Filter Presses are used in many product manufacturing processing systems as well as to efficiently manage waste material separations. They may be needed to recover water or other solutions from the process or waste stream, and/or to produce or recover dewatered solid materials. Because Filter Presses are used so broadly, many equipment options are available.
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Here are the most common options to consider when selecting a Filter Press system.
When Filter Presses were first utilized many years ago, they were composed of a series of simple rectangular frames sandwiched between flat solid plates with channels or grooves on the plate surfaces to allow liquid to be removed. This is where the term “plate and frame Filter Press” originated. Solid material accumulated on filtration cloths held between the frame and flat plates. When no more material could be pumped into the frame area, the plates were separated, allowing the solid material to fall out of the frames.
The plate and frame style press design is plagued with problems when it comes to removing the dewatered solid material from the frame. Although plate and frame press designs are still used in some industries/applications, today most Filter Press systems employ recessed plate or membrane plate designs.
Recessed plates have depressed surfaces inside the perimeter area of the plate face, eliminating the need for a frame. Filter cloths are installed on the plate surfaces. When a series of these plates are held tightly together, void spaces are created between them. A pump is used to force the solid and liquid slurry material between the plates, where solid material is captured in the recessed void spaces and the liquid is passed through the cloth and discharged through ports in the plate body.
Similar to recessed plates, membrane plates offer an additional means of separating solid and liquid materials that are compressible. The membrane plate is recessed, but the plate face surfaces are flexible, allowing them to be expanded into the void space created between the plates.
Similar to recessed plates, dewatering occurs initially by pumping solid and liquid material between the plates. Once no more material can be forced into the plate’s void spaces, the pump is stopped. The flexible membrane plate surface is then expanded with water or compressed air, forcing the flexible plate surface into the dewatered material. As the plate surface expands, the accumulated solid material is compressed, resulting in additional liquid removal.
Another basic Filter Press option involves how the filter plates are supported. There are two basic designs: overhead beam and side beam.
Overhead Beam Filter Presses support the filter plates from an overhead beam(s). This design allows complete access to both sides and the bottom of the filter plates. It eliminates any interferences with the dewatered solid cakes dropping out of the press and allows the best access for cleaning and maintaining the press. In addition, the mechanism that separates the filter plates in an overhead beam press is typically housed above the Filter Press. This minimizes equipment contamination from the materials being dewatered, optimizing equipment maintenance cost and equipment life.
In a side beam design, the filter plates are supported with long beams on the sides of the Filter Press. With this design, the plates opening mechanism is either accomplished by the stroke of the main hydraulic compression cylinder or by a separate plate opening mechanism mounted on the side beams. In either case, the plates are attached to one another with short chains or mechanical linkages, allowing multiple plates to be opened in a single movement. The side beam design is typically used where smaller Filter Presses have sufficient capacity and where space constraints limit the equipment size. The side beam design also allows open access above the plate stack for cloth changes and plate maintenance.
The liquid (filtrate) removed during dewatering on a Filter Press passes through the filter cloths mounted on the plate surfaces and exits the plate via internal ports or channels. On plates with closed filtrate discharge, the liquid passes from the plate surface into small ports in two or more plate corners.
From there, every plate has a corner hole milled through the entire plate thickness. With the plates clamped together, these corner holes act as a pipe pathway for the filtrate water to exit on one or both ends of the plate stack through the Filter Press’s structural plates. Piping mounted on the structural stationary and/or mobile plates directs the filtrate to a filtrate collection tank or to a thickener/clarifier tank.
Another option for removing liquid from the press is called open filtrate discharge. In this case, each individual filter plate has an internal port on both bottom corners, allowing liquid to discharge directly from each plate into a collection trough or directly onto a drip tray below the Filter Press.
Open Filtrate Discharge allows the operator to visually observe the quantity and cleanliness of the filtrate being discharged from each plate. When a filter cloth is damaged or worn excessively, very dirty filtrate will exit the press, and Open Filtrate Discharge allows the operator to immediately determine the location of the damaged cloth.
All Filter Presses are fed by some type of slurry feed pump to force the solids and liquid into the spaces between the filter plates. The pressure created by the pumping action creates the force necessary to separate the liquid and solids. The most common slurry feed pump is a centrifugal pump. One or more single-stage centrifugal pump(s) in series can be used or a single multi-stage centrifugal pump can be used.
Positive displacement pumps are also commonly used to feed Filter Presses. An individual positive displacement pump can be used, or a positive displacement pump can be used in series with a centrifugal pump. The pump equipment used will determine the maximum pressure and flow of the slurry being fed to the filter press. Multi-stage centrifugal pumps or positive displacement pumps can generate +400 psi and several thousand gallons per minute of flow rate. Higher pressures and flows will typically increase the capacity of the Filter Press.
Filter Presses can be fed from one or two ends with appropriately designed feed piping. Larger Filter Presses with more than 50-100 filter plates require a large flow of slurry during the initial filtration period. Feeding the press from both ends allows the press filtration time to decrease so the press can be filled faster. The dual feed reduces the amount of wear on the filter cloths, extending the life of the cloth.
When a press is designed to feed both ends, it is also possible to complete a “core blow.” At the end of the filtration stage when the feed pump is stopped, wet slurry still remains in the core hole of each plate. A core blow sequence is completed to remove the remaining core slurry before the filter plates are opened, preventing wet slurry from being discharged with the dewatered cakes.
The core blow is accomplished using a set of automatic valves connected to the slurry feed piping. The slurry is forced out of the core hole with pressurized water and/or compressed air. The water and/or compressed air are fed into one feed end of the press, and the slurry is forced out of the opposite end of the press, typically back to the slurry tank.
An animation of the core blow.
The filter cloth media allows solid particles to be captured between the filter plates while the liquid passes through the filter media. All cloths eventually become contaminated with some amount of solid material on the surface as well as in the spaces between the cloth weave. Eventually, the accumulation of material on the cloths will impact the filtration rate and the ability to seal the plates together to a point where the cloth will need to be replaced.
How quickly the contamination occurs is very dependent on the characteristics (particle size distribution, shape, makeup, abrasiveness, tackiness, etc.) of the material being dewatered. The cloth life can be extended by some amount of routine cleaning.
Manual cloth cleaning with water is the simplest and most typical way to prolong the efficient life of the filter cloths. Simple handheld force hoses with medium pressure and volumes are used to remove the excessive buildups on the filter cloth surfaces, especially in the sealing and stay boss areas. Fully automated cloth washing systems, shown in the video below, can also be used to more effectively clean the entire cloth surface as well as to remove some of the contamination material within the cloth weave. These washing systems use high-pressure cleaning showers that are automatically positioned for each filter plate and can be programmed to routinely clean the cloths at defined intervals of times or cycles.
Fully automated cloth washing system.
Drip trays are large moveable panels installed below the Filter Press to capture the cleaning water during cloth washing. The movement of the drip trays into the raised or lowered position is typically accomplished using hydraulic cylinders. With the drip trays in the raised position, the cleaning shower water is directed into a collection trough where it can be recycled. When the drip trays are in the lowered or in the open position, the dewatered cakes can discharge out of the press into a storage area or onto a conveyor.
Certain Filter Press applications require as much liquid removal as possible or require very dry cakes to be discharged. Similar to a membrane plate squeeze, cake dry is used to remove more liquid from the solid material after the filtration stage has been completed using the feed pump action. After the feed pump is stopped, the plates are held tightly together while compressed air is forced between the filter plates. The compressed air passes through the solid material captured between the plates and sweeps additional liquid out of the press.
Recessed plates and membrane plates can be used to complete the cake dry stage, but the plates will have additional ports to direct the compressed air through the cake material.
Some filtration applications require additional removal of soluble material out of the solid dewatered material captured between the plates. In this case, a liquid such as clean water or other solvents are pumped into the Filter Press to “wash” additional material out of the cakes. Similar to cake dry, the cake wash is accomplished by using recessed or membrane plates with ports that direct the liquid through the cakes after they have been formed by the pumping filtration action. Once the cake dry or cake wash has been completed, the Filter Press is opened to allow the cakes to discharge before another cycle is started.
Considering these seven Filter Press options ahead of time can help you have a better idea of what will work best for your application. If you need assistance, contact McLanahan.
A common objective in most wastewater treatment plants is to reduce the volume of sludge generated in both primary and secondary treatment as much as possible.
There are several technologies for dewatering these sludges, the most economical being mechanical systems, among which the following stand out: centrifugal decanters, belt filters, screw presses and filter presses, the latter being the ones that can achieve higher degrees of dewatering, in addition to having lower energy and reagent costs.
The filtering systemtro press system is the one that achieves the highest sludge dryness, but, unlike the other systems, it has discontinuous operation.
Filter presses have various applications in the industrial field, and in many cases, the dehydrated sludge obtained can be marketed, as in the following cases:
In this article we will focus on the use of filter presses for the treatment of sludge produced in wastewater treatment plants.
A filter press is a machine mounted on a support structure that houses a set of chambers with filter cloths inside.. The sludge is pumped by means of a pump, which is usually of the pneumatic type, to avoid operating problems due to the large amount of solids present in the liquid to be filtered.
A filter press consists of:
It is the structure that forms and supports the whole filter press.
The chambers, which are concave in shape and are opposed to each other, are the spaces between the filtering plates. They act as a mold for the sludge that forms inside them. The plate pack is kept closed and sealed at high pressures due to the action of an electro-hydraulic unit that uses a central piston to pressurize the chamber pack.
The plates can be of various dimensions. The most common range from 80 x 80 cm to 250 x 250 cm. The construction materials are usually plastic, such as polypropylene.
Xuda Filtration Product Page
Filter presses usually operate at pressures in the order of 6 bar, but there are also high pressure equipment up to 16 bar, and even higher, whose objective is to achieve the highest possible dehydration. The number of chambers depends on the volume of sludge to be obtained in each drying process and is usually between 12 and 50.
To calculate the number of chambers required, simply divide the amount of sludge to be dewatered by the volume of a chamber, taking into account the density of the sludge and the concentration that can be achieved with the filter. This concentration value is obtained in laboratory tests and depends on each type of sludge.
These fabrics are normally made of plastic materials (PP, polyester, polyamide, etc.). These fabrics are permeable to the passage of liquid and act as filters retaining suspended solids.
The sludge concentration increases with increasing pressure, up to the design limit obtained in the laboratory.
A collector is a part of the filtration system that is used to separate solids from liquids in the filtration process. In the context of a filter press, the collector may refer specifically to the system or components that collect the filtrate (the liquid that has passed through the filter media) after the suspension (the mixture of solid and liquid) has been pressed through the filter plates.
We develop an example with the following data:
We calculate the quantity of DM and liquid, based on the daily flow of discharges:
In summary, the u of sludge generated from cake is obtained as follows:
Let us assume that the filtration system operates 8 h/day and that a filtration process (including filling, filtering, opening and various options) takes 4 h. Under these conditions, 2 filtration operations will be necessary to dewater the sludge produced in one day, and each drying will produce = 785.7/2 = 392.9 liters of sludge at 30 % dryness.
Filter press volume = 785.7 liters/2 = 392.85 liters.
To determine the number of plates needed, we start from:
The number of plates for each drying is = 393 liters / 19.7= 19.9, i.e. 20 chambers. As the number of filter plates is n+1 chambers => 20 + 1 =21 filter plates. And the filtering surface of the filter press will be = 20 x 1,53 m = 30,3m2
The liquid sludge stream enters the chambers through a manifold from which a feed to each chamber is derived. This sludge is propelled into the chambers by a pneumatic pump.
The drained water is collected on the opposite side once it has been filtered. In this way, the filter cloths and the accumulating sludge itself facilitate the increase in pressure, the increase in concentration and the formation of sludge cakes.
The feed sludge must fill the filter chambers quickly in order to form uniform cakes (time approx. 5 - 6 minutes). For this reason, it is common to have low pressure pumps for support.
Depending on the type of solid to be separated, a specific flocculant, or lime slurry, is dosed into a sludge tank beforehand. In some cases, coagulant reagents, such as Fe(OH)3 or PAC, are also added beforehand.
While flocculant consumption does not usually exceed 4 kg/mt DM, lime consumption reaches values of 15% of the sludge DM, which translates into a higher degree of dehydration, at the expense of higher sludge (DM) production.
As the sludge cake forms, the pressure inside the chambers increases until it reaches a point where it becomes asymptotic and the pneumatic pump slows down its own rate until it practically stops. At this point the circulation of drained water also stops, which is detected by a flow sensor. The end of the pressing cycle is detected by a pressure switch signal, which alerts the operator of the situation. It is at this moment that the sludge cakes are ready to be discharged.
The filter press assembly is usually mounted on an elevated structure, so that a container can be placed under the chamber package in which the pressed solids can be collected. These containers usually have an emptying device in case water or malformed cakes get inside. These filters are usually placed outside and have a canopy to prevent rain from reaching the sludge tank.
Unloading the cakes is very simple. You only have to decompress the electro-hydraulic group that keeps them closed, and then open them and separate them from each other so that they fall into the tank under their own weight.
In small filters (with plates up to 1 m), discharge can be done manually, or even with the help of a lever to separate them from the plates to which they may remain attached. For larger sizes, automatic unloading systems are used that move the plates and accompany the openings with vibrations or rattles that cause the cakes to fall. In any case, it is imperative to check that no cakes remain adhered, either totally or partially on the plates, to avoid incorrect sealing and sludge leakage in successive drying processes. A common solution is to pass air currents to further dry the cakes and ensure that all the water has been filtered out.
To optimize the collection of dry sludge, cake breaking systems are installed to reduce the volume occupied inside the tanks.
The maintenance of this type of filter is very simple, since it is limited to keeping the cloths associated with the plates in good cleaning conditions. For this purpose, they must be cleaned with a certain frequency, which implies a closed-circuit washing operation, or they must be disassembled and cleaned separately, and then reassembled once they have been cleaned.
There are several types of filter presses, each with its own specific characteristics and applications. Some of the most common types are described below:
Recessed chamber filters: The plate and frame filter press uses a series of filter plates and frames, while the recessed chamber filter press uses filter plates to form recessed chambers. The plate and frame design allows the fabric to be easily covered, while the recessed chamber design requires the fabric to be firmly attached to the plates. The choice between these two designs depends on specific application requirements, filtration capacity and other operational considerations.
In a conventional wastewater treatment plant, the sludge separated by a settler will have a concentration varying between 0.6 and 1.5 % DM, depending on whether it is a secondary or primary sludge. This sludge concentration is too low to be dried in a filter press, so the installation of an intermediate thickener is necessary.
A thickener is a large and costly piece of equipment that requires a tank, mechanical systems and suitable transfer pumps. The sludge thickened in this equipment usually reaches concentrations of 3% to 4% DM, which is then considered suitable for treatment by mechanical drying.
This sludge is stored in an accumulation tank and introduced into the filter press by means of a pneumatic pump, after the addition of flocculant and coagulant reagents, if necessary.
If instead of sedimentation clarification equipment, the installation of a dissolved air flotation (DAF) system is chosen. a dissolved air flotation (DAF) system, the intermediate thickening system isThe intermediate thickening system is dispensable, since the sludge obtained by DAF has between 4% and 5% DM. A DAF system offers the following advantages:
If we make an economic balance, there is no great difference in investment and consumption between the two systems, since the DAF requires additional equipment for the pressurization and depressurization of the air provided.
Flow diagram of DAF + filter press system for sludge separation and drying
The solid wastes generated during wastewater treatment must be extracted, treated and dehydrated to reduce their volume to the maximum, reaching at least a level of reduction at which they can be accepted by an authorized landfill. For this purpose, there are several technologies, among which mechanical drying processes stand out, and among them, drying by means of filter presses, since it provides a high degree of dryness at a low energy and reagent cost.
An important point to take into account is that the sludge to be treated must arrive conditioned to the filter press, by adding the appropriate coagulant and flocculant reagents, and at a minimum concentration that makes the process viable (3% - 5%).
Sludge from sedimentation clarification systems rarely reaches these concentrations, so intermediate thickening treatment is required.
An effective alternative, which saves space and costs, and achieves a greater reduction of pollutants in wastewater, is dissolved air flotation (DAF), in which Sigmadaf has proven experience.
The sludge extracted from the DAF already has the minimum concentration necessary to be directly introduced into the filter press for dewatering.
What Is The Difference Between A Plate And Frame Filter Press And A Built-In Chamber Filter Press?
Filter Presses, the best alternative to sludge dewatering
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