Technologies and Costs for Removal of Arsenic From Drinking Water

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A survey of full-scale plants by McNeill and Edwards (1995) indicated that soluble As(V)removal is mediated primarily by sorption to magnesium and/or ferric hydroxide solids during watersoftening operations. At softening facilities precipitating only calcite, soluble As(V) removal wasbetween 0 and 10 percent, whereas soluble As(V) removal at plants precipitating calcite andmagnesium and/or ferric hydroxide was between 60 and 95 percent.McNeill and Edwards (1997b) performed bench-scale studies to investigate the role of ironaddition in optimizing the As(V) removal. At pH 9 without any iron addition, only a small amount ofAs(V) was removed. However, adding increasing amounts of iron at this pH improved As(V)removal, with 82 percent of the As(V) removed at an iron dose of 9 mg/L. At pH 9.7, a 38 percentAs(V) removal without iron addition was observed, versus 63±8.4 percent removal for iron dosagesbetween 0.25 and 9 mg/L.Effect of Other ConstituentsThe competitive effects of sulfate and carbonate for surface binding sites onto magnesiumhydroxide surfaces and the influence on the adsorption of arsenic was examined by McNeill andEdwards (1998). These effects were investigated in experiments with preformed magnesiumhydroxide by adding 20 mg/L Mg +2 and raising the pH to 12 after spiking the source water with 20mg/L of As(V). Samples were collected as pH was incrementally lowered at ten minute intervals.At pH 11 and above, no appreciable sulfate or carbonate interference was observed comparedto the control case. However, at pH 10 to 10.5, the system with carbonate exhibited significantlylower As(V) removal (78 percent versus 96 percent in the control and sulfate systems), and nearlytwice as much of the magnesium was measured as soluble (6.3 versus 3.3 mg/L). These resultssuggest that carbonate is somehow increasing the concentration of Mg +2 , leaving less solid availablefor As(V) sorption.McNeill and Edwards (1997b) investigated the interference of orthophosphate on As(V)removal by softening. Softening of raw water containing 15 µg/L As(V) at pH 12 indicated greaterthan 95 percent As(V) removal. After spiking raw water with 32 µg/L orthophosphate, As(V) removalwas slightly lower at intermediate pH values. Because the amount of calcium and magnesiumremoved during softening with and without orthophosphate was nearly equal, it seems thatorthophosphate interferes with arsenic removal by competing for sorption sites.2-12
Field StudiesSurveys of lime softening facilities by AWWARF (AWWARF 2000) show that the technologyshould be able to produce water with less than 3 Fg/L–all five of the surveyed plants that operatedat a pH of 10.2 or higher would be able to feasibly achieve these low arsenic levels. However,although lime softening processes are quite effective for arsenic removal when operated at high pH,lime softening facilities may not effectively remove arsenic to low levels when operated at or belowpH 10.0. For example, a field study conducted by Battelle Memorial Institute, with funding from EPA,found that a 10 mgd lime softening plant only reduced average total arsenic concentrations of 32.0Fg/L by about 45 percent to an average of 16.6 Fg/L in the finished water even though the treatmenttrain included pre-softening oxidation and post-softening filtration (EPA, June 2000). None of thesludge samples collected as part of this study qualified as hazardous waste based on TCLP testing.Optimization Hierarchy for Softening FacilitiesMcNeill and Edwards (1997b) developed a simple model for predicting As(V) duringsoftening. Using inputs of calcium carbonate, magnesium, and ferric hydroxide solid concentrationsformed during softening, the model can predict percentage As(V) removal.McNeill and Edwards (1997b) suggested an optimization hierarchy strategy for softeningfacilities which are unable to meet arsenic removal requirements with their existing treatment schemesimilar to optimization of coagulation hierarchy. If As(III) is present, the most cost-effective methodof improving arsenic removal is preoxidation of As(III) to As(V), since As(V) is more readilyremoved by precipitation of calcium carbonate and magnesium and ferric hydroxide. For facilitiesthat are currently precipitating only calcium carbonate, addition of iron can dramatically improvearsenic removal. A final option is to raise the softening pH in order to precipitate magnesiumhydroxide which strongly sorbs As(V). These removal trends should be quantitatively confirmed withjar testing for optimizing arsenic removal.SummarySoftening technology can be implemented by water systems to achieve greater than 90 percentremoval of As(V). As discussed above, a survey of softening facilities by AWWARF found that thosesurveyed facilities operating at a pH of 10.2 or higher could feasibly reduce arsenic concentrationsin the treated water below 3 Fg/L.2-13
Page 1: United StatesEnvironmental Protecti
Page 5: ACKNOWLEDGMENTSThis document was pr
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Page 10 and 11: 5.0 POINT-OF-ENTRY/POINT-OF-USE TRE
Page 12 and 13: LIST OF FIGURES2-1 Pressure Driven
Page 14 and 15: LIST OF ACRONYMSAAAWWAAWWARFBLSBVC/
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Page 23 and 24: ends up as ferric hydroxide. In alu
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Page 37 and 38: RegenerationRegeneration of AA beds
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Page 63 and 64: 2.6 ALTERNATIVE TECHNOLOGIES2.6.1 I
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Page 75 and 76: 3.2.3 Implementing TDP Recommended
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Page 81 and 82: 3.3 ADDITIONAL CAPITAL COSTSThe cos
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PilotingThe Technology Design Panel
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The 1993 Technology and Cost Docume
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cause disinfection by-product (DBP)
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Figure 3-1Pre-oxidation - 1.5 mg/L
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3.6 PRECIPITATIVE PROCESSES3.6.1 Co
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enhanced coagulation treatment plan
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Figure 3-4Enhanced Coagulation/Filt
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Small Systems (Less than 1 mgd)The
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Figure 3-6Coagulation Assisted Micr
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3.6.6 Enhanced Lime SofteningEnhanc
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Figure 3-8Enhanced Lime SofteningO&
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3. Empty Bed Contact Time (EBCT) is
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For design flows greater than 1 mgd
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Figure 3-9Activated AluminaCapital
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Figure 3-11Activated Alumina (pH 8
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Figure 3-13Activated Alumina (pH Ad
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3.7.2 Granular Ferric HydroxideGran
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6. The capital costs include a redu
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Figure 3-15Bed Volumes to Arsenic B
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Figure 3-17Anion Exchange (< 20 mg/
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Figure 3-19Anion Exchange (20-50 mg
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quality feed stream and often requi
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Figure 3-20Greensand FiltrationCapi
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3.11 COMPARISON OF COSTSThe April 1
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4.0 RESIDUALS HANDLING AND DISPOSAL
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mechanical dewatering processes. Wh
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Storage lagoons are best suited for
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the current Industrial Pretreatment
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4.3.3 Dewatered Sludge Land Applica
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usually determined by the Paint Fil
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for arsenic toxicity by a substanti
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The solids content of the backwash
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carbonate hardness removal produces
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Domestic sewage means untreated san
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4.4.10 Reverse OsmosisReverse osmos
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Figure 4-1Anion Exchange (< 20 mg/L
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Figure 4-3Anion Exchange (20-50 mg/
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Figure 4-9Activated Alumina (pH 7 -
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Figure 4-11Activated Alumina (pH Ad
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Figure 4-13Greensand FiltrationWast
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5.0 POINT-OF-ENTRY/POINT-OF-USE TRE
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chromatographic peaking, which occu
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5.3.1 Case Study 1: Fairbanks, Alas
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Manufacturer and laboratory data su
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Figure 5-2POU Reverse OsmosisO&M Co
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# Installation time - 1 hour unskil
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Figure 5-3POU Activated AluminaTota
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6.0 REFERENCESAmy, G.L. and P. Bran
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Cooperative Research Centres for Wa
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Fuller, C.C., J.A. Davis, G.W. Zell
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Le, X.C., and M. Ma (1998). “Dete
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Scott, K., J. Green, H.D. Do and S.
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Appendix AVery Small Systems Capita
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Table A4 - VSS Document Capital Cos
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Table B1.1 - Base Costs Obtained fr
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Table B11.1 - Base Costs Obtained f
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Table B13.1 - Base Costs Obtained f
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Appendix CW/W Cost Model Capital Co
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Table C2.1 - Base Costs Obtained fr
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Table C10.1 - Base Costs Obtained f
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APPENDIX D: BASIS FOR REVISED ACTIV
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Company case study is 18 minutes (2
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egressed against their respective v
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costs for small systems is as follo
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were based on $40/sq ft). The proce
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Basis. Capital cost components incl
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The source water at Plant D has the
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equation y = (2*10 -8 )x 2 + 0.0002
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The maximum run length is 18,500 be
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Since the available arsenic capacit
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If the first column can be operated
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which includes lighting, ventilatio
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disposal costs, if needed, are cove
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Appendix EBasis for Revised Anion E
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2. The estimated cost of the anion
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9. The capital costs have been esti
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Thus, the cost of a road and fence
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length when sulfate is at or below
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the labor rates for both large and
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2. US EPA. Technologies and Costs f
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Technologies and Costs for Removal of Arsenic From Drinking Water

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