Evaluation of Treatment Techniques for Selenium

Removal

IWC 09-05

Kyle Smith, Dow Water and Process Solutions

Antonio O. Lau, Ph.D., Infilco Degremont Inc.

Fredrick W. Vance, Ph.D., Dow Water and Process Solutions

Table of Contents Abstract ........................................................................................................................... 3

Background ..................................................................................................................... 3

Selenium Regulations ..................................................................................................... 3

Applications .................................................................................................................... 4

Treatment Options .......................................................................................................... 4

Zero Valent Iron .......................................................................................................... 5

Ion Exchange .............................................................................................................. 6

Reverse Osmosis ......................................................................................................... 7

Biological Reduction of Selenium .............................................................................. 7

Model Treatment Systems .............................................................................................. 9

Ion Exchange Systems ................................................................................................ 9

RO Systems ............................................................................................................... 10

Biological Treatment Systems .................................................................................. 12

Hybrid RO / Biological System ................................................................................ 14

Conclusions ................................................................................................................... 14

References ..................................................................................................................... 14

FiguresFigure 1. Eh-pH diagram for selenium species in water. (Takeno, 2005) ........................ 5

Figure 2. Equilibrium isotherm studies on Se(IV) and Se(VI) at various pH. ................... 7

TablesTable 1. Typical properties of waste water from mining, FGD, and agricultural runoff. .. 4

Table 2. Salts in regeneration stream for IX system treating mining waste. ..................... 9

Table 3. Effective flow rates, fresh water, and salt requirements for IX system treating

mining waste stream ........................................................................................................... 9

Table 4. RO system description for mining waste treatment. .......................................... 10

Table 5. Analyte concentrations (units of ppm) in streams from RO treatment of mining

waste. ................................................................................................................................ 11

Table 6. RO system description for FGD waste treatment. ............................................. 11

Table 7. Analyte concentrations (units of ppm) in streams from RO treatment of FGD

waste ................................................................................................................................. 11

Table 8. RO system description for agricultural waste treatment .................................... 12

Table 9. Analyte concentrations (units of ppm) in streams from RO treatment of

agricultural waste. ............................................................................................................. 12

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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

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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 g/l which due to bioaccumulating resulted in tissue concentrations at 510 1395 times greater levels

(Lemly, 2002).

The objective 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-1980s 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

1970s. 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.

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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)

FGD

(ppm)

Agricultural

(ppm)

Boron 100- 600

Calcium 130-1,000 300 10,000 550

Magnesium 89-174 1000 - 4000 300

Potassium 45

Sodium 160-1,260 500 2285

Chloride 3.6-481 10,000 25,000 1500

Nitrate 1 1 - 400

Selenium 0.015-0.050 1 - 10 0.35

Sulfate 525-6837 3,000 20,000 5000

Alkalinity 10 10 - 250 300

pH 2.1-6.6 4.5 - 5.5 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

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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

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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.

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y = 0.3795x + 0.6961

R2 = 0.9828

y = 0.3879x + 0.6287

R2 = 0.9905

y = 1.8859x - 3.8521

R2 = 0.7843

y = 0.9371x - 0.6844

R2 = 0.9998

-4

-3

-2

-1

0

1

2

-3 -2 -1 0 1 2 3

log [Se] ppm

log

Q (

mg

/g)

Se(IV) pH 7 Se(IV) pH 2.8 Se(IV) 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

ROs 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 of Selenium

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 SO42-

in drainage water does not appear to significantly interfere with nitrate or selenium

reduction (Drainage Water Treatment Technical Committee, 1999). This is a significant

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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).

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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 Amount Unit

Effective effluent flow 294 gpm

Effective waste flow 197 gpm

Effective clean water required 359 gpm

Salt required 60791 lb per day

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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) 500

Concentrate (gpm) 129.99

Permeate (gpm) 370.01

Membranes 126

Recovery (%) 74

Stage

# of Pressure

Vessels

1 14

2 7

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Table 5. Analyte concentrations (units of ppm) in streams from RO treatment of mining waste.

Parameter Feed Concentrate Permeate

K 5 19.03 0.07

Na 425 1670.5 4.63

Mg 100 383.27 0.48

Ca 250 958.25 1.17

CO3 0.01 0.23 0

HCO3 75 284.93 2.28

Cl 225 856.86 3.01

SO4 1550.09 5935.01 9.52

CO2 91 91.13 90.14

TDS 2630.1 10108.07 21.16

pH 6 6.44 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) 600

Concentrate (gpm) 373

Permeate (gpm) 227

Membranes 252

Recovery (%) 38

Table 7. Analyte concentrations (units of ppm) in streams from RO treatment of FGD waste

Parameter Feed Concentrate Permeate

K 45 134.3 0

Na 500 6448.63 0

Mg 1200 3584.28 0

NO3 1.1 3.14 0

SO4 9301 27779.43 0

Boron 350 575.26 136.42

TDS 13049 41240.09 780.29

pH 6 5.35 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

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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) 250

Concentrate

(gpm) 101.08

Permeate (gpm) 148.92

Membranes 90

Recovery (%) 59.5

Pass Stage

# of Pressure

Vessels

1 1 6

1 2 3

2 1 4

2 2 2

Table 9. Analyte concentrations (units of ppm) in streams from RO treatment of agricultural waste.

Parameter Feed Concentrate Permeate

Na 2285 7078.96 3.32

Mg 300 937.61 0

Ca 550 1718.99 0.01

Cl 1500 6579.38 0.05

SO4 5000 15460.88 8.92

TDS 10186.12 33498.8 12.33

pH 8 8 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

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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 100-130F range which makes it suitable for the anaerobic system because the ideal operating temperature is ~100F.

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 Feed

Effluent from

Physical- Chemical

Pre-Treatment

Effluent from

Biological

Treatment

Selenite 5 1

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.

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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 if f ic 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 if feren 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 if f ic 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 ef fec 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 42-

w 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 1 30-1 ,000

M ag n es iu m 8 9-1 7 4

Potas s iu m

S od iu m 1 6 0-1 ,26 0

Ch lorid e 3.6 -48 1

Nitrate 1

S elen iu m 0.01 5-0.050

S u lfate 525-6 8 37

A lk alin ity 1 0

pH 2.1 -6 .6

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IXSIM Simulating Selenate Removal

w ith T y p e I SB A (virgin)

L ow er T D S W ater

0

2

4

6

8

10

12

14

16

1 21 41 61 81 101 121 141 161 181 201 221 241 261 281

B ed V olumes

pp

b S

ele

nate

0

100

200

3 00

400

5 00

600

pp

m S

ulf

ate

S e le n a te

S u lfa te

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

Selenate C ap ac ity :

1 2 0 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

0

10

20

3 0

40

5 0

60

1 4 7 10 13 16 19 22 25 28 3 1 3 4 3 7 40 43 46 49

B ed V olumes

pp

b S

ele

nate

0

1000

2000

3 000

4000

5 000

6000

7 000

8000

pp

m S

ulf

ate

S e le n a te

S u lfa te

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

Selenate C ap ac ity :

1 2 B ed V olumes

Purolites 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.

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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.

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