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Unformatted text preview: Sand Filtration Sand
EVEG 3110 Version dated 2/12/03 Malone Overview We are here Overview Steele & McGee (1973) 5th edition Overview Metcalf & Eddy (1991) 3rd edition Solids Particle sizes
Settleable 10 10 10 10 1 10 100 -4 -3 -2 -1 Dissolved Colloidal Suspended Solids SOLIDS REMOVAL PROCESSES AND PARTICLE SIZES
Coarse Screens Plain Sedimentation Tube Settler Microscreen Granular Filter Foam Fractionation 100 75 Particle Size, microns 30 Cartridge Filter 10 Terminology Terminology-Granular filtration Terminology-Granular
Filtration- process of cleaning the water by process passing the water through the bed passing Backwashing-flow reversal and fluidization to -flow remove accumulated solids remove Fluidization-expansion of the bed by -expansion suspension of sands suspension Runtime- length of time between back washes Terminology Hydraulic Loading (or flux or superficial velocity) (or f = Q/A
Where Q = total flow to the filter in gpm (m3/sec) A = the approach surface area of the filtration bed in ft2 (m2) f = flux or hydraulic loading in gpm/ft2 or ft/sec (m/sec) Terminology Upflow filter – waters flows upward through the sand during the filtration mode Downflow Filter - water flows downward through the sand during filtration Anthracite-type of coal media that is used commonly for the low density (top) l layer in dual or multi-media filters (s.g 1.4-1.6) Garnite Sand – Typical sand type selected for filtration media (s.g. about 2.65) River Sand – Separated from river bottoms; typically well rounded Specific Gravity (s.g.) – ratio of material density to water Removal Mechanisms Removal
Straining - size of particle prohibits movement Straining
through openings through Sedimentation - Particles settles onto grain Sedimentation
of sand of Interception - momentum of particle causes it
to strike sand surface to Adsorption - Intra-particulate forces bring
particle out of flowstream to touch surface or to other particles other Removal Mechanisms Straining Metcalf & Eddy (1991) 3rd edition Removal Mechanisms Settling Metcalf & Eddy (1991) 3rd edition Removal Mechanisms Interception Metcalf & Eddy (1991) 3rd edition Removal Mechanisms Adsorption Metcalf & Eddy (1991) 3rd edition Engineering Issues Engineering
Sizing Sand selection/bed configurations Bed depths Backwash flows Backwash Design details Design Overview Conventional Sand (Gravity Sand Filtration) Davis & Cornwell (1991) 2nd edition Overview Rapid Sand Filtration (Pressurized) Sand Diego Zoo Overview Rapid Sand Filtration (Pressurized) Davis & Cornwell (1991) 2nd edition Overview Overview Hubb’s SeaWorld-San Diego Design Issues Fair, Geyer and Okun, 1968 Design Issues Fair, Geyer and Okun, 1968 Design issues Design Issues Fair, Geyer and Okun, 1968 Sand Characteristics Sand
Sand size : Dominate factor controls removal efficiencies, headlosses, and
flow requirements Sand Density: Controls flowrates required for backwashing Sand Porosity: Controls the amount of solids that can be held before
headloss buildup occurs, i.e. controls runtimes Sand Shape : Influences the backwashing behavior.
Uniformity Coefficient : measures uniformity of sand controls use of bed depth Standard Sieve analysis is used to characterize sand sizes Fair, Geyer and Okun, 1968 Sand size (simple rules) Sand
As the sand size decreases … Fine solids removal improves-best for turbidity Fine control control Headloss goes up Run times decrease Surface straining can become a problem Sensitivity to organic loading increases Sensitivity Backwash flows go down Backwash Sand size (simple rules) Sand
As the sand size increases … Fine solids removal declines Headloss decreases Run times dramatically increase Coarse solid removal through bed depth Backwashing abrasion improves increasing Backwashing resistance to biofouling resistance Backwash flows go up Backwash Terminology Effective Grain Size (d10) Effective
Diameter of the 10 percentile (by wt) sand Diameter grains grains Controls the filtration effectiveness of the Controls bed bed About 0.5 mm typical for gravity sand About filters filters Terminology Uniformity Coefficient
Ratio the 60 percentile size to the 10 percent percentile percent Characterizes the effectiveness of bed with Characterizes depth High values >1.7 will lead to an inefficient High inverted bed inverted Low values around 1 are desirable Low Surface Straining Surface
With a typical sand backwashing causes With the finer sands to end up on the surface the Causes surface straining of large particles Increase sand size Surface scrubbing Dual or multimedia (i.e. vary media density) Solutions Surface Straining Problem Fair, Geyer and Okun, 1968 Surface Straining Problem Surface Scrubbing Fair, Geyer and Okun, 1968 Surface Straining Problem Dual-Media Surface Straining Problem Metcalf & Eddy (1991) 3rd edition Surface Straining Problem Metcalf & Eddy (1991) 3rd edition Surface Straining Problem Metcalf & Eddy (1991) 3rd edition Sands River sands Crushed stone Wide variety of sand shapes available Fair, Geyer and Okun, 1968 These characteristics are quantified to facilitate calculations
Fair, Geyer and Okun, 1968 Metcalf & Eddy (1991) 3rd edition Downward Force Fg=m*g
s Gravity constant Particle Mass v Vi = Q/Ai Frictional forces Ff Davis & Cornwell (1991) 2nd edition v
1 V is sometimes represented by f Media Obstruction Area Water must move through the interstitial Area Since water must flow through the interstitial area: Zone of increasing Porosity Packed Bed mode Expanded Mode Failure mode 300
Percent Expansion Vi>vs Vi=vs Vi<vs 0 Q Malone and Burden 1988 Typical Values-Coventional Sand Typical
Flitration flux – 2-5 gpm/ft2 Maximum headloss – 9-12 ft Bed depths – 3-5 ft Backwash rate – 15-35 gpm/ft2 Sand sizes – 0.5-1.0 mm Downflow Metcalf & Eddy (1991) 3rd edition Pilot Studies Pilot
Required for applications where an Required extensive experience base is not required extensive Matches bed characteristics to source water Matches and objectives and Avoids costly mistakes Assignment Read Pages 284-322 Review for conceptual understanding only You will not be expected to use the detailed design equa We do not have that much time to dedicate to the topic. Homework: Will be posted ...
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This note was uploaded on 05/20/2010 for the course EVEG 3110 taught by Professor Malone during the Spring '10 term at LSU.
- Spring '10