Mechanical Filtration by OCrSjyK3

VIEWS: 0 PAGES: 58

									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.
 TOTAL SOLIDS ARE INDICATORS OF POLLUTION
   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
    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)
               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


                     FEED




   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
    disinfection
          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)


                                                   Settleable

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



Dissolved          Colloidal              Suspended
SOLIDS REMOVAL PROCESSES AND
        PARTICLE SIZES
 Coarse
 Screens

                                                                Cartridge Filter
Plain
Sedimentation

  Tube Settler

           Microscreen
                   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
                         Tilapia




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
              Sedimentation
 Advantages:
  – Inexpensive
  – Works by gravity and doesn’t require energy
 Disadvantages:
  – Only gets largest solids
  – Takes a lot of space
  – Labor intensive to clean
                     SEDIMENTATION

                            Vh                                        OUTFLOW
                      Vs




                                                        Outlet Zone
        Inlet Zone




                                        Settling Zone
INFLO
W                     (Vs > Overflow Rate
                       to settle)

                               Sludge Zone
 Sedimentation
Tanks and Basins
Sedimentation Tank
          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
   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
           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




Air
Circulation

                     Circulation Options

                                                Air




                                           Airlift
              Pump                   Air
                     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
                        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)
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
  situations
 Downside: cleaning
             O pen
             T op




                     O P EN                                             C LO S E D




                              oae
                         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
  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
Granular Filters
              Important Point
 Sand filters can be used in series to filter out
  different size particles so that they don’t clog
  quickly.
  – 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
Broodstock

                      Return


                         Anti-siphon
         Bypass
                            valve

                                                  Sludge View Port
                                 Pressure Gauge




                        Sludge

             Intake
   ADM System Prop-Washed Bead Filters

                     Motor and Backwash Propeller




3/12/2012
                   Pump
Filter Mode                                                                 Drop Filters :
                                                       (2 )               Low Water Loss
                         W a ter F low
                                                                     Floating Bead Bioclarifiers


                                                A


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

                                                                 A ir
                                                                 W a ter
Settled Backwash                                                                             Air Bleed
                                                D
Waters returned to
                                                     W a ter
                                                     S u d ge
                                                       l
                                                                                           Builds Charge
      system
                                                                                  (3 )
                                         P a ten t # 5 ,770 ,080
Backwash
                                            (2 )
  mode
                                                                  Drop Filters : Low Water
                                                                            Loss
                                                                 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
                                                                                          Capture

                                                                       (3 )
                              P a ten t # 5 ,770 ,080
Circulation
Aeration
Degassing
                Solids Capture
                Biofiltration

        Inlet



    Airlift
              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
  objective
 Integrates steps to develop a “treatment
  train”
         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

								
To top