Filtration(Slow sand, Rapid gravity and Roughing) Dr. Akepati S. Reddy Associate Professor, Thapar University Patiala (PUNJAB) – 147 001 INDIA Granular media filtration (rapid gravity, slow sand and roughing filters) Filtration types • Depth filtration and surface filtration • Granular media filters – Slow sand filters – Rapid (sand) gravity filters – open type – Closed type (pressure filters!) – High rate filters Duel media (sand and anthracite) and multi-media (sand, anthracite and garnet) filters Roughing filters (pre-filters) • Membrane filters – Micro-filters – Ultra-filters – Nano-filters (Reverse Osmosis) • Home water treatment options – Candle filters – Bio-sand filter Filtration Used to remove suspended particles, small flocs, turbidity and even pathogens • Rapid gravity filter for flocs and suspended particles removal • Slow sand filters for the removal of turbidity and pathogens • Roughing filter as pre-filter for reducing suspended solids, flocs and turbidity to acceptable levels and making water fit specially for slow sand filtration – VRF (vertical upflow) and HRF (horizontal flow) Mechanisms of treatment – Transport of solids to the filter medium grain surface by settling, inertial impaction/interception, diffusion, brownian motion, etc. – Retention of the transported solids by straining, electrochemical forces, vander Waals forces, physical adsorption, etc. – Bio-film and biological action (biodegradation) – Chemical destabilization (coagulation) and flocculation also contributes (Alum enhances pathogen removal in SSF) Particle Removal Mechanisms Suspension feeders Grazers Attachment to biofilms Capture by predators to medium to previously removed particles by medium by previously removed particles Straining (fluid and gravitational forces) Attachment (electrochemical forces) Physical-Chemical Biological Particle Removal Mechanisms • A bed of filtering medium supported on a gravel layer resting on a porous structure and under-drain system • Filter medium: sand (effective size & uniformity coefficient) • Water reservoir of desired height (depth) is maintained over the filter bed for higher rates of filtration – Filtration rates are in the range 4 to 12 m/hr. – In closed type, water is maintained under pressure over the bed • Continued use clogs the filter bed and reduces filtration rates, necessitates regular backwashing (every 24 to 72 hours once) – The filtration unit has a channel for feeding water and draining out the backwash water – Over-head backwash water supply reservoir with necessary piping and fittings is used in the backwashing (compressed air!) – Overflow weir and troughs are provided over the filter bed for the backwash water collection and conveyance to the channel – Necessary piping and fittings specially valves to control flows both during run and during backwashing (draining out of initial filtration water as wastewater) Rapid Gravity (open type) Filters Schematic diagram of a rapid gravity filter Here A, B, C, D and E are the flow control valves used Provisions for use ofc compressed air in the backwashing Gravel Backwash water Overhead tank Multi-media filter Filter media Commonly used media • Sand (SG: 2.6 typical bed depth 8 to 24 inch) – typical effective size 0.45 to 0.55 mm and uniformity coefficient of 1.65 • Anthracite coal (SG: 1.6; typical bed depth: 6.5 to 18 inch) • Garnet (SG: 4.2; typical bed depth: 4.5 inch) • Green sand (a natural resin specific to iron and manganese) – Needs regeneration (reappearance of punk colour) with potassium permanganate • Filtralite (baked clay!) (SG 1.6 to 1.8) – synthetic medium • Granular activated carbon Characteristics • Size and size distribution (effective size and uniformity coefficient) • Density and bulk density • Porosity (external!) and swelling properties • Ability to support microbial film development • Adsorption and ion exchange properties • Strength properties such as hardness Grain size and its distribution are important and affects clear water head loss and built-up head loss during filter run • Sieve analysis and plotting cumulative passing through the given sieve sizes on a log-probability plot • Sieve size is described by mesh number – 10 mesh has 2 mm pore size – 18 mesh has 1 mm pore size – 35 mesh has 0.5 mm pore size – 60 mesh has 0.25 mm pore size • 99% pass size and 1% pass size • Effective size: pass size for 10% of the material (indicated by D 10 ) – median size of the grain • Uniformity coefficient: ratio of D 60 /D 10 (similar to Stand. Dev.) • D 90 (needed in the backwash velocity calculations) Filter medium Log – probability plots of filter medium size The graded gravel layer • Supports the filter medium from below • The filter medium should not penetrate into it and gravel should not enter the under-drain system • Particles of a layer above should not penetrate the layer below it – Spherical particles allow penetration of particles of <1/3 rd the gravel size – Size of the particles in the above layer can be half the size of the particles in the layer below • 90 percentile size filter medium particles should not penetrate the top gravel layer • Gravel of the bottom most gravel layer should not penetrate the under drain system • Minimum size of the gravel should be twice the perforations size The graded gravel layer • Thickness of each of the gravel layers should be >6 times the largest size particle of the layer • Graded gravel layers for an under drain system with 5 mm perforations and for a filter medium of 1.0 mm (D 90 ) size – 10-20 mm size gravel of >120 mm thickness – 5-10 mm size gravel of >60 mm thickness – 2.5-5.0 mm size gravel of >30 mm thickness – 1.25-2.5 mm size gravel of >15 mm thickness • A newly assembled filter may be started with backwashing – Ensures proper stratification of the filter medium above • The graded gravel layer should ensure uniform distribution of the upward flowing backwash water and compressed air The under-drain system Under drain system should • Support the graded gravel, filter bed and water column • Allow filtered water to pass through, collected and conveyed out • Allow the incoming backwash water (and compressed air) to pass through and dispersed 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 • Typical filtration rates of 5 to 15 m/hr. are achieved through maintaining a water column (water reservoir) of up to 1.5 m height above the filter bed • In closed type rapid gravity filters, a virtual water column of the desired height is maintained through pressurizing the water • Size of the filter medium, extent of clogging of the filter bed, and height of water column over the filter bed 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 – Pizometers are used for the monitoring of head loss across the filter Water reservoir over the filter bed • 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 of the filter opens into this channel • The channel ensures submergence of the filter bed in water Water reservoir over the filter bed 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 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 Filter cycle Filter operation • 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 Filter cycle Air scouring during backwashing • Air scouring is very effective when water is introduced at sub- fluidization rates along with the compressed air – air creates additional turbulence without substantially increasing of filter bed expansion • Compressed air is used in three different modes – First only compressed air and then the backwash water – First only compressed air, then both air and backwash water (at sub- fluidization rate), and then only backwash water (at fluidization rates) – First compressed air and backwash water (at sub-fluidization rate) and then only backwash water (at fluidization rates) • Air scouring is a water conservation measure Filter to waste • Stop backwash water, 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 Typical filtration cycle 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 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 Filter hydraulics: During run Two separate categories: hydraulics of filtration process and hydraulics of filter backwashing Carmen-Kozeny equation is used for the hydraulic analysis of filter (Fair-Hatch equation, Rose equation and Hazen equation can also be used) g V d L f h s 2 3 1 o o | ÷ = 75 . 1 1 150 + ÷ = R N f o µ µ | s R dV N = h is head loss through filter bed f is friction factor (f i ) o is bed porosity L is depth of filter bed d is diameter of the media particle d gi is geometric mean between sieve sizes V s is superficial (approach) filtration velocity | is particle shape factor (1 for spherical particle 0.82 for rounded sand 0.75 for average sand 0.73 for crushed coal/angular sand) µ is density and µ is viscosity of water N R is Reynold number p i is fraction of solids ¿ ÷ = gi i i s d p f g LV h 2 3 1 1 o o | Equation for non-uniform bed is to be used – f value will be different for different dia. particles There are no acceptable mathematical equations for assessing head loss changes in clogged beds – The equations are applicable for clean filter beds Solids accumulation decrease porosity & increase head loss Head loss change depends on Nature of the suspension, Characteristics of the media, and Filter operation If constant head (driving force) is applied then filtration rate will diminish with decreasing porosity – For constant filtration rates head applied should be increased to match with the head loss increase Terminate filter run for backwash on sufficient solids accumulation – Storage capacity of the bed is exhausted – Driving force available is not sufficient for enough filtration rates Filter hydraulics: during run • 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 by – hydrodynamic shear forces – rubbing action among the grains • Expansion occurs when force applied by flowing water is greater than the buoyant weight of the grains Head required for expansion = weight of the packed bed • Depth of expanded bed – Greater than the packed bed depth – Assuming weight of packed bed = weight of fluidized bed ( ) w w m f b L h µ µ µ o ÷ ÷ = 1 f b f b L L o o ÷ ÷ = 1 1 L fb is depth of the fluidized bed o Is porosity and o fb is porosity of fluidized bed h fb is head loss need to initiate bed expansion µ m is density of the medium µ w is density of water Filter hydraulics: during backwash Porosity of the expanded is a function of terminal settling velocity of the particles and the backwash velocity and given as 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 o 22 . 0 1 1 | | . | \ | ÷ ÷ = t B f b V V L L o V B is back wash velocity V t is terminal settling velocity of particles Optimum porosity for backwashing is 0.65-0.70 ¿ | | | | | . | \ | | | . | \ | ÷ ÷ = 22 . 0 1 ) 1 ( ti B i f b V V x L L o Filter hydraulics: during backwash Optimum expansion for backwashing is 1.2 to 1.55 times of unexpanded bed 5 . 4 fB t B V V o = Flow Regime Defined on the basis of Reynolds number (Re) Four flow regimes – Darcy flow (Re<1) – encountered in slow sand filters! – Forchheimer flow (Re 1-100) – encountered in rapid sand filters! – Transition flow (Re 100-800) – not encountered in filtration – Fully turbulent flow (Re >800) – not encountered in filtration w f w e D V R µ µ = µ w is fluid density µ w is fluid dynamic viscosity D is dia. of media grain V f is superfecial velocity (filtration rate) Flow Regime Darcy flow – Occurs (when Re is <1) in slow sand filters and also in rapid sand filters (when the filtration rates are lower) – Flow is governed by Darcy’s law Forchheimer flow – Occurs in rapid sand filters when filtration rates are high and during backwashing (3>Re<25) – Laminar flow (influenced by both viscous & inertial forces) – Head loss is given by L h K V L f = K is hydraulic conductivity hL is head loss across the filter L is depth of the filter bed (granular media) 2 2 1 f f L V K V K L h + = Slow Sand Filters Slow Sand Ffilters (SSF) History • Use of SSF dates back to 1790 in Lancashire, England (used to filter municipal water in London in 1829) • SSF was shown to remove bacteria in 1885, and to remove Giardia in 1980s According to WHO, under suitable conditions, slow sand filtration is the cheapest, the simplest and the most efficient method of water treatment Used for Turbidity (colloidal particles) removal, for the reduction of bacteria, viruses, and protozoa, and also for organic levels reduction Used for treating low turbidity water (<20 NTU) • Water with >20 NTU turbidity requires pre-treatment (roughing filters!) - pre-treatment may also be needed for BOD reduction • Water with >200 NTU turbidity is not at all allowed Physical operations, and chemical and microbiological processes may be involved in the treatment SSF : Constituents • Includes three tanks (raw water, filter & filtered water tanks) • Filter tank 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 • Medium must be free from organic matter, loam and clay • 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 by 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, 6-32 mm) are used – Drainage system: bricks, concrete slabs, porous concrete, perforated pipes and screen system – Covering of filter to avoid winter freezing and algal growth A. Valve for raw water inlet and regulation of filtration rate B. Valve for draining unfiltered water C. Valve for back-filling the filter bed with clean water D. Valve for draining filter bed and outlet chamber E. Valve for delivering treated water to waste F. Valve for delivering treated water to the clear-water reservoir 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 development – Requires 2-7 days (even 2 to 3 weeks) for the redevelopment after each cleaning • Filters out or consumes and absorbs/adsorbs organic and inorganic contaminants and contributes to the reduction of bacteria, viruses and protozoa – Reported as good for the removal of <2 µm particles – Bacteriovory was reported as the significant biologically mediated particle removal mechanism – Breaks down organics, acts as fine mechanical filter • The sand filter (the top 20 cm, even upto 0.4 to 0.5 m depth) also shows biological activity Slow Sand Filter • Thickness of the sand layer: >0.6 m (every additional 0.3 m thick layer supports additional 3-4 years operation) • Oxfam filters: Geo-textile fabric is used on the top of the sand layer for retaining much of the suspended matter strained from the loaded water • 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 must always be 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 Slow Sand Filter • Water loaded should have <50 NTU turbidity – use pre-filter when it is >30 NTU • Filtration rate: – 50 to 100 times slower than the rate for rapid gravity filter – 100 to 300 L/m 2 hour (ideal: 0.2 m/hr & Max. rate:0.3 m/hr) • Filtered water has <0.3 NTU turbidity (goal is <0.1 NTU) – Filtered water may require chlorination for superior water quality • Anaerobic conditions in the filter bed can infuse lasting bad taste to water – Often necessitates water pre-treatment to remove organics – Water being filtered must have >3 mg/L DO • Start-up of a slow sand filter may take quite long time – Development of the biological mat ‘Schmutzdecke’ takes a few weeks time • In the water reservoir algal growth can occur – can add oxygen to water, but can interfere with the operation Cleaning of Slow Sand Filter • Initial head loss for a clean slow sand filter is <0.2 feet • Head loss >5 feet is avoided (can lead to air binding and uneven flow of water through the filter) through cleaning • Clogged filter (filtration rate dropped below acceptable levels) needs cleaning – 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 – After scrapping, refilling the filter with water should be done from the bottom for avoiding the air entrapment – Involves manual scraping of 2 to 5 cm of the top sand and dsicarding – 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 Design parameters Recommended range of values Filtration rate Area per filter bed 0.15 m 3 /m 2 •h (0.1–0.2 m 3 /m 2 •h) Less than 200 m 2 (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 Bio-sand Filter Bio-sand filter: Maintenance 1) Remove the lid and the colander/diffuser basin. 2) 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. 3) Make a small hole in the sand with the cup. Scoop out the water that accumulates in it until only wet sand remains. 4) Remove 3 to 5cm of the fine sand layer and set it aside. (After washing and drying in the sun, this sand may be reused next time maintenance is performed.) 5) Add clean, fine sand from previous maintenance. Level the surface of the sand. 6) Reinstall the colander/diffuser basin. 7) 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. 8) Again remove the lid and colander/diffuser basin 9) Level the surface of the sand again 10) Reinstall the colander/diffuser basin Roughing Filters (RF) (HRF and VRF) Roughing Filters • A pre-treatment unit used to remove/separate fine solids that could not be removed by sedimentation – May precede final treatment processes like SSF and chlorination – Required to reduce turbidity of water to <20 to 50 NTU prior to filtration in slow sand filters • Removal of suspended solids require laminar flow conditions within the filter bed (Reynold’s Number: <1.0) – Also be used to remove chemical flocs prior to biol. treatment and biological flocs prior to chlorination • Roughing filters can also support adsorption, absorption and chemical and biological processes – Vander waals forces and electrostatic forces attract the particles and hold on the medium surface – Biological water quality, dissolved organic matter, colour etc., parameters can get adjusted – Can handle very low organic loads – higher loads can clog the filter and reduce the hydraulic cleaning capacity Roughing Filters • Broken burnt bricks, charcoal, coconut fiber, quartz sand, gravel, charcoal, maize cobs or any other clean insoluble and mechanically resistant material can be used as filter medium – Filter media particle size varies from >20 mm to <2 mm – Use of multi-grade filter media, with size decreasing in the flow direction, can promote particle penetration through the filter bed • Filter incorporates a simple self cleaning (backwashing) mechanism – An under drainage system enables the filter flushing and cleaning – Flow direction is reversed through opening the downwash drains and higher rates of flows clean the filter – Unpacking the filter media and cleaning may often be required • Performance monitoring is done using the parameters: TSS, turbidity, colour, coliform count, iron and manganese, and algae • Mostly run in up-fow or horizontal flow regimes (VRF and HRF systems) • Have larger capacities to store the removed solids (HRFs have relatively larger storage capacity than VRFs) Roughing Filters: HRF • Filtration rates for HRF may be in the range of 0.3 to 1.5 m/hr • Shallow structure and hence and hence no structural problems • Unlimited length of the filter is possible – Usual length is 5 to 7 m • Filter is usually assembled in 3 compartments (coarse, medium and fine medium filter phases – Filter medium size ranges from <4 mm to >20 mm • Water is maintained below to the surface of the filter bed to shade and prevent the algal growth • HRFs are less susceptible to solids breakthrough and more sensitive to hydraulic short-circuiting – Can handle short time turbidity loads of 500 to 1000 NTU • Drainage facilities are placed at the bottom of the filter perpendicular to the flow direction – Drainage velocities of 60-90 m/hr are used for a good hydraulic cleaning of the filter Roughing Filters: VRF systems • Occupy relatively lesser floor space • Usually includes 3 or more filters arranged in series – VRF in layers (the 3 or more filters are stratified) are also used • Height of the filter bed may be 1 to 1.2 m and the filter medium size is 12-18 mm; 8-12 mm; and 4-8 mm – Filter bed is covered (by a layer of stones (100 mm size!) for shading the water and preventing algal growth – Bottom of the filter has drainage facilities (perforated pipes, false filter bottom, etc.) • Operated either as down-flow or as up-flow filters (upflow filters are recommended - VRF in layers are operated only in upflow mode) – 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 – Can handle water with turbidity 50-150 NTU – Filter resistance or head loss is <20 cm per filter Design of multistage multigrade roughing filter • Multistage (3 or more stages), multigrade filter (VRF in layers) – Divisible into the filter bed proper and the gravel support layers – Gravel support layers satisfy the condition of thickness >6 times the size of the largest medium particle in the layer – Thickness of the filter bed layer can be much higher • Inlet conveys water into the under drain system and uniformly distributes water for the upflow filtration – Air bubble entry into the under-drain system is avoided • Reservoir for holding enough water for the backwashing – Filtered water is drained out while keeping the filter bed submerged • Under drain system allowing backwashing (at the rate of 40-60 m/hr) • Filtration rate (0.3 to 1.0 m/hr) – Laminar flow conditions are ensured within the filter bed (Rynold’s number <1.0) – Scour velocities are avoided with the filter bed Typical design of a Multistage Multigrade Vertical flow Roughing filter
Report "Granular Media Filtration for Water and Wastewater treatment"