Mechanical FiltrationMechanical Filtration

Hugh S. Hammer, PhD GSCCHugh S. Hammer, PhD GSCC

Ron Malone, PhD LSU Ron Malone, PhD LSU

Joe Fox, PhD Texas A&MJoe Fox, PhD Texas A&M

Total SolidsTotal Solids The amount of solid material left in a container after the The amount of solid material left in a container after the

water has evaporated.water has evaporated. Total Solids = Total Suspended Solids (TSS) + Total Total Solids = Total Suspended Solids (TSS) + Total

Dissolved Solids (TDS)Dissolved Solids (TDS) Total Suspended Solids (TSS) are solids that can be Total Suspended Solids (TSS) are solids that can be

trapped by a filter. Examples: silt, decaying organic trapped by a filter. Examples: silt, decaying organic material, industrial wastes, sewagematerial, industrial wastes, sewage

Total Dissolved Solids (TDS) are solids that pass through a Total Dissolved Solids (TDS) are solids that pass through a filter (0.45 microns). Examples: carbonates, bicarbonate, filter (0.45 microns). Examples: carbonates, bicarbonate, chloride, sulfate, phosphate, nitrate, calcium, magnesium, chloride, sulfate, phosphate, nitrate, calcium, magnesium, sodium and other ions.sodium and other ions.

TOTAL SOLIDS ARE INDICATORS OF POLLUTIONTOTAL SOLIDS ARE INDICATORS OF POLLUTION

Sources of Total Suspended SolidsSources of Total Suspended Solids

High flow rates from fast moving water, silt, High flow rates from fast moving water, silt, sand, clay, organicssand, clay, organics

Soil erosion (non-point source)Soil erosion (non-point source) Urban runoff (non-point source)Urban runoff (non-point source) Waste water and septic effluentWaste water and septic effluent Decaying organic matterDecaying organic matter Fish that stir up sediments (carps)Fish that stir up sediments (carps)

Problems with TSSProblems with TSS Increased biotic and abiotic turbidityIncreased biotic and abiotic turbidity

– Reduced light transmittance and photosynthesisReduced light transmittance and photosynthesis– Unstable dissolved oxygenUnstable dissolved oxygen– Increase water temperatureIncrease water temperature– Abiotic sources can clog gills and increase Abiotic sources can clog gills and increase

diseasedisease– Smother eggs, filter feeding animals, and Smother eggs, filter feeding animals, and

aquatic insectsaquatic insects– High TSS is often an indicator of other types of High TSS is often an indicator of other types of

pollutants and toxins (mercury and PCB)pollutants and toxins (mercury and PCB)

Testing TSSTesting TSS

A water sample is filtered through a pre-A water sample is filtered through a pre-weighed filter (0.45 microns)weighed filter (0.45 microns)

The residue retained in the filter is dried in The residue retained in the filter is dried in an oven at 103 to 105 Can oven at 103 to 105 C

The sample is dried to constant weight and The sample is dried to constant weight and the weight is recordedthe weight is recorded

Reported as grams per liter (ppt)Reported as grams per liter (ppt)

Total Dissolved SolidsTotal Dissolved Solids The water sample is passed through a 0.45 The water sample is passed through a 0.45

micron filtermicron filter The water that passes through the filter is The water that passes through the filter is

dried in a pre-weighed dish at 180 Cdried in a pre-weighed dish at 180 C The sample is dried to constant weightThe sample is dried to constant weight TDS is reported as milligrams per liter (ppm)TDS is reported as milligrams per liter (ppm) This is directly related to the conductance of This is directly related to the conductance of

water (dissolved ions)water (dissolved ions) EPA standard of 500 ppm for drinking waterEPA standard of 500 ppm for drinking water

Sources of TDSSources of TDS

Geology and sediment compositionGeology and sediment composition Fertilizer run-offFertilizer run-off Waste-water and septic effluentWaste-water and septic effluent Soil erosionSoil erosion Urban run-offUrban run-off

*** The TDS frequently includes *** The TDS frequently includes phosphorous, nitrate, and other nutrientsphosphorous, nitrate, and other nutrients

Aquaculture SolidsAquaculture Solids

FEED

FECES Uneaten Feed

Mechanical FiltrationMechanical Filtration Solids removal employs systems from the Solids removal employs systems from the

wastewater treatment industrywastewater treatment industry Screening, gravity separation (sedimentation, Screening, gravity separation (sedimentation,

centrifuging, hydrocycloning) or adsorption centrifuging, hydrocycloning) or adsorption between particulate bedsbetween particulate beds

Processes designations for RASProcesses designations for RAS– PrimaryPrimary: one or more gravity methods: one or more gravity methods– SecondarySecondary: biological filtration: biological filtration– TertiaryTertiary: ion exchange, reverse osmosis, foam : ion exchange, reverse osmosis, foam

fractionation, carbon adsorption, sometimes fractionation, carbon adsorption, sometimes disinfectiondisinfection

Solids CharacterizationSolids Characterization Three means of classification:Three means of classification: Solid materials are further classified as being either Solid materials are further classified as being either

settleable, suspended, dissolved or colloidalsettleable, suspended, dissolved or colloidal Difference between settleable and suspended solids is Difference between settleable and suspended solids is

a matter of practicalitya matter of practicality Most settleable: > 10 Most settleable: > 10 µM (settle in an Imhoff cone in µM (settle in an Imhoff cone in

less than 1 hr)less than 1 hr) Particles passing through a 1.2 µM membrane filter Particles passing through a 1.2 µM membrane filter

are dissolved, suspended are trappedare dissolved, suspended are trapped Dissolved particles consist of some organic and Dissolved particles consist of some organic and

inorganic ions and molecules present in solutioninorganic ions and molecules present in solution

Particle Size Distribution (microns)Particle Size Distribution (microns)Particle Size Distribution (microns)Particle Size Distribution (microns)

10-4 10-3 10-2 10-1 1 10 100

Dissolved Colloidal Suspended

Settleable

Foam Fractionation

Granular Filter

Microscreen

Tube Settler

Cartridge Filter

CoarseScreens

PlainSedimentation

100 75 30 10Particle Size, microns

SOLIDS REMOVAL PROCESSES AND PARTICLE SIZES

Impact of Solids on Recirculating SystemsImpact of Solids on Recirculating SystemsImpact of Solids on Recirculating SystemsImpact of Solids on Recirculating Systems

Increased BOD: causes oxygen availability Increased BOD: causes oxygen availability problems with animals and biofiltersproblems with animals and biofilters

Organic wastes (feces) build up increasing Organic wastes (feces) build up increasing ammonia and nitrite levels (toxic)ammonia and nitrite levels (toxic)

Increased system turbidity, decreased water Increased system turbidity, decreased water clarity (fine particles)clarity (fine particles)

Gill damage in fish (fine particles) can create Gill damage in fish (fine particles) can create opportunities for diseasesopportunities for diseases

Waste Solids Become Chemical ProblemsWaste Solids Become Chemical Problems

Both uneaten feed and fecal material Both uneaten feed and fecal material become become toxic ammoniatoxic ammonia through the action of through the action of decomposing bacteria.decomposing bacteria.

Uneaten Feed Feces

Heterotrophic Bacteria

Ammonia NH3/NH4

Increased Biochemical Increased Biochemical Oxygen Demand (BOD)Oxygen Demand (BOD)

Oxygen

Oxygen Oxygen

Oxygen

Oxygen

Oxygen

No Fine Solids Capture

Tilapia

Tiger Barbs

SettleableSettleable Solids Removal Solids Removal

If screens aren’t used, wastewater is first treated by If screens aren’t used, wastewater is first treated by simple sedimentation (primary treatment)simple sedimentation (primary treatment)

Separation is via gravity settlingSeparation is via gravity settling As with ponds, the principle design criteria are the As with ponds, the principle design criteria are the

basin’s cross-sectional area, detention time, depth and basin’s cross-sectional area, detention time, depth and overflow rate (refer to previous notes)overflow rate (refer to previous notes)

Ideal sedimentation basins don’t exist in the real world Ideal sedimentation basins don’t exist in the real world due to a variety of particle sizes, composition, etc.due to a variety of particle sizes, composition, etc.

Once settling velocity is known, basic dimensions can Once settling velocity is known, basic dimensions can be estimatedbe estimated

SedimentationSedimentation

Advantages:Advantages:– InexpensiveInexpensive– Works by gravity and doesn’t require energyWorks by gravity and doesn’t require energy

Disadvantages:Disadvantages:– Only gets largest solidsOnly gets largest solids– Takes a lot of spaceTakes a lot of space– Labor intensive to cleanLabor intensive to clean

INFLOW

OUTFLOW

Inle

t Zon

e

Out

let Z

oneVs

Vh

Sludge Zone

Settling Zone

(Vs > Overflow Rate to settle)

SEDIMENTATIONSEDIMENTATION

Sedimentation Sedimentation Tanks and BasinsTanks and Basins

Sedimentation TankSedimentation Tank

PPlate and Tube Separatorslate and Tube Separators

Also work on principle of Also work on principle of gravitygravity

Actually enhance settling Actually enhance settling capacity of basinscapacity of basins

Typically shallow settling Typically shallow settling devices consisting of devices consisting of modules of flat parallel modules of flat parallel plates or inclined tubes of plates or inclined tubes of various geometric designvarious geometric design

Used in primary thru Used in primary thru tertiary treatment tertiary treatment

Limited successLimited success

CCentrifuges and cyclonic separatorsentrifuges and cyclonic separators

Increase gravitational force on Increase gravitational force on particles via spinning motion (i.e., particles via spinning motion (i.e., settling rate increases)settling rate increases)

Many devices rated at different Many devices rated at different gg forcesforces

Work best on freshwater systems due Work best on freshwater systems due to many particles having similar to many particles having similar densities to that of seawaterdensities to that of seawater

Most practical are hydrocyclones or Most practical are hydrocyclones or cyclonic separatorscyclonic separators

Heavy particles are moved by higher Heavy particles are moved by higher outside velocity to outside and outside velocity to outside and downwarddownward

Underflow exiting unit is very small and Underflow exiting unit is very small and high density, “cleaner” water exits tophigh density, “cleaner” water exits top

Under-gravel FiltersUnder-gravel Filters

Advantages:Advantages:– Easy to build and operateEasy to build and operate– InexpensiveInexpensive– Does both mechanical and biological filtrationDoes both mechanical and biological filtration

Disadvantages:Disadvantages:– Needs to be vacuumed regularly (lots of maintenance)Needs to be vacuumed regularly (lots of maintenance)– Clog easilyClog easily– Can’t handle big loads (mainly for aquariums and not Can’t handle big loads (mainly for aquariums and not

practical for aquaculture production)practical for aquaculture production)

Air

Air

Airlifts Perform Several FunctionsAirlifts Perform Several Functions– CirculationCirculation– AerationAeration– C02 strippingC02 stripping– Foam controlFoam control

Circulation OptionsCirculation Options

Airlift

Air

Air

Pump

Circulation

ScreensScreens

Simplest, oldest method, pre-treatment prior to Simplest, oldest method, pre-treatment prior to primary treatmentprimary treatment

Placed across flow path of RAS waterPlaced across flow path of RAS water Coarse screens handle raw effluent, biofloc; fine Coarse screens handle raw effluent, biofloc; fine

screens for tertiary treatmentscreens for tertiary treatment Many materials: fibers to A/C filters; cost increases Many materials: fibers to A/C filters; cost increases

with decreased mesh sizewith decreased mesh size Static vs. rotary screens (0.25 to 1.5 mm; about 4-16 Static vs. rotary screens (0.25 to 1.5 mm; about 4-16

gpm flow per square inch of screen; removal efficiency gpm flow per square inch of screen; removal efficiency around 5-25%around 5-25%

Rotary screens for fine solids removal are 50-70% Rotary screens for fine solids removal are 50-70% efficient; 15-60 efficient; 15-60 µMµM

ScreensScreens Disadvantages:Disadvantages:

– May be difficult to remove and cleanMay be difficult to remove and clean– Labor intensive to cleanLabor intensive to clean– Auto wash micro-screen filters use a lot of waterAuto wash micro-screen filters use a lot of water– Some Units very expensive ($10,000)Some Units very expensive ($10,000)– Get mainly large solids and clog quicklyGet mainly large solids and clog quickly

Advantages:Advantages:– Simple conceptSimple concept– Can be inexpensive and simple to build (socks, panti-Can be inexpensive and simple to build (socks, panti-

hose, furnace filters, mesh bags)hose, furnace filters, mesh bags)

Micro-screen FiltersMicro-screen Filters

Over-Drain Flow

Captured Solids

Microscreen Cleaning Jets

Granular Media FiltersGranular Media Filters Commonly referred to as “sand” or “bead” filtersCommonly referred to as “sand” or “bead” filters Two types “slow” and “rapid” filtersTwo types “slow” and “rapid” filters Advantages:Advantages:

– Less labor is required (typically only to backwash)Less labor is required (typically only to backwash)– Gets a wide variety of solid sizes (down to 20 microns)Gets a wide variety of solid sizes (down to 20 microns)– Require less water than some unitsRequire less water than some units– Mechanical and Biological filters (depending on the media)Mechanical and Biological filters (depending on the media)– Best all-around mechanical filtersBest all-around mechanical filters– Capable of handling large loads (production aquaculture)Capable of handling large loads (production aquaculture)

Disadvantages:Disadvantages:– Requires a lot of pressure for some (pumps)Requires a lot of pressure for some (pumps)– ExpensiveExpensive– Can be more complex to operateCan be more complex to operate– Can clog quickly depending on the mediaCan clog quickly depending on the media

Slow Sand FiltersSlow Sand Filters Usually custom-built, open Usually custom-built, open

to atmto atm Loading rates are slow, Loading rates are slow,

0.6-0.7 lps/m0.6-0.7 lps/m22

Particle size: 30 Particle size: 30 µM maxµM max For this reason require For this reason require

more floor spacemore floor space Used in gravity flow Used in gravity flow

situationssituations Downside: cleaningDownside: cleaning

OpenTop

Gravel

FILTRATION MODE BACKWASHING MODE

PerforatedSupport Plate

CLOSEDOPEN

Rapid Sand FiltersRapid Sand Filters

Typically closed, Typically closed, pressurized unitspressurized units

Handle high flow rates: 20 Handle high flow rates: 20 gpm/ftgpm/ft22

Downside: very high head Downside: very high head loss (30-90 ft)loss (30-90 ft)

Only really good for low Only really good for low solids process streams with solids process streams with some sort of pre-trtsome sort of pre-trt

Backwashing can be made Backwashing can be made automaticautomatic

Granular FiltersGranular Filters

Important Point Important Point Sand filters can be used in series to filter out Sand filters can be used in series to filter out

different size particles so that they don’t clog different size particles so that they don’t clog quickly.quickly.– Large gravel Small gravel Large gravel Small gravel sand filtersand filter– This is frequently used for facilities that bring in This is frequently used for facilities that bring in

natural water (such as seawater) natural water (such as seawater)

(a) Propeller-washed (b) Bubble-washed

BEAD FILTERSBEAD FILTERS

Propeller-washed Floating Bead FiltersPropeller-washed Floating Bead Filters

Filtration Backwashing

Sludge

Return

Bypass

Pressure GaugeSludge View Port

Anti-siphon valve

Intake

Broodstock

04/11/2304/11/23

ADM System Prop-Washed Bead FiltersADM System Prop-Washed Bead Filters

Motor and Backwash PropellerMotor and Backwash Propeller

PumpPump

B

D

E

(1)

(3)

(6)

A

Water

(2)

(7)

AirWater

Patent # 5,770,080

Water Flow

(4)C

(5)

Air Flow

Sludge

Water Flow

Air Bleed Builds Charge

Settled BackwashWaters returned to

system

Filter Mode Drop Filters : Low Water Loss

Floating Bead Bioclarifiers

E

(1)

(3)

(4)

(6)C

(2)

(7)

Water

(5)

Air

Patent # 5,770,080

Air Flow

Water Flow

Internal Sludge Capture

Drop Filters : Low Water Loss

Floating Bead Bioclarifiers

Released Air Washes Beads

Backwashmode

Airlift

Inlet

CirculationAerationDegassing

Solids CaptureBiofiltration

Cartridge FiltersCartridge Filters Consist of cannister and replaceable cartridgeConsist of cannister and replaceable cartridge Advantages:Advantages:

– Removes very small particlesRemoves very small particles– Max particle retention is 0.01 Max particle retention is 0.01 µM (0.00001 mm)µM (0.00001 mm)– Very high water clarityVery high water clarity– Great for aquariumsGreat for aquariums

Disadvantages:Disadvantages:– Can be expensiveCan be expensive– Can clog quicklyCan clog quickly– Can’t handle large volumesCan’t handle large volumes– Not practical for production aquacultureNot practical for production aquaculture

Sock and Canister FiltersSock and Canister Filters

Diatomaceous Earth (DE) FiltersDiatomaceous Earth (DE) Filters

Granular material Granular material composed of diatom composed of diatom skeletons (frustules)skeletons (frustules)

Can serve as Can serve as replacement for replacement for cartridge filters, but cartridge filters, but require pre-filtrationrequire pre-filtration

Fine grade DE can Fine grade DE can filter down to 0.1 filter down to 0.1 µMµM

Factors to ConsiderFactors to Consider

Particle size to be removedParticle size to be removed Amount of energy required to operateAmount of energy required to operate Labor and maintenanceLabor and maintenance Amount of bio-load the filter can handle Amount of bio-load the filter can handle

(pounds of fish and pounds of feed)(pounds of fish and pounds of feed)

Separate Units StrategySeparate Units Strategy

Partitions water treatment into a series of Partitions water treatment into a series of individually stepsindividually steps

Optimizes each step to meet the narrow Optimizes each step to meet the narrow objectiveobjective

Integrates steps to develop a “treatment Integrates steps to develop a “treatment train”train”

Consolidation StrategyConsolidation Strategy

Utilize multi-functioning components to:Utilize multi-functioning components to: Minimize the number of componentsMinimize the number of components

Improve the stabilityImprove the stability Reduce costs of components and energyReduce costs of components and energy Smaller footprint (less space)Smaller footprint (less space)

Disadvantage is that neither process is Disadvantage is that neither process is optimizedoptimized

If you have space and money the separate If you have space and money the separate units strategy is betterunits strategy is better