Download Evaluation of Treatment Techniques for Selenium Removal

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.
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.3
Background.
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.3
Seleni
um Regul
at
ions.
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.3
Appl
i
cat
i
ons.
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.4
TreatmentOpti
ons.
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.4
ZeroValentIron.
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.5
IonExchange.
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.6
ReverseOsmosi
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.7
Bi
ol
ogi
calReduct
i
onofSel
eni
um .
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.7
M odelTreat
mentSyst
ems.
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.9
IonExchangeSyst
ems.
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.9
RO Syst
ems.
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.10
Bi
ol
ogi
calTreat
mentSyst
ems.
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.12
HybridRO /Bi
ol
ogi
calSystem .
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.14
Concl
usions.
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.14
References.
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.14
Figures
Figure1.Eh-pH di
agram forsel
eni
um speci
esi
nwat
er.(Takeno,2005).
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.5
Figure2.Equilibrium isotherm studiesonSe(IV)andSe(VI)atvari
ouspH.
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.7
Tables
Tabl
e1.Typi
calpropert
iesofwastewaterfrom mining,FGD,andagri
culturalrunoff.
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.4
Tabl
e2.Sal
tsi
nregenerati
onst
ream forIX syst
em t
reat
i
ngmi
ni
ngwast
e..
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.9
Table3.Effectiveflow rates,freshwater,andsal
trequirement
sforIX syst
em t
reat
i
ng
mini
ngwastest
ream .
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.9
Tabl
e4.RO syst
em descript
i
onformini
ngwast
etreat
ment..
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.10
Table5.Anal
yt
econcent
rat
i
ons(uni
tsofppm)instreamsfrom RO treatmentofmining
waste..
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.11
Table6.RO syst
em descri
pt
i
onforFGD wast
et
reat
ment
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.11
Table7.Anal
yt
econcent
rat
i
ons(unitsofppm)instreamsfrom RO treatmentofFGD
waste.
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.11
Table8.RO syst
em descri
pt
i
onforagri
cult
uralwast
et
reat
ment.
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.12
Table9.Analyteconcentrati
ons(unitsofppm)instreamsfrom RO treatmentof
agri
cul
t
uralwaste..
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.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.
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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)
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
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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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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).
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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
Effective effluent flow
Effective waste flow
Effective clean water required
Salt required
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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
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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
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Guidance for the Great Lakes System; Proposed Rule.” 40 CFR Part 132 Federal
Register Vol. 61, No. 221 1996.
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“Effect of zero-valent iron and a redox mediator on removal of selenium in
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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 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
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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.
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