by BF Press · 2000 — Belt filter presses are used to remove water from liquid wastewater residuals and produce a non-liquid material referred to as “cake”.
7 pages
140 KB – 7 Pages
PAGE – 1 ============
United StatesEnvironmental ProtectionAgencyOffice of WaterWashington, D.C.EPA 832-F-00-057September 2000BiosolidsTechnology Fact SheetBelt Filter PressDESCRIPTIONBelt filter presses are used to remove water fromliquid wastewater residuals and produce anon-liquid material referred to as ficakefl.Dewatered residuals, or cake, vary in consistencyfrom that of custard to moist soil. Dewateringserves the following purposes:CReducing the volume, thus reducing storageand transportation costs.CEliminating free liquids before landfilldisposal.CReducing fuel requirements if residuals areto be incinerated or dried.CProducing a material which will havesufficient void space and volatile solids forcomposting when blended with a bulkingagent.CAvoiding the potential of biosolids poolingand runoff associated with liquid landapplication.COptimizing subsequent processes such asthermal drying.A belt filter dewaters by applying pressure to thebiosolids to squeeze out the water. Biosolidssandwiched between two tensioned porous belts arepassed over and under rollers of various diameters.Increased pressure is created as the belt passes overrollers which decrease in diameter. Many designsof belt filtration processes are available, but allincorporate the following basic features: polymerconditioning zone, gravity drainage zones, lowpressure squeezing zone, and high pressuresqueezing zones. Advanced designs provide a largefiltration area, additional rollers, and variable beltspeeds that can increase cake solids by five percent.The general mechanical components of a belt filterpress include dewatering belts, rollers and bearings,belt tracking and tensioning system, controls anddrives, and a belt washing system. Figure 1 depictsa typical belt filter press.APPLICABILITYBelt filter presses can be used to dewater mostbiosolids generated at municipal wastewatertreatment plants and are a common type ofmechanical dewatering equipment. Usingmechanical equipment to dewater solids may not bethe most cost effective alternative for wastewatertreatment plants operating at less than about 4 mgd.The selection of dewatering equipment should bebased on the results of a site specific biosolidsmanagement plan which identifies both processingand end use alternatives and estimates costs. It maybe less expensive to haul liquid to an applicationsite or pay a processing facility to dewater andprocess or landfill the dewatered cake. Smallerfacilities should also evaluate non-mechanicaldewatering methods, such as drying beds or reedbeds. ADVANTAGES AND DISADVANTAGESAdvantages and disadvantages of belt filter pressesfor dewatering wastewater solids are summarizedbelow:
PAGE – 2 ============
Source: U.S. EPA, 1987.FIGURE 1 SCHEMATIC OF A BELT FILTER PRESSAdvantagesCCStaffing requirements are low, especially ifthe equipment is large enough to process thesolids in one shift (USEPA, 1987).CMaintenance is relatively simple and canusually be completed by a wastewatertreatment plant maintenance crew.Replacing the belt is the major maintenancecost.CBelt presses can be started and shut downquickly compared to centrifuges, whichrequire up to an hour to build up speed(Henderson and Schultz, 1999).CThere is less noise associated with beltpresses compared to centrifuges (Hendersonand Schultz, 1999).DisadvantagesCOdors may be a problem, but can becontrolled with good ventilation systemsand chemicals, such as potassiumpermanganate, to neutralize odor-causingcompounds (Rudolf, 1992). Somemanufacturers offer fully enclosed equipment to minimize odors and reducevapors in the operating room air (Bain et al.,1999).CBelt presses require more operator attentionif the feed solids vary in their solidsconcentration or organic matter. Thisshould not be a problem if the belt pressesare fed from well-mixed digesters(Henderson and Schultz, 1999).CWastewater solids with higherconcentrations of oil and grease can resultin blinding the belt filter and lower solidscontent cake.CWastewater solids must be screened and/orground to minimize the risk of sharp objectsdamaging the belt.CBelt washing at the end of each shift, ormore frequently, can be time consumingand require large amounts of water
PAGE – 3 ============
(Henderson and Schultz, 1999). Anautomatic belt washing system and the useof effluent can minimize these costs.DESIGN CRITERIABelt presses are sized on the basis of weight orvolume of solids to be dewatered rather than thewastewater flow to the plant. To determine howmany presses are needed, the wastewater treatmentplant must:CDetermine the amount of primary solids thatwill flow through the plant per day. CDetermine the amount of waste-activated ortrickling filter solids produced per day.CDetermine the volume of thickened solids tobe dewatered per day.CEstimate the range of dry solidsconcentration in the feed.CEstimate future increases in solidsproduction.CAnticipate changes in sewer discharges oroperation that could change solids quality ororganic matter content.An effective biosolids management plan willinclude the above information. It is important todesign for excess capacity so that the anticipatedamount of incoming solids can be easily dewateredduring operating hours. Allowing for excesscapacity also ensures that the plant will notexperience a build-up of solids if a unit is out ofservice. If only one unit is required, the plantshould have an alternate program to remove solidsin liquid form.The polymer conditioning zone can be a small tank,approximately 265 to 379 liters (70 to 100 gallons)located 0.6 to 1.8 meters (2 to 6 feet) from thepress, a rotating drum attached to the top of thepress, or an in-line injector. The pressmanufacturer usually supplies this zone along withthe belt filter press (USEPA, 1986).The gravity drainage zone is a flat or slightlyinclined belt unique to each model. Solids aredewatered by the gravity drainage of the free water.A 5 to 10 percent increase in solids concentrationfrom the original biosolids should occur in this zone(USEPA, 1987). The free water drainage is afunction of wastewater solids type, quality,conditioning, screen mesh, and design of thedrainage zone.The low-pressure zone is the area where the upperand lower belts come together with the wastewatersolids in between. This is sometimes called thefiwedge zone,fl because the feed solids aresandwiched between the upper and lower belts.The low-pressure zone prepares the biosolids byforming a firm cake which can withstand the forcesof the high pressure zone.In the high-pressure zone, forces are exerted on thesolids by the movement of the upper and lowerbelts as they move over and under a series of rollersof decreasing diameter. Some belt filter pressmodels separate from the rest of the unit to increasepressure on the biosolids. This produces a driercake, an important factor for plants that incineratethe final product or face high end use or disposalcosts. A biosolids management plan shouldevaluate the advantages and disadvantages of a highperformance belt filter press.An additional design feature is a self-enclosedfacility to reduce odors and protect worker health(Bain et al., 1999). Workers in the belt press areasare exposed to aerosols from wash spray nozzlesand pathogens and hazardous gasses such ashydrogen sulfide. Enclosing the press reducesvisibility to the operators and produces a corrosiveenvironment for the rollers and bearings, butautomating the system can alleviate these problems.The automation of belt presses is the subject of aWater Environment Research Foundation project.Benefits of automation include optimization of non-linear variables which was rarely possible withmanual or semi-automated operation, and the abilityto produce dewatered cake at a constant rate.Automation generally increases capital costs by 10percent. Manufacturers claim that this extraexpense is worthwhile because it lowers labor costs,
PAGE – 4 ============
reduces polymer use, and maximizes the solidscontent of the cake, reducing disposal and end usecosts (Gillette et al., 2000). The choice of dewatering technique and chemicalpolymer or salts impacts dewaterability as well asthe potential for odor during further processing orrecycling to land. Ancillary equipment for efficient operation of a beltpress includes:CPolymer.CMixing, aging, feed, liquid feed day tank.CLiquid residuals feed pump.COdor control and ventilation.CConveyor and/or pump to move dewateredcake.CAn enclosed area to load trucks orcontainers.PERFORMANCEManufacturers should be consulted for design andperformance data early in the planning stage. Datashould be confirmed with other operatinginstallations and/or thrash pilot testing. Evaluationof equipment should consider capital and operatingcosts, including polymer, electricity, wash water,solids capture, and ventilation and odor controlduring dewatering and further processing orrecycling. The operator can ensure systemintegration by requiring that the self-enclosed beltpress, ventilation, and polymer system is suppliedby a single provider. Since solids characteristicsand quantity vary from plant to plant, it is importantto evaluate different weaves, permeability, andsolids retention abilities of dewatering belts toensure optimum performance. Surveys of similarplants or testing of wastewater solids can be helpfulin the decision-making process.Table 1 displays the range of performance of a highpressure belt press on various types of wastewatersolids.Odor ControlOdor complaints at wastewater treatment plants andbiosolids end use sites can interfere withimplementation of the most cost effective biosolidsmanagement options. Odor control measuresshould be included when designing dewateringfacilities. Odor control is addressed in more detailin another fact sheet, but briefly, the methodsinclude:TABLE 1 TYPICAL DATA FOR VARIOUS TYPES OF SLUDGES DEWATERED ON BELTFILTER PRESSESType of Wastewater SludgeTotal Feed Solids(percent)Polymer(g/kg)Total CakeSolids (percent)Raw Primary3 to 101 to 528 to 44Raw WAS0.5 to 41 to 1020 to 35Raw Primary + WAS3 to 61 to 1020 to 35Anaerobically Digested Primary3 to 101 to 525 to 36Anaerobically Digested WAS3 to 42 to 1012 to 22Anaerobically Digested Primary + WAS3 to 92 to 818 to 44Aerobically Digested Primary + WAS1 to 32 to 812 to 20Oxygen Activated WAS1 to 34 to 1015 to 23Thermally Conditioned Primary + WAS4 to 8025 to 50Source: U.S. EPA, 1987.
PAGE – 5 ============
Source: Dr. Peter Wright, Cornell University, 1996.FIGURE 2 DEWATERED SOLIDS CAKEDROPPING FROM BELT FILTER PRESSAFTER PROCESSINGŁUsing a self enclosed belt press.ŁAdding potassium permanganate or otheroxidizing agent to minimize odors in thesolids.ŁMinimizing liquid storage prior to beltpressing to less than 24 hours. The longerthe solids are stored, the lower the pH, thehigher the liquid ammonia concentration,and the higher the organic sulfide emissions(Hentz et al., 2000).ŁConducting bench-scale and full-scaletesting of liquid sludge to determine ifcombined storage of primary and wasteactivated sludges accelerates thedeterioration of biosolids (Hentz et al.,2000).ŁSpecifying polymers that are stable atelevated temperatures and pH. This isespecially important at facilities using limestabilization or high temperature processingsuch as heat drying, thermophyllicdigestion, or composting.Self-Enclosed Belt PressesThe main purpose of a self-enclosed system is tominimize the amount of foul air needing treatmentin an odor control system. An induced draft fanprovides a slight negative pressure (typically 100cubic feet per meter per meter of belt width.) Thesystem design should:CMinimize gaps in the enclosure.CMinimize enclosure volume.CLocate mechanical and electricalcomponents requiring maintenance outsidethe enclosed area for easy access andreduced maintenance.CInclude automation to optimizeperformance of the belt press.CUse stainless steel materials.CProvide multiple access hatches to allowoperator viewing and clean up. CProvide for easy removal of the belt forreplacement (Bain et al., 1999).Chemical AdditionSolids must be conditioned with polymer to ensureoptimum performance. Polymer feed points shouldbe designed at several locations to ensure flexibilityand optimum performance. The solids/polymermixture should be subject to gentle mixing asturbulent conditions can sheer the floc, minimizingpolymer effectiveness. Polymer dilution and agingsystems should be large enough to optimizepolymer usage.Potassium permanganate or other oxidizing agentsare often added to solids prior to dewatering. Thesehave been shown to reduce odors caused bysulfides, reduce the amount of polymer needed, andincrease cake solids content (Rudolf, 1992).Figure 2 shows dewatered solids from a belt filterpress after processing.OPERATION AND MAINTENANCEIt is important to monitor operating parameters toachieve optimum performance and ensure thatsolids are properly conditioned and that goodgravity drainage occurs. The manufacturer should
PAGE – 6 ============
provide operation and maintenance training afterinstallation as well as ongoing training to maintainskills.Dewatering belts should be designed for easyreplacement with minimum downtime. Beltwashing should occur daily after the cake isremoved.Replacement of filter belts is a commonmaintenance requirement. Belt life averages about2,700 running hours, with a range of 400 to 12,000hours.A belt press operator is responsible for polymermixing, dosing and monitoring usage, andobserving the feed and cake several times per day,making adjustments as necessary. Rollers andbearings require frequent lubrication. It is important for the operator to keep records of allpress performance parameters, including thevolume of biosolids fed to the press, polymerdosage, and potassium permanganate or otherchemical usage. A sample of the biosolids to thepress, cake discharge, and filtrate should be taken atleast once per shift and analyzed for total solids. Atthe end of each shift, the belt should be cleanedwith high-pressure wash water. Labor is relative toplant size. A plant with a single belt press needsfour to eight staff hours per day (including labtesting), whereas six to eight presses can beoperated with eight to ten staff hours per day.Large plants use less operating effort per belt press.Highly automated systems reduce laborrequirements, but require an instrumentationspecialist to maintain the system.COSTSCapital costs for belt filter presses vary with thesize of the equipment. Vendor estimates vary from$47,500 (0.5 meter belt, approximate capacity of500 dry pounds per hour) to $115,000 (1.5 meterbelt with approximate capacity of 1,625 dry poundsper hour). These estimates are based on a feedmaterial which is 5 percent solids. These prices donot include the cost of installation, shipping, orancillary equipment, such as flow control andcentrate management.Overall operation and maintenance costs range from$80 per dry ton of solids (DTS) to $200 per DTS.Typical polymer conditioning costs for belt filterpress dewatering range from $2.65 per milliongallons to $91.15 per million gallons, and average$24.38 per million gallons. Permanganate addsabout $1 per million gallons to the cost ofdewatering the biosolids. These costs vary widely,depending on the source of the residuals. Thepolymer costs for raw primary may cost $12 perDTS, but may be as high as $80 per DTS forresiduals that are difficult to dewater.REFERENCESOther Related Fact SheetsOdor Management in Biosolids ManagementEPA 832-F-00-067 September 2000Alkaline Stabilization of BiosolidsEPA 832-F-00-052 September 2000Land Application of BiosolidsEPA 832-F-00-064 September 2000In-Vessel CompostingEPA 832-F-00-061 September 2000Other EPA Fact Sheets can be found at thefollowing web address:http://www.epa.gov/owmitnet/mtbfact.htm1.Bain, R.E., Brady, P., and Torpey, P., 1999.fiExperience With 70+ Self-Enclosed BeltPresses and Thickeners.fl In Proceedings ofthe WEF/AWWA Joint Residuals andBiosolids Management Conference:Strategic Networking for the 21st Century.Arlington, VA. Water EnvironmentFederation.
140 KB – 7 Pages