What is a Filter Press?

This post will answer the following questions: What is a filter press and how does it work? We’ll also give you some advice on sizing your equipment (including your feed pump). Our Sales and Service Team is looking forward to answering any other questions you might have.

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M.W. Watermark 800mm Filter Press

What is a filter press?

A filter press is a batch operation, fixed volume machine that separates liquids and solids using pressure filtration. A slurry is pumped into the filter press and dewatered under pressure. It is used for water and wastewater treatment in a variety of different applications ranging from industrial to municipal.

M.W. Watermark manufacturers filter presses ranging from .06-600 cubic feet.


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There are four main components to a filter press

A) Frame (Skeleton): The steel filter press frame acts as a clamping device for the filter plates.

B) Filter Plates: There are three basic categories of filter plates all with different functions – plate and frame, recessed chamber (or recessed plate) and diaphragm squeeze (also known as membrane squeeze).

C) Manifold: Our standard manifold consists of piping and valves which control the slurry inlet and connect the four corner filtrate discharge ports into a common discharge pipe.

D) Filter Cloth: A cloth filter that is attached to the filter plate. Solids build up on cloth to form a filter cake, separating liquids from solids.

*M.W. Watermark manufactures filter cloths in-house.*

M.W. Watermark - What is a Filter Press? How Does a Filter Press Work? M.W. Watermark - What is a Filter Press? How Does a Filter Press Work? Filter Press FAQ, Filter Cloth

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How does a filter press work?

Slurry is pumped into the filter press. The solids are distributed evenly during the feed (fill) cycle.

Solids begin to build on the filter cloth. Most of the solid/liquid separation is done by the filter cake building on the cloths. At first some fines may pass through the cloth (1), but eventually the solids begin to form a layer on the filter cloth (2) much like a pre-coat. That layer traps the fine particles and forms a filter cake (3).

M.W. Watermark - What is a Filter Press? How Does a Filter Press Work? solids-build-on-filter-cloth

As the filter press builds pressure, the solids build within the chambers until they are completely full of filter cake. When the chambers are full, the fill cycle is complete. The filtrate (liquid) exits the filter pack (plates) through the corner ports into the manifold; when the correct valves in the manifold are open, the filtrate exits the press through one single point, the filtrate outlet.

M.W. Watermark - filter press manifold

How long does a filter press cycle take?

The Total Cycle time is the Fill Cycle time plus a constant. For presses of 125 cubic feet and under this constant is usually around 45 minutes. This is the time required to close/open the press, perform the Air Blow Down and discharge the filter cake. If the particular application requires operations such as Core Blow or Cake Wash, for example, this constant is longer.

How long does a Fill Cycle take?

The Fill Cycle is dependent on many parameters. The most important parameter is the nature of material to be dewatered. A sand slurry releases its water readily and dewaters quickly. On the other hand, an Aluminum Hydroxide waste slurry from beverage can manufacture does not readily release its water and dewaters slowly.

The next most important parameter is the concentration of the solids by weight in the slurry. The Fill Cycle for a 5% solids slurry is about twice as long as a 10% solids slurry (with all other parameters being equal). This is because the press has to process half of the water to fill with solids.

Other parameters include the thickness of the filter cake, the maximum feed pressure which the slurry is fed to the press, and the filter cloth selection. These parameters are typically fixed during the proposal process.

Can you give some examples of Fill Cycles?

With 32mm (1.25”) cake chamber thickness, 100 psi max feed to the press and a 3-5 SCFM filter cloth, a 5% sand slurry would be expected to dewater in 20-30 minutes and a 10% sand slurry in 10-15 minutes.

Conversely, the 5% Aluminum Hydroxide slurry may take 4-6 hours to dewater, while the 10% slurry would dewater in 2-3 hours.

We have an in-house laboratory where we can test a sample of your slurry to determine the Fill Cycle time as well as the other outputs from pressure filtration testing. Give us a call.

Will a small press fill faster than a larger press?

The Fill Cycle times for a 1 cubic foot, 10 cubic foot and 100 cubic foot press are approximately the same. Press volume is the ability to remove solids. Associated with this volume is the square feet of surface area in the press.

Square footage is the ability to process fluid. As volume is added, square feet of surface area is proportionally added and the cubic foot to square foot ratio remains (roughly) constant. Therefore, the Fill Cycle Time is basically the same.


A 1 cubic foot press at 32mm cake thickness has 22 square feet of surface area for a 0.045 cf/sf ratio. A 10 cubic foot press at 32mm cake has 211 square feet of surface area for a 0.047 cf/sf ratio.

What type of pump should I use to feed my press?

For press capacities of 125 cubic feet or less, the double Air Operated Diaphragm pump (AOD) is uniquely suited for filter press operations. The filter press is not a constant flow device. As the solids build up within the press, the resistance to flow increases and the flow rate through the press decreases. At a given air pressure supply, the time between pump strokes for an AOD pump, then, constantly increases with no harm done to the pump.

On presses larger than 125 cubic feet in capacity, AOD pumps become impractical because three or more large (3”) pumps would be required.

On large presses pumps such as progressive cavity, centrifugal and piston membrane pumps are often used. Control of these constant flow pumps is through a PLC that requires input from a pressure transducer and a flow meter to control a VFD (Variable Frequency Drive) on the pump motor. For a small press the control system for a constant flow pump is generally more costly than the press itself.

So, are there any Rules of Thumb on press feed flow rates that can be used to select a pump size?

For presses 125 cubic feet and less, an AOD pump that can deliver 0.1 gallons per minute per square foot of surface area after the initial fill should be selected.


800mm, 20 cubic foot press has about 420 square feet of surface area. A pump that can deliver 42 gpm should be selected. This can be either a 1.5” or 2” AOD pump.

A 1200mm, 100 cubic foot press has 2030 square feet of surface area. While one 3” AOD pump can deliver 203 gpm, the operating envelope would be on the edge of the curves. Here, two 3” AOD pumps in parallel would be recommended.

For the very large presses a constant flow pump would be sized to be able to fill the void of the press in about 4-6 minutes. For example, a 300 cubic foot press is about 2,250 gallons. To initially fill this press in 5 minutes requires a pump that can deliver 450 gpm.

Is there any way to quickly relate this Rule of Thumb information to press capacity?

The vast majority of filter presses sold are 125 cubic feet in capacity and below. The table below gives quick guidelines. However, there are always overlaps at the ends of the range. A 15 (or16) cubic foot press can be fed with a 1.5” or 2” pump.

Filter Press SizePump Size
<5 cubic feet1” AOD pump
5-15 cuft1.5” AOD pump
15-25 cuft2” AOD pump
25-50 cuft3” AOD pump
> 50 cuft, <125 cuftMultiple 2” or 3”pumps

We furnish a full line of feed pumps. Please call our Sales and Service Team at (616) 399-8850 for your best sizing and price.

Now that the correct size AOD pump has been selected, how do I know when the Press Fill cycle is completed and it’s time to turn the pump off?

For feed pumping purposes, the filter press can be viewed as an open system. There can always be one more stroke of the AOD pump. It may take days to get it, but there will be one more stroke. A practical end of the dewatering cycle has to be determined.

For all practical purposes, the Fill Cycle is finished when the flow rate through the press at terminal pressure is 0.01 gpm per square foot of surface area. For an 800mm, 20 cf press with 420 square feet of surface area, this is 4.2 gpm. There is a correlation between this terminal flow rate and the time between pump strokes at terminal feed pressure, typically 100 psi. Depending on the nature of the slurry being dewatered, the interval between pump strokes at terminal pressure is 30-60 seconds.

Do I give the feed pump 100 psi of air pressure from the start and stand there timing the interval between pump strokes or is there some way to automate this?

The press Fill Cycle can be started at full blast by giving the AOD pump a 100 psi air supply, walking away and coming back in a couple of hours to check on the interval between pump strokes. However, to save on wear and tear on the AOD feed pump and prolong the useful life of the filter cloths, it would be preferable to ramp up the feed pressure to the press.

The feed pressure can be manually ramped up by installing a pressure regulator in the air supply line to the AOD pump and ramping up in 25 psi increments for example. The ramp point for the 25, 50 and 75 psi stages would be when there is a 5-10 second interval between pump strokes. On the 100 psi stage the termination point is 30-60 seconds between pump strokes. This ramp up and termination can be done automatically with the M.W. Watermark Automatic Feed Pump Control System (AFPCS).


If you have any questions, please contact our Sales & Service Team. They will be happy to assist you with your current equipment, or, sizing new equipment.

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What is a Filter Press?