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Evaluation of Treatment Techniques for Selenium Removal IWC 09-05 KyleSmith,Dow WaterandProcessSoluti ons Antoni oO.Lau,Ph. D. ,Infi l coDegremontInc. Fredri ckW.Vance,Ph. D. ,Dow Wat erandProcessSol ut ions Table of Contents Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Seleni um Regul at ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Appl i cat i ons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 TreatmentOpti ons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 ZeroValentIron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 IonExchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 ReverseOsmosi s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Bi ol ogi calReduct i onofSel eni um . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 M odelTreat mentSyst ems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 IonExchangeSyst ems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 RO Syst ems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Bi ol ogi calTreat mentSyst ems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 HybridRO /Bi ol ogi calSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Concl usions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Figures Figure1.Eh-pH di agram forsel eni um speci esi nwat er.(Takeno,2005). . . . . . . . . . . . . . . . . . . . . . . .5 Figure2.Equilibrium isotherm studiesonSe(IV)andSe(VI)atvari ouspH. . . . . . . . . . . . . . . . . . . .7 Tables Tabl e1.Typi calpropert iesofwastewaterfrom mining,FGD,andagri culturalrunoff. . .4 Tabl e2.Sal tsi nregenerati onst ream forIX syst em t reat i ngmi ni ngwast e.. . . . . . . . . . . . . . . . . . . . .9 Table3.Effectiveflow rates,freshwater,andsal trequirement sforIX syst em t reat i ng mini ngwastest ream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Tabl e4.RO syst em descript i onformini ngwast etreat ment.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Table5.Anal yt econcent rat i ons(uni tsofppm)instreamsfrom RO treatmentofmining waste.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Table6.RO syst em descri pt i onforFGD wast et reat ment .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Table7.Anal yt econcent rat i ons(unitsofppm)instreamsfrom RO treatmentofFGD waste. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Table8.RO syst em descri pt i onforagri cult uralwast et reat ment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Table9.Analyteconcentrati ons(unitsofppm)instreamsfrom RO treatmentof agri cul t uralwaste.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 75 of957 Table 10. Analyte concentrations (units of ppm) in streams from Biological Treatment of FGD wastewaters .............................................................................................................. 13 Table 11. Analyte concentrations (units of ppm) in streams from Biological Treatment of Agricultural wastewaters .................................................................................................. 13 76 of 957 Abstract Several techniques were evaluated for the treatment of selenium, including the use of anion exchange resins, selective adsorbents, zero valent iron, reverse osmosis, and biological reduction. The similarities and differences will be discussed in terms of waste streams produced and ultimate fate of the selenium. Background Due to increased enforcement of selenium regulations and an increased understanding of health and environmental effects the need to be able to efficiently treat selenium has taken on an increased importance. Elemental selenium is relatively nontoxic and is considered to be an essential trace element however it quickly becomes toxic at higher than recommended levels. Additionally, hydrogen selenide (H2Se) and several other selenium compounds are known to be extremely toxic. Physical deformities and reproductive failure have been noted in several aquatic species exposed to selenium at 10 ȝg/l which due to bioaccumulating resulted in tissue concentrations at 510 –1395 times greater levels (Lemly, 2002). The obj ective of this study was to develop a broad understanding of the existing treatment techniques and provide general guidance on the most effective methods for selenium removal. Several treatment techniques for selenium removal were evaluated which included:treatment with standard strong base anion resin, Copper form selective resin, biological, and reverse osmosis. Treatment methods were evaluated based on their economic feasibility, technical feasibility and difficulty to operate, and amount of waste generated. Selenium Regulations In the mid-1980’s the U.S. Environmental Protection Agency (EPA) established a national freshwater criterion of 5 ppb largely based on a case study done at Belews Lake in North Carolina which was contaminated from a coal-fired power plant during the 1970’s. The study has lasted over twenty years and is one of the most studied sites for selenium contamination. As more data has been obtained over the case of the study several experts have even begun recommending that the criterion be lowered to 2 ppb or less. Selenium concentrations as low as 1.5 ppb have accumulated to toxic concentrations in the aquatic food chain (Lemly, 2004). In November of 1996 the EPA released a new proposed regulation for acute aquatic life criterion for selenium based on tissue concentrations in specific aquatic species (USEPA 1996). This was based on the field data from Belews Lake in North Carolina and has been presented as an alternative to the 5 ppb regulation to account for the bioaccumulating nature of selenium. 77 of 957 Applications One of the major sources of selenium is coal. Much of the coal in the eastern United States is high in selenium and is a major source of selenium for industries heavily involved in the use of coal. This includes mining, refining, coal-fired power and ash landfill. Selenium contamination is not limited to coal applications and has been identified as an issue in agricultural drainage and municipal wastewater applications as well. The major components of a typical water analysis for some of the applications are shown in Table 1, where the coal application has been represented by the waste stream associated with flue gas desulfurization (FGD). Table 1.Typical properties of waste water from mining,FGD,and agricultural runoff. Mining (ppm) Boron Calcium Magnesium Potassium Sodium Chloride Nitrate Selenium Sulfate Alkalinity pH 130-1,000 89-174 160-1,260 3.6-481 1 0.015-0.050 525-6837 10 2.1-6.6 FGD (ppm) 100- 600 300 – 10,000 1000 - 4000 45 500 10,000 – 25,000 1 - 400 1 - 10 3,000 – 20,000 10 - 250 4.5 - 5.5 Agricultural (ppm) 550 300 2285 1500 0.35 5000 300 8.1 While the water quality of all these applications varies significantly what is typical is that they tend to contain high levels of total dissolved solids (TDS)particularly relative to the amount of selenium present. For example, for FGD wastewaters the TDS concentration ranges from 15,000 to 45,000 mg/L. This makes selectively removing the selenium very difficult and often requires systems to be large enough to treat a significant portion of the TDS before being able to reach an acceptable selenium concentration. Numerous treatment options have been proposed for removing selenium with advantages and disadvantages to each. As discussed previously selenium found above the recommended discharge levels is present in a wide range of waste streams each requiring a unique approach to removal. Treatment Options The speciation of selenium plays a critical role in the effectiveness of any approach for removal, especially to low levels. While several species of selenium are stable, in aqueous environments, it is most often found as the oxygenated anions of selenite, Se(IV) and selenate, Se(VI). This is illustrated in Figure 1 which shows that selenite is present as the single charged anion, HSeO3- below pH 7, but as the double charged anion, SeO32- above pH 7. In contrast, the more oxidized selenate carries a 78 of 957 double charge whenever the pH is higher than about 2. The result of this complexity is that both the speciation and the pH of the water must be taken into account when attempting to remediate the selenium. This was demonstrated in early studies focusing on coagulation filtration and lime softening, which concluded that neither approach was effective for complete selenium removal, but that they could be optimized with pH adjustments (Sorg, 1978). Figure 1. Eh-pH diagram for selenium species in water. (Takeno, 2005) Zero Valent Iron One of the most widely known treatment techniques is the use of zero valent iron (ZVI). ZVI when added to the waste stream is oxidized to soluble Fe2+ which then reacts with OH- to form “green rust”. The green rust serves as a reducing agent to reduce selenium, Se(VI) and Se(IV) to insoluble selenite, Se(0) (Hansen et. al., 1996) ZVI is one of the most economic means of reducing selenium and has shown the potential for removal to very low levels. However, the effectiveness can vary significantly depending on the oxidation state of the selenium as well as the presence of certain additional salts, particularly phosphates and nitrates. As the level of salts increases the removal of selenium is diminished as more electron acceptors are competing with selenium for oxidation. Removal efficiencies have ranged from 43%, with 10 mM PO43- Se(VI), to almost 100% for Se(IV) with mock solutions in laboratory testing (Zhang 2005). To overcome the effects of competitive oxidation a recent study evaluated the idea of combing ZVI along with biological reduction which significant potential for 79 of 957 treating selenium in high salt streams (Zhang, 2008). Generation of a relatively large amount of hydroxide sludge is a disadvantage of this treatment system. Ion Exchange Selenium, when present as selenate, is in many ways is very similar to sulfate, in fact they are most often found together, and as such are fairly easily removed with a strong base anion exchange resin. The similarities however create another issue. The levels of sulfate are often several factors of a thousand times greater than the selenate concentration. Ion exchange (IX) is based on selectivity, removing more selective ions first and less selective later. Because the sulfate and selenium are removed congruently to reach low ppb levels of selenium the IX unit needs remove all of the sulfate as well as the selenium. This significantly increases the economics particularly the cost of regenerant chemicals needed to regenerate the resin bed. It also creates the need to dispose of the regenerant stream which contains excess chemical as well as the selenium and sulfate that were removed. This additional cost often makes ion exchange uneconomical for selenium removal in high salt streams. For low salt streams, such as drinking water applications, ion exchange remains an effective and economical treatment choice. There are some more selective approaches for removing selenite from a high sulfate background, where the difference in the charge carried by the anion can be exploited. For example, titanium based media, which is well suited for arsenate removal, has been shown to remove selenite selectively with up to a 1000X excess of sulfate present. Figure 2 illustrates the results of an equilibrium isotherm study conducted in water prepared by following NSF/ANSI 53, which contains 20 ppm of sulfate. It shows that the capacity (Q) for the monovalent anion Se(IV) is in the range of 1 to 10 g / kg of media when the pH is 7 or 2.8, but drops by two orders of magnitude for the divalent anion at pH 11. Figure 2 also shows that the divalent anion Se(VI) is adsorbed with a capacity of 0.4 to 4 g / kg of media at pH 2.8. Studies at higher pH showed no measurable adsorption for Se(VI). Taken together, these indicate that the binding mechanism for this media is likely very similar to that for arsenic, where the monovalent H2AsO4- species is adsorbed with higher efficiency than the divalent or uncharged species. 80 of 957 2 log Q (mg/g) 1 y = 0.3795x + 0.6961 R2 = 0.9828 0 y = 0.3879x + 0.6287 R2 = 0.9905 -1 y = 0.9371x -0.6844 R2 = 0.9998 y = 1.8859x -3.8521 R2 = 0.7843 -2 -3 -4 -3 -2 -1 0 1 2 3 log [Se] ppm Se(I V)pH 7 Se(I V)pH 2.8 Se(I V)pH 11 Se(VI )pH 2.8 Figure 2. Equilibrium isotherm studies on Se(IV) and Se(VI) at various pH. Similar to the titanium based media example, a copper impregnated ion exchange resin has been shown to be effective at both selenite and arsenate removal from a high sulfate background, apparently due to the similarity of the anions (Zhu, 1992). Reverse Osmosis An alternative to reduction and ion exchange is the use of reverse osmosis (RO). As with ion exchange the selenate anion behaves very similarly to sulfate which is easily rejected with recovery rates typically in the 60 to 70% range. Additional benefits include RO’s ability to handle variable water quality, reject additional undesirable salts, as well as maintain continuous operation. In high salt streams membranes can suffer from significant fouling issues due to the concentrating effect that takes place as water passes through the membrane concentrating the remaining salts (McCutchan, 1976). Advancements in thin film composites have significantly reduced fouling compared to original cellulose acetate membranes. Along with antiscalants fouling in most applications would most likely be manageable. Biological Reduction ofSelenium One of the areas of significant study has been the use of biological treatment. Typically, the use of Sulfate Reducing Bacteria (SBR) under anaerobic conditions has been the common method to reduce selenite and selenate to insoluble selenium. One of the most significant advantages of this form of treatment is that it has shown to be independent of “high” sulfate concentration. For example, 2,000 to 4,000 mg/L as SO42in drainage water does not appear to significantly interfere with nitrate or selenium reduction” (Drainage Water Treatment Technical Committee, 1999). This is a significant 81 of 957 advantage due to the high levels of sulfate that are almost always present along with selenium. Two types of biological systems for selenium removal are currently commercially marketed. One system, marketed as iBIO™ by Infilco Degremont, is a typical suspended growth (activated sludge type) reactor system (Stover et. al. 2007, Stover and Pudvay 2007). The other system marketed by GE/Zenon is the ABMet process which consists of a packed-bed activated carbon system impregnated with their proprietary selenium reducing bacteria (Sonstegard, et. al 2008 and Wylie et. al, 2008). For both systems, a carbon source and essential macronutrients (nitrogen, phosphorus, etc.) are added to the wastewater as necessary ingredients for the proper metabolism of the microorganisms. For the suspended growth system, the anaerobic reactor is loaded with typical anaerobic sludge from municipal or industrial wastewater treatment plants and a short acclimation process allows the SBR to be selected. For the pack-bed system, the vendor supplies the selenium specific bacteria which are isolated from naturally occurring sources and impregnated onto the activated carbon substrate. The biological treatment system, in most cases, consist of a series of reaction vessels to accomplish the desired removals. For example, for wastewaters that contain nitrates, the first step is typically the removal of nitrates by “denitrification microorganisms” which reduce the nitrates to nitrogen gas which is vented to the atmosphere. The denitrification step is followed by the selenium reduction process where the SRB microcorganims reduce the selenite and selenate to elemental selenium as evidenced by the formation of a red precipitate in the reactor. One of the most important conditions that control the growth of the microorganism is the presence or oxygen or oxygen containing compounds. One parameter used to monitor the reactor is the Oxidation Reduction Potential (ORP) measured in millivolts (mV). The denitrification processes occur in the ORP range of +50 to -200 mV where bacteria will utilize nitrates and sulfates if present to generate nitrogen gas and sulfides. Bacteria will preferentially use the nitrates before the sulfates because they can extract more energy from the denitrification reaction. At much lower ORP values, the anaerobic and fermentative bacteria predominate producing methane gas. The final step for the suspended growth system is the use of a clarifier to separate the treated effluent from the biosolids which are settled and returned to the reactor. In the pack-bed system, a series of backwashes are performed on the carbon bed to remove the biomass on a periodic basis. One variant on this approach is the bio-film reactor, where a film is supported on a membrane which supplies H2 gas as the reducing medium, i.e., fuel for the microorganisms. The autotrophs supported in this manner are claimed to produce less biomass as waste than their heterotrophic counterparts which are fed a carbon source (sugar, acetic acid, methanol, etc.) instead of hydrogen (Rittmann, 2007). 82 of 957 Model Treatment Systems As a means of illustration, the model waste streams from the earlier section were used as the basis for constructing model treatment systems. Our focus was not to determine actual capital or operating costs for each system, but rather to assess in a qualitative sense what the relative size of the system may be, and perhaps more importantly, what the residuals from each system may look like to determine whether a suitable disposal outlet may be available. Since selective treatment is available and more effective for selenite removal, these systems focused on the removal of selenate from the sulfate background where only the less selective alternatives seem viable. Ion Exchange Systems The first system modeled used a strong base anion exchange resin to treat the mining waste described in Table 1. Calculations were performed using CADIX v 6.1 modeling software. The average values for most analytes were used, with the exception of the pH which was assumed to be 6.0 for the purpose of more straightforward comparison to other treatment options. (IX could effectively treat a low pH stream, whereas RO could not.) To treat a flow of 500 gpm with a reasonable regeneration schedule, about 5,300 ft3 of resin would be required. The resin was used in the chloride form and regenerated with a 5% solution of NaCl. The quantities of major salts in the regeneration stream are shown in Table 2, where the selenate is assumed to follow the concentration factor for sulfate although it is not explicitly calculated in the model. Table 2. Salts in regeneration stream for IX system treating mining waste. Analyte Concentration (ppm) NaCl 19946 NaNO3 0 Na2SO4 6221 NaHCO3 749 Although IX is truly a batch operation with a finite usage and regeneration stage, we have attempted to describe it in Table 2 as at least a quasi continuous process for comparisons to RO cases. This shows that although the waste stream has been reduced, the system would in fact have to be larger in order to produce 500 gpm of clean effluent. More importantly, it also shows that it would require a fresh water source nearly as large as the waste stream itself, and that the salt consumption would be more than 40 lb per hour. Table 3. Effective flow rates, fresh water, and salt requirements for IX system treating mining waste stream Parameter Effective effluent flow Effective waste flow Effective clean water required Salt required 83 of 957 Amount 294 197 359 60791 Unit gpm gpm gpm lb per day Clearly, anion exchange is not an efficient process for dealing with this mining waste stream, where the sulfate load controls the size of the system, waste streams, and required water and salt for regeneration. Since both the FGD and agricultural waste streams have similar sulfate levels, neither of these was modeled since they would be similarly inefficient. RO Systems For each waste stream, ROSA v6.1.5 projection software was used to calculate required system sizing and water quality parameters for concentrate and permeate streams. For each case, a brackish water element with 400 ft2 of surface area incorporating a 34 mil feed spacer was used, and six elements were fit to each pressure vessel. The projections were run to treat to a selenium level of less than 5 ppb, which is suitable for discharge in many locations. Thus, the remaining selenium will be in the concentrate stream, which must be disposed of accordingly. As for the IX calculations, the selenium is not explicitly accounted for in the projections, but is assumed to follow the sulfate rejection which was used for the basis. In the tabulations below, all parameters with no quantified values have been omitted for clarity. Case 1: Mining Waste To treat the mining waste stream, a two stage system using 126 elements could be used with the general features found in Table 4. The waste stream from this system is somewhat smaller in size than that produced from the IX system, but without the additional requirements for fresh water and salt utilization. Water quality parameters for the concentrate and permeate streams are found in Table 5. Table 4. RO system description for mining waste treatment. Feed (gpm) Concentrate (gpm) Permeate (gpm) Membranes Recovery (%) Stage 1 2 84 of 957 500 129.99 370.01 126 74 #of Pressure Vessels 14 7 Table 5. Analyte concentrations (units of ppm) in streams from RO treatment of mining waste. Parameter K Na Mg Ca CO3 HCO3 Cl SO4 CO2 TDS pH Feed 5 425 100 250 0.01 75 225 1550.09 91 2630.1 6 Concentrate 19.03 1670.5 383.27 958.25 0.23 284.93 856.86 5935.01 91.13 10108.07 6.44 Permeate 0.07 4.63 0.48 1.17 0 2.28 3.01 9.52 90.14 21.16 4.69 Case 2: FGD Waste The FGD waste stream is more challenging to treat than the mining stream, largely due to the much higher selenium levels present. A four pass system would be required using 252 elements with the general features found in Table 6. Note the considerable added complexity for only a moderately higher flow volume when compared to the mining case. This is quantified in the lower recovery rate of only 38%, or about half that for the mining case. Water quality parameters for the concentrate and permeate streams are found in Table 7. Table 6. RO system description for FGD waste treatment. Feed (gpm) Concentrate (gpm) Permeate (gpm) Membranes Recovery (%) 600 373 227 252 38 Table 7. Analyte concentrations (units of ppm) in streams from RO treatment of FGD waste Parameter K Na Mg NO3 SO4 Boron TDS pH Feed 45 500 1200 1.1 9301 350 13049 6 Concentrate 134.3 6448.63 3584.28 3.14 27779.43 575.26 41240.09 5.35 Permeate 0 0 0 0 0 136.42 780.29 5.22 Case 3: Agricultural Waste The agricultural waste stream represents an intermediate case between the mining and FGD examples due to its similar sulfate load and intermediate selenium level. A two pass, two stage system would be required using 90 elements with the general features 85 of 957 found in Table 8. Water quality parameters for the concentrate and permeate streams are found in Table 8. RO system description for agricultural waste treatment Feed (gpm) Concentrate (gpm) Permeate (gpm) Membranes Recovery (%) 250 101.08 148.92 90 59.5 Pass 1 1 2 2 Stage 1 2 1 2 # of Pressure Vessels 6 3 4 2 Table 9. Analyte concentrations (units of ppm) in streams from RO treatment of agricultural waste. Parameter Na Mg Ca Cl SO4 TDS pH Feed 2285 300 550 1500 5000 10186.12 8 Concentrate 7078.96 937.61 1718.99 6579.38 15460.88 33498.8 8 Permeate 3.32 0 0.01 0.05 8.92 12.33 8 Biological Treatment Systems Case 1: Mining Wastewater Mining wastewaters containing 15 – 50 ppb selenate are most likely not amenable for biological treatment because of the low concentration range for bacterial reduction. If the selenium is in the selenite form, physical-chemical treatment is the recommended treatment process. Case 2: FGD Wastewater FGD wastewaters have unique characteristics that make them very amenable to anaerobic treatment for selenium removal. These wastewater contain many regulated metals (Pb, Ni, Hg, etc.) in addition to selenium, thus, they first require a treatment process for metals removal. The most common process for FGD wastewater metals removal is physical-chemical treatment via precipitation of the metal hydroxides. A cost-effective method is to raise the pH of the wastewater with the addition of lime (Ca(OH)2) from 4 5 up to 9 - 10 in order to form the insoluble metal hydroxides. Addition of 86 of 957 organosulfides and a coagulant (e.g., FeCl3) can be made to enhance the formation of insoluble metal sulfides and ferric hydroxides. In this pre-treatment process, selenium in the “selenite” form is also removed effectively (50 -90%), however, the oxidized “selenate” form passes through the system. The temperature of the FGD wastewater is normally in the 100-130°F range which makes it suitable for the anaerobic system because the ideal operating temperature is ~100°F. FGD wastewaters also contain nitrates (NO3) which must be removed prior to selenium reduction. Nitrate concentrations vary depending on whether the power plant has the Selective Catalytic Reduction Systems (SCR) for nitrogen oxides emission control. Plants with SCR systems can result in nitrate concentrations up to 500 mg/L. The anaerobic systems an easily degrade all the nitrates to levels < 1 mg/L using the facultative “denitrification” bacteria. Table 10. Analyte concentrations (units of ppm) in streams from Biological Treatment of FGD wastewaters Parameter Selenite Selenate Nitrates pH Feed 5 5 500 4-5 Effluent from Physical- Chemical Pre-Treatment 1 5 500 8-10 Effluent from Biological Treatment <0.05 <0.05 <1 7 Case 3: Agricultural Wastewater Agricultural wastewaters containing “selenate” can be easily treated by the anaerobic biological process. These wastewaters contain much less contaminants than the FGD type wastewaters. If selenium in the “selenite” form is present, then physical-chemical treatment is the most cost-effective treatment process. Table 11. Analyte concentrations (units of ppm) in streams from Biological Treatment of Agricultural wastewaters Parameter Selenite Selenate Nitrates pH 87 of 957 Feed 0.5 0.5 10 8 Effluent from Physical- Chemical Treatment <0.1 <0.5 10 9-10 Effluent from Biological Treatment <0.05 <0.05 <1 7 Hybrid RO / Biological System To address disposal issues RO has been suggested as a possible pretreatment to biological reduction. This has two benefits in that it removes selenium left in the concentrate stream but by utilizing the RO treatment the size of the biological treatment can be greatly reduced. Conclusions Several technologies have proven utility in the treatment of selenium. When in the reduced form, selenite can be removed through selective adsorption media or through the use of zero valent iron with high efficiency, provided the pH is adjusted for maximum effectiveness. However, when selenium is oxidized, sulfate strongly competes for selenate adsorption, making these methods much less effective. IX may be useful in applications with low sulfate present, such as drinking water, but for the waste applications studied here it suffers from high demands for fresh water and salt required for regeneration. RO can be useful for concentrating the waste streams, but these smaller streams must still be disposed of in some manner. Biological treatment can be very effective for selenium removal, since sulfate does not interfere and it is usually present in high concentrations. Therefore, it is only limited by nitrate loads which can be effectively treated. A combined system using RO to concentrate the stream followed by biological reduction may prove to be the best solution for completely converting selenium to a solid form for disposal and providing a stream which is easily discharged. References Drainage Water Treatment Technical Committee. “Drainage Water Treatment: Final Report.” February 1999 Drbal, Lawrence F., Patricia G. Boston, Kayla L. Westra. Power plant engineering. Springer, 1995 p. 528 ISBN 0412064014, 9780412064012 Hansen, H.C.B., C.B. Koch, H. Nancke-Krogh, O.K. Borggaard and J. Sorensen. “Abiotic nitrate reduction to ammonium: key role of green rust.” Environ. Sci. Technology 1996, 30:2053-2056 Lemly, A. Dennis. “Symptoms and implications of selenium toxicity in fish: the Belews Lake case example.” Aquatic Toxicology 57 (2002) 39-49 Lemly, A. Dennis. “Implications of Selenium in Proposed Wastewater Discharges to Great Salt Lake.” 2004 88 of 957 McCutchan, J.W.; V. Goel; D.B. Bryce; and K. Yamamoto. “Reclamation of field drainage water.” Desalination. 19:153-160, 1976. Rittmann, B. “The membrane biofilm reactor is a versatile platform for water and wastewater treatment.” Environmental Engineering Research (2007) Sonstegard, J., Pickett, T., Harwood, J., Johnson, D., “Full Scale Operation of GE ABMet® Biological Technology for the Removal of Selenium from FGD Wastewaters”, 2008. Sorg, T.J. and Logsdon, G.S. “Treatment Technology to Meet the Interim Primary Drinking Water Regulations for Inorganics: Part 2.” Jour. AWWA, July 1978, 379-393. Stover, E.L, Pudvay, M, Kelly, R.F., Lau, A.O., Togna, A.P., “Biological Treatment of Flue Gas Desulfurization Wastewater”, International Water Conference, 2007. Stover, E.L., Pudvay, M.L., “Biological Treatment of Flue Gas Desulfurization Wastewater”, FGD Systems and Operating Guidelines Workshop, International Water Conference, 2007. Takeno, N. “Atlas of Eh-pH diagrams: Intercomparison of thermodynamic databases” Geological Survey of Japan Open File Report No. 419, May 2005. USEPA. “Proposed Selenium Criterion Maximum Concentration for Water Quality Guidance for the Great Lakes System; Proposed Rule.” 40 CFR Part 132 Federal Register Vol. 61, No. 221 1996. Wylie, R., Baker, R., Kennedy, W., Riffe, M., Heimbigner, B., Pickett, T., “Duke Energy Carolina LLC’s Strategy and Initial Experience of FGD Wastewater Treatment Systems”, IWC-08-32, 2008. Zhang, Yiqiang and William T. Frankenberger, Jr. “Removal of Selenate in Simulated Agricultural Drainage Water by a Rice Straw Bioreactor Channel System.” J. Environ. Qual. 32:1650-1657, 2003. Zhang, Yiqiang; Juanfang Wang; Chris Amrhein; William T. Frankenberger, Jr. “Removal of Selenate from Water by Zero valent Iron.” J. Environ. Qual. 34:487495, 2005. Zhang, Yiqiang; Christopher Amrhein; Andre Chang, William T. Frankenberger Jr. “Effect of zero-valent iron and a redox mediator on removal of selenium in agricultural drainage water.” Science of the Total Environment. 407 89-96, 2008 Zhu, Y.; Sengupta, A.R.; Ramana, A. “Ligand Exchange for Anionic Solutes” Reactive Polymers, 1992, 17, 229-237. 89 of 957 Evaluation of Treatment Techniques for S elenium R emoval P rep ared D iscussion IWC Paper No. 09-05 Clau d e G au th ier P.E n g . T h e Pu rolite Com pan y I m u s t c om m en d th e au th ors for prov id in g an ex c ellen t s u m m ary of th e reg u lation s an d tec h n olog ies for th e rem ov al of s elen iu m from v ariou s ty pes of w ater. O n e on ly h as to g oog le as ex am ple: s elen iu m rem ov al – 1 ,37 0,000 h its , s elen iu m rem ov al from w ater – 26 1 ,000, s elen ite rem ov al – 28 6 ,000, s elen ate rem ov al 93,900 h its to b e ov erw h elm ed . T h e au th ors h av e c h arac teriz ed th e m ore d iffic u lt to treat w aters s u c h as m in in g , F G D – F lu e G as D es u lph u riz ation , ag ric u ltu ral ru n off. T h e c h em is try for th e rem ov al of s elen iu m is m u c h d ifferen t from th e rem ov al of ars en ic w h ic h h as b een m u c h m ore in th e s potlig h t for d rin k in g w ater applic ation s . In c om paris on ars en ic is eas ier to treat th an s elen iu m as ars en ate is relativ ely eas y to rem ov e an d ars en ite c an b e eas ily ox id iz ed to ars en ate. T h e c h em is try c h allen g es for s elen iu m rem ov al are ju s t th e oppos ite w h ereas s elen ite is relativ ely eas y to rem ov e an d s elen ate is m os t d iffic u lt to red u c e to s elen ite. O u r ex perien c e is th at n an o partic les of iron im preg n ated an ion res in is effec tiv e in rem ov in g s elen itie h ow ev er n ot effec tiv e in rem ov in g s elen ate. H ex av alen t s elen ate, S e(V I) is pres en t as a th e d ou b le c h arg ed an ion , S eO 32- ab ov e pH 7 . T h e ox y an ion s elen ate h as h ig h er s elec tiv ity pu b lis h ed v alu e of 1 7 w h ic h c om pared to s u lfate ion S O 42w h ic h h as a v alu e of 1 1 . T ab le No. 1 Water an aly s is u s ed for S B A – S tron g B as e A n ion M in in g (ppm ) B oron Calc iu m M ag n es iu m Potas s iu m S od iu m Ch lorid e Nitrate S elen iu m S u lfate A lk alin ity pH 90 of 957 1 30-1 ,000 8 9-1 7 4 1 6 0-1 ,26 0 3.6 -48 1 1 0.01 5-0.050 525-6 8 37 10 2.1 -6 .6 Purolite’s modeling software provided the following capacity estimates for selenate when treating the lower and higher TDS ranges of the water q uality given in Table No. 1, showing selenate ranging from 0.015 to 0.050 ppm while sulfate ranged from 525 to 6837 ppm. We find that this type of data presentation and modeling most effective for evaluating treatment options. IXSIM Simulating Selenate Removal w ith T y p e I SB A (virgin) L ow er T D S W ater 16 14 S u lfa te 5 00 12 400 10 8 3 00 6 200 4 100 Selenate C ap ac ity : 1 2 0 B ed V olumes 2 0 p p m Sulfate p p b Selenate S e le n a te 600 0 In flu e n t: S u lfa te 5 25 p p m N itra te 1 p p m A lk a lin ity 10 p p m C h lo rid e 3 .6 p p m S e le n a te 15 p p b 1 21 41 61 81 101 121 141 161 181 201 221 241 261 281 B ed V olumes IXSIM Simulating Selenate Removal w ith T y p e I SB A (virgin) H igh er T D S W ater p p b Selenate 50 S e le n a te 7 000 S u lfa te 6000 40 5 000 30 4000 Selenate C ap ac ity : 1 2 B ed V olumes 20 10 3 000 2000 1000 0 0 1 4 7 10 13 16 19 22 25 28 3 1 3 4 3 7 40 43 46 49 B ed V olumes 91 of 957 8000 p p m Sulfate 60 In flu e n t: S u lfa te 683 7 p p m N itra te 1 p p m A lk a lin ity 10 p p m C h lo rid e 481 p p m S e le n a te 5 0 p p b It is difficult for myself to comment on the RO – Reverse Osmosis technology as I do not have an extensive background in this area. My only comment would be for the authors in their presentation should show an example projection supported by actual field test data if possible. I found the RO projection summary tables numerical data to be over done by going to two decimal places as for example for the concentrate streams with tens of thousand ppm concentration levels. Rounding off the numbers to zero decimal points would be more appropriate. The reductive chemistry approach to detoxify selenite and selenate to elemental selenium by biological approach appears to very cost effective in treating large volumes of water. The hybrid approach of utilizing chemical pretreatment combined RO / Biological Treatment to concentrate selenium up to higher concentration levels for biological reduction is novel and innovative thought approach in dealing with a difficult and challenging application. In conclusion I again must commend the authors on a well thought out paper and would encourage them in the future to present real world case histories on the innovative approaches to selenium removal. 92 of 957