Upload
lamcong
View
227
Download
3
Embed Size (px)
Citation preview
FUNDAMENTALS OFGRANULAR MEDIAFILTRATIONGordon Williams, PhD, PEEast Bay Municipal Utility District
1
OVERVIEWFiltration basicsFiltration theoryFilter media selection and designMonitoring and Troubleshooting
2
FILTRATION BASICS ANDTHEORY
3
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)
4
Filtration Type Coagulation Flocculation Sedimentation Filtration NTU
Conventional Any
Direct <15 NTU
Contact <10 NTU
TYPICAL FILTRATION PRE-TREATMENT
5
A TYPICAL GRAVITY DEPTH GMF
ComponentsFlow controlMediaUnderdrain and supportBackwash system
Picture from MWH (2005) Water Treatment Principles and Design
6
GRANULAR MEDIA TYPES
ConfigurationMono mediaDual mediaMultimedia
TypesSilica sandAnthracite coalGACOther (e.g. garnet)
7
FILTER UNDERDRAINS AND SUPPORT
8
UNDERSTANDING A FILTER RUN
Ripening
Breakthrough
Terminal head loss
Time to breakthrough
Operating Turbidity
Clean bed head loss
99
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)
10
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
13
ACTUAL DATA MATCHES UP WITH THEORY!
From Williams 2009
MS2
Giardia
Crypto
E. Coli
14
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!
15
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)
16
FILTRATION MEDIASELECTION AND DESIGN
17
HOW DOES THEORY AFFECT DESIGN?
from Lawler and Benjamin 2006
Increased media depth Increased media size
18
HEADLESS ACROSS FILTER BED
19
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
20
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
21
APPROACH TO MEDIA SELECTION
Begin with good understanding of treatment goals and water quality
Approaches to Design
Follow industry design standardsModeling approachFilter piloting
22
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
23
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
24
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
25
25
EXAMPLE OF MODELING RESULTS
26
26
FILTEROPERATIONS
27
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
28
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
29
Old school turbidity by
Jackson candle (JTU)
Slide from Ben Stanford, AWWA IPRS 2016, Long Beach, CA
30
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
31
QUESTIONS?Gordon Williams, PhD, PEEast Bay Municipal Water [email protected]
Virus
Bacteria
From Williams 2009
32
UNDERSTANDING THE EXISTING MEDIA
Filter Coring of Existing Media
Sieve Analysis
33
34
35
36
37
38