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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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Project Conducted at the

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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/