox is mounted on a plate with four legs that are welded to the inlet hopper (A-plate) of the press. It would cost a lot less to simply bolt the box to the A-plate.

The reason for the separation is to provide for leakage at the shaft seal. There are a pair of Johns Manville seals located in a seal housing that is bolted to the A-plate. The four legs are used to make a large physical separation between this seal and the gearbox. That way, when the seals eventually fail, leakage will be to the structure below the press.

Without this provision the leakage would be against the gearbox shaft seal. Gearbox seals are designed to keep oil in, not to keep manure out. It is common for competitors to have bearing and gearbox failures due to manure contamination since provision is not made to drain away leakage. It is all but impossible for this to happen with a Vincent press.


Inclined Discharge


Screw presses used to dewater manure are notorious for "purging" or "blowing the plug". The condition is one where all the manure being pumped to the press simply flows out through the cake discharge, with no liquid being separated through the screen. Whether it is 20 gpm or 600 gpm does not matter: if it starts at midnight and no one notices until dawn, the mess is terrible.

Improving the screw press design to address this problem has been difficult because a press will operate for weeks or months without an occurrence. Gradually a number of different factors have been observed that contribute to blowing the plug.

  1. In the winter the cake being discharged can freeze and jam the discharge door open.
  2. When the screw becomes worn, manure can flow between the screw and the screen, wetting the plug and allowing it to wash away.
  3. When the manure pit is near empty, the flow from a centrifugal pump can go down by 75% compared to when the pit is full. The plug will tend to blow when the pit is low.
  4. A change in feed or bedding (like going from sawdust to manure) results in reduced dewatering and wetter cake.

The latest improvements in press design that address the problem are (a) Extending the screw shaft through the discharge so that the cake door is breaking up a donut, rather than a solid plug, of manure, and (b) Double flighting the screw over the first half of the screen where the manure is wettest. This produces drier cake, earlier in the dewatering process, which is less likely to purge.

Another useful remedy is to elevate the discharge end of the press. This has been found to be particularly effective when the pit is low and the flow of solids to the press is reduced. At this time the cake discharge becomes so slow that water wicks (soaks) into the cake on the lower side of the screw, allowing part of the plug to become soft and to blow out. Without sufficient solids being fed into the press, the plug does not re-form. A 5º incline to the press can make a world of difference, and it can cause no harm.

The photo at the top right hand corner of our brochure shows an installation with an elevated discharge. This was retrofit, during start-up, in a successful effort to eliminate blowing the plug.


Installation Quick Tips

May 9, 2001, Revised April 2003                                                                                                                                                                             ISSUE #M16

There are a number of details relating to the installation of screw presses as manure separators that should be noted:

Unless a piston pump is used, it is necessary to have a line that allows manure to recirculate from the press inlet back to the manure pit. This will prevent pressurizing the inlet of the press, which can be a cause for purging.

It is best to have a vent line at the inlet to the press. A 1" line, 5' tall, is adequate for breaking a suction in the recirculation line. Such a suction can lead to reduced press capacity.

The drain line from the press should go below the surface of the pit or pond into which it drains. If this line is relatively small in diameter and has a steady downward slope, a vacuum will be induced around the screen of the screw press. This will increase press capacity.

The amount of vacuum is a function of the elevation between the press and the drain pond. Where convenient presses should be mounted on 20' or higher stands.

The control panel for the manure pump and the press should have a timer. This timer should be set to have the press run for two minutes after the pump shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started.

Minimize the time that the press is run with no material being fed into it. Running dry will allow abrasive rubbing. The last manure admitted to the press will dry to an abrasive powder.

When bolting the frame of a manure separator to its platform, look into the cake discharge end. Note if the screw is being pulled into the screen. This is an indication that the frame is racking. If this occurs, shims should be used so that the mounting bolts tighten evenly. The screw must be kept centered in the screen.

Elevate the discharge end of the press a few inches to minimize any tendency for the press to purge. This has been found to be particularly effective when the pit is low and the flow of solids to the press is reduced. At this time the cake discharge becomes so slow that water wicks (soaks) into the cake on the lower side of the screw, allowing part of the plug to become soft and to blow out. Without sufficient solids being fed into the press, the plug does not re-form. A 5º incline to the press can make a world of difference, and it can cause no harm.

Where the manure is very thin, it may be necessary to pre-thicken the flow to the screw press. This is best done with a sidehill (inclined) screen. Without pre-thickening, some dilute flows will flush the solids through the screen. This makes it difficult to form a plug, and the capture rate is reduced excessively.


Iowa State Digester

DECEMBER 5, 2001, Revised September 2010                                                                                                                                                          ISSUE #M21

The Dairy Foundation in cooperation with the Iowa State University has started up a new dairy manure digester system.  The installation is at the model farm located at Northeast Iowa Community College in Calmer, Iowa.

The school has opened a new campus with a herd of 170 cows.  The manure and bedding from one hundred of these cows are disposed of in a digester.  The biogas generated is used to produce hot water used on the farm.

The manure digester is the plug flow type.  This uses a concrete tank 10' deep by 13' wide by 68' long, holding a nominal 50,000 gallons.  Foam insulation helps keep the liquid at 95o F, which is well suited for producing methane.

Biogas from the digester is burned in a boiler system designed and manufactured by Perennial Energy of West Plains, Missouri (417-256-2002).  Rated at 150,000 BTU/hour, this heats water that is used to maintain the digester at temperature and to supply floor heat and hot wash water for the milk pipeline.

The manure sludge from the digester is pumped to a Vincent Model KP-6 screw press.  The cake from this press is composted and used for bedding, while the press liquor flows to a treatment pond.  Normal cake production is in the range of 150 to 250 pounds per hour, and press liquor flow of 34 gpm was measured recently.

Daniel Meyer of Fayette IA (319-425-3331) has been actively involved the $200,000 project since its inception.  He favors the plug-flow type digester for dairy manure.  He has also noted that scrape barn manure works better than flush barn for digesters because of the greater concentration of solids.

Mr. Meyer describes biogas production as a two stage process.  In the first stage the solids are broken down with acid forming bacteria.  In the second stage methane forming bacteria cause gasification.  The pH must be kept about neutral during the process.  The average electric production is one kilowatt of generator capacity per 10 cows.

Ray Crammond of Ankeny IA (515-965-8301) was a consultant on the design of the system.

September 2010
Like most manure digester projects, this one has been shut down. 
When the digested plugged with solids (sand?) which settled out, a group of students were drafted into digging it out.  The system was altered so that the manure was pressed ahead of the digester.  Only the press liquor was put into the digester.  This should have worked.

Layer Manure

December 10, 2014
ISSUE #269

Layers (egg-laying chickens) are kept in wire cages which allow the manure to fall through. The manure can either fall into a pool of water or onto a belt conveyor.

In late 2013 we were approached by inventor Woody Cronin in regards to a system he developed for layer manure. Years before, we had tried dewatering the manure from water-bottom barns, with poor results. Because of this we were cool to the idea of running a screw press on the material from a conveyor-bottom barn.

To our surprise, with Woody's technology, it works well. Press cake with only 65% moisture content can be produced. This material is undigested food. It has very high nutritional value, and it makes an excellent feed additive for farm animals. Conceivably it could qualify as biomass fuel for a boiler.

We were told that layers digest only 40% of their feed. The operation of the screw press system results in a recovery of half of the feed found in the manure. Mathematically that calculates to a 30% recovery of the poultry feed.

Overall the operating system is as follows. The manure is first mixed with press liquor in order to fluidize and dilute it. (The system is started up with water since there is no press liquor.)

Next the flow goes through equipment which separates out the calcium (it looks and feels like sand) and the feathers. That is a gravity separation process.

From there the flow is dosed with organic polymer. Most polymers are made from natural gas, and they are not good as feed additives. However natural polymers, like those made from shrimp waste, are acceptable in feed rations.

Next the flow goes to the screw press. The flow is dilute, so pre-thickening ahead of the press may be required. This pre-thickening can be achieved with either a rotary drum or a sidehill (parabolic) screen. Prethickening can greatly increase the throughput capacity of the screw press.

The as-received manure typically had 75% to 80% moisture content. The press cake produced typically had a moisture content of 64% to 68%.

The press liquor from the press goes to the mixing pit where it is added to incoming chicken manure. High in urea, the excess press liquor is drained off for use as a liquid fertilizer.

Alternatively, with heavier dosage of polymer, the press liquor can be made very clear, with as low as 0.5% total solids.

Manure Brix

April 24, 2010                                                                                                                                                                                                    ISSUE # 221
                                                                                                             MANURE BRIX

There is a tremendous focus on animal waste. This is true around the world, especially in Europe and the United States. Dairy manure gets the most attention, closely followed by swine. University studies and EPA pronouncements come out on a monthly basis. Much of this work is labeled "nutrient management". This has to do with the how, when, and where of disposition of the various nutrients in manure.

If I were going for a PhD in Ag Engineering, I would do my dissertation on "The Role of Dissolved Solids in Manure". A basic characteristic of manure is that the moisture
fraction in it contains dissolved solids. This characteristic deserves special attention because the dissolved solids in a mass cannot be separated by mechanical filtration. That
is, if there are 3% dissolved solids in the water in manure, there will be 3% dissolved solids in the moisture in the both the fiber and liquid which are separated by separation
equipment. This is basic to all conventional equipment such as sidehill screens, rotary drum screens, screw presses, belt presses, centrifuges, etc.

This means that a dissolved nutrient in manure cannot be concentrated (let alone separated) by running the manure through any of the aforementioned equipment.

Vincent engineers have gathered some interesting data in regards to dissolved solids:
Scrape barn manure measured 6º to 7º Brix.
Pond water measured 3º Brix.
Manure from a flush barn measured 4º Brix
Effluent from a dairy manure digester read about 3º Brix.
Human urine reads a real sharp 2º Bx.

This has an important effect on the moisture content of the press cake produced when manure is run through a screw press. We have seen manure press cake from a scrape
barn that contains 68% moisture and 7 Bx. In round numbers, if before pressing this  same manure were diluted (by flushing) with pond water to where it had 4º Brix, then the press cake would measure 70% moisture.

In other words, the cake from flush barn manure will typically be two percentage points higher in moisture content than that from a scrape barn. This is because the moisture
does not carry with it as many dissolved solids.

Separately, we queried Dr. Robert Burns of Iowa State University about the dissolved solids in manure. His reply has good insight on digester operation:

"You are on the right track in thinking about the bugs eating sugars. In reality, they are consuming carbon as a food source. Some carbon sources (like sugar) are very easy for bugs to utilize, while other, more complex carbon sources are harder to digest, and some may even be recalcitrant to the point that they can not be digested within the HRT that a given digester operates at. Unlike fruit juice (where most of the dissolved solids are sucrose), manure contains lots of complex compounds (like proteins, carbohydrates and lipids), so I believe that your hypothesis that the dissolved solids you are seeing with your refractometer are a mix of digestible and undigestible dissolved solids is the correct one.

"The question of should you press manure before or after digestion is one that can be answered pretty easily. My lab regularly runs Biochemical Methane Potentials (BMPs)
which are an effective method to determine the anaerobic degradability of a given substrate. We run these on a fee basis for companies all of the time to determine what
mix of substrates will yield the most biogas. You can take a look under the AD portion of our webpage ( for more info. It would be pretty simple to run some BMPs on raw manure and then run some on the press liquor to see which yields more biogas."

Robert T. Burns, Ph.D., P.E.
Agricultural & Biosystems Engineering
Iowa State University
3224 NSRIC, Ames, Iowa 50011
Email -

Note from Pressing News #190:
Brix is a unit of measurement named after Adolph Brix. It is used commonly by food technologists to measure the amount of sugar dissolved in water. It can be calculated
by dividing the dissolved solids by the sum of the dissolved solids plus the water, all multiplied by 100. That is, Bx = (Ds x 100)/(Ds + w), where Ds is the weight of
dissolved solids and w is weight of water. ((Note that suspended (or insoluble) solids do not enter into the equation for calculating Brix.))

Manure Digester Bio-Gas

SEPTEMBER 6, 2001                                                                                                                                                                                              ISSUE #M18

Due to renewed energy awareness there has been a surge in interest in manure digesters.  In these digesters the manure decomposes anaerobically (without Oxygen), releasing bio-gas. This bio-gas is collected and burned to release its energy.

These dairy systems have environmentally attractive features.  Energy is produced from a waste source.  Also, the digester action treats the manure, which addresses other problems:  run-off is controlled and odor from the dairy is reduced.

The bio-gas (methane with CO2, SO2 and other gasses) can be burned in a boiler to produce either steam or hot water for the farm.  Alternatively, this low BTU gas can be burned in a Waukesha or Caterpillar internal combustion engine.  These engines are coupled to induction generators that more than fill the farm's electricity requirements.

Besides internal combustion engines, mini gas turbines are being offered for use on biogas.  The metallurgy of the engine and other components is critical because of corrosive gases.

Two types of digesters predominate.  In both the gas is collected off the top.  The most common, a plug flow digester, is a long pit where the manure enters at one end and progresses slowly to the discharge.  Our presses are used with this type of digester at Fairgrove Farms in Michigan, High Plains Dairy in Kansas, and Iowa State University.

Mixed flow digesters using tanks are a second type of digester where extensive development has been done.  A.O. Smith Harvestore leads in this technology, and Cushman Farms in Franklin, Connecticut is a good installation to visit.  Manure solids flow off the top of the digester into our Model KP-10. 

Another type of mixed flow digester uses a pit instead of a tank.  A good example is under construction at Matlink Dairy in Clymer, New York.  This 660,000 gallon pit digester has propeller agitators on two sides.  Digested manure will flow over a weir to be pumped to a Vincent press.

The manure can be pressed in a screw press ahead of the digester, with the press liquor being directed into the digester.  However the more common arrangement has the press dewatering the sludge that comes from the digester.  The press cake, from either raw or digested manure, has about 70% moisture.  This is ideal for composting and producing a rich soil amendment, or for immediate use as barn bedding.

It is surprising how accurate this report is.  The trend has been toward mixed flow instead of plug flow, and gas generation and recovery technology has improved.  However politically incorrect unstated facts from ten years ago remain true:  No digester project goes ahead without a government grant because the investments do not pencil out.  And, most digester projects get abandoned.  For example, all five showcase digesters mentioned in this Pressing News have been shut down.

Manure Incineration

July 28, 2006


Van Der Geest Dairy, located near Wausau, Wisconsin, has developed a unique technology for manure disposal. The farm has 3,800 cows and uses a flush-barn manure collection system.

Bedding, in the form of dried manure, is produced by burning about half of the manure generated by the herd. To do this, the manure from the dairy is first dried to a low moisture content so that it can either be incinerated or used as bedding. The heat released in the incineration process is what is used to dry the manure to low moisture content.

Initial dewatering of the manure is achieved by pumping the manure across two sidehill screens, one curved surface Houle and one flat panel Agpro. Both have 0.080" slot widths. The narrower, but taller, Agpro is preferred.

Prior to selecting the sidehills, the farm operated both Integrity roller drum machines and the Accent internally fed rotary drum screen. Sidehills won out because of their high capacity and simplicity.

Solids from the sidehills fall into a pair of KP-16 screw presses. The Vincent press was selected, after a nine-month testing program, over FAN, Manure Monster, and Boldt screw presses. Both Vincent and FAN modified their presses numerous times. Efforts by both companies failed to adequately press the manure without prethickening.

Key features that led to the selection of the Vincent press are as follows:

    • The KP-16, with its larger screw diameter, has greater capacity.
    • There is ample separation between the gearbox and the inlet hopper seal, assuring long gearbox life.
    • Vincent presses are available with standard NEMA motors which are readily available throughout North America.
    • The Vincent press has removable covers over the screen, making it convenient to pressure and acid wash the screen.
    • The screw of the Vincent press is supported at both ends, at the gearbox and at the cake discharge, which assures the screw-screen alignment that is necessary for long screen life.

The cake from the screw presses is conveyed to a system developed by Energy Unlimited. The key component is a Heil triple pass rotary drum dryer. Here the manure press cake, at 70% moisture content, is dried down to 10%.

Next, the dried manure is separated from the gas stream in a cyclone separator. The solids are blown into a storage bin, from which they a blown into a vertical combustion chamber. The solids are burned in the combustion chamber without the need for auxiliary fuel. The products of combustion (hot gasses) are directed into the dryer drum, where they dry the manure press cake.

An induced draft fan draws the gasses through the dryer and combustion chamber. From the dryer, the gasses go to a covered trench. Filtrate from the sidehill screens and press liquor from the two screw presses flows through this 450' long trench. Particulate and gas pollutants are captured and oxidized, and aeration is achieved. Another induced draft fan draws the gasses through the trench and pushes them to a smoke stack. A white plume of water vapor is generally visible from the stack.

Water from the trench is directed into a treatment pond. The treated water is used for barn flushing.

About half of the dried manure is used as bedding for the cows, with the other half being used as fuel for the combustion chamber.

This unique system for manure disposal is receiving a great deal of attention from environmental specialists.

Issue 176

Manure Installations


Ohio State University in Wooster, Ohio is running at 40 gpm press liquor from their KP-10. This is the machine in the video that Harvestore made; we have a copy if you want it. The new barn manager is Kevin Miller, 330-263-3924, Dairy Research Center Manager, Krauss Dairy Center. They have 120 cows, scrape barn, feeding our press. They run only 16 hours a week, making bedding. They put the press cake fresh into the stalls, without composting.

The original KP-10 at the University of Wisconsin goes great. We are not sure who the farm manager is, so the best contact would be Dr. Dick Koegel ("Cagle") at 608-264-5149 or 608- 264-5355. He is at the U.S. Dairy Forage Research Center. The KP-10 processes 50 gpm of dairy manure, with a separation of 43 gpm of liquid. The inbound solids were split evenly, with 50% in the press liquid and 50% in the press cake. Press cake moisture content measured 72%. Scraped barn.

One we have not heard from for a year is Warren Hatcher, mobile 423-309-8108, farm 423-338-2780 at Riverside Dairy Farm in Benton, Tennessee. They have a KP-16 on 800 head, flush barn. I saw a good 800 gpm going through the machine.

A good for sure KP-16 is at McArthur Farms in Okeechobee, Florida. They run 1,500 cows in a flush barn. Their pump was limited to 600 gpm, and the press took it all easily. The contact would be Maintenance Manager Rick Amadon 941-763- 8986. This farm has at least 5,000 head. Bob Rydzewski is the VP-GM, 941-763-4719 or 941-763-4373.

The KP-10 machine at Fairgrove Farms is a winner. Our Pressing News #1 describes this biogas system for electricity generation. At the farm, Warren Kelly's numbers are 616-651-6646, 616-651-8903, and cell 616-268-3747; John Pueschel's cell phone is 616-341-3053. The farm is two miles west of Sturgis, Michigan.

As at Fairgrove Farms, a KP-10 is used on digester sludge on an A.O. Smith Harvestore dairy manure biogas system at Cushman Farm. The biogas is used as boiler fuel. Our contact is Dave Smith at 860-234-0285 and Nathen Cushman, 860-642-4711, 120 Kahn Road, North Franklin, CT 06254.

Our little KP-6 is well loved at the University of Tennessee. Contacts would be Dr. Robert Burns, 423-974-7266, 865-974- 7237 and farm manager Henry Dowlen, 931-270-2240. They have 150 to 200 cows, scrape barn.

We have sold three KP-6's to pig farmers in Costa Rica. Since the sales came one year apart, we know the machines are working well. These farmers are bagging the cake and selling it as cattle and goat feed. The only person to call would be our distributor, Wilfred Korte at Industrias Bendig, 011-506- 259-7379. Wilfred's English is good.

Gary Hodgson, cell 403-340-9727, of United Livestock in Alberta, Canada sold a KP-10 to the Pibroch Colony, part of the Hutterite or Hutterian Brethren. The farm is north of Edmonton. George Walter is the Minister; Ely takes care of the manure system. They installed a tiny static screen over the inlet to the press. The press liquor runs a good 60 gpm. They have 4,000 pigs in the finishing barns. They produce 22 tons per week of press cake.

A KP-6 has been sold for the Futura Dairy in Central City, Iowa. This unit will dewater sludge from a biogas digester. Start-up is scheduled for later this year.


Manure Separator Inquiry

April 18, 2001, Revised October 2002                                                                                                                                                                      ISSUE #M15

Recently an inquiry was received that highlighted the need for information in quoting a manure separator. The e-mailed inquiry read: "Please quote the price for a manure separator to handle 100,000 liters of manure." 

Here are questions that we asked in order to select the proper size machine, with the right screen and drive motor:

What animals does the manure come from? Cows or pigs? 
We avoid poultry layer manure, although broiler grow-out house litter can be pressed.

By chance, is the manure actually sludge from a bio-gas digester?
Capacity goes down significantly on such sludge.

Assuming it is dairy cow manure, how many cows are on the farm? Are they confined (100% of the manure is captured), or is the manure only from a milking parlor or feeding barn?
Is a flush barn or scraped barn system used to collect the manure? This will tell us the dilution to expect.

What is the gpm capacity of the manure pump? 
How many hours a day is the manure separator expected to run? Some farms want to run only four hours a day, while others run up to twenty four hours a day.

What kind of bedding is used? 
Sawdust will require more horsepower than straw. Sand must be separated before the manure is pumped to the separator.

What is the power supply at the farm, single phase or three phase? What voltage? 
Most small farms are single phase 220 volt, while some large farms have 440 volt three phase power.

Is compressed air available for use with the manure separator? 
The large Model KP-16 has a discharge door that is actuated by an air cylinder. This feature is an option with the smaller 6" and 10" machines.

Is the manure currently being separated? If so, how? What is the cake used for? 
Reviewing these questions will result in a much more satisfactory machine selection.



Manure Separator Screens

May 24, 2000                                                                                                                                                                                                        ISSUE #M7

All categories of manure separators use metal screens made of stainless steel. These screens all fall into one of two categories: either perforated metal or wedgewire.

The perforated metal screens are made of medium gauge stainless sheet. The smaller the hole, the thinner the sheet. The most popular size is 3/32" perf (0.093" or 2-1/4 millimeter holes). This is generally made of 14 gauge metal, which is 0.075" thick. Another popular size, used in the small KP-6 separators, is 0.050" perf (1-1/4 mm); it is made from 24 gauge sheet which is 0.024" thick.

Wedgewire is the name given to a screen that is made from wires that are roll formed into a truncated pyramid cross section (a triangle with the top cut off). These wires are welded into flat panels with a fixed distance between each wire. By putting the broad base of the triangle against the liquid, a self-relieving passage is formed that has a minimal tendency to plug. Any solids getting through the narrow entrance between two wires will be swept away with the filtrate.

The flat panels thus produced can be used at the bottom of drag flight separators. Or, the panels may be curved to a gentle radius to make a sidehill (gravity) screen. The panels can be rolled to a cylindrical shape for use in a screw press, but they are expensive and tend to lack durability and burst strength.

The opening between adjacent wires is referred to as a slot. Typical slot widths used on manure range from 0.020" (1/2 mm) to 0.040" (1 mm).

The relative amounts of free area are of interest. For comparison purposes, 3/32" perf has 23% open area, while .030" wedgewire has only 15% open area.

Extensive testing has shown that changing the size of the opening has little impact on the solids capture rate. Oversimplifying a little, manure seems to be split between big particles and little particles. That is, almost all the little particles are small enough to get through any opening 0.010" or larger, while most big particles will be caught by an opening as long as it is under 3/32" inch (2-1/4 mm). (We tested 5/32" perf, and abandoned the effort.)


Manure Variations

September 22, 2000                                                                                                                                                                                               ISSUE M11

It is the goal of Vincent Corporation to have one basic press design that will work in all manure applications. We feel that it is reasonable to expect to separate all manure types with a single screw configuration, one screen selection, and one discharge plate mechanism. It is because of this goal that each production run of Series KP presses shows some improvement over its predecessors.

A wide variety of manure is being run through KP presses. The machines have proven themselves in four broad areas: cow, pig, biogas digester sludge, and slaughterhouse pauch manure. The varieties within these categories are surprising.

Cow manure general is split into scrape and flush barn systems. Obviously one has thicker solids than the other, and consistent agitation is required for consistent feed. However an equally important variable is the type of bedding that is in use. After all, the bedding can be a major component going into the pit. Straw and shredded paper make a great press aid and facilitate operation of the separator. Sawdust will press well, but it will draw more horsepower. (Of course, sawdust can range from sawdust to wood shavings or sanding dust; they behave differently in the press.) When manure is the only bedding used, we must count on the screen to pass the tiny digested solids into the press liquor flow; otherwise they tend to blind a screen.

Pig manure and sludge from a biogas manure digester have an important characteristic in common. Both have a high percentage of micron size particles that pass through the screen of the press. Thus, the solids capture rate is lower than when there is a preponderance of large particles of undigested food.

Definite variations exist in pig manure. For example, in Alberta, barley is fed to pigs. This is a coarse grain that puts fiber in the manure that facilitates press operation. On the other hand, in the States the feed is milled to 750 micron average particle size. This makes a manure whose tiny solids are hard to capture.

The absence of bedding in pig manure is normal, as is a lack of agitation. Flow starts with thick solids, material that has floated to the top. Then there is a prolonged flow of highly clarified water, followed by bottom sludge. To handle this range of conditions requires careful press design.


Manure as Bedding

October 20, 2001                                                                                                                                                                                                 ISSUE #M20

For many years leading agricultural universities have recommended dewatered manure as a dairy bedding material. Until relatively recently the recommendation has been that the manure be composted prior to use in the stalls.

The dewatering requirement gave rise to a number of pieces of equipment that are commonly called manure separators. They separate free water from the manure.

The reason for the composting requirement was twofold. Firstly, some manure separators leave a visible amount of moisture with the manure solids. This material is too soupy or slushy for use as bedding. Composting allows removal of the excess moisture.

The second reason for composting had to do with sanitation and disease control. It seemed obvious that fresh manure was full of bacteria that could cause mastitis in the herd. And it was shown that the heat generated in composting killed off the great majority of these bacteria.

Composting is no longer being regarded as an absolute requirement. Two observations have led to this. One observation was that cows in southern states, like Florida, wallow in ponds in order to keep cool. These ponds contain a great deal of manure, and yet mastitis is a manageable problem.

Another observation was that the few bacteria left after composting multiply exponentially. Composted manure ends up in the barn with the same bacteria count as the original material. Despite this, mastitis was shown to have low incidence in dairies using composted manure as bedding.

This has led to the use as bedding of material straight from the manure separator. This practice has been used at the Ohio State farm in Wooster, Ohio. Press cake from a Vincent Model KP-10 press has been used immediately as bedding for over a year.

The excellent dewatering characteristic of screw presses is recognized. This is resulting in increasing popularity of these machines on the farm.


Matlink Digester (Continued)

MAY 28, 2002, Revised September 2010                                                                                                                                                                 ISSUE #M23

Three months ago we wrote about an extremely smooth start-up that had taken place at the Matlink Farm in Clymer, New York.  This involved a manure digester system designed by RCM of Berkeley, California, 510-658-4466. 

The Vincent screw press worked very well dewatering the digester sludge.  However this only lasted for a few months during which sawdust was used as bedding.  When the farm switched over to using composted press cake, troubles were encountered.  The gpm capacity of the press went down by about 50%. 

A worse problem was that the press would occasionally go into a purge condition.  This condition is commonly referred to as having the press "blow the plug" at the cake discharge.  At this time unpressed manure flows through the press without any dewatering, flooding the area where press cake normally forms a pile.

It was observed that the purge condition occurred when firm, solid press cake held open one side of the discharge door.  This reduced the pressure against the cake on the other side of the door.  The cake would become more and more moist, eventually reaching a condition where it flowed, without being pressed, past the door.

After two visits and testing a number of experimental doors, a very satisfactory solution was developed.  The shaft of the screw was extended an additional 14".  This makes the screw extend well past the cake discharge of the press.  A door with a hole in the center, to allow for the extended screw length, was required.  The result was absolute:  there is no longer sufficient door area for cake to hold the door open.  The purging characteristic has been totally eliminated.

Retrofit kits are being produced to modify manure presses that have had problems with purging.  The kit consists of an extension that is welded to the end of the screw shaft along with a new door and yoke style actuator arms.  The kits can be installed without the need to disassemble the machine. 

At the same time a screw design has been developed that drastically increases the capacity of the press on digester sludge.  This design, operating at Matlink, includes double flighting in the inlet portion of the press where the manure is thin.  This design has also been found to significantly improve screw press capacity on flush barn systems where press cake is used for bedding.

September 2010
The farm entered bankruptcy and the digester is no longer being operated.  The Vincent press was purchased by Noblehurst Farms.  Today we would eliminate the purging problem by using half pitch (not double pitch) flighting and the rotating cone feature.

Matlink Digester

February 13, 2002                                                                                                                                                                                                    ISSUE #M22

An extremely smooth start-up has taken place at the Matlink Farm in Clymer, New York. Owned and operated by Ted Mathews, the farm has installed a manure digester and electricity generating system designed by RCM (Resource Conservation Management, Berkeley, California, 510-658-4466). The system employs a 660,000 gallon stirred pit digester. Manure from the 1,200 head dairy is scraped and pumped to the concrete pit.

Digested manure floats over a weir and into a secondary pit. The manure in this secondary pit is pumped to a Vincent Model KP-10 screw press. In normal operation the press dewaters the manure at the rate of 120 cows per hour.

Methane from the digester is burned in a Waukesha internal combustion engine. This engine is coupled to an induction type electric generator. The rating of this generator is 130 kilowatts (175 hp), at which rate the system is currently operating.

Waste heat from the motor that drives the generator is used to heat the digester. Cooling water from the radiators is circulated through the pit so as to maintain a temperature of approximately 100° F.

Circulation is maintained in the digester with a pair of sidethruster propeller pumps. These are positioned diagonally opposite in the sides of the pit. 

Pig Manure

October 30, 1998; Update August, 2013

Trials running pig manure through our KP presses have been run in the States and Canada, and KP-6 presses have been sold to pig farmers in Costa Rica and Sri Lanka.  In all cases the press has done its job of separating solids and liquids, but machine sales did not result in the States.

Pig Manure is highly digested and contains a large portion of the solids in the form of very small particles.  The larger particles are basically undigested food. Typically pig manure is pumped to a KP press with a dilute inbound solids consistency in the range of 1% to 6%.  At the lower solids content, the gpm capacity of the press goes up, reaching 100 gpm in the KP-10.

However there are two problems that can occur when very low consistency flows are pumped to a press.  Either the solids capture rate goes down, or the screen of the screw press tends to blind.  For that reason, Vincent insists that flows of pig manure be pre-thickened with a sidehill screen before being admitted to the press.

Regardless of inbound solids content, the cake produced by the press will have a moisture content of about 70%.  This cake material is quite dry to the touch, and it composts very readily.  Remarkably, it has almost no odor.

Since the cake is essentially undigested animal feed, it provides a good feed additive.  Our Costa Rican customer sells his press cake, in 50-pound bags, to nearby cattle ranchers. The investment in the press has significantly reduced a pond odor problem while paying for itself in by-product sales.

Testing sponsored by A.O. Smith ESPC (Harvestore) was conducted by the Dairy Forage Research Department at the University of Wisconsin.  They found that the capture rate for the KP press was in the range of 20% to 25%.  This means that at least three quarters of the inbound solids go through the screen and are carried away with the filtered liquid.

Advanced animal husbandry practices in North America result in a higher proportion of the feed being digested.  Overseas it is typical for coarser ground material to be feed, and for less complete digestion to take place.  Thus the solids capture rate is higher outside of North America.

The low capture rate is the reason that sales were not achieved in the States and Canada.  In these countries the reason for pressing manure was the hope that most of the solids would be separated from the liquid and that the odor and wastewater treatment problems associated with hog farms would be addressed.  A belt press would be better suited for this task than a screw press.

Issue 85


Sand Bedding

October 24, 2000, Revised April 2003                                                                                                                                                                      ISSUE #M12

Questions about sand bedding as it relates to manure separators came up many times at this year's World Dairy Show. It is clear that sand bedding is very popular, and dairy farmers are well aware of the abrasion it causes.

Earlier this year a Vincent KP-10 was tested at the Everett Williams 400 head dairy in Pennington, Georgia. Good results were expected because of minimal wear experienced at McArthur Farms in Okeechobee, Florida. McArthur's has a great deal of coral sand in their manure, their press has held up very well over a period of years. (Although McArthur's is currently adding settling basins.)

The opposite occurred at Williams' farm. In only a few weeks the Georgia sand wore out a screw. The screw was replaced with one using improved technology: the screw flights were made of abrasion resistant plate, with two layers of MIG applied hardsurfacing followed by TIG applied Stellite or Colmonoy. This was a notable improvement and minimal wear ensued.

However in no time at all the screen was worn through. (The latest Vincent manure presses use sleeve insert screens that are inexpensive to replace.)

It was concluded that sand bedding and screw presses do not mix.

This same knowledge applies to other manure separators exhibited at the World Dairy Show. A drag flight conveyor separator is vulnerable because of chain and sprockets that must operate in the sand/manure environment. The same goes for the wringer/roller type machines.

Our advise to farmers who use sand bedding was to either have good separation of the sand and manure ahead of the separator, or to avoid manure separation. Sand can be separated by flowing the manure through a trench with velocity such that the sand settles out and the manure stays in the flow stream.

If this separation is not possible, then the farmer is better off draining the manureand sand into a pond, which will have to dredged periodically. If they buy a mechanical separator based on savings of avoided dredging costs, they are likely to face repair and parts expenses before the payback period is complete.


Screen Patching

It is not uncommon for the screen of a screw press to wear out. Abrasion from dry manure is a cause. Also, operating with a worn screw puts increased loading on the machine. This can cause the screw to shift sideways, leading to rubbing between the screw and the screen. 

When a screw is tight against a screen, the screen material bows outward, minimizing wear. Consequently wear tends to occur where there are reenforcing rings on the screen that prevent it from bowing away from the screw.

Screen Patch Kits are available for all the Series KP presses. The patches supplied by Vincent are made of 11 gauge steel (1/8" thick) with 3/8" hole perforations. This assures plenty of draining surface even when extensive patching is required.

The patches can be welded in place without having to remove a screen from a press.

The kits can be used only if the screen is still relatively intact. If it has been torn into two or more pieces or if it has been beer-canned, it is likely beyond repair.

The kits consist of a series of pairs of half-cylinder sleeves that fit around the outside of the screen. Each sleeve is about 180º so that the two pieces of a pair can be wrapped around a worn or torn part of a screen.

Once the sleeves are in position they should be TIG welded in place. (Stick welding can be made to work.) The width of the patch sleeves is such that they fit between two adjacent reinforcing rings of the original screen. The patches are welded to these rings and to each other.

The patches, like the original screens, are made of stainless steel.

In a bind, patches can be made of any sheet metal. Because of the excess of open area in the original screens, patches need not be made of perforated material.



Screw Removal and Replacement

Removing and replacing the screw of a Series KP press is a matter of pushing or pulling against an internal snap ring. This snap ring is located in the hollow shaft of the gearbox. A large washer, almost as large in diameter as the hollow shaft of the gearbox, is seated on one side or the other of the snap ring. All-thread rod and a nut are used to push or pull against this washer in order to move the screw in the desired direction.

Complete tool kits and drawings of these kits are available upon request. The Operating Hints section of the O&M manual has a number of helpful suggestions:

The snap ring, washer, and all-thread rod combination must be used to remove the screw from the press. Using a wheel puller can destroy the gearbox.

It is common that heavy force may be required in either removing or reinstalling a screw.

Be sure to lubricate the screen when reinstalling a screw, and be sure to apply Never Seize to the portion of the screw shaft that goes into the gearbox.

Heavy industrial snap ring pliers will be required for use with the internal snap ring.

A special nut with a lug that catches in the keyway is required when pushing the screw out of the press.

When pushing a screw out of the press a steel disc should be placed over the end of the all-thread rod in order to prevent it from screwing into the threaded hole in the end of the screw.

When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. Be careful when guiding this step in the shaft through the shaft seals. It is best to remove the four bolts holding the seal housing during the screw installation.

When pulling a screw into the press the all-thread rod must be screwed into the end of the screw. This can be done before starting to put the screw into the press.


Types of Manure Separation

March 8, 2000                                                                                                                                                                                                        ISSUE #M4

There are four different types of machines commonly seen separating dairy manure: sidehill screens, drum screens, drag flight conveyors, and screw presses.

Sidehill screens are the least expensive and require the least maintenance. These are simple sloped screens (alternatively called static and gravity screens) with a weir box at the top. The manure is pumped to flow over the dam of the weir box and down the screen. The filtered liquid goes through the screen, while the solids (tailings) fall off at the bottom.

The disadvantage of a sidehill screen is that even under the best of operating conditions the solids come off as a wet sludge. This sludge is almost too wet to compost, and it is difficult to handle for spreading on the field. Additionally, sidehills tend to blind over, requiring a farm hand to wash down the screen. A sidehill will keep a lot of solids out of the pond, but it is definitely not a strong manure separator.

Drum screens use a drum made of perforated metal to separate the solids from the liquid. Alfa Laval previously offered one, and Houle currently has a unit in their line. These will produce drier cake than a sidehill. However they are rarely popular. Maintenance is frequent because of all the moving parts and adjustment; screens can be damaged by large solid tramp materials; and they are exceptionally dirty.

Drag flight conveyor separators come in two varieties. The Blossom and Agpro machines are single pass: the lower end is submerged in the manure pit, and the solids are dragged up perforated or wedgewire screens to where they fall into a small screw squeezer. The Albers unit is a double pass design. The manure starts by being dragged down a sidehill. It makes a U-turn in a sump and is dragged back up to where it falls into a roller squeezer.

Drag flight separators have the disadvantage of many moving parts: chain, sprockets, drag flights, bushings, besides an optional squeezer at the discharge. Leakage and large size are negatives. The manure cake is dryer than achieved with a sidehill screen, but not up to that produced by a screw press.

The screw press, which produces the driest cake, is the fourth category of manure separator. These are designed to operate intermittently, unattended, with a minimum of maintenance. They will be further described in later Pressing News issues.


Vacuum Testing

March 1, 2001, Revised August 2001                                                                                                                                                                           ISSUE #M14

In February tests were run to test vacuum on the outside of the screens of a screw press. The testing was done with a Model KP-6 press operating at the Lewisburg dairy of the University of Tennessee. Manure is pumped to the press from the pit into which it is scraped. There is a recirculation line from the inlet of the press back the pit, which assures that the inlet hopper will not become pressurized. The press liquor drains though a hose, going down 12' to a point where the drain line could be either open-ended or sealed under liquid in a 5-gallon pail.

The tests were facilitated by the fact that the screen of the KP-6 is surrounded by an integral, airtight, combination cover and press liquor collection pan. A hole was drilled through this cover, and a manometer was inserted to measure the vacuum surrounding the screen.

We were in for a number of surprises. While there are several unanswered questions, some facts were established and best-performance conditions were observed. The facts are that a vacuum can be formed on the outside of the screen, and the throughput capacity of the press easily went up 25% to 100% with vacuum conditions. The vacuum ranged from 8" to 42" water column (28" w.c. equals one psi). The best results where when we had the vent pipe open and the end of the drain hose submerged.

Dave DeWaard at DariTech in Washington is insistent that there must be an open vent line on the inlet hopper of the press in order obtain maximum capacity. Previously we thought the vent line was needed to let air out of the press on start-up. However it is now hypothesized that press capacity goes up with the vent line open because the recirculation line carrying manure back to the pit draws a vacuum in the inlet hopper, counteracting the vacuum in the drain line. An intermittent gurgling sound like a flushing toilet was heard from the press during our tests with an open vent line.


Why Separate Dairy Manure?

April 5, 2000                                                                                                                                                                                                          ISSUE #M5

The use of manure separators is practically unknown in many areas, while in other regions no dairy is without a device of some sort. There are reasons why this is true, and it is equally true that the trend is toward the expanded use of manure separators.

There are different justifications for the use of manure separators. Probably the strongest reason is to keep solids out of the manure ponds and to get the manure solids into a form that can be land applied without building a cake layer on the pasture or field. If un-separated manure is sent to the pond, tank or Slurrystore structure, settling can become a problem. This is especially true in flush barn ponds where a source of clarified water is requisite. The repeated cost of dredging a pond can be sufficient to justify installation of a separator.

Spraying the raw manure on a field or pasture is not a foolproof alternative. Flush barn farms usually need the recycled water. Besides, the wet manure can build up a mat layer so dense that grass will not grow through it. In contrast, composted manure solids are readily tilled into the soil, with strong benefits from fertilizer and organic addition.

There are areas where an altogether other justification exists for using a manure separator. Many farmers, especially in the north central States, use the composted manure as bedding for their cows. This represents a direct savings of cash that would be spent on sawdust, sand, ground paper, straw, or other bedding. (Farmers in other areas fear that, because of the climate, disease outbreak may occur with the use of manure bedding, so the practice is not used among them.)

Many other reasons are given for using a manure separator. In every county there is the story of the farmer who is getting rich raising worms in the compost, selling potting soil to nurseries, or selling bags of compost for $4.00 each. There is less discussion of the practice of blending press cake (which is mostly undigested feed, 50% protein) in the ratio of 5% for heifer and non-lactating cow ration.

Odor reduction is probably the greatest fringe benefit of using a manure press. However, environmental regulations have not reached the point of mandating the use of these machines.


Wobble Stick Switch

October 24, 2002                                                                                                                                                                                                    ISSUE #M25


The screws of screw presses used in manure are subject to abrasive wear. This wear does not occur in normal operation nearly as much as it does in a condition of "running dry". If a manure press is allowed to continue running when there is little or no cake being produced, the cake remaining in the machine becomes bone dry. This dry material, even without sand being present, is extremely abrasive. Because of this it is recommended that a wobble-stick switch be incorporated in the electrical control system.

The switch is mounted someplace below the cake discharge so that the wobble stick is in the path of the falling press cake. This is illustrated in the sketch below. The switch should be connected to a timer such that, if the switch does not detect activity for a while, like three minutes, the manure pump and press are shut down.

Shutting down the system when cake is not being produced is advantageous for several reasons:

1. Even though the press is not producing cake, as long as the screw is turning the flights are being worn down. In this stalled condition, the cake at the discharge remains stationary, tends to get very dry and even hot, and the amount of abrasion occurring to the circumference of the flights increases substantially.

2. If cake is not being produced, it is likely that the pit has been drawn down to where the capacity of the manure pump has been seriously reduced. This comes about because of the increased height that the manure must be pumped. (The pumped height is from the level of the pit to the inlet of the separator, not from the pump suction to the inlet of the separator.) It is time to shut down the system.

3. If very little cake is being produced, water on the inlet end of the screen will soak into the dry cake at the discharge. In this manner the lower portion of the plug at the discharge becomes sopping wet to the point where it breaks loose and flows out. The discharge door is held open by the dry material on the upper portion of the plug, and purging will begin. Shutting down the system before this happens can prevent the mess that comes with blowing the plug.

Square D offers wobble-stick switches and timers, as do many other companies. They can be bought through McMaster-Carr, Grainger, and Gulf Controls among others.

Wobble-Stick Switch

October 24, 2002                                                                                                                                                                                                 ISSUE #M25

The screws of screw presses used in manure are subject to abrasive wear. This wear does not occur in normal operation nearly as much as it does in a condition of "running dry". If a manure press is allowed to continue running when there is little or no cake being produced, the cake remaining in the machine becomes bone dry. This dry material, even without sand being present, is extremely abrasive.

Because of this it is recommended that a wobble-stick switch be incorporated in the electrical control system. The switch is mounted someplace below the cake discharge so that the wobble stick is in the path of the falling press cake. This is illustrated in the sketch below.

The switch should be connected to a timer such that, if the switch does not detect activity for a while, like three minutes, the manure pump and press are shut down.

Shutting down the system when cake is not being produced is advantageous for several reasons:

1. Even though the press is not producing cake, as long as the screw is turning the flights are being worn down. In this stalled condition, the cake at the discharge remains stationary, tends to get very dry and even hot, and the amount of abrasion occurring to the circumference of the flights increases substantially.

2. If cake is not being produced, it is likely that the pit has been drawn down to where the capacity of the manure pump has been seriously reduced. This comes about because of the increased height that the manure must be pumped. (The pumped height is from the level of the pit to the inlet of the separator, not from the pump suction to the inlet of the separator.) It is time to shut down the system.

3. If very little cake is being produced, water on the inlet end of the screen will soak into the dry cake at the discharge. In this manner the lower portion of the plug at the discharge becomes sopping wet to the point where it breaks loose and flows out. The discharge door is held open by the dry material on the upper portion of the plug, and purging will begin. Shutting down the system before this happens can prevent the mess that comes with blowing the plug.

Square D offers wobble-stick switches and timers, as do many other companies. They can be bought through McMaster-Carr, Grainger, and Gulf Controls among others.