filter design, operation and treatment optimization rev3

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Filter Design, Operation and Treatment

OptimizationNew Jersey Water Association Annual Conference‐ 10/22/15PSI Process and EquipmentDavid J. Silverman, P.E.

Presentation Outline

• Definition of Filtration

• Filter types and selection

• Filter design

• Filter operation

• Filter (plant) optimization

• Filter troubleshooting case study

Definition of filtration

• Filtration is defined as “the separation of colloidal and larger particles from water by passage through a porous medium, usually sand, granular coal, or granular activated carbon”.

• The suspended particles removed during filtration range from 0.001 to 50 microns and larger.

Filter types and selectionAn array of options

• Gravity vs. Pressure

• Filtration rate is measured in gpm/sf– “Slow” sand filters 0.015‐0.15 gpm/sf

– Rapid sand filters 2‐8 gpm/sf, typically 3‐5 gpm/sf

– High rate up to 16 gpm/sf (requires energy)

– A low rate does not guarantee better water

– Rate depends on water quality, pretreatment

• Upflow vs. downflow

Filter types and selectionA complex decision

• Filter selection will depend upon:

–Water quality

– Flow capacity, variability

– Site conditions (available space)

– Hydraulics (available head)

– Operator preference/skills

– Other treatment needs

• Disinfection byproducts

• Seasonal algae taste/odor

• Fe/Mn, reservoir turnover

Gravity Filters

• Advantages– Low energy requirements

– Cost effective, especially for large plants (>7 MGD)

– Accessible for visual inspection, maintenance

• Disadvantages– Require relatively large area

–May be sensitive to variation in flow rates

– Pretreatment is critical

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“Slow” sand filters

• Oldest type of large‐scale filter

• Water passes first through about 36 inches of sand, then through a layer of gravel, before entering the underdrain.

• The sand removes particles from the water through adsorption and straining.

• Also removes contaminants through biodegradation in the “schmutzdecke”, a layer which breaks down organics and improves straining.

• Maintenance consists of raking the sand periodically, removing the top two inches of sand

• Due to the large area requirements, not considered economical in most cases

Rapid Sand Filters

• Filtration rate: 2‐10 gal/min‐ft2• Media depth: 2‐3 ft• Basin depth: 10 ft• Square tank• Surface area: <2,100 ft2• flow through filter: 350‐3,500 gpm• Backwash frequency: every 24 hours• Backwash rate: 8‐20 gal/min‐ft2• Backwash period: 5‐10 minutes• Backwash water: 1‐5% of filtered water• Filter rise rate: 12‐36 in/min• Bed expansion: 50%• Backwash trough 3 ft above media• Backwash water piped to raw water intake

Pressure Filters

• Flow capacity from 20‐2000 gpm• Fabricated steel vessel• Small footprint• Can be fitted with a variety of media for various applications (Fe, Mn, As, ion exchange)

• Cost competitive with gravity filtration for small plants

Filtration mechanisms

• Mechanical straining accounts for a minor part of the filter’s action. A filter is able to remove particles much smaller than the spaces between the media grains

• Adsorption‐ particles in the water stick either to the media or to previously deposited contaminants

• Attachment‐ magnetic forces causes particles in the water to stick to previously removed particles. Coagulants and polymers can neutralize the surface charge, facilitating attachment

• Transport‐ the processes of interception, sedimentation and diffusion bring water particles into contact with media or previously removed particles

Pretreatment for filtration

• When particles in water have the same surface charge, they are “stable”, meaning they will not attract each other

• Coagulant is added to destabilize particles

• The repulsive layer is neutralized by counterions in the coagulant, destabilizing the particles

• Flocculation (gentle mixing) facilitates transport and attachment, forming floc particles

Filter Media

• Traditional‐ gravel, sand and anthracite• Others – activated carbon, greensand,

garnet sand, pyrolucite• Washed, sized, naturally occurring

silica rock• Rounded shape stones in various sizes• Required Filter Media Properties

– Inertness in water– Attrition resistance in a filter application– Proper size and compatibility– Particle shape– Particle Density

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Filter Media Properties

• Filter media is specified by grain size distribution which is based on a sieve analysis

• Effective size d10 is the size sieve that 10% of the grains will pass

• Uniformity Coefficient or UC is d60/d10 (Max 1.5)

• Media is sometimes specified by top size (d95), bottom size (d5), over size (% grain size >d95) and under size (% grain size <d5)

• Filter media is governed by AWWA B100‐09, Granular filter material

Media Properties‐ Hydraulic Size

• The hydraulic size of a filter media can be calculated directly from the sieve analysis.

• The result gives an averaged area size and is used in the Carman and Kozeny equations to calculate media flow and backwash expansion.

Filter Gravel and Sand

• Gravel– Commonly used as support media– Purpose is to retain the fluidizable filter media

above it (sand and/or anthracite)– Provide diffusion of backwash flow– Should remain level to prevent filter channeling– Gravel depth may range from 6 to 24 inches

• Sand– Naturally occurring silica sand– Washed and sized for filter applications– Hydraulically leveled during backwash– Specific Gravity 2.65– Shape – round to angular– Polishing Zone– Depth – process dependent

Round vs. Angular Sand

• Round sand has larger pore spaces and less compaction

• Angular sand has a smaller surface area to volume ratio, but rougher texture, rough surface and microporous void spaces

• The angularity of the granules and the tapered internal pore spaces allow for increased removal of dirt, silt and organic matter suspended in water by bridging, straining and adhesion.

Sand Properties

Media properties vary considerablyMedia uniformity (high Uniformity Coefficient on left, low UC on right)

Agglomerated media (left) vs. whole grain media (right)

Filter coal (Anthracite)

• Anthracite generic term for coal used in filters

• Typically referred to as Filter anthracite

• First used over filter sand in 1911, referred to as dual‐media

• Functions:– Roughing filter

– Flocculating zone

– Solids holding layer

– Depth – process dependent

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Filter coal (Anthracite)

• Color: black• Bulk Density: 50 lbs./cu. ft.• Specific Gravity: 1.3‐1.7• Effective Size: .60 – 2.30 mm• Uniformity Coefficient: 1.3 – 1.7• Hardness: 3.0 – 3.8 (Mohs scale)• Bed depth: 24 – 36 in., 10 – 18 in. for

multimedia filters• Freeboard: 50% of bed depth (min.)• Service flow rate: 5 gpm/sq. ft. or higher

depending upon local conditions• Backwash flow rate: 12 – 25 gpm/sq. ft.

depending upon mesh size• Backwash expansion rate: 20 – 40% of bed

depth

Backwash Considerations• Backwash Rate – a function of media size, specific

gravity and water temperature – 50% bed expansion• Filter Media compatibility• Backwash duration, 15‐20 min typ.

• Backwash frequency• Backwash Rate‐sand 12‐15 gpm/sf, anthracite 8‐12

gpm/sf• Air scouring with low‐rate backwashing can break up

the surface crust without producing random currents, if the underdrain system is designed to distribute air uniformly. – Solids removed from the media collect in the layer of water

between the media surface and wash channels. – After the air stops, the dirty water is normally flushed out by

increased backwash water flow rate or by surface draining. – Wash water consumption is approximately the same whether

water‐only or air/water backwashing is employed.

Backwash Rates Effect of Density on Backwash Rates

Media Selection and L/D

• No ideal media configuration applies to all water sources and pretreatment schemes.

• There is a growing acceptance of some minimum summation of the ratio L/D [depth (L)/diameter (D)] as a guideline.

• A summation L/D ratio of 1200 (dimensionless L/D using ES for D) has been a common guide.

• For conventional pretreatment with sedimentation, a common dual media uses 18 to 24 inches of anthracite of 0.90 mm effective size over 9 to 12 inches of sand of 0.45 mm effective size.

• Granular activated carbon (GAC) can remove organics and DBP Precursors

Filter Media and L/D

• A coarser top size is needed to accommodate higher solids load and algae blooms for direct filtration without sedimentation.

• A tri‐media for a conventional plant may use 16 inches of anthracite 1 mm ES, 9 inches of sand with 0.5 mm ES and 3 to 4.5 inches of garnet with 0.24 mm ES. This would provide an L/D ratio summation from 1180 to 1339.

• It is important to select a top size appropriate to the source water so that Unit Filter Run Volume production goals are achieved.

• The choice of the grain size of the media at the top, where the water enters the filter bed, influences the depth of penetration of solids into the bed. The bigger the grain size, the better the solids penetration and thereby the better the bed utilization.

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Filter design

• Evaluate regulatory requirements for pathogen removal/inactivation

• Evaluate raw water quality and water quality goals

• Compare alternative processes and filter configurations

• Develop list of media configurations• Perform pilot testing• Select media that provides best performance

and meets Unit Filter Run Volume (UFRV) criteria

• Select appropriate filter components‐underdrains, troughs, weirs etc.

Filter underdrain types

• Conventional underdrain– HDPE Plastic modules snap together– Grouted at bottom of filter and in between modules

– Furnished with caps or plates to support media and prevent media intrusion

– Most types require support gravel – Mass produced for minimum capital cost

• Wheeler type underdrains• Clay tile underdrains

Custom fabricated underdrains

• Complete, custom underdrain system; panels or laterals

• Self cleaning design

• Guaranteed uniform distribution

• Rapid, low cost installation

• Integral air scour chamber

• Optimizes filter performance

• Durable stainless steel construction

• Long service life

Filter Control

• Constant Rate– Each filter is equipped with a rate‐of‐flow control valve.– The valve maintains a constant rate of water flow through the filter.– As filter clogs the valve slowly opens to maintain the flow rate.

• Declining Rate– The filter controller maintains a constant level of water above the

media– Filtration rate declines as filter clogs– A loss of head gauge on a filter is used to measure the drop in

pressure through a filter bed

Filter operation

• Stages of the filtration cycle:– Ripening

• Turbidity breakthrough• Filter to waste

– Production• Headloss increases as solids build up in the filter• Turbidity decreases and particles are captured• Variation in flow rates will cause shearing of flocs

– Termination• Cycles can be terminated based on head loss or time

• Filter should not exhibit turbidity breakthrough at end of run

– Backwash• Monitor turbidity, do not overwash filter, it wastes water and lengthens ripening time

Filter performance measures

• Run length‐ length of time between backwashes

• Hydraulic loading rate‐ flow in gpm divided by area in sf gpm/sf

• Unit filter run volume (UFRV)‐flow per cycle divided by filter area gal/sf (7,500‐10,000)

• Unit backwash waste volume backwash flow divided by filter area gal/sf (<100)

• Washwater consumption (<5%)

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Treatment plant optimization

• Carefully monitor data• Learn to think of the individual

plant COMPONENTS as a SYSTEM • Consider the effect that a

modification to COMPONENT operation could have on the entire SYSTEM

• Recognize that COMPONENToptimization and SYSTEMoptimization may be mutually exclusive

• Optimizing COMPONENT and SYSTEM is a never ending process

Treatment plant optimization• Sedimentation

– Settled water turbidity is less than 1.0 ntu 95 percent of the time when daily average raw water turbidity is less than or equal to 10.0 ntu during the same period

– Settled water turbidity is less than 2.0 ntu. 95 percent of the time when daily average raw water turbidity is greater than 10.0 ntu. during the same period

• Filtration:– Filtered water turbidity is less than 0.1 ntu 95

percent of the time based on the maximum values recorded during 2‐hour time increments

– Maximum turbidity of any filtered water measurement is never greater than 0.2 ntu

Treatment Plant Optimization• Monitoring Requirements

– Daily raw water turbidity is determined at 2 hour increments

– Settled water turbidity is determined at 2 hour increments from each sedimentation basin

– Filtered water turbidity is determined at 2 hour increments from each filter

– One filter backwash turbidity profile is performed each month for each filter

• Recommended Instrumentation: – Each filter effluent is equipped such that turbidity is

continuously monitored and recorded – The pH of raw and filtered water is continuously

monitored and recorded – Plant is equipped with an adequately sized PC for

recording and electronically transmitting raw, settled and filtered water data, and for generating turbidity vs. time graphs

Common filtration problems• Mudballs‐ high rate backwash pulls

surface crust solids down into the filter• Short runs

– improper coagulation, polymer type or dosage causes blinding of the filter

– insufficient backwashing leaves filter dirty– excessive biological growth in filter

• Turbidity breakthrough– improper flocculation or flow changes causing shearing of floc particles

• Mounting, craters or channeling in media– these are usually equipment‐related issues

Filter evaluation techniques

• Filter inspections– Media‐ look for mounding, cracking, media pulling away from walls, inconsistent flow distribution, backwash turbulence or “boiling”

– Monitor backwash pressures

– Media bed depth measurement

– Media grain size distribution• Compare with original specifications

Backwash Evaluation

• Backwashing

• Bed expansion measurement

• Water temperature (density) correction

• Backwash turbidity analysis– Measure every 30 seconds

– Avoid over‐ or under‐washing media

• Solids retention analysis– Take samples at 0‐2 inches, 2‐6, 6‐12, 12‐18,18‐24, etc. until all strata are sampled

– Sample before and after washing the bed

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Floc retention profile Filter Troubleshooting‐ Case Study

• New England Location• Surface Water• Conventional Treatment• Avg. 5.5 MGD – Peak 13.3 MGD

• Temp. 33 – 55 deg. F• Limited Finished Water Storage

• 2 Shifts (14 – 24 hr/day)• Phase III Directors Cert. (6 yrs)

Filter Troubleshooting‐ Case Study

• Optimization problem ‐ filters

– First filters installed in 1906

– Filters replaced in 1936

• 6 filters

– Underdrains replaced in 1970

– Converted to dual media 1970

– Noted sand infiltration into the underdrains in late 1970’s

– Began a filter maintenance program about 1980


• Filter Maintenance Program– One Filter Rebuilt Each Year– Remove Media and Gravel– Flush Sand From Underdrain

– Re‐screen and Re‐install Gravel

– Re‐Install Media– Add Top‐up Anthracite as Req.

– Minimized Sand Infiltration


• Began using Partnership for Safe Water (PSW) software and continuous monitoring on individual filters in mid‐1990’s

• PSW software shows filter performance problems prior to scheduled maintenance

• Inspections of problem filters show gravel migration followed by infiltration of sand into underdrains


Filter gravel mounding

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• Plant staff decide to replace existing underdrains with new ones that do not require support gravel

• Filter media is scheduled for replacement with underdrains

• Replacement of filter effluent valves included in project

• Project starts early 2002


• Filter Replacement Project

• Project begins in March

• One filter to be replaced at a time

• Single filter rehab takes 2 weeks

• Project must be completed by end of May

• Halfway through project, rebuilt filters exhibit short runs and reduced flow


• State the problem clearly and concisely– Upgraded filters operating short filter runs, reduced flow capacity. Run time is less than 50% of filters waiting for upgrade.

– Operating headloss data not available – no headloss gauges.

• Review existing data– Both original and rebuilt filters producing equal water quality

– Turbidity breakthrough not occurring in any filters

– Significantly greater quantity of solids appearing on the surface of the rebuilt filters

– Similar surface solids had been seen on old filters when polymer addition was increased


• List Assumptions – Effluent flow meters working properly– Build loss of head instrument for all filters

– Turbidity instrument working– Operators recording data correctly

• Ask questions – What’s different? – Underdrains (fine screens for direct retention of fine media)

– Filter media old (24” sand, 7” anthracite) vs. new (11” sand, 20” anthracite)

– Filter effluent valves– Develop hypothesis– New gravel‐less underdrains are plugging


• Get Input from Others

– Consult with contractor

– Consult with engineer

– Consult with underdrain supplier

– Consult with media supplier

• Develop Alternative Hypotheses

–Media blinding quicker ‐ sand not properly skimmed

– Media blinding quicker ‐anthracite fines present


• Collect Data– Collect media core samples from old and rebuilt filters

– Run sieve analysis and flocretention tests on media samples

– Inspect screens on new underdrains

– Review headloss data for old and rebuilt filters (Build headloss gauges first)

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Filter Troubleshooting‐ Case Study• Review all data

– Inspection shows underdrain screens are not plugged

– Headloss data show same clean bed headloss and backwash headloss for both old and rebuilt filters

– New gravel‐less underdrains plugging? NO

– Sieve analyses show no fines in first rebuilt filter

– Second and third rebuilt filter have similar headloss profile to first filter

– Media blinding quicker – sand not properly skimmed? NO


• Sieve analyses show anthracite fines on all three rebuilt filters

• Sieve analysis show ES of anthracite to be significantly smaller for rebuilt filters

• Sieve analysis for new media matches ES for specification and specification of media installed in 1970

• Anthracite in old filters ES larger ES than specification

• Floc retention tests show most solids removed in top 6” of filter media of rebuilt filters

• Media blinding quicker anthracite fines present? – SORT OF


• Data does not match any of the hypotheses

• However, the data do point to problems with excess solids collection in the top layers of the media bed

• Go back to Step 2, Review Existing Data– Both original and rebuilt filters producing equal water quality

– Turbidity breakthrough not occurring in any filters

– Significantly greater quantity of solids appearing on the surface of the rebuilt filters

Filter Troubleshooting‐ Case Study• Obervations:

– Sieve analyses show anthracite fines on all three rebuilt filters

– Floc retention tests show most solids removed in top 6” of filter media of rebuilt filters

– Similar surface solids had been seen on old filters when polymer addition was increased

• Develop Alternative Hypotheses– Media in old filters has gotten larger over time

– Plant adjusted treatment chemicals for larger media size

– Finer replacement media is blinding more quickly – no depth filtration

– Anthracite fines at very top layer causing rapid blinding


• Collect Data– Skim anthracite in one of the rebuilt

filters and review operating data– Reduce polymer addition and monitor

operation of rebuilt and old filters

• Review all Data– Performance of rebuilt filter with

skimmed anthracite and rebuilt filters without skimming identical

– Reduced polymer addition greatly extend run times of rebuilt filters

– Rebuilt filters take now take 30% greater load than old filters without compromising performance

– Old filters perform well with reduced polymer addition however they require a 15% reduction in capacity

Conclusions

• There are many different types of filtration systems‐ there is no such thing as “one size fits all”

• Optimize your treatment process and it will take good care of you in good times and bad

• Develop a filter surveillance program and perform regular filter evaluations

• Consult with manufacturers, engineers and your operations professional cohorts on your treatment issues

• Every filtration application is different, select your components carefully, invest in good equipment, and do not try to “keep up with the Joneses”

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Questions and Answers

1. Name three types of granular filter media– Sand, anthracite, garnet, gravel, perlite

2. Names three types of filters– Pressure, gravity, slow sand, upflow

3. Name three types of filter underdrains– Pipe lateral, block, Wheeler, panel, fabricated

4. Name three filter evaluation techniques– Filter inspection, media grain size analysis, backwash

turbidity analysis, floc retention profile

5. Explain how to optimize a filter– Trick question, you cannot optimize only one component

of a plant, it must be viewed as a complete system

Thank You

David J. Silverman, P.E.

PSI Process and Equipment

[email protected]

(347) 563‐0766

www.psiprocess.com

filter design, operation and treatment optimization rev3

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