Mechanical Filtration by OCrSjyK3


									Mechanical Filtration
  Hugh S. Hammer, PhD GSCC
     Ron Malone, PhD LSU
   Joe Fox, PhD Texas A&M
                   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.
   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)
          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
  – Smother eggs, filter feeding animals, and
    aquatic insects
  – High TSS is often an indicator of other types of
    pollutants and toxins (mercury and PCB)
               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)
        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
            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
Aquaculture Solids


   FECES      Uneaten Feed
         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
          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: > 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
Particle Size Distribution (microns)


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

Dissolved          Colloidal              Suspended

                                                                Cartridge Filter

  Tube Settler

                   Granular Filter
                                                            Foam Fractionation

             100             75                            30          10
                                  Particle Size, microns
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
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
    Increased Biochemical
    Oxygen Demand (BOD)

Oxygen      Oxygen      Oxygen

Oxygen       Oxygen         Oxygen

No Fine Solids Capture
Tiger Barbs
       Settleable Solids Removal
 If screens aren’t used, wastewater is first treated by
  simple sedimentation (primary treatment)
 Separation is via gravity settling
 As with ponds, the principle design criteria are the
  basin’s cross-sectional area, detention time, depth and
  overflow rate (refer to previous notes)
 Ideal sedimentation basins don’t exist in the real world
  due to a variety of particle sizes, composition, etc.
 Once settling velocity is known, basic dimensions can
  be estimated
 Advantages:
  – Inexpensive
  – Works by gravity and doesn’t require energy
 Disadvantages:
  – Only gets largest solids
  – Takes a lot of space
  – Labor intensive to clean

                            Vh                                        OUTFLOW

                                                        Outlet Zone
        Inlet Zone

                                        Settling Zone
W                     (Vs > Overflow Rate
                       to settle)

                               Sludge Zone
Tanks and Basins
Sedimentation Tank
          Plate and Tube Separators

 Also work on principle of
 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
   Centrifuges and cyclonic separators
 Increase gravitational force on particles
  via spinning motion (i.e., settling rate
 Many devices rated at different g
 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
 Underflow exiting unit is very small and
  high density, “cleaner” water exits top
           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
  – Can’t handle big loads (mainly for aquariums and not
    practical for aquaculture production)
Airlifts Perform Several Functions
          Air      –   Circulation
                   –   Aeration
                   –   C02 stripping
                   –   Foam control


                     Circulation Options


              Pump                   Air
 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
 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)
Micro-screen Filters
Over-Drain Flow
Captured Solids
Microscreen Cleaning Jets
             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
              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
 Downside: cleaning
             O pen
             T op

                     O P EN                                             C LO S E D

                         P e rf r t d       a
                                          G r ve l
                                      l e
                         S uppo r tP a t

  L        I
F I TR A T O N M O D E                                            I
                                                     B AC KW A SH NG M O D E
               Rapid Sand Filters
 Typically closed,
  pressurized units
 Handle high flow rates: 20
 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
Granular Filters
              Important Point
 Sand filters can be used in series to filter out
  different size particles so that they don’t clog
  – Large gravel       Small gravel         sand filter
  – This is frequently used for facilities that bring in
    natural water (such as seawater)
               BEAD FILTERS

(a) Propeller-washed          (b) Bubble-washed
   Propeller-washed Floating Bead Filters

     a i
F iltr ton                    w     i
                        B a ck a sh ng



                                                  Sludge View Port
                                 Pressure Gauge


   ADM System Prop-Washed Bead Filters

                     Motor and Backwash Propeller

Filter Mode                                                                 Drop Filters :
                                                       (2 )               Low Water Loss
                         W a ter F low
                                                                     Floating Bead Bioclarifiers


                              (7 )
                       (1 )
                                                          (5 )
              W a ter F low   (6 )
                                                          C                (4 )                  w
                                                                                         A ir F lo

                                                                 A ir
                                                                 W a ter
Settled Backwash                                                                             Air Bleed
Waters returned to
                                                     W a ter
                                                     S u d ge
                                                                                           Builds Charge
                                                                                  (3 )
                                         P a ten t # 5 ,770 ,080
                                            (2 )
                                                                  Drop Filters : Low Water
                                                                 Floating Bead Bioclarifiers

                                                                       Released Air Washes Beads
        (1 )           (7 )                    (5 )
W a ter F low   (6 )
                                               C                (4 )
                E                                                                     w
                                                                              A ir F lo
                              A ir
                                                      W a ter

                                                                                          Internal Sludge

                                                                       (3 )
                              P a ten t # 5 ,770 ,080
                Solids Capture


              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
  – Can’t handle large volumes
  – Not practical for production aquaculture
Sock and Canister Filters
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
            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)
        Separate Units Strategy

 Partitions water treatment into a series of
  individually steps
 Optimizes each step to meet the narrow
 Integrates steps to develop a “treatment
         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
 If you have space and money the separate
  units strategy is better

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