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

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OVERVIEWFiltration basicsFiltration theoryFilter media selection and designMonitoring and Troubleshooting

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FILTRATION BASICS ANDTHEORY

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WHAT IS FILTRATION?Definition: any process that removes suspended particles through a porous mediumTypes of Filters

Granular media filters (GMF)Slow sand filtersRapid depth filters

Membrane filters (MF/UF/NF/RO)

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Filtration Type Coagulation Flocculation Sedimentation Filtration NTU

Conventional Any

Direct <15 NTU

Contact <10 NTU

TYPICAL FILTRATION PRE-TREATMENT

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A TYPICAL GRAVITY DEPTH GMF

ComponentsFlow controlMediaUnderdrain and supportBackwash system

Picture from MWH (2005) Water Treatment Principles and Design

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GRANULAR MEDIA TYPES

ConfigurationMono mediaDual mediaMultimedia

TypesSilica sandAnthracite coalGACOther (e.g. garnet)

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FILTER UNDERDRAINS AND SUPPORT

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UNDERSTANDING A FILTER RUN

Ripening

Breakthrough

Terminal head loss

Time to breakthrough

Operating Turbidity

Clean bed head loss

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SURFACE VS. RECYCLED WATER FILTRATIONTypical Surface Water Recycled Water

Drivers sediment,pathogens, organic

solids

pathogens,organic solids

Pretreatment Any direct/contactSource Water 1 to 100 NTU 1 to 10 NTUTurbidity Req’d 0.3 NTU/0.1 NTU 2 NTUFiltration Rate (gal/ft2-min)

3/6 (mono/dual) 5 (up to 7.5)

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FILTRATION THEORY11

FILTRATION THEORYModeled as 2-step process

1. Particle transport2. Particle attachment

Particle transport mechanismsSedimentationInterceptionBrownian motion (diffusion)

Straining not desirableOverall removal is sum of all mechanisms

Picture from Lawler and Benjamin 200612

PARTICLE TRANSPORT EFFICIENCY

Transport efficiency driven by:

Filtration rateMedia designParticle sizeTemperatureDensity (sed. only)

Graph from Lawler and Benjamin 2006

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ACTUAL DATA MATCHES UP WITH THEORY!

From Williams 2009

MS2

Giardia

Crypto

E. Coli

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WHY IS CHEMISTRY SO IMPORTANT?Chemistry needs to be right for particles to “stick”Both particles and media are naturally negatively charged and thus repelled by each otherFor excellent filtration, the surface chemistry of target particles must be modified – coagulation!

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PARTICLE ATTACHMENT EFFICIENCY

Attachment is driven by chemistryAttraction/repulsive forces

London-van der Waals forceElectrical double-layer interactionBorn repulsive forceHydration force

From V. Jegatheesan and S. Vigneswaran(2005)

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FILTRATION MEDIASELECTION AND DESIGN

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HOW DOES THEORY AFFECT DESIGN?

from Lawler and Benjamin 2006

Increased media depth Increased media size

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HEADLESS ACROSS FILTER BED

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WHAT MAKES DESIGN GMF COMPLICATED? Two competing goals:

1. Particle removal (i.e., effluent quality)

2. Filter run length (i.e., head loss accumulation)

Head loss

Particle Removal

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HOW DO WE BALANCE COMPETINGVARIABLES?

Modifying three basic design parametersMedia selectionDepth of mediaFiltration rate (flow velocity)

In terms of costBiggest cost driver will be filtration rate

Filter area is function of filtration rate6 gpm

1 ft2

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APPROACH TO MEDIA SELECTION

Begin with good understanding of treatment goals and water quality

Approaches to Design

Follow industry design standardsModeling approachFilter piloting

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SPECIFYING GMF MEDIAEffective grain size (d10)Uniformity coefficient (UC)

UC = d60/d10

Smaller the betterTypically <1.5

Media depth (L)L/d10 > 1000Trends over time

Deeper and coarserHigher filtration rates

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DEFINE FILTER MEDIA GOALS

What does better performance mean?Reduce head loss accumulationImprove effluent water qualityReduce risk of breakthroughManage a combination of goals for multiple source waters

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MODELING APPROACH TO MEDIA SELECTION

Model Calibration(historical data)

• Water quality

• Chemical use

• Breakthrough

• Head loss changes

Media SelectionPerformance Modeling(under various conditions)

• Feed water qualities

• Media sizes and depths Time to

breakthrough Time to Max. head loss

Particle removal

Optimal balance

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EXAMPLE OF MODELING RESULTS

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FILTEROPERATIONS

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CLEAN-BED REMOVAL AND RIPENINGSurface chemistry is improved through chemical conditioning (coagulants and polymers)Particle-particle attachment is more efficient than particle-media attachmentRipening needed for excellent removalTypical practice to have a filter-to-waste step

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FILTER PERFORMANCEMONITORING

Turbidity standard performance metricNTU = Nephelometric Turbidity UnitBased on light scattering

Particle counters – based on light blockageSurrogate parameter for treatment performanceRegulatory limits of 0.3 or 0.1 NTU

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Old school turbidity by

Jackson candle (JTU)

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

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GMF FILTER O&M TROUBLESHOOTING

Excellent influent water quality = ease of operationBalance turbidity and head loss using chemical doseBackwash management When issues arise - filter surveillance!

Visual observationUFRVFilter coring – media changes/mudballsWatch head loss – air bindingBackwash profile – improved sequence

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

Virus

Bacteria

From Williams 2009

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UNDERSTANDING THE EXISTING MEDIA

Filter Coring of Existing Media

Sieve Analysis

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