Fundamentals Of Granular Media Filtration

FUNDAMENTALS OFGRANULAR MEDIAFILTRATIONGordon Williams, PhD, PEEast Bay Municipal Utility District1

OVERVIEWFiltration basics Filtration theory Filter media selection and design Monitoring and Troubleshooting 2

FILTRATION BASICS ANDTHEORY3

WHAT IS FILTRATION?Definition: any process that removes suspendedparticles through a porous medium Types of Filters yGranular media filters (GMF)Slow sand filters Rapid depth filters yMembrane filters (MF/UF/NF/RO)4

TYPICAL FILTRATION PRE-TREATMENTFiltration NTUConventionalAnyDirect 15NTUContact 10NTU5

A TYPICAL GRAVITY DEPTH GMFComponents Flow control Media Underdrain andsupport Backwash systemPicture from MWH (2005) Water Treatment Principles and Design6

GRANULAR MEDIA TYPESConfiguration Mono media Dual media MultimediaTypes Silica sand Anthracite coal GAC Other (e.g. garnet)7

FILTER UNDERDRAINS AND SUPPORT8

UNDERSTANDING A FILTER RUNRipeningOperating TurbidityBreakthroughTerminal head lossClean bed head loss9Time to breakthrough9

SURFACE VS. RECYCLED WATER FILTRATIONTypicalSurface WaterRecycled WaterDriverssediment,pathogens, organicsolidspathogens,organic solidsPretreatmentAnydirect/contactSource Water1 to 100 NTU1 to 10 NTUTurbidity Req’d0.3 NTU/0.1 NTU2 NTUFiltration Rate(gal/ft2-min)3/6 (mono/dual)5 (up to 7.5)10

11FILTRATION THEORY

FILTRATION THEORY Modeled as 2-step process1.2. Particle transportParticle attachmentParticle transport mechanismsSedimentationy Interceptiony Brownian motion (diffusion)yStraining not desirable Overall removal is sum of allmechanisms Picture from Lawler and Benjamin 200612

PARTICLE TRANSPORT EFFICIENCYTransport efficiencydriven by: Filtration rate Media design Particle size Temperature Density (sed. only)Graph from Lawler and Benjamin 200613

ACTUAL DATA MATCHES UP WITH THEORY!MS2GiardiaCryptoE. ColiFrom Williams 200914

WHY IS CHEMISTRY SO IMPORTANT? Chemistryneeds to be right for particles to “stick” Both particles and media are naturally negativelycharged and thus repelled by each other For excellent filtration, the surface chemistry oftarget particles must be modified – coagulation!15

PARTICLE ATTACHMENT EFFICIENCYAttachment is driven bychemistry Attraction/repulsive forces London-van der Waals forcey Electrical double-layerinteractiony Born repulsive forcey Hydration forceyFrom V. Jegatheesan and S. Vigneswaran(2005)16

17FILTRATION MEDIASELECTION AND DESIGN

HOW DOES THEORY AFFECT DESIGN?Increased media depthIncreased media size18from Lawler and Benjamin 2006

HEADLESS ACROSS FILTER BED19

WHAT MAKES DESIGN GMFCOMPLICATED?Two competing goals:1.Particle removal (i.e., effluent quality)2.Filter run length (i.e., head loss accumulation)Head lossParticleRemoval20

HOW DO WE BALANCE COMPETINGVARIABLES?1 ft2Modifying three basic design parameters Mediaselection Depth of media Filtration rate (flow velocity)In terms of cost Biggest cost driver will be filtration rateyFilter area is function of filtration rate6 gpm21

APPROACH TO MEDIA SELECTIONBegin with good understandingof treatment goals and waterqualityApproaches to Design Followindustry design standards Modeling approach Filter piloting22

SPECIFYING GMF MEDIA Effectivegrain size (d10) Uniformity coefficient(UC)yyyUC d60/d10Smaller the betterTypically 1.5 Mediadepth (L) L/d10 1000 Trends over timeyyDeeper and coarserHigher filtration rates23

DEFINE FILTER MEDIA GOALS Whatyyyydoes better performance mean?Reduce head loss accumulationImprove effluent water qualityReduce risk of breakthroughManage a combination of goals for multiple sourcewaters24

MODELING APPROACH TO MEDIA SELECTIONModel Calibration(historical data) Water quality Chemical use Breakthrough Head loss changesPerformance Modeling(under various conditions) Feed water qualities Media sizes anddepthsMedia SelectionParticleremovalTime tobreakthroughTime toMax. head lossOptimal balance2525

EXAMPLE OF MODELING RESULTS2626

27FILTEROPERATIONS

CLEAN-BED REMOVAL AND RIPENINGSurface chemistry is improved through chemicalconditioning (coagulants and polymers) Particle-particle attachment is more efficient thanparticle-media attachment Ripening needed for excellent removal Typical practice to have a filter-to-waste step 28

FILTER PERFORMANCEMONITORING Turbidity standard performance metricy NTU Nephelometric Turbidity Unity Based on light scattering Particle counters – based on light blockage Surrogate parameter for treatment performance Regulatory limits of 0.3 or 0.1 NTUOld schoolturbidity byJackson candle(JTU)29

30Slide from Ben Stanford, AWWA IPRS 2016, Long Beach, CA

GMF FILTER O&M TROUBLESHOOTINGExcellent influent water quality ease ofoperation Balance turbidity and head loss usingchemical dose Backwash management When issues arise - filter surveillance! yyyyyVisual observationUFRVFilter coring – media changes/mudballsWatch head loss – air bindingBackwash profile – improved sequence31

VirusBacteriaFrom Williams 200932QUESTIONS?Gordon Williams, PhD, PEEast Bay Municipal Water Districtgjwillia@ebmud.com

UNDERSTANDING THE EXISTING MEDIASieve AnalysisFilter Coring of Existing Media33

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Typical Surface Water Recycled Water Drivers sediment, pathogens, organic solids pathogens, organic solids Pretreatment Any direct/contact Source Water 1 to 100 NTU 1 to 10 NTU Turbidity Req’d 0.3 NTU/0.1 NTU 2 NTU Filtration Rate (gal/ft2-min) 3/6 (mono/dual) 5 (up to 7.5) 10