Presentation Outline Definition of Filtration Gravity Filters

10/23/2015

Presentation Outline

Filter Design, • Definition of Filtration Operation and • Filter types and selection Treatment • Filter design Optimization • Filter operation New Jersey Water Association Annual Conference‐ 10/22/15 • Filter (plant) optimization PSI Process and Equipment • Filter troubleshooting case study David J. Silverman, P.E.

Definition of filtration Filter types and selection An array of options • Filtration is defined as “the • Gravity vs. Pressure separation of colloidal and • Filtration rate is measured in gpm/sf larger particles from water by – “Slow” sand filters 0.015‐0.15 gpm/sf passage through a porous – Rapid sand filters 2‐8 gpm/sf, typically medium, usually sand, 3‐5 gpm/sf granular coal, or granular – High rate up to 16 gpm/sf (requires activated carbon”. energy) – A low rate does not guarantee better • The suspended particles water removed during filtration – Rate depends on water quality, range from 0.001 to 50 pretreatment microns and larger. • Upflow vs. downflow

Filter types and selection Gravity Filters A complex decision • Filter selection will depend upon: • Advantages – Water quality – Low energy requirements – Flow capacity, variability – Cost effective, especially for large plants (>7 MGD) – Site conditions (available space) – Accessible for visual – Hydraulics (available head) inspection, maintenance – Operator preference/skills • Disadvantages – Other treatment needs – Require relatively large area • Disinfection byproducts – May be sensitive to • Seasonal algae taste/odor variation in flow rates • Fe/Mn, reservoir turnover – Pretreatment is critical

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“Slow” sand filters Rapid Sand Filters

• Oldest type of large‐scale filter • Filtration rate: 2‐10 gal/min‐ft2 • Water passes first through about 36 inches of • Media depth: 2‐3 ft sand, then through a layer of gravel, before • Basin depth: 10 ft entering the underdrain. • Square tank • The sand removes particles from the water • Surface area: <2,100 ft2 through adsorption and straining. • flow through filter: 350‐3,500 gpm • Also removes contaminants through • Backwash frequency: every 24 hours biodegradation in the “schmutzdecke”, a layer • Backwash rate: 8‐20 gal/min‐ft2 which breaks down organics and improves • Backwash period: 5‐10 minutes straining. • Backwash water: 1‐5% of filtered water • Maintenance consists of raking the sand • Filter rise rate: 12‐36 in/min periodically, removing the top two inches of • Bed expansion: 50% sand • Backwash trough 3 ft above media • Due to the large area requirements, not • Backwash water piped to raw water intake considered economical in most cases

Pressure Filters Filtration mechanisms

• Flow capacity from 20‐2000 gpm • Mechanical straining accounts for a minor part of the filter’s action. A filter is able to remove • Fabricated steel vessel particles much smaller than the spaces between • Small footprint the media grains • Can be fitted with a variety of media for various • Adsorption‐ particles in the water stick either to the applications (Fe, Mn, As, ion exchange) media or to previously deposited contaminants • Cost competitive with gravity filtration for small • Attachment‐ magnetic forces causes particles in the plants 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 Filter Media

• Traditional‐ gravel, sand and anthracite • When particles in water have the same surface charge, they are • Others – activated carbon, greensand, garnet sand, pyrolucite “stable”, meaning they will not attract each other • Washed, sized, naturally occurring silica rock • Coagulant is added to destabilize particles • Rounded shape stones in various sizes • The repulsive layer is neutralized • Required Filter Media Properties by counterions in the coagulant, – Inertness in water destabilizing the particles – Attrition resistance in a filter application • Flocculation (gentle mixing) – Proper size and compatibility facilitates transport and – Particle shape attachment, forming floc particles – Particle Density

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Filter Media Properties Media Properties‐ Hydraulic Size

• Filter media is specified by grain • The hydraulic size of a size distribution which is based on a sieve analysis filter media can be • Effective size d10 is the size calculated directly sieve that 10% of the grains will from the sieve pass analysis. • Uniformity Coefficient or UC is d60/d10 (Max 1.5) • The result gives an • Media is sometimes specified by averaged area size and top size (d95), bottom size (d5), is used in the Carman over size (% grain size >d95) and under size (% grain size

Filter Gravel and Sand Round vs. Angular Sand

• Gravel • Round sand has larger pore – Commonly used as support media spaces and less compaction – Purpose is to retain the fluidizable filter media • Angular sand has a smaller above it (sand and/or anthracite) surface area to volume ratio, – Provide diffusion of backwash flow but rougher texture, rough – Should remain level to prevent filter channeling – Gravel depth may range from 6 to 24 inches surface and microporous void • Sand spaces – Naturally occurring silica sand • The angularity of the granules – Washed and sized for filter applications and the tapered internal pore – Hydraulically leveled during backwash spaces allow for increased – Specific Gravity 2.65 removal of dirt, silt and organic – Shape – round to angular matter suspended in water by – Polishing Zone bridging, straining and – Depth – process dependent adhesion.

Sand Properties Filter coal (Anthracite)

Media properties vary considerably Media uniformity (high Uniformity Coefficient • Anthracite generic term for coal on left, low UC on right) used in filters • Typically referred to as Filter anthracite • First used over filter sand in 1911, referred to as dual‐media Agglomerated media (left) vs. whole • Functions: grain media (right) – Roughing filter – Flocculating zone – Solids holding layer – Depth – process dependent

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Filter coal (Anthracite) Backwash Considerations • Backwash Rate – a function of media size, specific • Color: black gravity and water temperature – 50% bed expansion • Bulk Density: 50 lbs./cu. ft. • Filter Media compatibility • Specific Gravity: 1.3‐1.7 • Backwash duration, 15‐20 min typ. • Effective Size: .60 – 2.30 mm • Backwash frequency • Uniformity Coefficient: 1.3 – 1.7 • Backwash Rate‐sand 12‐15 gpm/sf, anthracite 8‐12 • Hardness: 3.0 – 3.8 (Mohs scale) gpm/sf • Bed depth: 24 – 36 in., 10 – 18 in. for • Air scouring with low‐rate backwashing can break up multimedia filters the surface crust without producing random • Freeboard: 50% of bed depth (min.) currents, if the underdrain system is designed to • Service flow rate: 5 gpm/sq. ft. or higher distribute air uniformly. depending upon local conditions – Solids removed from the media collect in the layer of water • Backwash flow rate: 12 – 25 gpm/sq. ft. between the media surface and wash channels. depending upon mesh size – After the air stops, the dirty water is normally flushed out by increased backwash water flow rate or by surface draining. • Backwash expansion rate: 20 – 40% of bed – Wash water consumption is approximately the same whether depth water‐only or air/water backwashing is employed.

Backwash Rates Effect of Density on Backwash Rates

Media Selection and L/D Filter Media and L/D

• No ideal media configuration applies to all water • A coarser top size is needed to accommodate higher sources and pretreatment schemes. solids load and algae blooms for direct filtration without sedimentation. • There is a growing acceptance of some minimum • A tri‐media for a conventional plant may use 16 summation of the ratio L/D [depth (L)/diameter (D)] inches of anthracite 1 mm ES, 9 inches of sand with as a guideline. 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 • A summation L/D ratio of 1200 (dimensionless L/D from 1180 to 1339. using ES for D) has been a common guide. • It is important to select a top size appropriate to the • For conventional pretreatment with sedimentation, source water so that Unit Filter Run Volume a common dual media uses 18 to 24 inches of production goals are achieved. anthracite of 0.90 mm effective size over 9 to 12 • The choice of the grain size of the media at the top, where the water enters the filter bed, influences the inches of sand of 0.45 mm effective size. depth of penetration of solids into the bed. The • Granular activated carbon (GAC) can remove bigger the grain size, the better the solids organics and DBP Precursors penetration and thereby the better the bed utilization.

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Filter design Filter underdrain types

• Evaluate regulatory requirements for pathogen • Conventional underdrain removal/inactivation – HDPE Plastic modules snap together • Evaluate raw water quality and water quality – Grouted at bottom of filter and in goals between modules • Compare alternative processes and filter – Furnished with caps or plates to configurations support media and prevent media • Develop list of media configurations intrusion • Perform pilot testing – Most types require support gravel • Select media that provides best performance – Mass produced for minimum capital and meets Unit Filter Run Volume (UFRV) cost criteria • Wheeler type underdrains • Select appropriate filter components‐ • Clay tile underdrains underdrains, troughs, weirs etc.

Custom fabricated underdrains Filter Control

• Complete, custom underdrain • Constant Rate system; panels or laterals – Each filter is equipped with a rate‐of‐flow control valve. – The valve maintains a constant rate of water flow through the filter. • Self cleaning design – As filter clogs the valve slowly opens to maintain the flow rate. • Guaranteed uniform distribution • Declining Rate – The filter controller maintains a constant level of water above the • Rapid, low cost installation media – Filtration rate declines as filter clogs • Integral air scour chamber – A loss of head gauge on a filter is used to measure the drop in • Optimizes filter performance pressure through a filter bed • Durable stainless steel construction • Long service life

Filter operation Filter performance measures

• Stages of the filtration cycle: • Run length‐ length of time – Ripening • Turbidity breakthrough between backwashes • Filter to waste • Hydraulic loading rate‐ flow in – Production gpm divided by area in sf • Headloss increases as solids build up in the filter gpm/sf • Turbidity decreases and particles are captured • Variation in flow rates will cause shearing of flocs • Unit filter run volume (UFRV)‐ – Termination flow per cycle divided by filter • Cycles can be terminated based on head loss or area gal/sf (7,500‐10,000) time • Filter should not exhibit turbidity breakthrough at • Unit backwash waste volume end of run backwash flow divided by – Backwash filter area gal/sf (<100) • Monitor turbidity, do not overwash filter, it wastes water and lengthens ripening time • Washwater consumption (<5%)

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Treatment plant optimization Treatment plant optimization • Sedimentation • Carefully monitor data – Settled water turbidity is less than 1.0 ntu 95 • Learn to think of the individual percent of the time when daily average raw plant COMPONENTS as a SYSTEM water turbidity is less than or equal to 10.0 ntu during the same period • Consider the effect that a – Settled water turbidity is less than 2.0 ntu. 95 modification to COMPONENT percent of the time when daily average raw operation could have on the entire water turbidity is greater than 10.0 ntu. SYSTEM during the same period • Recognize that COMPONENT optimization and SYSTEM • Filtration: – Filtered water turbidity is less than 0.1 ntu 95 optimization may be mutually percent of the time based on the maximum exclusive values recorded during 2‐hour time • Optimizing COMPONENT and increments SYSTEM is a never ending process – Maximum turbidity of any filtered water measurement is never greater than 0.2 ntu

Treatment Plant Optimization Common filtration problems

• Monitoring Requirements • Mudballs‐ high rate backwash pulls – Daily raw water turbidity is determined at 2 hour surface crust solids down into the filter increments • – Settled water turbidity is determined at 2 hour Short runs increments from each sedimentation basin – improper coagulation, polymer type or – Filtered water turbidity is determined at 2 hour dosage causes blinding of the filter increments from each filter – insufficient backwashing leaves filter dirty – One filter backwash turbidity profile is performed each month for each filter – excessive biological growth in filter • Recommended Instrumentation: • Turbidity breakthrough – Each filter effluent is equipped such that turbidity is – improper flocculation or flow changes continuously monitored and recorded causing shearing of floc particles – The pH of raw and filtered water is continuously monitored and recorded • Mounting, craters or channeling in – Plant is equipped with an adequately sized PC for media recording and electronically transmitting raw, settled – these are usually equipment‐related and filtered water data, and for generating turbidity vs. time graphs issues

Filter evaluation techniques Backwash Evaluation

• Filter inspections • Backwashing – Media‐ look for • Bed expansion measurement mounding, cracking, media pulling away from • Water temperature (density) walls, inconsistent flow correction distribution, backwash turbulence or “boiling” • Backwash turbidity analysis – Monitor backwash – Measure every 30 seconds pressures – Avoid over‐ or under‐washing media – Media bed depth measurement • Solids retention analysis – Media grain size – Take samples at 0‐2 inches, 2‐6, 6‐12, 12‐ distribution 18,18‐24, etc. until all strata are sampled • Compare with original – Sample before and after washing the bed specifications

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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)