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08 filtration

Deals with rapid gravity filtration, slow sand filtration and Roughing filters.
Aspects covered include
Filter media and their characterization is described.
Filter running and filter backwashing,
Filter hydraulics.
Suspended solids, turbidity and bacterial removal from water.

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08 filtration

  1. 1. Filtration
  2. 2. Filtration types • Surface filtration and Depth filtration • Granular media filters – Slow sand filters, Rapid (sand) gravity filters and roughing filters – Open type and Closed type (pressure filters!) – Duel media (sand and anthracite) and multi-media (sand, anthracite and garnet) filters • Home water filtration options: Candle filters, Bio-sand filters • Membrane filters: Micro-filters. Ultra-filters, Nano-filters and Reverse Osmosis Water treatment and filtration • Filtration for removing the residual flocs/TSS (after coagulation-flocculation-settling) • Residual flocs/TSS are ~20 mg/L and need reduction to <5 mg/L • Rapid gravity filters (open type or closed type) can be used • Pre-filtration followed by slow sand filtration in place of coagulation-flocculation-settling, rapid gravity filtration and chlorination • Replacing chemical treatment (chemical free water treatment)
  3. 3. Granular rapid gravity filters Components • Bed of filter medium – 0.2 to 0.9 m thick bed of natural or synthetic medium – Sand, Anthracite coal, Garnet, Green sand, Filtralite (baked clay!), Granular activated carbon, etc. • Graded gravel layers supporting the filter medium bed and preventing entry of filter medium into the under-drain system • Under-drain system – Supports the graded gravel layers and the filter medium bed from below – Permeable to water (filtered water from above and backwash water from below) • Water reservoir over the filter medium bed – Includes backwash water trough • The support structure (filter tank) hosing the filter • Backwash water reservoir to supply water for backwashing • Piping/ducting/drains with necessary valves and other fittings
  4. 4. Multi-media filter
  5. 5. Filter media Characteristics • Size and size distribution (Effective Size and Uniformity Coefficient) – D10, D60/D10 and D90 – Sieve analysis and plotting the % passing through the given sieve sizes on a log-probability plot for finding D10, D60/D10 and D90 – Shape factor (1.0 for perfect sphere) • Density, bulk density and porosity Effective size, uniformity coefficient and porosity affect the resistance offered to flow (head loss) during filtration Density and grain size influence backwashing of the filter medium • Clay content and (calcium and magnesium) carbonate content – Clay can be lost and bed thickness can be reduced – Water can dissolve the carbonates over time and the medium grains can become porous and weak (can be crushed and size reduction can occur) • Strength properties such as hardness • Adsorption and ion exchange properties, and ability to support microbial film development
  6. 6. Graded gravel layer • Supports the bed of filter medium from below • Graded in such a way that – The filter medium does not penetrate into the gravel layer (D90 should not penetrate into the top gravel layer) – The gravel does not enter into the under-drain system (gravel of size greater than 2 times the under-drain perforations size) – Gravel of one layer does not penetrate the next layer below – In case of spherical particles, only the particles of size <1/3rd the diameter can penetrate • Thickness of each of the gravel layers should be > 4 to 6 times the largest size particle of the layer • Graded gravel layer of sufficient thickness ensures uniform distribution of backwash water and compressed air flows • A newly assembled filter should be started with a backwash – Ensures proper stratification of the filter medium above
  7. 7. The under-drain system Under drain system should • Supports graded gravel layers, filter bed and water column • Allows filtered water to pass through, collected and conveyed out • Allows incoming backwash water (and compressed air) to pass through and disperse for the filter bed backwashing The under drain system can be • A water box with porous concrete roof • Concrete slabs with slots supported on concrete ribs • A water manifold with perforated laterals The graded gravel and the under drain system may better be considered together as integral components Geo-textiles and geo-nets can also be used in assembling the under drain system
  8. 8. • Typical filtration rates are achieved through maintaining a water column (water reservoir) of up to 1.5 m above the filter bed • In closed type, a virtual water column is maintained • Filter medium size, extent of filter bed clogging, and water column height determine the filtration rates • Gravity filters are operated either at constant rate (but variable head) or at constant head (but variable rate) • Head loss/filtration rate or turbidity break through are used as the basis for filter backwash • Backwash water overflow weirs and troughs are provided in the reservoir zone above the filter bed • Level difference between the overflow weir and water level in the backwash water reservoir is maximized • Overflow weir is provided above the fluidized filter bed (during backwashing) for avoiding filter medium washout • The Filtration unit is provided with a channel either in the middle or to one side • Backwash water troughs drain into this channel • Has a wash water drain, and water inlet opens into this channel Water reservoir over the filter bed
  9. 9. Overhead reservoir of backwash water • Holds filtered water and supplies for the filter backwashing – Almost constant head of water is maintained in the reservoir to ensure constant backwash velocities in the filters • The reservoir, and the piping and fittings are designed to ensure the needed water supply for achieving the desired backwash velocity – Excess backwash velocities can washout the filter medium and lower velocities can result in inefficient cleaning of the filter – Sizing of the overhead reservoir (capacity and water depth) – reliability (against pump and power failures) is given importance – Deciding on the relative elevation of the reservoir • Filtered water is pumped and maintained in the overhead reservoir – Pumping system for the reliable pumping of filtered water into the overhead reservoir is important
  10. 10. • Particle removal mechanisms include straining, settling, adsorption/ion exchange, and Inertial impaction/ interception – Diffusion and Brownian motion may help in the transportation of particles to filter medium surface – Chemical destabilization, VanderWaal forces of attraction, action of the bio-films developed over the filter medium – Retention/holding/storage of the removed particles till the filters are cleaned • Clogged filter (once its sludge storage capacity is exhausted) is cleaned through regular backwashing – Upward flow of water at backwash velocity lifts and expands the filter bed and fluidizes the bed material – Hydrodynamic shear cleans the medium particle surface and the dirt enters into the backwash water – Use of compressed air can enhance the cleaning process and minimize the backwash water requirement – Alternate ways of using compressed air in the backwashing: first air and then water; first air, then air and water, and then only water; first air and water and then only water Mechanisms of filtration and cleaning
  11. 11. Filter run and backwash cycle Here A, B, C, D and E are the flow control valves used Provision for use of compressed air in the backwashing Gravel Backwash water Overhead tank
  12. 12. Filter cycle Filter run • Once filtered water reaches the acceptable quality, stop wasting and collect filtered water • Filter water either at constant rate or at constant head • Backwash the filter when – Head loss across the filter crosses a desired set value (2.5 m) – Filtration rate drops below a set value – Turbidity breaks through the filter Stop filter operation • Head loss or turbidity are used as the basis for filter backwash • Close filter inlet and allow filtration till water level drops below the wash water overflow weir, but the sand bed remains submerged Filter backwashing • Close filtered water outlet and introduce compressed air for air scouring the bed – run the compressed air for a specified duration • Open wash water outlet and introduce backwash water – stop compressed air injection (air injection and backwash water introduction, at sub-fluidizing water flows, can overlap) – continue backwash water flow till clear water overflows the weir
  13. 13. Filter cycle Air scouring during backwashing • Very effective when the backwash water is introduced at sub- fluidization rates along with the compressed air – Air creates additional turbulence without substantially increasing the filter bed expansion • Must be stopped much before stopping the backwash water flow (needed to drive out air bubbles and to avoid air entrapment) • It is a water conservation measure – Lowers backwash water flow velocities and shortens the duration of backwash water run Filter to waste • Stop backwash water run, and allow draining out of the washwater • Close wash water drain, open water inlet and open filtered water wasting drain • Allow wastage of filtered water till desired water quality is achieved • Close the filtered water waste drain and start collecting the filtered water
  14. 14. Filter problems • Mud balls: Deposition of solids during backwashing instead of washout with backwash water – Can be from poor coagulation-flocculation-settling – Can be due to improper filter backwashing • Surface cracking: compressible matter around the media surface causes the surface cracking • Media boils: can be caused by – Too rapid backwash (higher backwash velocities!) – Displacement or uneven distribution of the gravel layer • Air binding: – Excessive head loss during filter run leading to negative pressures in the under drain system can result in air suction – Running the filter dry (filter bed exposed to air) • Improper backwashing (from shorter backwash durations, lower backwash velocities, etc. ) – Can be a cause for media boiling, loss of filter media and inefficient filter cleaning
  15. 15. Hydraulics of filtration • Carmen-Kozeny equation is used for the filter hydraulic analysis – Fair-Hatch equation, Rose equation and Hazen equation can also be used g V d Lf h s 2 3 1      75.1 1 150    RN f    s R dV N  h is head loss through filter bed (meters) f is friction factor (fi)  is bed porosity L is depth of filter bed (meters) d is diameter of the media particle (meters) dgi is geometric mean between sieve sizes Vs is superficial (approach) velocity (m/sec.)  is particle shape factor (1 for spherical particle, 0.82 for rounded sand, 0.75 for average sand and 0.73 for crushed coal/angular sand)  is density and  is viscosity of water NR is Reynolds number pi is fraction of solids    gi i i s d p f g LV h 2 3 11   
  16. 16. • Filter beds are stratified, and hydraulic analysis involves considering the filters as non-uniform beds • Filter hydraulics equations are applicable for clean beds (not for clogged beds) – Acceptable equations are not available for clogged beds • With filtration, solids accumulate in the filter bed and decrease the bed porosity, and this inturn increases the head loss • The head loss increase depends on – Nature of the suspension – Characteristics of the media – Filter operation • Running a pilot scale filter (at constant filtration rates) till the turbidity breakthrough, and monitoring the head loss across the filter can help in understanding the clogged filter hydraulics Hydraulics of filtration
  17. 17. • Direction of flow is reversed (upwards through the media) • Media bed is expanded (contact among the grains is broken) and grain surfaces are exposed for cleaning – hydrodynamic shear and rubbing action among the grains help • Expansion occurs when force applied by flowing water is greater than the net weight force of filter medium Head required for expansion = Net weight of the filter bed • Depth of expanded bed weight of the packed bed = weight of the fluidized bed fb fb LL      1 1 Lfb is depth of the fluidized bed  Is porosity and fb is porosity of fluidized bed   w wm fb Lh      1 hfb is head loss need to initiate bed expansion m is density of the medium w is density of water Filter backwash hydraulics
  18. 18. Porosity of the expanded bed is a function of terminal settling velocity of the particles and the backwash velocity This relation on incorporation into the expression for expanded bed depth For a stratified non-uniform bed the expression will become 22.0        t B fb V V  VB is back wash velocity Vt is terminal settling velocity of particles Optimum porosity for backwashing is 0.65-0.70                        22.0 1 )1( ti B i fb V V x LL  Filter hydraulics: during backwash 22.0 1 1          t B fb V V LL  Optimum expansion for backwashing is 1.2 to 1.55 times of unexpanded bed
  19. 19. Slow Sand Filters (SSF) • Used in rural areas in place of a rapid gravity filter – Filtration rate is 50 to 100 times slower than that of a rapid gravity filter (0.1 to 0.3 m/hour - 0.2 m/hr is the typical rate) – Filter cleaning through backwashing is not practiced – the top clogged sand layer is scrapped and removed • Used for the removal of turbidity (colloidal particles), suspended solids and pathogens – Replaces the coagulation-flocculation-settling, the filtration and the disinfection by chlorination treatments in rural areas – Filtered water has < 0.3 NTU turbidity (the goal is < 0.1 NTU) – Output water may require chlorination (for quality improvement) – A pre-treatment in roughing filters may be needed specially when the turbidity is high (greater than 20-50 NTU) – Oxfam filters (use of geo-textile fabric on the top of the sand layer for straining out the suspended matter (pre-treatment!)
  20. 20. SSF : Constituents • Includes three tanks (the raw water tank, the filter and the filtered water tank) • The filter includes – Supernatant water (0.5 to 1.5 m depth) – Sand (filter) bed • Granular filter medium of 0.15-0.35 effective size and 2-3 uniformity coefficient • Depth of the bed is >0.6 m (upto 1.6 m) – Schmudzdecke layer (a biological film or mat!) • Develops on the top of the sand bed within a few weeks, and disturbed during cleaning, but redevelops within a few days • Filters out and/or consumes and absorbs/adsorbs organic and inorganic contaminants including bacteria, viruses, etc. – Gravel layer: 3 grades of gravel (fine, 2-8 mm; medium size, 8-16 mm; and bottom coarse size, 16-32 mm) are used – Drainage system: bricks, concrete slabs, porous concrete, perforated pipes and screen system – Covering of the filter to avoid winter freezing and algal growth
  21. 21. SSF: Schmutzdecke layer • A bio film/mat (0.5 to 2 cm) formed on the sand bed surface – Made up of algae, bacteria, fungi and other microbes and accumulated particulates – Full development may take a few weeks time (>4 weeks) - Proper water temp. and sufficient nutrients support the development – Requires 2-7 days (even 2 to 3 weeks) for the redevelopment after each cleaning • Filters out and absorbs/adsorbs organic and inorganic contaminants (removes particles of <2 µm size) – contributes to the biological water quality (MPN reduction) improvement (bacteriovory) – Breaks down organics, acts as fine mechanical filter • The sand filter (the top 20 cm, and even the top of 0.4 or 0.5 m depth layer) also shows biological activity
  22. 22. Slow Sand Filter • Depth of the sand bed is maintained upto 1.6 m (>0.6 m) to support filter cleaning (scrapping removal of top sand layer) • The gravel layers can be replaced by a synthetic fabric – Below the gravel for protecting the filter tank lining, 50 mm thick sand layer may be used • Filter is always kept submerged in water for maintaining the biological mat – Must not be run dry (unless complete draining out is needed) – Outlet should be slightly (50 mm) above the top of the sand layer for keeping the filter wet and submerged • Provisions should be made to – dissipate the energy of the water loaded to the filter – drain out the supernatant water – drain out the filter bed – backfill the filter with filtered water
  23. 23. Cleaning of Slow Sand Filter • Head loss for a clean slow sand filter is <0.06 m • Head loss >1.5 m is avoided through cleaning • Higher head loss can lead to air binding and uneven flow of water through the filter • Clogged filter is cleaned once in every 20 to 90 days • Turbidity of water and filtration rate determine the cleaning interval • Supernatant from the sand bed is drained out to below 20 cm depth of the sand bed prior to cleaning through scraping • Involves manual scraping of 2 to 5 cm of the top sand and discarding • After scrapping, refilling the filter with water should be done from the bottom for avoiding air entrapment • New sand is added when the sand depth drops to <24 inch (may be once in 10 years) • Cleaning affects the filter performance for a few days (ripening period) – After the ripening period returns to normal performance
  24. 24. Slow Sand Filter • Start-up of a slow sand filter may take quite long time – Development of the ‘Schmutzdecke’ takes a few weeks time • Refilling the filter with water after the scrap removal of the top sand layer is associated with the risk of air entrapment • Refilling from the bottom through the underdrain system with filtered water can avoid the air entrainment • Cleaning affects the filter performance for a few days (ripening period) – The filtered water during this period should not be used – instead should be drained out • In the water reservoir algal growth can occur – can add oxygen to water, but can interfere with the operation • Anaerobic conditions in the filter bed can infuse lasting bad taste to water – Pre-treatment of water to remove organics is often required – Water being filtered must have >3 mg/L DO
  25. 25. Design parameters Recommended range of values Filtration rate Area per filter bed 0.15 m3/m2/h (0.1–0.2 m3/m2/h) Less than 200 m2 (in small community water supplies to ease manual filter cleaning) Number of filter beds Minimum of two beds Depth of filter bed 1 m (minmum of 0.7 m of sand depth) Filter media Effective size (ES) = 0.15–0.35 mm; uniformity coefficient (UC) = 2-3 Height of supernatant water 0.7–1 m (maximum 1.5 m) Underdrain system Standard bricks Precast concrete slabs Precast concrete blocks with holes on top Porous concrete Perforated pipes Generally no need for further hydraulic calculations. Maximum velocity in the manifolds and in laterals = 0.3 m/s Spacing between laterals = 1.5 m Spacing of holes in laterals = 0.15 m Size of holes in laterals =3 mm Design parameters for typical slow sand filter
  26. 26. Bio-sand Filter
  27. 27. Bio-sand filter: Maintenance • Remove the lid and the colander/diffuser basin and lower the water level inside the filter by using a small cup to scoop out the water that has not drained through the outlet pipe. • Make a small hole in the sand with the cup and scoop out the water that accumulates in it until only wet sand remains. • Remove 3 to 5 cm of the fine sand layer and set it aside (after washing and drying in the sun, this sand may be reused in the next time maintenance) • Add clean, fine sand from previous maintenance and level surface of the sand and reinstall the colander/diffuser basin • Slowly add water to the filter until water begins to flow through the outlet pipe again and water is 5 cm above fine sand layer. • Again remove the lid and colander/diffuser basin, and again level the surface of the sand and reinstall the colander/diffuser basin
  28. 28. Roughing Filters • A pre-treatment unit for slow sand filtration, chlorination, and biological treatment process • Used to reduce turbidity to <20 to 50 NTU • Used to separate fine solids/chemical flocs that escape sedimentation (prior to biological treatment or prior to chlorination) • Roughing filters, in addition to filtration, can also support adsorption, ion-exchange and even biological processes – Can however handle very low loads • Materials like gravel, quartz sand, burnt bricks, coal/charcoal are used as the filter medium – Particle size varies from >20 mm to <2 mm – Used as graded layers - Size decreases in the filtration direction – promotes solids penetration – Has higher solids storage capacity • Run in up-flow (VRF) or horizontal flow (HRF) regimes • Incorporates a simple backwashing mechanism – Flow direction is reversed by opening the under drains and higher rates of flows are allowed through the filter
  29. 29. Horizontal flow roughing filters (HRF) • A shallow structure of limited length and unlimited width – Usually assembled in 3 compartments (coarse, medium size and fine size medium) – Drainage facilities are placed at the bottom(perpendicular to flow) • Less susceptible to solids breakthrough, but more sensitive to hydraulic short-circuiting • Filtration rates are 0.3 to 1.5 m/hr • Water is maintained below to the surface of the filter bed (to shade and prevent the algal growth) Vertical flow roughing filters (VRF) • Occupy relatively lesser floor space • 3 or more filters are arranged in series or are stratified – Filter bed is shaded from top to prevent algal growth – Bottom of the filter has the under drainage system – Filter material is maintained completely submerged in water (10 cm layer of water is maintained above the filter) – Filtration rate is usually o.3 to 1.0 m/hr
  30. 30. Multi-stage multi-grade vertical flow roughing filter
  31. 31. Reservoir is provided for holding enough water for the backwashing Under drain system allows backwashing at the rate of 40-60 m/hr