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Mechanical Filtration

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  1. Mechanical Filtration Hugh S. Hammer, PhD GSCC Ron Malone, PhD LSU Joe Fox, PhD Texas A&M

  2. 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

  3. 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)

  4. 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)

  5. 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)

  6. 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

  7. 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

  8. Aquaculture Solids Solids FEED FECES Uneaten Feed

  9. 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

  10. 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

  11. Particle Size Distribution (microns) Settleable 10-4 10-3 10-2 10-1 1 10 100 Dissolved Colloidal Suspended

  12. Coarse Screens Cartridge Filter Plain Sedimentation Tube Settler Microscreen Granular Filter Foam Fractionation 100 30 75 10 Particle Size, microns SOLIDS REMOVAL PROCESSES AND PARTICLE SIZES

  13. 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

  14. 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

  15. Increased Biochemical Oxygen Demand (BOD) Oxygen Oxygen Oxygen Heterotrophic Bacteria Oxygen Oxygen Oxygen

  16. Tilapia No Fine Solids Capture

  17. Tiger Barbs

  18. 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

  19. 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

  20. SEDIMENTATION Vh OUTFLOW Vs Settling Zone INFLOW Inlet Zone Outlet Zone (Vs> Overflow Rate to settle) Sludge Zone

  21. Sedimentation Tanks and Basins

  22. Sedimentation Tank

  23. 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

  24. 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

  25. 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)

  26. Circulation Aeration C02 stripping Foam control Airlifts Perform Several Functions

  27. Circulation Air Airlift Air Circulation Options Pump

  28. 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

  29. 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)

  30. Micro-screen Filters

  31. Over-Drain Flow

  32. Captured Solids

  33. Microscreen Cleaning Jets

  34. 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

  35. 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

  36. 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

  37. Granular Filters

  38. 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)

  39. BEAD FILTERS (a) Propeller-washed (b) Bubble-washed

  40. Propeller-washed Floating Bead Filters

  41. Broodstock Return Anti-siphon valve Bypass Sludge View Port Pressure Gauge Sludge Intake

  42. ADM System Prop-Washed Bead Filters Motor and Backwash Propeller Pump

  43. Filter Mode Drop Filters : Low Water Loss Floating Bead Bioclarifiers Air Bleed Builds Charge Settled Backwash Waters returned to system