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Mechanical Filtration

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Presentation on theme: "Mechanical Filtration"— Presentation transcript:

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 Dissolved Colloidal Suspended

Foam Fractionation Granular Filter Microscreen Tube Settler Cartridge Filter Coarse Screens Plain Sedimentation 100 75 30 10 Particle Size, microns

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 Aside from their direct physical impact, suspended solids generated in culture systems are highly organic. Most of the suspended solids generated in a recirculating system are feces, reflecting the undigested residuals of feed. Additional solids are generated as bacterial colonies (biofloc) grow from dissolved organic materials found in the water. Both feces and biofloc are mostly organic matter and are thus are subject to further breakdown by bacteria. Over two-thirds of the Biochemical Oxygen Demand (BOD) generated is attributed to suspended solids. It is not surprising, therefore, that clarification was the first wastewater treatment component added to extend water re-use and the first to be used to treat effluents. If allowed to remain in a culture system, solids encourage heavy bacterial growths that can clog (biofoul) a wide variety of treatment units. Excessive growths of bacteria can rapidly deplete oxygen in isolated pockets within a system. Heavy organic loads can inhibit critical nitrification processes in biofilters (Bovendeur et al., 1990). These localized, oxygen depleted zones can stimulate the development of a wide variety of anaerobic reactions that produce odor and contribute to off-flavor development. The bacteria will utilize large amounts of oxygen and produce TAN as they breakdown the solids. Additionally, these enriched conditions tend to cause population explosions of less desirable bacteria, leading to disease outbreaks.

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

Outlet Zone Settling basins have been the clarifier of choice for a number of years. A settling basin is designed to provide an area of quiescent water where the solids can settle out into a cone, from which they can then be removed. More advanced configurations, tube settlers, use a media to shorten settling distances, reducing the size of the settling basin. Performing superbly on systems with high water replacement rates, settling provides poor control of fine suspended solids (< 80 microns in diameter), which tend to accumulate as replacement rates are reduced. This problem, coupled with labor issues, has prompted some to search for a more efficient, yet cost-effective approach. (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: Disadvantages:
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)



28 Airlifts Perform Several Functions
Circulation Aeration C02 stripping Foam control

29 Airlift Pump Circulation Options Circulation Air
Low head centrifugal or airlift pumps are the most common choices for recirculation of waters through the water treatment block in a recirculating system. Centrifugal pumps are extremely efficient at water movement, but, they must matched with the treatment components through their pump curve that relates flow deliver (liter per minute) with the expected headloss (meters of water). The best pumps are expensive but realize considerable savings over the long run generally drawing less than half the amperage of less expensive units commonly marketed for pool applications. Most recirculating systems operate well with head delivery pressures below 5 meters. Additional savings in energy can be realized by use of 220 or 440 V units. Airlift pumps consist in their simplest form of a partially submerged piece of pipe fitted with a air injection line near the bottom. When air is injected the density of the air/water mixture drops allowing water to enter from the bottom of the pipe eventually overflowing the above water lip of the pipe. Airlift pumps are capable of delivering large quantities of water, but, usually only at a few inches of lift. The lift is usually limited to about 30 percent of the submerged depth. Serially placed airlift pumps can lift water several feet but they are not particularly efficient. Airlift pumps are generally used only in systems specifically designed for their use. Their principle advantage is their substantial contribution to the reaeration needs of the system. Once their design variables are understood, they are easily constructed from standard PVC piping. Pump

30 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; µM

31 Screens Disadvantages: Advantages:
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)

32 Micro-screen Filters


34 Over-Drain Flow

35 Captured Solids

36 Microscreen Cleaning Jets

37 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

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


40 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

41 Granular Filters

42 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)

43 BEAD FILTERS (a) Propeller-washed (b) Bubble-washed
Bead filters perform well in the control of suspended solids across a broad spectrum of conditions. Bead filters capture solids through four identifiable mechanisms. With the exception of adsorption, the solids capture mechanisms are physical in nature and are common to all types of granular media filters. As a general observation, the filters seem to control fine colloidal particles best with some biofilm development. This suggests that the biofilm absorption process is an important mechanism in the control of fine suspended solids and thus water clarity. The bead filter designs are preferred over the upflow sand filters for larger installations since they are more compact and lose less water when washed. Bead filters are superior to microscreens in their ability to capture fine solids, but generally operate at higher headlosses and lower flowrates than microscreens. Both bead filters and microscreens demonstrate similiar advantages in space and labor savings when compared to settling basins. There are a few clarification applications for which the PBF series filters are generally not recommended. The most notable is the treatment of wastewaters containing a high concentration of particulate fats or other floatable material. Separation of captured solids from the bead bed is accomplished by sedimentation of released sludge after the bed is agitated. Materials such as fats or wood chips merely float upward with the beads and are not removed. In sufficient quantity, these materials will eventually foul the bed requiring media replacement. Bead filters are also not well suited for the clarification of waters suffering from mineral turbidity problems caused by fine clays or other colloidal particles. Lacking good biofilm development, the mechanisms for the capture efficiencies are unacceptably low. Finally, the bead filters will impact but can not control planktonic algal blooms. Although some capture occurs as a general rule, the algae can grow faster than they can be caught and thus little progress towards clarification is made. Application of the bead filter technology to the problem of colloidal mineral turbidity or algal blooms requires the use of supplemental treatments (chemical flocculation or U.V. disinfection, respectively). (a) Propeller-washed (b) Bubble-washed



46 Propeller-washed Floating Bead Filters

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

48 ADM System Prop-Washed Bead Filters
Motor and Backwash Propeller Pump 4/1/2017


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

51 Drop Filters : Low Water Loss Floating Bead Bioclarifiers
Backwash mode Drop Filters : Low Water Loss Floating Bead Bioclarifiers Released Air Washes Beads Internal Sludge Capture

52 Circulation Aeration Degassing Solids Capture Biofiltration Inlet Airlift

53 Cartridge Filters Consist of cannister and replaceable cartridge
Advantages: Removes very small particles Max particle retention is 0.01 µM ( 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

54 Sock and Canister Filters

55 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

56 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)

57 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”

58 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

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