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Filter Presses
Slurry Dewatering System
Find out more about filter press and slurry dewatering system here at FFP Systems Inc. We are a company in Mississauga, ON that provides an extensive selection of industrial filtering systems. Continue reading through our website to learn more about the filter press feed cycle and its important phases.
Sludge dewatering using filter presses has become accepted as a dependable and effective method of dewatering effluents and sludge from industrial and municipal waste treatment processes. Some typical applications include:
- Brine Sludge
- Metal Hydroxide Sludge
- Oily Sludge
- Secondary Biological Sludge
- Water Treatment Alum Sludge
One of the most difficult problems today is the disposal of sludge in waste treatment. Dewatered sludge from traditional dewatering equipment (i.e., rotary vacuum drum filters, centrifuges and belt presses) are less acceptable for disposal in landfills, and thanks to their high moisture content, they're not economically feasible. The filter press process leads to drier sludge that has proven to be an efficient solution to the present problem.
For example – sludge (such as alum sludge and waste activated sludge) that were previously considered difficult to dewater on traditional equipment can now be dewatered in a filter press sufficiently to supply a tough, dry, easily handled and autogenous material for incineration.
The filter press fundamentally comprises various chamber filter plates (also referred to as recessed filter plate pack) mounted vertically on and between two sidebars or suspended from an overhead support shaft. The support beams or sidebars are linked toward one side to a fixed head, otherwise called a feed head, and at the opposite end to an end head. Through a closing arrangement on the end head, generally a hydraulic slam, the recessed filter plate pack is compacted firmly together between the fixed feed head and a third head known as the moving head. This forms a compact filtering unit utilizing recessed chamber filter plates or flush plates and frames.
The filter cake chambers are shaped in both of two different ways: by mating two recessed chamber plates or by two flush plates with a cake outline (much like a photo frame) sandwiched in the middle.
The two faces of the filter plate feature a drainage surface as ribs, grooves or pips to permit filtrate to drain behind the fabric to the drainage ports situated on each side of the plates. These ports, thus, linked with the corner eyes, which transfer the filtrate drainage to the fixed end of the press.
Filter material is mounted over every one of the two faces of the plate. The fabric is joined at the feed eye by an impenetrable sleeve or cylinder otherwise called a barrel neck.
Filter Press Feed Cycle
Stage 1
Sludge is fed into the filter press by an appropriate pumping method and goes through the feed eye of the next plates along the length of the plate pack until all chambers are brimming with slurry. This is known as the fast fill portion of the filter cycle.
Stage 2
As the solid particles stream under pressure, they start to store on the outside of the filter fabric forming the first layer of the filter cake often called as the pre-coat. This then becomes the actual filtering medium when applied.
Stage 3
The cake thickness slowly increases as the filtering process continues. Later on, the adjacent filter cake in each chamber will bridge. During this phase of the cycle, the dewatering stage goes into the final cake consolidation to attain concentrated cake dryness (refer to figures 1 to 6).
 Figure 1 |  Figure 2 |  Figure 3 |
 Figure 4 |  Figure 5 |  Figure 6 |
Final Cake Consolidation
Extra solids are being pumped into the filter chamber during the consolidation step. The result of this is a dryer, firmer and denser filter cake. This cycle typically proceeds until the liquid flow has been reduced to virtually nil. Now, the feed pump is stopped and the internal pressure inside the plate is relieved.
Figure 7
Before releasing the filter cake, the cake might be further washed in place for impurity extraction or neutralization purposes and/or the cake might be blown with air to remove free moisture and to dry the filter cake (see Section 9 for further explanation).
Figure 8
The hydraulic closure system is actuated to withdraw the moving head to its completely open position once the pressured is relieved inside the filter plate pack. When open, every filter plate is isolated from its neighbor to permit the filter cake to fall freely from between the isolated chambers. The plates might be moved manually or by a semi or completely motorized equipment depending on the size of the press.
Figure 9
Mechanized filter presses are usually used from sludge dewatering because of the huge number of pressing cycles required in a specified timeframe.
The transport speed is customizable using completely mechanized plate shifting devices. But this only averages six to eight plates per minute.
The cycle is finished once the filter cake has been completely released. By this time, the press hydraulic system will close off and the filter press will go into standby mode until another cycle is prepared to start.
To begin the next filter cycle, the hydraulic closure system is initiated to stretch out the moving head back to its closed position. Subsequently, the total filter plate pack is pushed back and sealed in an operating position. Now, the press is ready to begin another cycle.
The basic filter press dewatering system consists of the filter press, sludge feed pumps, sludge storage and/or sludge conditioning tanks. The latter includes slow speed mixers, sludge transfer pumps, chemical feed, storage equipment, necessary piping, valves and controls. There are two kinds of filter press systems. These are:
- High Pressure (For Example 225 PSI or 16 Bar)
- Low Pressure (For Example 100 PSI or 7 Bar)
Numerous sludge-dewatering filter press systems are intended for automatic system operation from the main control board. A basic flowchart is illustrated in Figure 10.
Figure 10
During the first step of the filter press dewatering cycle, the filtrate flow from a huge press can be as much as 10,000 to 20,000 gallons for every hour. This part of the process is usually called the fast fill. During this phase, the cake chambers of the filter press gather a significant amount of sludge solids. As the chambers have increasingly been filled with sludge solids, the pressure inside the filter press plate pack rises and the filtrate flow quickly declines.
By the end of the cycle, the filtrate flow steadily drops to virtually nothing. This part of the cycle is called the cake consolidation step. During this phase, more sludge solids are being forced under tension into the cake chambers, which thus displaces more water from the loosely formed sludge cake. This allows the filter press to produce harder and drier cakes than other strategies for dewatering.
Factors affecting cycle time are the type of sludge and sludge feed concentration. Generally speaking, the higher the sludge feed concentration, the shorter the cycle time.
An average cycle duration for a typical metal hydroxide sludge would be around one to three hours, but it can be as much as four to six hours for more difficult sludge like an alum sludge.
In certain applications like base metal concentrate slurries, where the feed concentration can be as high as 40% to half by weight, the cycles take only 5 to 10 minutes.
Typical results from the filter press are expressed in percent cake dry solids by weight in Table 1 below.
A wide scope of sludge can be successfully dewatered at pressures as low as 7bar (100 PSI). For difficult applications like Biological water-activated sludge, alum sludge, or where maximum cake dryness is required, high-pressure 16bar (225 PSI) filter presses are commonly used.
Filter Cloths
The filter cloths are woven fabrics using monofilament or multifilament synthetic fibers or a blend of both. The most regularly utilized materials are polypropylene, polyester and nylon. Filter cloths are chosen for filtration, strength and cake release properties. With the best possible material choice, good cloth life (ordinarily 1500 to 1800 cycles) and good cake release from the filter cloth can be anticipated.
Filter cloth style usually depends on the kind of filter plate chosen for the application. Shown below are two kinds of filter plates. Caulked and gasket recess filter plate (Figure 11) is the type where a cord is sewn into the filter cloth to protect the fabric into the cloth-retaining groove in the filter plate. On the other hand, recessed filter plate (Figure 12) is where the filter cloth is secured by cloth dog pins or electrical ties.
 Figure 11 |  Figure 12 |
Numerous kinds of industrial sludge like metal hydroxide sludge can be dewatered in a filter press with no pre-treatment other than that required for pH correction.
In many cases, biological waste-activated sludge, where a huge level of organic materials is available, require chemical treatment or conditioning to render the sludge suitable for dewatering in a filter press.
Efficient sludge conditioning or pre-treatment is characterized by any modification to the raw sludge that adequately reduces viscosity, resistance to flow, increased filtration rates or increased solids concentration in the sludge feed. These changes can be made physically or chemically.
Chemically conditioning the sludge includes coagulation and flocculation of the sludge fine particles to create a filterable floc structure.
The physical procedures for conditioning sludge are heat treating and freezing.
A third technique for rendering sludge more filterable is the introduction of bulking agents: filter aid (diatomaceous earth), fly ash or any other solid, which would give porosity in the sludge cake to improve filterability.
Chemical conditioning is proven to be the most economical when costs of capital equipment, conditioning agents, transportation and treatment of conditioning agents are considered. This is also the most utilized method of filtration treatment.
Essential coagulants of the trivalent salt arrangement tend to produce the most stable flocs for filter pressing. High molecular weight polyelectrolytes can be used with good outcomes through proper pH control.
Good floc stability and strength are important. A weak floc will easily collapse under filtration pressure or will shear within the filter press and impair filtration performance.
Commonly used conditioners for sludge treatment are: (a) Ferric Chloride, (b) Alum, (c) Lime and (d) Polyelectrolyte.
Conditioning Agents
Ferric Chloride
Ferric chloride is usually delivered in fluid form (39% solution) and kept in corrosion-resistant storage tanks. Water is regularly added and mixed to give a diluted solution of around 10% to 15% strength.
Alum
Alum is usually delivered in fluid form (48% solution) and kept in corrosion-resistant storage tanks. Water is regularly added and mixed to give a diluted solution of around 10% strength.
Lime
Typically, lime would be in a slaked or hydrated form. This is normally kept in bulk (lime silo) and/or dispensed out of 50 lbs. bags. Estimated amounts are then fed to a tank where a 10% strength milk of lime mixture is made by adding water and mixing thoroughly.
Polyelectrolyte
Polyelectrolyte (polymer) can be provided in three different ways: a dry powder, a liquid solution, or a liquid emulsion form. The polymer must be diluted and blended before use. The mixed stock (neat) solution at 0.1% to 0.2% strength is kept in a corrosion-resistant tank and afterward can be diluted at the point of application.
Usually, most types of sludge can be treated with any of the abovementioned or a blend of conditioners. However, a specific sludge will promptly react to a particular conditioner or a mix of conditioners to give ideal dewatering properties.
Treatment chemicals can be added to the sludge through various techniques.
- Conditioning agents can be added immediately to the sludge-conditioning tank.
- Properly controlled polymer can be included at the suction side of the filter press feed pump. This strategy is also an effective technique of introducing the treatment chemical in the "inline" prior the filter press.
- The preferred technique includes introducing the conditioning agent directly into the line and mixed with the sludge by a static inline mixer prior to the conditioning tank.
The size of the conditioning tank is another significant thing to be considered. The detention time in the sludge tank and tank mixer selection has an impact on the floc's structure and strength. This must be considered when measuring the tank and mixer so as not to give excess shear on the sludge floc because of prolonged detention time and mixing. Usually, a tank size with a limit of 20 minutes pumping at the fast fill rate is chosen. The tank mixer should be at the low speed with a low shear impeller intended for a maximum tip speed of under 500 ft./minute.
A quality pumping system doesn't impart excess shear or deteriorate the sludge particles or chemically formed floc. It is significant in the structure of a filter press dewatering system.
The feed pump’s qualities, required for appropriate filter press operation, should be capable of delivering high volume/low pressure at first, and afterward a declining low volume/consistent high pressure during the last phase of the filter cycle.
The four most basic kinds of pumps utilized are:
- Ram or Piston Pumps, Usually Hydraulically Driven
- Double Diaphragm Pumps, Air Driven
- Piston Membrane Pumps
- Progressive Cavity Pumps With Speed Control
The pumps of the first three types should consistently incorporate a surge suppression tank mounted on the discharge side of the pump to dampen pressure surges on the filter.
Pre-coating the filter press with a filter aid (like diatomaceous earth) is usually not required when dewatering in a filter press at low pressure. Given that the initial fill rate to the filter press is appropriately chosen with the correct feed pump system, then the impingement of the solids in the filter cloth interstices causing cloth blinding. This is a normal process of the filter press and doesn't present a problem.
With the more difficult waste activated sludge like the oily and gelatinous type, a pre-coating might be needed to protect the filter cloth from premature blinding and to promote a more economical (shorter) press cycle. The pre-coat also guarantees better sludge cake release from the filter cloth during the cake discharge process.
Pre-coating with a filter aid needs the preparation of the pre-coat solution in a pre-coat tank. This arrangement must be recalculated through the filter channel and back to the tank until the whole filter has been stored on the fiber cloths. This procedure usually takes 10 to15 minutes.
When pre-coating huge capacity filter presses, provisions for double-feeding (feeding from both ends) should be considered to guarantee a uniform pre-coat throughout the filter.
The filter press is regularly elevated to permit the filter cakes to fall freely. The cakes can be dealt within one of the following ways for removal to landfill or incineration.
- Cakes Discharge Into Trucks
- Cakes Discharged Into Conveyors (Screw, Belt, or Drag Chain) Under the Press For Transportation Elsewhere
- Cakes Discharged Into Dumper Boxes (i.e. Dumpster Type for Truck Pick Up)
The third strategy referenced is commonly utilized within bigger plants where the amount of sludge cake or end disposal technique justifies the additional cost.
Recessed plate filter press has become more sophisticated (with completely mechanized plate shifters, automatic drip trays, gasket style plates, etc.). Filter presses are being specified instead of pressure leafs and pressure rounded tubular type filters for process polishing applications.
Pre-coated and properly operated, a filter press will create an effluent quality equal to any other conventional liquid/solid separation device.
The filter press offers a few essential and operational perks.
- Simple to operate and maintain. Very few moving parts. Filter elements and media are easily accessible.
- No unfiltered heel at end of cycle. No fluid to reprocess. All heel is blown forward as filtrate. No danger of dropping filter cakes during heel blowback.
- Thorough cake washing or extraction can be accomplished very simply, again with no danger of cake sloughing during emptying process fluid and refilling with wash liquor.
- No pressure vessel to contend with. Recessed plates form their own pressure chamber.
- Cake can be discharged wet or dry, as desired.
- Cake may be either blown dry with air or a cake consolidation cycle can be run for maximum cake dryness.
- No exotic materials of construction. In most standard filter presses, all wetted parts are polypropylene. For high temperature applications 90 degrees C (194 °F) and above, FRP and metallic plates can be provided.
- No danger of damaging presses due to “bridging” of plates.
- The entire operation may be fully automated.
- Very cost-competitive compared to tank type pressure leaf, pressure tube or vacuum filters.
The third strategy referenced is commonly employed within bigger plants where the amount of sludge cake or end disposal technique justifies the additional cost.
Typical Polishing Applications
- Carbon Removal From Corn Syrups
- Edible Oils (for Bleaching Clay or Nickel Catalyst Recovery)
- Evaporated 50% Caustic
- Saturated Brine for Chlor-Alkali Production
- Swarf Removal From Cutting Fluids
- Various Acids (Sulfuric, Acetic, Hydrochloric)
| Impurities can be extracted or leached from the filter cake by washing it after it has been formed. The washing medium, water or other liquor/solvent, is forced through the filter cake by pumping at a pressure marginally over the terminating filter pressure. For best washing results, the filter cake should be of uniform thickness and consistency. There are two sorts of washing techniques: simple washing and thorough washing. In simple washing, the wash liquor is introduced to the filter in the same direction as the feed, entering directly into the slurry inlet. Simple washing is usually the best strategy to practice when there is a wide range of particle size distribution or when the frame or cake chamber isn't completely filled with filter cake. In thorough washing using recessed chamber plates, the washing liquor is fed into one of the filtrate outlet connections on the filter press and goes into the drainage area behind the cloth of the washing plate. It is then forced to stream back through the filter cloth of the wash plate, through the cake and the filter cloth of the adjacent non-wash plate and afterward discharges from the non-wash plate (Refer to figure 13). |  Figure 13 |
 Figure 14 | A filter using flush plates and frames set up for thorough washing has separate slurry feed and wash feed ports, and either the same or separated discharge ports for filtrate and wash liquor outlet. During thorough washing, the slurry feed inlet and wash plate outlets are closed, thus forcing the wash liquor to enter the washing plates to stream back through the filter cloth, through the cake, through the cloth of the adjacent non-washing plate, and discharge from the outlet of this plate (Refer to figure 14). |
Blowing the filter cake with air or some different gas can be used to diminish the free moisture held in the filter cake or the filtrate outlet channels. As in the washing operation, air blowing can be achieved through simple or thorough blowing.
Wash plates are normally marked by three buttons on the upper outside corner or with one button in the non-wash plates. In a plate and frame style filter, the cake frame is marked with a two-button designation.
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FFP Systems Inc.
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