removal of unknown selenium species and other ... company removal of unknown selenium species and...
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WaterTech 2016 – April 7, 2016
Jay E. Renew, P.E.1, Dr. Chethan Acharya2, Kristen Jenkins1, and Keith Hendershot1
1Southern Research2Southern Company
Removal of Unknown Selenium Species and other Constituents from Flue Gas Desulfurization Wastewater through Nanofiltration
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Key Waters of Concern at Coal-Fired Power Plants
Source - ("Proposed Effluent Guidelines for the Steam Electric Power Generating Category," 2015)
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Flue Gas Desulfurization (FGD) Wastewater
4/8/2016 Southern Research Engineering
Limestone Slurry (Ca2+
or Ca2+-Mg2+ Rich)
Air Injection(Forced –
Ox)
Make-Up Water
FGD SlurryFGD Slurry Blowdown
Solids-Lean StreamSolids
Separation
Solids-Rich Stream Solids Dewatering
Ash Pond or Other Settling FGD
Wastewater
Purge
CaSO4.2H2O
(Gypsum) for Sale or
Disposal
Wastewater for Disposal
(Low Volume)
FGD
Slu
rry
Rec
ycle
Flue Gas
FGD Slurry Spray
Treated, Saturated Flue Gas to Plant Stack
Forced-Oxidation FGD Scrubber
Pump
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FGD Scrubbers
Source - ("USEPA Website," 2012)
• Removes SOx plus other components from flue gas.
• Heavy metals accumulate in FGD slurry including As, Cd, Cr, Hg, and Se.
• Salts in the form of Ca2+, Mg2+, Na+, Cl-, and SO4
2- also accumulate in FGD slurry.
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FGD Oxidation Modes1. Forced Oxidation FGD
Most common FGD and most new FGDs are expected to be this type (Impact of Air Emissions Controls on Coal Combustion Products, 2008).
Air is pumped through FGD slurry to oxidize CaSO3 to CaSO4.2H2O (gypsum) –
beneficially used in wallboard and agriculture.
Heavy metals especially Cr and Se can also be oxidized to more mobile and toxic forms.
This project concerns forced-oxidation FGDs.
2. Inhibited-Oxidation FGD
Na2S2O3 or S(0) is usually added to prevent oxidation of CaSO3 (Impact of Air Emissions Controls on Coal Combustion Products, 2008).
Does not typically oxidize heavy metals.
Sludge mixed with coal fly ash (CFA) and CaO or Ca(OH)2 to form a pozzolanicmaterial called “fixated scrubber sludge”.
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Se Speciation in FGD Wastewaters• Forced-oxidation systems can oxidize
SeO32-/HSeO3
- to SeO42-.
• Se Speciation in FGD wastewater is more complex than SeO3
2-/HSeO3- and SeO4
2-
(Petrov, Charters, & Wallschläger, 2012).
• SeCN- and SeSO32- along with 11
unknown Se species have been identified in FGD wastewater (Petrov, Charters, & Wallschläger, 2012).
• Presence of these unknown Se species could complicate FGD wastewater treatment.
4/8/2016 Southern Research Engineering
Source - ("Selenium," 2016)
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Nanofiltration (NF)• NF is a pressure driven membrane process,
intermediate between ultrafiltration (UF) and reverse osmosis (RO) (An Integrated Framework for Treatment and Management of Produced Water - Technical Assessment of
Water Treatment Technologies, 2009; Murthy & Choudhary, 2011).
• NF rejects solutes by two key mechanisms (Murthy & Chaudhari, 2008; Z. V. P. Murthy & A. Choudhary, 2011;
Z.V.P. Murthy & A. Choudhary, 2011):1. Electrostatic interactions between solutes and
membrane surface. 2. Sieve effects based on solute size.
• NF has removed Se from seleniferousagricultural drainage waters with greater effectiveness in removing SeVI than SeIV (Kharaka,
Ambats, Presser, & Davis, 1996).
4/8/2016 Southern Research Engineering
Source - ("Nanofiltration Plants," 2015)
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Project Objectives1. Evaluate NF and select UF membranes for FGD
wastewater treatment including removal of:A. SeB. HgC. Ca2+an Mg2+ (softening)D. Cl- and SO4
2-
2. Evaluate impact pre-treatment with KMnO4 on Se removal from FGD wastewater.
3. Evaluate NF and UF membrane molecular weight cut-offs (MWCOs) on FGD wastewater treatment.
4/8/2016 Southern Research Engineering
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1. NF and UF membranes were utilized to treat FGD wastewater in two bench-scale scenarios:
A. No pretreatment.B. Pretreatment with KMnO4 (50 mg/L) to oxidize Se.
2. 75% permeate recovery was targeted, but not achievable for all membranes (41% to 79%).
3. The following were measured in in the feed, permeate, and concentrate:A. Se speciation: ion chromatography-inductively coupled
plasma-mass spectrometry (IC-ICP-MS)B. Total and dissolved Hg: cold vapor (CV)-ICP-MSC. Total and dissolved metals: ICP-MSD. Anions: IC
Experimental Procedure
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Bench-Scale NF and UF SetupSimulated Geothermal Brine Reservoir
F1 P1 P2V1 V2
V3
V4
Permeate Collection
Concentrate Line
Plate and Frame Membrane Module
Permeate
F1 – Flowmeter No. 1P1 – Pressure Gauge No. 1P2 – Pressure Gauge No. 2P3 – Pressure Gauge No. 3V1 – Valve No. 1V2 – Valve No. 2V3 – Valve No. 3 – Regulating ValveV4 – Valve No. 4
P3
FGD Wastewater Reservoir
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NF and UF Membranes
Membrane PolymerMembrane
TypeMWCO
(Daltons)
Midpoint MWCO
(Daltons)Snyder NFX Polyamide-TFC NF ~150-300 225
Dow NF270 Polyamide NF ~200-400 300
Dow NF90 Polyamide NF ~200-400 300
Snyder NFW Polyamide-TFC NF ~300-500 400
Snyder NFG Polyamide-TFC NF ~600-800 700
Snyder NDX Polyamide-TFC NF ~800-1,000 900
GE UF GE Composite Polyamide UF 1,000 1,000
GE UF GH Thin Film UF 2,000 2,000
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Feed FGD Wastewater Composition
MetalTotal Conc.
(µg/L)Dissolved
Conc. (µg/L) AnionsDissolved
Conc. (mg/L)As <5.56 <5.56 Br- <0.625Ba 47 45 Cl- 111B 62,084 58,534 F- 6.2Ca 505,064 462,760 NO2
- (as N) 1.7Cd 3 <2.78 NO3
- (as N) 8.0Cr 3,180 18 SO4
2- 1,658Hg 0.42 0.22 PO4
3- <0.25Mg 157,258 150,675Mn 342 <11.11Na 88,092 82,928Ni 1,708 35Se 3,430 3,370Si 17,859 15,120
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FGD Wastewater Se Speciation
No Pretreatment Pretreatment with KMnO4
SeVI - 88.1%
Other (<MDL) -1.0%
Unknown Se Species - 1.5%
SeIV -9.2%MeSeIV,
Methylselenic Acid - 0.2%
SeVI - 90.5%
Other (<MDL) - 7.8%
Unknown Se Species - 1.7%
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Total and Dissolved Se Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Total Se - No Pretreatment
Dissolved Se - No Pretreatment
Total Se - Pretreatment
Dissolved Se - Pretreatment
UFNF
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Total and Dissolved Se Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Total Se - No Pretreatment
Dissolved Se - No Pretreatment
Total Se - Pretreatment
Dissolved Se - Pretreatment
UFNF
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Dissolved SeIV and MeSeIV
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
MeSe(IV)
Se(IV)
UFNF
MeSeIV
SeIV
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Dissolved SeVI Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Se(VI) - No Pretreatment
Se(VI) - Pretreatment
UFNF
SeVI - No Pretreatment
SeVI - Pretreatment
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Dissolved SeVI Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Se(VI) - No Pretreatment
Se(VI) - Pretreatment
UFNF
SeVI - No Pretreatment
SeVI - Pretreatment
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Dissolved Unknown Se Species Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Unknown Se Species - No Pretreatment
Unknown Se Species - Pretreatment
UFNF
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Dissolved Ca2+, Mg2+, and Na+ Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
CaMgNa
UFNF
Ca2+
Mg2+
Na+
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Dissolved Ca2+, Mg2+, and Na+ Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
CaMgNa
UFNF
Ca2+
Mg2+
Na+
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Cl-, F-, and SO42- Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Cl-
F-
SO42-
UFNF
Cl-
F-
SO42-
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Cl-, F-, and SO42- Removal
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Rej
ectio
n E
ffic
ienc
y (%
)
Midpoint MWCO (Daltons)
Cl-
F-
SO42-
UFNF
Cl-
F-
SO42-
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1. NF membranes with MWCO < 400 Daltons removed more than:A. 80% - Se (total and dissolved), SeVI (dissolved).B. 70% - MeSeIV (dissolved), SeIV (dissolved), and
unknown Se species.2. NF membranes with MWCO < 400 Daltons softened
FGD wastewater by more than 80%.3. All membranes removed more than 80% of Hg (total and
dissolved). 4. NF membranes with MWCOs < 400 Daltons removed
more than 90% of SO42-.
Conclusions
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5. Cl- and F- were not effectively removed except at membranes with the very lowest MWCOs.
6. Pretreatment of FGD wastewater with KMnO4:A. Converted all SeIV and MeSeIV through oxidation to
SeVI.B. Did not remove all unknown Se species.C. Did not significantly enhance Se removal.
Conclusions (Continued)
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References
4/8/2016 Southern Research Engineering
Impact of Air Emissions Controls on Coal Combustion Products (1015544). (2008). Retrieved from Palo Alto, California: An Integrated Framework for Treatment and Management of Produced Water - Technical Assessment of Water Treatment
Technologies (RPSEA Project 07122-12). (2009). Retrieved from Kharaka, Y. K., Ambats, G., Presser, T. S., & Davis, R. A. (1996). Removal of Selenium from Contaminated Agricultural
Drainage Water by Nanofiltration Membranes. Applied Geochemistry, 11(6), 797-802. doi:http://dx.doi.org/10.1016/S0883-2927(96)00044-3
Murthy, Z. V. P., & Chaudhari, L. B. (2008). Application of Nanofiltration for the Rejection of Nickel Ions from Aqueous Solutions and Estimation of Membrane Transport Parameters. Journal of Hazardous Materials, 160(1), 70-77. doi:http://dx.doi.org/10.1016/j.jhazmat.2008.02.085
Murthy, Z. V. P., & Choudhary, A. (2011). Application of Nanofiltration to Treat Rare Earth Element (Neodymium) Containing Water. Journal of Rare Earths, 29(10), 974-978. doi:http://dx.doi.org/10.1016/S1002-0721(10)60581-9
Murthy, Z. V. P., & Choudhary, A. (2011). Separation of Cerium from Feed Solution by Nanofiltration. Desalination, 279(1–3), 428-432. doi:http://dx.doi.org/10.1016/j.desal.2011.06.014
Nanofiltration Plants. (2015). Retrieved from http://www.eurowater.com/products/standard_products/nanofiltration_plants.aspx
Petrov, P. K., Charters, J. W., & Wallschläger, D. (2012). Identification and Determination of Selenosulfate and Selenocyanate in Flue Gas Desulfurization Waters. Environmental Science & Technology, 46(3), 1716-1723. doi:10.1021/es202529w
Proposed Effluent Guidelines for the Steam Electric Power Generating Category. (2015). Retrieved from http://water.epa.gov/scitech/wastetech/guide/steam-electric/proposed.cfm
Selenium. (2016). Retrieved from http://periodictable.com/Elements/034/USEPA Website. (2012). Retrieved from http://water.epa.gov/scitech/wastetech/guide/steam-electric/