GRIT HAPPENS â YOU DON'T KNOW WHAT YOU ARE MISSING

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generally provide the plant with adequate protection by removing 80-90% of the grit entering thetreatment plant.Other considerations in this cost analysis would include upstream screening requirements, maintenancerequirements, available space, and anticipated headloss/hydraulic gradient through the process and thelevel of protection to be afforded to downstream equipment and processes. Table 2 outlines importantrecommended guidelines to use when designing or evaluating a grit removal system.TABLE 2. <strong>GRIT</strong> REMOVAL SYSTEM CRITICAL DESIGN FACTORSDesign GuidelineDefine Design Requirements:ü Grit Particle Size Analysisü Settling velocity or Specific Gravityü Required System Removal Efficiencyü Screening RequirementsRequired Downstream Protection:ü Biological Processesü Sludge Processing EquipmentEvaluate Equipment:ü Removal Efficiency/Performanceü Equipment Design/Featuresü Space Requirementsü Headloss Requirementsü Cost: Capital, Installed, Operationalü Maintenance RequirementsEMERGING TRENDHeadworks screening and grit removal is the primary protection for all treatment processes andequipment in a wastewater treatment plant, yet it has been the most neglected part of the plant. Toimprove solids removal screen openings on influent screens have trended progressively smaller over thepast 10-15 years. Years ago screen openings were frequently 1’ and larger. Today, screens arecommonly supplied with ¼” openings. It is logical that improving grit removal processes to effectivelyremove incoming grit is becoming a higher priority in plant designs.Biological processes have evolved to become more efficient and produce a cleaner effluent inprogressively smaller areas. As plants move toward higher performing processes, effective grit removalbecomes more important. The growing acceptance of Membrane Bio-Reactor (MBR) technology bringsthe need for advanced grit management systems into consideration. MBR technology requires extensivescreening pretreatment which protects the membrane and often allows elimination of primary clarification.Without the protection of primary clarification, advanced grit removal should also be part of effective MBRpretreatment system design (Andoh 2009). Grit entering a MBR plant can cause abrasive wear anddamage the membranes, which is often the most expensive equipment in the plant.Fine Bubble aeration is not a particularly advanced treatment process yet when a plant upgrades fromcoarse bubble aeration to fine bubble aeration, and in the process eliminates their primary clarifier, theimpact of grit deposition increases for two reasons. First, the velocity in the aeration basin is reduced asis the deposit limit, making it the first likely location in the process train for grit to accumulate. Secondly,the deposited grit creates new maintenance challenges as diffusers cover the full floor of the basin whichrestricts the ability to clean them and makes the cleaning process more operator intensive and expensive.Diffusers covered with grit are less effective and additional horsepower may be required to achievedesired results.Today many plants are requiring increased effectiveness of their grit removal systems and their designengineers are taking a closer look at the size and settling velocity distributions of grit entering theseplants. Newer editions of design manuals are recommending targeting particles smaller than 212 µm ingrit system design. In 2009, WEF MOP #8 issued a new chapter on Grit Removal stating that in 2008, aGrit Happens – You Don’t Know What You Are Missing
WEF member survey reported grit density ranged from 1100 to 2200 kg/m3 (70 – 140 lb/ft 3 ) and averaged1,400 kg/m3 (90 lb/ft 3 ). Metcalf & Eddy states that the specific gravity of clean grit reaches 2.7 for inertsbut can be as low as 1.3 when a substantial amount of organic material is agglomerated with inerts. Bulkdensity and specific gravity of wastewater grit is lower than these factors are for clean silica sandindicating a shift in design considerations. Grit is not clean sand and many grit removal systems beingdesigned today are targeting removal of particles in the 75-106-150 µm size range in order to remove theparticles that cause abrasive wear and deposit throughout the process.CONCLUSIONMany installed grit systems fail to keep depositable grit out of the plant. In fact, they fail to remove thesizes and amounts of grit they were designed to capture. Grit system failure happens primarily due to afaulty assumption that municipal grit behaves like clean silica sand particles in clean water. The failure ofmany traditional grit removal systems has led to the misconception that grit removal systems cannotwork, and that the only option is to deal with grit deposits in process basins downstream of the headworksand to deal with the abrasive wear from grit, both increasing maintenance and operational budgets.In order to design an effective system, design guidelines should be more comprehensive examining sizeand settling velocity of endemic grit, impact grit will have on downstream equipment and processes, gritsystem efficiency, headloss, space, cost, etc. A clear understanding of the grit entering the plant includesgrit concentration, size distribution, effective specific gravity and/or settling velocity. Only with a clearunderstanding of the material to be removed can a system be designed to achieve specified results.When a complete characterization of the endemic grit is not available a conservative approach canprovide effective grit removal. Targeting particles in the 75-100 micron range will typically remove 80-90% of the grit entering the treatment plant.Effective grit management requires a system approach. All components of the system must be effective inorder for the overall system to yield the desired results. Improving grit collection only to lose a majorportion of it back to the process in the washing and dewatering step is detrimental to overall removalefficiency and reduces the value of the entire system. Capturing a high percentage of the incoming gritload along with a high concentration of organics yields a product that is both difficult and expensive tolandfill and can starve downstream biological processes. Each step of the process is important andshould be optimally designed.Grit systems can work as intended when designed with an accurate understanding of the nature andcharacteristics of the grit arriving at the treatment plant and how this grit actually behaves in wastewater.An effective system addresses size as well as settling velocity or SG and produces a clean, dry productfor landfill. The result is abrasive wear on equipment and deposits and accumulations in the plant areminimized. In turn, money is saved on maintenance and operational costs and equipment and processesperform better.REFERENCESWilson, G., Tchobanoglous, G., and Griffiths, J. (2007) The Grit Book. Eutek Systems, Inc.: Hillsboro,Oregon.Massart, N., O’Kelley, S., Neun, G. (2010) Sustainable Biosolids Handling – Digester Cleaning, FloridaWater Resources Conference. May 2010Herrick, P. (2009) A Portable Solution for Degritting Aeration Basins. Pollution Equipment News. January2009, pp 17-18.Griffiths, J. (2004) Fox Lake Regional WRF Grit Testing Results. Grit Solutions. May 7, 2004 and August19, 2003.Boldt, J. (2005) Eliminating Grit Deposition Problems through Objective Grit System Design, A CaseStudy at the Fox lake NRWRP – Fox Lake, IL. Illinois WEA Conference. January 2005.Grit Happens – You Don’t Know What You Are Missing
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GRIT HAPPENS â YOU DON'T KNOW WHAT YOU ARE MISSING