Mechanical Filtration

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Mechanical Filtration

• Hugh S. Hammer, PhD GSCC
• Ron Malone, PhD LSU
• Joe Fox, PhD Texas AM

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Total Solids

• The amount of solid material left in a container
  after the water has evaporated.
• Total Solids Total Suspended Solids (TSS)
  Total Dissolved Solids (TDS)
• Total Suspended Solids (TSS) are solids that can
  be trapped by a filter. Examples silt, decaying
  organic material, industrial wastes, sewage
• Total Dissolved Solids (TDS) are solids that pass
  through a filter (0.45 microns). Examples
  carbonates, bicarbonate, chloride, sulfate,
  phosphate, nitrate, calcium, magnesium, sodium
  and other ions.
• TOTAL SOLIDS ARE INDICATORS OF POLLUTION

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Sources of Total Suspended Solids

• High flow rates from fast moving water, silt,
  sand, clay, organics
• Soil erosion (non-point source)
• Urban runoff (non-point source)
• Waste water and septic effluent
• Decaying organic matter
• Fish that stir up sediments (carps)

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Problems with TSS

• Increased biotic and abiotic turbidity
• Reduced light transmittance and photosynthesis
• Unstable dissolved oxygen
• Increase water temperature
• Abiotic sources can clog gills and increase
  disease
• Smother eggs, filter feeding animals, and aquatic
  insects
• High TSS is often an indicator of other types of
  pollutants and toxins (mercury and PCB)

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Testing TSS

• A water sample is filtered through a pre-weighed
  filter (0.45 microns)
• The residue retained in the filter is dried in an
  oven at 103 to 105 C
• The sample is dried to constant weight and the
  weight is recorded
• Reported as grams per liter (ppt)

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Total Dissolved Solids

• The water sample is passed through a 0.45 micron
  filter
• The water that passes through the filter is dried
  in a pre-weighed dish at 180 C
• The sample is dried to constant weight
• TDS is reported as milligrams per liter (ppm)
• This is directly related to the conductance of
  water (dissolved ions)
• EPA standard of 500 ppm for drinking water

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Sources of TDS

• Geology and sediment composition
• Fertilizer run-off
• Waste-water and septic effluent
• Soil erosion
• Urban run-off
• The TDS frequently includes phosphorous,
  nitrate, and other nutrients

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Aquaculture Solids
Solids
FEED
FECES
Uneaten Feed
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Mechanical Filtration

• Solids removal employs systems from the
  wastewater treatment industry
• Screening, gravity separation (sedimentation,
  centrifuging, hydrocycloning) or adsorption
  between particulate beds
• Processes designations for RAS
• Primary one or more gravity methods
• Secondary biological filtration
• Tertiary ion exchange, reverse osmosis, foam
  fractionation, carbon adsorption, sometimes
  disinfection

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Solids Characterization

• Three means of classification
• Solid materials are further classified as being
  either settleable, suspended, dissolved or
  colloidal
• Difference between settleable and suspended
  solids is a matter of practicality
• Most settleable gt 10 µM (settle in an Imhoff
  cone in less than 1 hr)
• Particles passing through a 1.2 µM membrane
  filter are dissolved, suspended are trapped
• Dissolved particles consist of some organic and
  inorganic ions and molecules present in solution

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Particle Size Distribution (microns)
Settleable
10-4 10-3 10-2
10-1 1 10
100
Dissolved
Colloidal
Suspended
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SOLIDS REMOVAL PROCESSES AND PARTICLE SIZES
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Impact of Solids on Recirculating Systems

• Increased BOD causes oxygen availability
  problems with animals and biofilters
• Organic wastes (feces) build up increasing
  ammonia and nitrite levels (toxic)
• Increased system turbidity, decreased water
  clarity (fine particles)
• Gill damage in fish (fine particles) can create
  opportunities for diseases

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Waste Solids Become Chemical Problems

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

Uneaten Feed
Feces
Heterotrophic Bacteria
Ammonia NH3/NH4
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Increased Biochemical Oxygen Demand (BOD)
Oxygen
Oxygen
Oxygen
Heterotrophic Bacteria
Oxygen
Oxygen
Oxygen
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Tilapia
No Fine Solids Capture
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Tiger Barbs
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Settleable Solids Removal

• If screens arent used, wastewater is first
  treated by simple sedimentation (primary
  treatment)
• Separation is via gravity settling
• As with ponds, the principle design criteria are
  the basins cross-sectional area, detention time,
  depth and overflow rate (refer to previous notes)
• Ideal sedimentation basins dont exist in the
  real world due to a variety of particle sizes,
  composition, etc.
• Once settling velocity is known, basic dimensions
  can be estimated

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Sedimentation

• Advantages
• Inexpensive
• Works by gravity and doesnt require energy
• Disadvantages
• Only gets largest solids
• Takes a lot of space
• Labor intensive to clean

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SEDIMENTATION
Vh
OUTFLOW
Vs
Settling Zone
INFLOW
Inlet Zone
Outlet Zone
(Vs gt Overflow Rate to settle)
Sludge Zone
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Sedimentation Tanks and Basins
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Sedimentation Tank
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Plate and Tube Separators

• Also work on principle of gravity
• Actually enhance settling capacity of basins
• Typically shallow settling devices consisting of
  modules of flat parallel plates or inclined tubes
  of various geometric design
• Used in primary thru tertiary treatment
• Limited success

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Centrifuges and cyclonic separators

• Increase gravitational force on particles via
  spinning motion (i.e., settling rate increases)
• Many devices rated at different g forces
• Work best on freshwater systems due to many
  particles having similar densities to that of
  seawater
• Most practical are hydrocyclones or cyclonic
  separators
• Heavy particles are moved by higher outside
  velocity to outside and downward
• Underflow exiting unit is very small and high
  density, cleaner water exits top

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Under-gravel Filters

• Advantages
• Easy to build and operate
• Inexpensive
• Does both mechanical and biological filtration
• Disadvantages
• Needs to be vacuumed regularly (lots of
  maintenance)
• Clog easily
• Cant handle big loads (mainly for aquariums and
  not practical for aquaculture production)

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Airlifts Perform Several Functions

• Circulation
• Aeration
• C02 stripping
• Foam control

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Circulation Options
Circulation
Pump
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Screens

• Simplest, oldest method, pre-treatment prior to
  primary treatment
• Placed across flow path of RAS water
• Coarse screens handle raw effluent, biofloc fine
  screens for tertiary treatment
• Many materials fibers to A/C filters cost
  increases with decreased mesh size
• Static vs. rotary screens (0.25 to 1.5 mm about
  4-16 gpm flow per square inch of screen removal
  efficiency around 5-25
• Rotary screens for fine solids removal are 50-70
  efficient 15-60 µM

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Screens

• Disadvantages
• May be difficult to remove and clean
• Labor intensive to clean
• Auto wash micro-screen filters use a lot of water
• Some Units very expensive (10,000)
• Get mainly large solids and clog quickly
• Advantages
• Simple concept
• Can be inexpensive and simple to build (socks,
  panti-hose, furnace filters, mesh bags)

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Micro-screen Filters
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Over-Drain Flow
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Captured Solids
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Microscreen Cleaning Jets
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Granular Media Filters

• Commonly referred to as sand or bead filters
• Two types slow and rapid filters
• Advantages
• Less labor is required (typically only to
  backwash)
• Gets a wide variety of solid sizes (down to 20
  microns)
• Require less water than some units
• Mechanical and Biological filters (depending on
  the media)
• Best all-around mechanical filters
• Capable of handling large loads (production
  aquaculture)
• Disadvantages
• Requires a lot of pressure for some (pumps)
• Expensive
• Can be more complex to operate
• Can clog quickly depending on the media

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Slow Sand Filters

• Usually custom-built, open to atm
• Loading rates are slow, 0.6-0.7 lps/m2
• Particle size 30 µM max
• For this reason require more floor space
• Used in gravity flow situations
• Downside cleaning

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Rapid Sand Filters

• Typically closed, pressurized units
• Handle high flow rates 20 gpm/ft2
• Downside very high head loss (30-90 ft)
• Only really good for low solids process streams
  with some sort of pre-trt
• Backwashing can be made automatic

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Granular Filters
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Important Point

• Sand filters can be used in series to filter out
  different size particles so that they dont clog
  quickly.
• Large gravel Small gravel sand filter
• This is frequently used for facilities that bring
  in natural water (such as seawater)

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BEAD FILTERS
(a) Propeller-washed
(b) Bubble-washed
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Propeller-washed Floating Bead Filters
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Broodstock
Return
Anti-siphon valve
Bypass
Sludge View Port
Pressure Gauge
Sludge
Intake
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ADM System Prop-Washed Bead Filters
Motor and Backwash Propeller
Pump
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Filter Mode
Drop Filters Low Water Loss Floating Bead
Bioclarifiers
Air Bleed Builds Charge
Settled Backwash Waters returned to system
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Backwash mode
Drop Filters Low Water Loss Floating Bead
Bioclarifiers
Released Air Washes Beads
Internal Sludge Capture
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Circulation Aeration Degassing
Solids Capture Biofiltration
Inlet
Airlift
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Cartridge Filters

• Consist of cannister and replaceable cartridge
• Advantages
• Removes very small particles
• Max particle retention is 0.01 µM (0.00001 mm)
• Very high water clarity
• Great for aquariums
• Disadvantages
• Can be expensive
• Can clog quickly
• Cant handle large volumes
• Not practical for production aquaculture

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Sock and Canister Filters
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Diatomaceous Earth (DE) Filters

• Granular material composed of diatom skeletons
  (frustules)
• Can serve as replacement for cartridge filters,
  but require pre-filtration
• Fine grade DE can filter down to 0.1 µM

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Factors to Consider

• Particle size to be removed
• Amount of energy required to operate
• Labor and maintenance
• Amount of bio-load the filter can handle (pounds
  of fish and pounds of feed)

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Separate Units Strategy

• Partitions water treatment into a series of
  individually steps
• Optimizes each step to meet the narrow objective
• Integrates steps to develop a treatment train

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Consolidation Strategy

• Utilize multi-functioning components to
• Minimize the number of components
• Improve the stability
• Reduce costs of components and energy
• Smaller footprint (less space)
• Disadvantage is that neither process is optimized
• If you have space and money the separate units
  strategy is better