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A BASIC INTRODUCTION TO FILTER PRESSESFOR SLUDGE DEWATERING, FILTRATION

AND PROCESS FLUID POLISHING

The intention is to provide an introduction to Filter Press technology as it applies to sludge dewatering and filtration and providethe basic facts for an understanding of these processes.IntroductionThe Filter PressBasic Sludge Dewatering System And PerformanceSludge ConditioningFeed Pumps For Filter PressesPrecoating The Filter PressHandling Dewatered Filter CakesProcess Fluid PolishingFilter Cake Washing And Blowing
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es ng on s e n our a . weare able to offer varioustypes of filter press pilot

press units for rent.

Introduction

Sludge dewatering using fi lter presses has become accepted as a reliable and efficient method of dewatering eff luents andsludges from industrial and municipal waste treatment processes. Some typical applications include:

Metal hydroxide sludgeBrine sludgeSecondary biological sludgeWater treatment alum sludge

Oily sludge

One of the most difficult problems today is the disposal of sludges in waste treatment. Dewatered sludges from traditionaldewatering equipment, (i.e. rotary vacuum drum filters, centrifuges and belt presses), are less acceptable for disposal inlandfills and due to their high moisture content they are not economical feasible. The filter press process results in drier sludgethat has proven to be an effective solution to this problem.

For example sludges (such as alum sludge and waste activated sludge) that were previously considered difficult to dewater ontraditional equipment can now be dewatered in a fi lter press sufficiently to produce a hard, dry, eas ily handled and autogenousmaterial for incineration.

The Filter Press

The filter press basically consists of a number of chamber filter plates (also referred to as recessed filter plate pack) mountedvertically on and between two sidebars or sus pended from an overhead support beam. The support beam or s idebars areconnected at one end to a fixed head, also known as a feed head, and at the other end to a closing head. Through a closingarrangement on the closing head, usually a hydraulic ram, the recessed filter plate pack is compressed tightly together betweenthe fixed feed head an a third head know as the moving or follower head, thus forming a compact filtering unit using recessedchamber filter plates or flush plates and frames.


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The filter cake chambers are formed in either of two ways: by mating two recessed chamber plates or by two flush plates with acake frame (much like a picture frame) sandwiched in between.

The two faces of the filter plate have a drainage surface in the form of ribs, grooves or pips to allow filtrate to drain behind thecloth to the drainage ports located in each corner of the filter plates. These ports, in turn, connect to the corner eyes, whichcarry the filtrate drainage to the fixed end of the filter press.

A filter cloth is mounted over each of the two faces of the filter plate. The cloth is joined at the feed eye by an impervioussleeve or tube also known as a barrel neck.

Filter Press Feed Cycle

Step 1

Sludge is fed into the filter press by a suitable pumping system and passes through the feed eye of the succeeding plates alongthe length of the plate pack until all chambers are full of slurry. This is known as the fast fill portion of the fi lter cycle.


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Step 2

Flowing under pressure, the solid particles begin to deposit on the surface of the filter cloth forming the initial layer of filtercake referred to as the pre-coat. Once applied, this pre-coat layer becomes the actual filtering medium.

Step 3

As filtering continues the cake thickness gradually increases, until the adjacent filter cake in each chamber touch or bridge. Atthis point of the filter cycle, the dewatering phase enters into final cake consolidation to achieve maximum cake dryness (Referto figures 1 to 6).

Figure 1 Figure 2

Figure 3 Figure 4


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Figure 5 Figure 6

Final Cake Consolidation

During the consolidation step of the cycle, additional solids are being pumped into the filter chamber to displace more liquid.This results in a dryer, firmer and denser filter cake. Thi s cycle normally continues until liquid flow from the press is reduced to

virtually nil. At this point, the feed pump is stopped and the internal pressure within the filter plate pack is relieved.

Figure 7

Prior to discharging the filter cake, the cake may be further washed in place for extraction of impurities or neutralizationpurposes and/or the cake may be blown with air to further remove free moisture and to dry the filter cake (See Section 9 forfurther explanation).


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Figure 8

With pressure relieved within the filter plate pack, the hydraulic closure system is actuated to retract the moving head to itsfully open position. Once open, each individual filter plate is separated from its adjacent neighbor allowing the filter cake to fallfreely from between the separated chambers. Depending upon the size of the press , the plates may be moved manually or by asemi or fully mechanized plate-shifting device.

Figure 9

In most cases mechanized filter presses are preferred for sludge dewatering due to the large number of pressing cycles requiredin a given period of time.

With fully mechanized plate shifting devices, the transport speed is adjustable, but averages 6 to 8 plates per minute.

After the filter cake has been fully discharged the filter cycle is complete. At this point the press hydraulic system will shut offand the filter press will go into the stand-by mode until another fi lter cycle is ready to begin.


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To begin the next filter cycle, the hydraulic closure system is activated to extend the moving head back to its closed position.As a result, the complete filter plate pack is pushed back and sealed in an operating position. At this point, the filter press isready to begin another cycle.

Basic Sludge Dewatering System And Performance

The typical filter press dewatering system includes the filter press, sludge feed pumps as well as sludge storage and/or sludgeconditioning tanks with slow speed mixers, sludge transfer pumps, chemical feed, storage equipment, necessary piping, valves,and controls. Two types of filter press systems are available:

a) High Pressure (i.e. 225 PSI or 16 Bar)b) Low Pressure (i.e. 100 PSI or 7 Bar)

Many sludge-dewatering filter press systems are designed for automatic system operation from a main control panel. A typicalline flow diagram is shown in Figure 10.

Figure 10

During the initial step of the filter press dewatering cycle, the filtrate flow from a large press can be as much as 10,000 to


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20,000 gallons per hour. This part of the cycle is often referred to as the fast fill. During this period the cake chambers of thefilter press collect the major amount of sludge solids. As the chambers become progressively more filled with sludge solids, thepressure ins ide the filter press plate pack rises and the filtrate flow rapidly decreases.

Towards the end of the cycle, the filtrate flow gradually drops to virtually nothing. This portion of the cycle is referred to as thecake consolidation step. During the cake consolidation step, more sludge solids are being forced under pressure into the cakechambers, which in turn displaces more water from the loosely formed sludge cake. This enables the filter press to produceharder and drier sludge cakes than other methods of dewatering.

Factors effecting cycle time are: the type of sludge and s ludge feed concentration. Generally speaking, the higher the s ludgefeed concentration, the shorter the cycle time.

An average cycle time for a typical metal hydroxide sludge would be in the range of 1 to 3 hours, but can be as much as 4 to 6hours for more difficult sludges such as an alum sludge.

In some applications, i.e. base metal concentrate slurries, where the feed concentration can be as high as 40% to 50% by weight,the cycles can be as short as 5 to 10 minutes.

Typical results from the filter press are expressed in percent cake dry solids by weight in Table 1 below.

TYPE OF SLUDGE % CAKE SOLIDS

Biological waste activated sludge 30% to 40%

Metal hydroxide sludge 30% to 55%

Alum water treatment sludge 25% to 35%Oil refinery sludge 40% to 50%

Lime Sludge 30% to 60%

Brine Sludge 60% to 70%

A wide range of sludges can be effectively dewatered at pressures as low as 7bar (100 PSI). For more difficult applications likeBiological water activated sludge, alum sludge, or where maximum cake dryness is required, high-pressure 16bar (225 PSI) filterpresses are generally utilized.

Filter Cloths

The filter cloths are woven fabrics using monofilament or multifilament synthetic fibers or a combination of both. The most

commonly used materials are polypropylene, polyester, and nylon. Filter cloths are primarily selected for fi ltration, strength, andcake release properties. With the proper cloth selection, good cloth life (typically 1500 to 1800 cycles) and good cake releasefrom the filter cloth can be expected.

Filter cloth style is typically dependent on the type of filter plate selected for the application. Illustrated below are two types offilter plates; Caulked and gasket recess filter plate (Figure 11) where a cord is sewn into the filter cloth to secure the cloth intothe cloth-retaining groove in the filter plate. Recessed filter plate (Figure 12) where the filter cloth is secured by cloth dog pinsand/or electrical ties.


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Figure 11 Figure 12

Sludge Conditioning

Many types of industrial sludge, such as metal hydroxide sludge, can be dewatered in a filter press without any pretreatmentother than that required for pH correction.

In most instances, biological waste activated sludge (sludges) where a large percentage of organic materials are present, requirechemical treatment or conditioning to render the s ludge suitable for dewatering in a f ilter press.

Effective sludge conditioning or pretreatment is defined by any changes to the raw sludge that effectively reduce viscosity,resistance to flow, increase filtration rates or increase solids concentration in the sludge feed. These changes can be madechemically or physically.

Chemically conditioning the sludge involves coagulation and flocculation of the sludge fine particles to produce a filterable flocstructure.

The physical means of conditioning sludge include processes such as heat treating and freezing.

A third method of rendering sludge more filterable is the introduction of bulking agents; filter aid (diatomaceous earth), fly ashor any other solid, which would provide porosity in the sludge cake to enhance filterability.

When considering costs such as capital equipment, conditioning agents, transportation and handling of conditioning agents.Chemical conditioning, in most cases, will prove to be the most economical and by far is the most generally used method oftreatment.

Primary coagulants of the trivalent salt classification tend to produce the most stable flocs for filter pressing, although withproper pH control, high molecular weight polyelectrolytes can be used with good results.

Good floc stability and strength is of the utmost importance. A weak floc will collapse easily under filtration pressure or willshear 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


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a) Ferric Chloride

Ferr ic chloride is normally delivered in liquid form (39% solution) and stored in corrosion-resis tant storage tanks . Water isnormally added and mixed in to g ive a diluted solution of approximately10% to 15% strength.

b) Alum

Alum is normally delivered in liquid form (48% solution) and stored in corrosion-resistant storage tanks. Water is normally addedto give diluted solution of approximately 10% strength.

c) Lime

Normally lime would be provided in a slaked or hydrated form. Stored in bulk (lime silo) and/or dispensed out of 50 lbs bags.Measured quantities are then fed to a tank where a Milk of Lime mixture of about 10% strength is made by adding water andmixing thoroughly.

d) Polyelectrolyte

Polyelectrolyte (polymer) can be provided in three ways: a dry powder, a liquid solution or in a liquid emulsion form. The polymermust be diluted and mixed before use. The mixed stock (neat) solution at 0.1% to 0.2% strength is stored in a corrosion-resistant tank and then can be further diluted at the point of application.

Generally, most kinds of sludge can be treated with any of the above or a combination of conditioners, but a particular sludge

will often respond more readily to a specific conditioner or a combination of conditioners to provide ideal dewatering properties.

TYPE OF SLUDGEFERRICCHLORIDE

ALUM LIME POLYMER

Biological Waste Activated X X

Biological Waste Activated X X

Biological Waste Activated

Oil Refinery X X

Water Treatment Plant -Alum

X

Mineral - Clay X

Treatment chemicals can be added to the sludge using different methods.

Conditioning agents can be added directly to the sludge-conditioning tank.Properly controlled Polymer can be added at the suction s ide of the fi lter press feed pump. This method is also aneffective method of introducing the treatment chemical in the inline prior to the filter press.The preferred method involves introducing the conditioning agent directly into the line and blended with the sludge by astatic inline mixer prior to the conditioning tank.

The size of the conditioning tank is another important item to be considered. The detention time in the sludge tank and tankmixer selection has an influence on the flocs structure and strength. This must be considered when sizing the tank and mixer soas not to im art excess shear on the slud e floc due to rolon ed detention time and mixin . Normall a tank size with a


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maximum of 20 minutes pumping capacity at the fast fill rate is selected. The tank mixer should be low speed with a low shearimpeller designed for a maximum tip speed of less than 500 ft./minute.

Feed Pumps for Filter Presses

A proper pumping system which does not impart excess shear, or deteriorate the sludge particles or chemically formed floc isvery important in the design of a filter press dewatering system.

The feed pumps characteristics, required for proper filter press operation, should be capable of delivering high volume/lowpressure initially, and then a declining low volume/constant high pressure during the latter portion of the fi lter cycle.

The four mos t common types of pumps employed are:

Ram or piston pumps, usually hydraulically drivenDouble diaphragm pumps, air drivenPiston membrane pumpsProgressive cavity pumps with speed control

Pumps of the first three categories should always include a surge suppression tank mounted on the discharge side of the pump todampen pressure surges on the filter.

Precoating the Filter Press

Pre-coating the filter press with a filter aid (i.e. diatomaceous earth) is normally not required when dewatering in a filter pressat low pressure. Provided that the initial fill rate to the filter press is properly selected with the correct feed pump system, thenthe impingement of the solids in the filter cloth interstices causing cloth blinding. This is a normal process of the filter press anddoes not present a problem.

With the more difficult waste activated sludges, such as the oily and gelatinous type, a pre-coating may be desirable to protectthe filter cloth from premature blinding and to promote a more economical (shorter) press cycle. The pre-coat also ensuresbetter sludge cake release from the filter cloth during cake discharge process.

Pre-coating with a fi lter aid requires the preparation of the pre-coat solution in a pre-coat tank. Recalculating this solutionthrough the filter and back to the tank until the entire filter aid has been deposited on the filter cloths. This process normallytakes 10 to15 minutes.

When pre-coating large capacity filter presses, provisions for double-feeding (feeding from both ends) the filter press should bestrongly considered to ensure a uniform pre-coat throughout the filter.

Handling Dewatered Filter Cakes

The filter press is normally elevated to allow the filter cakes to fall freely. The cakes can be handled in one of the following waysfor disposal to landfill or incineration.

Cakes discharge into trucksCakes discharged into dumper boxes (i.e. dumpster type for truck pick up)


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Cakes discharged into conveyors (screw, belt, or drag chain) under the press for transportation elsewhere

The third method mentioned is generally employed within larger plants where the quantity of s ludge cake or end disposal methodjustifies the added expense.

Process Fluid Polishing

Recently the recessed plate filter press has become more sophisticated (with fully mechanized plate shifters, automatic driptrays, gasket style plates, etc.). Filter presses are being specified in place of pressure leafs and pressure tubular type filters forprocess polishing applications.

Pre-coated and properly operated, a filter press will produce an effluent quality equal to any other conventional liquid/solidseparation device.

The filter press offers several inherent operating and maintenance advantages. Listed below are just a few of these advantages.

Simple to operate and maintain. Very few moving parts. Filter elements and media are easi ly accessible.No unfiltered heel at end of cycle. No fluid to reprocess. All heel is blown forward as filtrate. No danger of dropping filtercakes during heel blowback.Thorough cake washing or extraction can be accomplished very simply, again with no danger of cake s loughing duringemptying process f luid 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 hightemperature 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.Entire operation may be fully automated.Very cost competitive compared to tank type pressure leaf, pressure tube, or vacuum filters.

The third method mentioned is generally employed within larger plants where the quantity of s ludge cake or end disposal methodjustifies the added expense.

Typical Polishing Applications

Saturated brine for chlor-alkali productionEvaporated 50% causticEdible oils (for bleaching clay or nickel catalyst recovery)Swarf removal from cutting fluidsCarbon removal from corn syrupsVarious acids (sulfuric, acetic, hydrochloric)

Filter Cake Washing and Blowing

Impurities or soluble matter can be extracted or leached from the filter cake by washing the fi lter cake after it has beenformed. The washing medium, water or other liquor/solvent, is forced through the filter cake by pumping at a pressure slightly


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in excess of the terminating filter pressure. For best washing results, the filter cake should be of uniform thickness andconsistency.

There are two types of washing methods practiced; 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 slurryinlet. Simple washing is sometimes the best method to practice when there is a wide range of particle size distribution or whenthe frame or cake chamber is not 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 thefilter press and enters into the drainage area behind the cloth of the washing plate. It is then forced to flow back through thefilter cloth of the wash plate, through the cake and through the filter cloth of the adjacent non-wash plate and then dischargesfrom the non-wash plate (Refer to figure 13).

Figure 13

A filter using flush plates and frames set up for through washing has separate slurry feed and wash feed ports, and either thesame or separated discharge ports for filtrate and wash liquor outlet. During thorough washing the slurry feed inlet and washplate outlets are closed thus forcing the wash liquor entering the washing plates to flow back through the filter cloth, through thecake, through the cloth of the adjacent non-washing plate and discharge from the outlet of this plate (Refer to figure 14).


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Figure 14

Blowing the filter cake with air or some other gas can be used to reduce the free moisture retained in the filter cake or in thefiltrate outlet channels. As in the washing operation, air blowing can be accomplished either by simple blowing or thoroughblowing.

Wash plates are normally marked by three buttons on the upper outside corner or with one button in the non-wash plates. In aplate and frame style filter, the cake frame is marked with a two-button designation.

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