Presentation on theme: "On removing little particles with big particles"— Presentation transcript:
1 On removing little particles with big particles Filtration TheoryOn removing little particles with big particles
2 Filtration Outline Filters galore Particle Capture theory Filters Range of applicabilityParticle Capture theoryTransportDimensional AnalysisModel predictionsFiltersRapidSlow“BioSand”PotsRoughingMultistage Filtration
3 Filters Galore Slow Sand Bag Rapid Sand Pot Cartridge “Bio” Sand Diatomaceous earth filterRoughCandle
4 Categorizing Filters Straining Depth Filtration Particles to be removed are larger than the pore sizeClog rapidlyDepth FiltrationParticles to be removed may be much smaller than the pore sizeRequire attachmentCan handle more solids before developing excessive head lossFiltration model coming…All filters remove more particles near the filter inlet
5 The “if it is dirty, filter it” Myth The common misconception is that if the water is dirty then you should filter it to clean itBut filters can’t handle very dirty water without clogging quickly
6 Filter range of applicability SSFRSF+CartridgeBagPotCandleDE1000NTU110100on DE filters
7 Developing a Filtration Model Iwasaki (1937) developed relationships describing the performance of deep bed filters.C is the particle concentration [number/L3]l0 is the initial filter coefficient [1/L]z is the media depth [L]Iwasaki, T. (1937). "Some Notes on Sand Filtration." Journal American Water Works Association 29: 1591.The particle’s chances of being caught are the same at all depths in the filter; pC* is proportional to depth
8 Graphing Filter Performance This graph gives the impression that you can reach 100% removalWhere is 99.9% removal?
9 Particle Removal Mechanisms in Filters collectorTransport to a surfaceMolecular diffusionInertiaGravityInterceptionAttachmentStrainingLondon van der Waals
10 Filtration Performance: Dimensional Analysis What is the parameter we are interested in measuring? _________________How could we make performance dimensionless? ____________What are the important forces?Effluent concentrationC/C0 or pC*InertiaLondon van der WaalsElectrostaticViscousGravitationalThermalNeed to create dimensionless force ratios!
11 Dimensionless Force Ratios Reynolds NumberFroude NumberWeber NumberMach NumberPressure/Drag Coefficients(dependent parameters that we measure experimentally)
12 What is the Reynolds number for filtration flow? What are the possible length scales?Void size (collector size) max of 0.7 mm in RSFParticle sizeVelocitiesV0 varies between 0.1 m/hr (SSF) and 10 m/hr (RSF)Take the largest length scale and highest velocity to find max ReFor particle transport the length scale is the particle size and that is much smaller than the collector size
13 Choose viscosity!In Fluid Mechanics inertia is a significant “force” for most problemsIn porous media filtration viscosity is more important that inertia.We will use viscosity as the repeating parameter and get a different set of dimensionless force ratiosInertiaGravitationalViscousThermalViscous
14 GravityvelocitiesforcesvporeGravity only helps when the streamline has a _________ component.horizontalUse this definition
15 Diffusion (Brownian Motion) vporeDiffusion velocity is high when the particle diameter is ________.kB=1.38 x J/°KT = absolute temperaturesmalldc is diameter of the collector
16 London van der WaalsThe London Group is a measure of the attractive forceIt is only effective at extremely short range (less than 1 nm) and thus is NOT responsible for transport to the collectorH is the Hamaker’s constantVan der Waals forceViscous force
17 What about Electrostatic repulsion/attraction? Modelers have not succeeded in describing filter performance when electrostatic repulsion is significantModels tend to predict no particle removal if electrostatic repulsion is significant.Electrostatic repulsion/attraction is only effective at very short distances and thus is involved in attachment, not transport
18 Geometric ParametersWhat are the length scales that are related to particle capture by a filter?________________________________________Porosity (void volume/filter volume) (e)Create dimensionless groupsChoose the repeating length ________Filter depth (z)Collector diameter (media size) (dc)Particle diameter (dp)(dc)Number of collectors!Definition used in model
19 Write the functional relationship Length ratiosForce ratiosIf we double depth of filter what does pC* do? ___________doublesHow do we get more detail on this functional relationship?Empirical measurementsNumerical models
20 Numerical Models Trajectory analysis A series of modeling attempts with refinements over the past decadesBegan with a “single collector” model that modeled London and electrostatic forces as an attachment efficiency term (a)Yao, K.-M., M. T. Habibian, et al. (1971). "Water and Waste Water Filtration: Concepts and Applications." Environmental Science and Technology 5(11): 1105.InterceptionSedimentationDiffusiona
22 Transport Equations Brownian motion Interception Gravity Total is sum of partsTransport is additive
23 Filtration Technologies Slow (Filters→English→Slow sand→“Biosand”)First filters used for municipal water treatmentWere unable to treat the turbid waters of the Ohio and Mississippi RiversCan be used after Roughing filtersRapid (Mechanical→American→Rapid sand)Used in Conventional Water Treatment FacilitiesUsed after coagulation/flocculation/sedimentationHigh flow rates→clog daily→hydraulic cleaningCeramic
24 Rapid Sand Filter (Conventional US Treatment) SpecificGravity1.62.65Depth(cm)3045Size(mm)0.705 - 60AnthraciteInfluentSandGravelDrainEffluentWash water
25 Filter Design Filter media Flow rates smaller Backwash rates silica sand and anthracite coalnon-uniform media will stratify with _______ particles at the topFlow ratesm/dayBackwash ratesset to obtain a bed porosity of 0.65 to 0.70typically 1200 m/daysmallerCompare with sedimentation
26 Backwash Wash water is treated water! WHY? Anthracite Only clean water should ever be on bottom of filter!SandInfluentGravelDrainEffluentWash water
27 Rapid Sand predicted performance Interception is very importantLousy at removing pathogens if they haven’t been flocculatedA 0.1mm particle has a pC* of 100!!!!!!!!!!Either particles haven’t been flocculated or attachment is poorNot very good at removing particles that haven’t been flocculated
28 Slow Sand Filtration filter cake First filters to be used on a widespread basisFine sand with an effective size of 0.2 mmLow flow rates ( m/day)Schmutzdecke (_____ ____) forms on top of the filtercauses high head lossmust be removed periodicallyUsed without coagulation/flocculation!Turbidity should always be less than 50 NTU with a much lower average to prevent rapid cloggingCompare with sedimentationfilter cake
29 Slow Sand Filtration Mechanisms Protozoan predators (only effective for bacteria removal, not virus or protozoan removal)Aluminum (natural sticky coatings)Attachment to previously removed particlesNo evidence of removal by biofilms
30 Typical Performance of SSF Fed Cayuga Lake Water 1Fraction of influent E. coli remaining in the effluent0.10.0512345Time (days)(Daily samples)Filter performance doesn’t improve if the filter only receives distilled water
31 Particle Removal by Size 1control3 mM azide0.1Fraction of influent particles remaining in the effluentEffect of the Chrysophyte0.01What is the physical-chemical mechanism?0.0010.81Particle diameter (µm)10
32 Techniques to Increase Particle Attachment Efficiency Make the particles stickierThe technique used in conventional water treatment plantsControl coagulant dose and other coagulant aids (cationic polymers)Make the filter media stickierBiofilms in slow sand filters?Mystery sticky agent present in surface waters that is imported into slow sand filters?
33 Cayuga Lake Seston Extract Concentrate particles from Cayuga LakeAcidify with 1 N HClCentrifugeCentrate contains polymerNeutralize to form flocs
34 Seston Extract Analysis I discovered aluminum!carbon16%How much Aluminum should be added to a filter?
35 E. coli Removal as a Function of Time and Al Application Rate No E. coli detected20 cm deep filter columnspC* is proportional to accumulated mass of Aluminum in filter
37 How deep must a filter (SSF) be to remove 99.9999% of bacteria? Assume a is 1 and dc is 0.2 mm, V0 = 10 cm/hrpC* is ____z is ________________What does this mean?6for z of 1 m23 cm for pC* of 6Suggests that the 20 cm deep experimental filter was operating at theoretical limitTypical SSF performance is 95% bacteria removalOnly about 5 cm of the filters are doing anything!
39 Aluminum feed methodsAlum must be dissolved until it is blended with the main filter feed above the filter columnAlum flocs are ineffective at enhancing filter performanceThe diffusion dilemma (alum microflocs will diffuse efficiently and be removed at the top of the filter)
40 Performance Deterioration after Al feed stops? HypothesesDecays with timeSites are used upWashes out of filterResearch resultsNot yet clear which mechanism is responsible – further testing required
41 Sticky Media vs. Sticky Particles Potentially treat filter media at the beginning of each filter runNo need to add coagulants to water for low turbidity watersFilter will capture particles much more efficientlySticky ParticlesEasier to add coagulant to water than to coat the filter media
42 The BioSand Filter Craze Patented “new idea” of slow sand filtration without flow control and called it “BioSand”Filters are being installed around the world as Point of Use treatment devicesCost is somewhere between $25 and $150 per household ($13/person based on project near Copan Ruins, Honduras)The per person cost is comparable to the cost to build centralized treatment using the AguaClara model
43 “BioSand” Performance The operation, flow conditions and microbial reductions of an intermittently operated, household-scale slow sand filterM.A. Elliott*, C.E. Stauber, F. Koksal, K.R. Liang, D.K. Huslage, F.A. DiGiano, M.D. Sobsey.*University of North Carolina, CB 7431, Chapel Hill, NC, 27514, USA.Long ripening periodAfter pore volume is flushed has poor performance
44 “BioSand” Performance Pore volume is 18 LitersVolume of a bucket is ____________Highly variable field performance even after initial ripening periodField tests on 8 NTU water in the DR
45 Field Performance of “BioSand” Table 2 pH, turbidity and E. coli levels in raw and BSF filter waters in the fieldParameter raw filteredMean pH (n =47)Mean turbidity (NTU) (n=47)Mean log10 E. coli MPN/100mL (n=55)
46 Potters for Peace PotsColloidal silver-enhanced ceramic water purifier (CWP)After firing the filter is coated with colloidal silver.This combination of fine pore size, and the bactericidal properties of colloidal silver produce an effective filterFilter units are sold for about $10-15 with the basic plastic receptacleReplacement filter elements cost about $4.00What is the turbidity range that these filters can handle?How do you wash the filter? What water do you use?
47 Horizontal Roughing Filters 1m/hr filtration rate (through 5+ m of media)Usage of HRFs for large schemes has been limited due to high capital cost and operational problems in cleaning the filters.Equivalent surface loading =10 m/dayGravity roughing filter for pre-treatmentJ.M.J.C. Jayalath and J.P. Padmasiri, Sri LankaPicture fromfor filtration rate
48 Roughing FiltersFiltration through roughing gravity filters at low filtration rates (12-48 m/day) produces water with low particulate concentrations, which allow for further treatment in slow sand filters without the danger of solids overload.In large-scale horizontal-flow filter plants, the large pores enable particles to be most efficiently transported downward, although particle transport causes part of the agglomerated solids to move down towards the filter bottom. Thus, the pore space at the bottom starts to act as a sludge storage basin, and the roughing filters need to be drained periodically. Further development of drainage methods is needed to improve efficiency in this area.Filter Mechanisms in Roughing Filters Boller, M Aqua AQUAAA, Vol. 42, No. 3, p , June fig, 1 tab, 13 ref.
49 Roughing Filters Size comparison to floc/sed systems? Roughing filters remove particulate of colloidal size without addition of flocculants, large solids storage capacity at low head loss, and a simple technology.But there are only 11 articles on the topic listed in(see articles per year)They have not devised a cleaning method that worksFilter Mechanisms in Roughing Filters Boller, M Aqua AQUAAA, Vol. 42, No. 3, p , June fig, 1 tab, 13 ref.Size comparison to floc/sed systems?
50 Multistage Filtration The “Other” low tech option for communities using surface watersUses no coagulantsGravel roughing filtersPolished with slow sand filtersLarge capital costs for constructionNo chemical costsLabor intensive operationWhat is the tank area of a multistage filtration plant in comparison with an AguaClara plant?
51 Conclusions…Many different filtration technologies are available, especially for POUFilters are well suited for taking clean water and making it cleaner. They are not able to treat very turbid surface watersPretreat using flocculation/sedimentation (AguaClara) or roughing filters (high capital cost and maintenance problems)
52 ConclusionsFilters could remove particles more efficiently if the _________ efficiency were increasedSSF remove particles by two mechanisms__________________________________________________Completely at the mercy of the raw water!We need to learn what is required to make ALL of the filter media “sticky” in SSF and in RSFattachmentPredationSticky aluminum polymer that coats the sand
53 ReferencesTufenkji, N. and M. Elimelech (2004). "Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media." Environmental-Science-and-Technology 38(2):Cushing, R. S. and D. F. Lawler (1998). "Depth Filtration: Fundamental Investigation through Three-Dimensional Trajectory Analysis." Environmental Science and Technology 32(23):Tobiason, J. E. and C. R. O'Melia (1988). "Physicochemical Aspects of Particle Removal in Depth Filtration." Journal American Water Works Association 80(12):Yao, K.-M., M. T. Habibian, et al. (1971). "Water and Waste Water Filtration: Concepts and Applications." Environmental Science and Technology 5(11): 1105.M.A. Elliott*, C.E. Stauber, F. Koksal, K.R. Liang, D.K. Huslage, F.A. DiGiano, M.D. Sobsey. (2006) The operation, flow conditions and microbial reductions of an intermittently operated, household-scale slow sand filter