Coagulation-Filtration - Chwirka & Thomson 1 Arsenic Removal by Coagulation & Precipitation Processes Presentation Prepared by: Joe Chwirka - Camp Dresser

Coagulation-Filtration - Chwirka & Thomson

1

Arsenic Removal by Coagulation & Precipitation Processes

Presentation Prepared by:Joe Chwirka - Camp Dresser & McKee

([email protected])Bruce Thomson - University of New Mexico

([email protected])Presented by: YuJung Chang – HDR Engineering, Inc.


2

Introduction

• Arsenic removal by coagulation & filtration is effective for many applications

• Presentation will discuss:

• Chemistry of the process

• Variables affecting process performance (especially pH & coagulant dose)

• Process variations

• Coagulation & granular media filtration

• Coagulation & membrane filtration

• Design considerations


3

Acid-Base Chemistry of As(V)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fra

ctio

n

H3AsO4 H2AsO4- HAsO4

2-AsO4

3-


4

Acid-Base Chemistry of As(III)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fra

ctio

n

H3AsO3

H2AsO3-

HAsO32-

AsO33-


5

Redox Chemistry of As

0

5

10

15

20

-5

-10

-15

p e

0 2 4 6 8 10 12 14p H

0

H AsO3 4

H AsO2 4-

HAsO 42-

AsO 43-

H AsO3 3

HAsO 32-

AsO 33-

As S2 3(s)

.5

1.0

-.5

Eh (vo lts)

H AsO2 3-


6

Solubility of As(III) Compounds

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 2 4 6 8 10 12 14

pH

log

Co

nce

ntr

ati

on

Arsenolite (As2O3)

Claudetite (As2O3)

Orpiment (As2S3)


7

Solubility of As(V) Species

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14

pH

log

Co

nce

ntr

atio

n

AlAsO4Ca3(AsO4)2CrAsO4Cu3(AsO4)2Fe3(AsO4)2FeAsO4Mn3(AsO4)2LaAsO4

CrAsO4

FeAsO4

Ca3(AsO4)2

Fe3(AsO4)2

Mn3(AsO4)2

Cu3(AsO4)2

AlAsO4

LaAsO4


8

Important Points

• As has two oxidation states - As(III) & As(V)• As(III)

• Non-ionic (H3AsO3) at neutral pH

• High solubility• More toxic to many organisms

• As(V)• Ionic (H2AsO4

-/HAsO42-) at neutral pH

• Some phases are less soluble• More reactive in solution:

• Membranes• IX• Adsorption


9

Coprecipitation

• Coprecipitation involves removal of two or more constituents by a precipitation reaction. Coprecipitation of As with Fe(OH)3 is an effective treatment process:

FeCl3 + 3H2O = Fe(OH)3(s) + 3H+ + 3Cl-

Points:

• Produces HCl which will lower pH

• Typical Fe dose ~10-4 M, whereas As conc. ~10-7 M, hence As is minor component within precipitate

• As likely removed by adsorption onto Fe(OH)3 surface with subsequent enmeshment as floc particle grows

• Al(OH)3 also effective


10

Solubility of Fe(OH)3

-14

-12

-10

-8

-6

-4

-2

0

0 2 4 6 8 10 12 14

pH

log

Co

nc.

Fe(OH)4-

Fe(OH)2+FeOH2+

Fe3+

Solubility of Fe(OH)3


11

Effect of pH on Surface Charge of Fe(OH)3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fra

cti

on

FeOH2+

FeOHo

FeO-


12

Covalent Bond Formation(Grossl et al., 1997)

As

OO

O O H

Fe

O

Fe

O

O

Fe

O

Fe

O

O

H

H

H

As

O H

O

O

+

+ H O2

As

O

O H

k1

k-1

k-2

k2

+ O H-

Fe

O

Fe

O

O

G o e thite Surfa c e

M o no d e nta teSurfa c e C o m p le x

Bid e nta teSurfa c e C o m p le x


13

pH of Zero Point of Charge

• Electrostatic attraction is important first step in adsorption

• pHzpc = pH at which net surface charge = 0

• Surface is positive at pH < pHzpc

• Most clay minerals have pHzpc < 6

• Hence poor adsorption• Clays dominate surface chemistry of soils

• Fe(OH)3 and Al2O3 have relatively high pHzpc

• Good adsorbents of As(V)


14

As Removal by Conventional Treatment (McNeill & Edwards 1997)

• Survey of conventional coagulation-flocculation water treatment plants

• Correlate As removal to removal of Fe, Mn, & Al

• [Fe] = Iron Precip. Formed (mM)

K[Fe]1K[Fe] RemovedFraction As


15

As Removal by Conventional Trt. - 2

0

10

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06 0.08 0.1

Precipitate Formed (mM)

As

Re

mo

va

l (%

)

[Fe]

[Fe]+[Al]


16

As Removal by Conventional Trt. - 3

• Strong correlation to removal of Fe & use of FeCl3 as coagulant

• Weaker correlation to removal of Al & use of Alum

• Possible sorption onto colloidal Al(OH)3 which passes through granular media filters

• Improved As removal achieved by minimizing effluent total Al concentration

• Note the importance of particulate As


17

0

10

20

30

40

50

0 5 10 15 20

FeCl3 Dose, mg Fe/L

Res

idu

al A

s, u

g/L

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

Filt

rate

pH

Residual Arsenic

Filtrate pH

Arsenic Removal vs FeCl3 Dose, Albuquerque NM


18

Ambient pH: FeCl3 vs As Leakage, NAS Fallon

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 20 40 60 80 100 120

FeCl3, mg/L

Ars

enic, u

g/L

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

pH As

pH


19

El Paso Jar Testing

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40

FeCl3 Dose, mg/L

Ars

enic

Lea

kag

e, u

g/L

5

5.5

6

6.5

7

7.5

8

8.5

9

pH

ArsenicpH


20

pH Adjustment with CO2, NAS Fallon, NV

pH Reduction with CO2

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

FeCl3 Dose, mg/L

Res

idual

As, u

g/L

pH 6.0

pH 6.8


21

Silica Impacts Arsenic Removal at pH 7.0 and Above

As Removal as a Function of Silicate and pH

0

5

10

15

20

25

0 10 20 30 40 50 60

Silicate, mg/L

Eff

lue

nt

As

, u

g/L

pH 6.5

pH 7.5

pH 8.5

pH 8.5

pH 7.5

pH 6.5

FeCl3 Dose: 4 mg/L

After Clifford and Ghurye


22

Silica Speciation with pH

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

pH

Fra

ctio

nH4SiO4 H3SiO4

-

H2SiO42-


23

Silica in US Water Supplies (NAOS)

0

20

40

60

80

100

0 10 20 30 40 50 60 70

Soluble SiO2 (mg/L)

Cum

ulat

ive

Per

cent

age

(%)

Surface Waters

Ground Waters

After Davis and Edwards


24

Polymeric Silica

0

20

40

60

80

100

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5pH

[Si] p

olym

er/[

Si] t

otal (

%)

After Davis and Edwards

50 mg/L17 mg/L

5 mg/L


25

Coagulation/ Filtration

• Use pressure filters

• Direct Filtration frequently used for iron and manganese removal.

• Limited to low ferric dose applications.

• High coagulant dose will result in frequent backwash requirements

• Increased residuals production & handling costs

• Increased production of wastewater


26

Schematic of Coagulation/ Filtration

Aeration

FeCl3

Rapid Mix

Solids to Dewatering

Treated Water

Pressure Filter

Solids to Landfill

Raw Water

CO2

CO2


27

Calculation of Filter Loading Limitation

• Rule of Thumb, no more than 10 mg/L of FeCl3

• Limit Solids Loading to 0.1 lbs/SF

• May need to add sedimentation


28

Vertical Pressure Filter


29

Direct Filtration Performance(Based on 0.1 lbs Solids/sf)

Filter Performance at 3 gpm/sf

0102030405060708090

100110120130

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Ferric Chloride, mg/L

Filte

r R

un T

ime,

Hrs


30

Backwash Water as Percent of Production(Based on 250 gal/sf)

Wastewater Production at 3 gpm/sf

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

14.0%

16.0%

0 2 4 6 8 10 12 14 16

Ferric Chloride Dose, mg/L

Wa

ste

wa

ter,

%


31

C/F O&M Issues

• Large backwash volume (20 gpm/sf for 10 minutes)

• Tanks may need internal painting, 10 yr intervals. Use 316 SST.

• Standby filters, typically provided, but need to evaluate.

• Pneumatic or electric valve operators.


32

Coagulation/ Pressure Filtration

• Particle size

• Particle breakthrough

• backwash requirements

• filter ripening

• Backup Filters


33

Coagulation/ Microfiltration

• Pilot tested in Albuquerque, 1998

• Pilot tested in NAS Fallon, NV, 2001

• Pilot tested in El Paso, TX, 2001/2002

• Fallon Paiute Shoshone Tribe: 0.5 mgd

• City of Albuquerque: 2.3 mgd


34

Microfiltration General Concepts

• Low Operating Pressure, 5 - 30 psi

• 0.1 to 0.2 micron pore size

• Water flow from Outside to the Inside

• Air-Water Backwash

• Backwash Every 25 to 30 minutes (95% recovery)

• Flux rate defined as Gallons/SF/Day (GFD)

• Chemical Cleaning Frequency > 30 days


35

What is C/MF?

Aeration

FeCl3

Rapid Mix


Treated Water

Microfiltration Unit

Solids to Landfill

Raw Water

CO2

CO2


36

Pressure Driven Membranes

0.001µ0.001µ 0.01µ0.01µ 0. 1µ0. 1µ 1.0µ1.0µ 10µ10µ 100µ100µ 1000µ1000µ

Dissolved OrganicsDissolved Organics

BacteriaBacteria

SandSand

CystsCysts

VirusesViruses

SaltsSalts ColloidsColloids

Media FiltrationMedia Filtration

MicrofiltrationMicrofiltration

UltrafiltrationUltrafiltration

NanofiltrationNanofiltration

Reverse OsmosisReverse Osmosis


37

MF Process Operates in Direct Filtration Mode

FeedFeed

FiltrateFiltrate

Feedwater Feedwater (with (with contaminants)contaminants)

FiltrateFiltrate


38

Solids are Removed from Module by an Air-Water Backwash

Air Air (100 (100 psig)psig)

AirAir

FeedwaterFeedwater

11 22 33


39

Coagulation/ Microfiltration

• Pilot tested in Albuquerque, 1998

• Pilot tested in NAS Fallon, NV, 2001

• Pilot tested in El Paso, TX, 2001/2002

• Fallon Paiute Shoshone Tribe: 0.5 mgd

• City of Albuquerque: 2.3 mgd


41

What is C/MF?

Aeration

FeCl3

Rapid Mix


Treated Water

Microfiltration Unit

Solids to Landfill

Raw Water

CO2

CO2


42

Pressure Driven Membranes

0.001µ0.001µ 0.01µ0.01µ 0. 1µ0. 1µ 1.0µ1.0µ 10µ10µ 100µ100µ 1000µ1000µ

Dissolved OrganicsDissolved Organics

BacteriaBacteria

SandSand

CystsCysts

VirusesViruses

SaltsSalts ColloidsColloids

Media FiltrationMedia Filtration

MicrofiltrationMicrofiltration

UltrafiltrationUltrafiltration

NanofiltrationNanofiltration

Reverse OsmosisReverse Osmosis


43

MF Process Operates in Direct Filtration Mode

FeedFeed

FiltrateFiltrate

Feedwater Feedwater (with (with contaminants)contaminants)

FiltrateFiltrate


44

Solids are Removed from Module by an Air-Water Backwash

Air Air (100 (100 psig)psig)

AirAir

FeedwaterFeedwater

11 22 33


45

Pall Microfilter


46

Memcor PerformanceNAS Fallon, 15 mg/L FeCl3

Transmembrane Pressure

0

5

10

15

20

25

0 20 40 60 80 100 120Elapsed Time [days]

psi

25 GFD (No FeCl3)

27 GFD

38 GFD(No FeCl3)

32 GFD

27 GFD


47

Memcor Cleaning EfficiencyNAS Fallon, Citric Acid

TMP vs. Normalized Flux

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50

Normalized Flux [gfd @ 20°C]

TMP

[ps

i]

Start-Up

Post Clean


48

Pall PerformanceNAS Fallon, FeCl3 45 mg/L

0

5

10

15

20

0 10 20 30 40 50 60 70 80 90

Elapsed Time, Days

TM

P, p

si

42G FD54 G FD

54 G FD


49

El Paso C/MF Pilot Studies


50

El Paso Pilot Studies

• Only Pall MF tested

• Ferric dose 10 mg/L

• pH lowered to 6.8 with CO2


51

El Paso Pall Performance

15 mg/L FeCl3, pH 6.8,

89.8 gfd, >100 Day CIP



0

4

8

12

16

20

24

0 500 1000 1500 2000 2500 3000 3500 4000

Operational Hours

TM

P (p

si)



Well Shutdown for Repair


52

Fallon Paiute Shoshone Tribe C/MF PFDCO2

CO2 M

Rapid Mix

Static Mixer

Well 2

LowryAerationSystem

Well 1

Solids

DewateredSolids toLandfill

FilterBottom

Dumpster

RecycleSump

FinishedWaterSump

Recycle

TreatedWater to

DistributionSystem

FerricChloride

Cl2

Microfilter

LamellaPlate

Thickener

Liquids toSewer


53

Fallon Paiute Shoshone Tribe C/MF


54

Fallon Paiute Shoshone Tribe As Treatment Faciliy


55

Fallon Paiute Shoshone Tribe Start-upDecember 2004

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50

Ferric Chloride Dose

Fin

ish

ed

Wa

ter

Ars

en

ic

pH = 6.8

pH = 7.4

pH = 8.0


56

C/MF Summary

• Emerging Technology for Arsenic Treatment

• Can be designed for high flux rates with Low TOC groundwater

• Optimize solids loading by pH pre-treatment

• Cost competitive with other technologies


57

Recent Studies on Particle Size Filtration and Arsenic Removal


58

C/MF O&M Issues

• Membrane Replacement: Pall warrantees membranes for 10 years, prorated.

• Chemical cleaning with citric acid, can not be recycled, must be disposed of.

• Provide sufficient replacement parts, not system redundancy.


59

Comparison of C/MF to Pressure Filters at the Fallon Paiute Shoshone Tribe

Capital Cost Summary Pressure Filters C/MFTotal Estimated C/MF Facility Cost $1,252,998 $987,898Summary of Annual O&M CostsTotal Estimateded O&M for Treatment, $/yr $71,436 $82,392Unit O&M Costs for C/MF $0.77 $0.89Present Worth AnalysisTotal Present Value of Facitlities $2,087,000 $1,948,000Annual Amortized Cost of Capital & O&M $125,220 $116,880Total Unit Cost of Water Produced, $/1,000 gals 1.38 1.29


60

Residuals Characteristics for C/MF and C/F

• C/MF around 4% to 5 % Backwash.

• C/F around 5% to 10% Backwash.

• Recycle the backwash water to minimize wastewater.

• Ferric residuals will pass TCLP, however, may not pass the Cal WET.


61

Residuals Handling

• Mechanical dewatering will be complicated: Ferric sludge is difficult to dewater

• Need body additives, Diatomaceous Earth

• Filter Bottom Dumpsters and polymer for small applications.

• Solids drying ponds:

• Ponds need to be lined.

• Anaerobic conditions may release the As.• Provide access for sludge removal equipment.


62

Concurrent Iron, Manganese, and ArsenicDrinking Water Standards

• Fe: Secondary Standard of 0.30 mg/L

• Mn: Secondary Standard of 0.05 mg/L


63

Iron and Arsenic Removal

• Oxidize Fe with Cl2 or O3

• Adsorb As onto Fe(OH)3 precipitate

• pH needs to be around 7.3


64

Manganese and Arsenic Removal

• As requires low pH for adsorption

• Mn requires high pH (>10) for oxidation with Cl2

• Mn oxidation by ClO2 is rapid & appears to be independent of pH

• ClO2 reported to be ineffective for As(III) oxidation.

• May need to add Cl2 in addition

Documents

Coagulation-Filtration - Chwirka & Thomson 1 Arsenic Removal by Coagulation & Precipitation Processes Presentation Prepared by: Joe Chwirka - Camp Dresser