Aeration Approaches and Activated Iron Solids (AIS) for AMD Treatment

ByJon Dietz, Ph.D.Environmental Engineering & ScienceIron Oxide Technologies, LLCdietzetal@adelphia.netwww.DGengr.com

Iron Removal from Acid Mine Drainage

A Two Step Process

1. Ferrous Iron (Fe2+) Oxidation to Ferric Iron (Fe3+) – the rate limiting step in ALL treatment technologies

2. Precipitation of Ferric Iron (Fe2+) to a hydroxide solid – very fast but the conditions (e.g., pH) determine solids quality

Oxidation & Hydrolysis(overall equations)

FeFe2+2+ + ¼O + ¼O22 + H + H++ => Fe => Fe3+3+ + ½H + ½H22OO

FeFe3+3+ + 3H + 3H22O => Fe(OH)O => Fe(OH)33 + 3H + 3H++

1 mg/L of D.O. = 7 mg/L Fe2+

1.8 mg/L as CaCO3 = 1 mg/L Fe2+

Ferrous Iron OxidationFerrous Iron OxidationProcesses In AMD TreatmentProcesses In AMD Treatment

Homogeneous Ferrous Iron OxidationA solution-based oxidation process whereby Ferrous Ions and

hydroxide complexes (Fe2+, Fe(OH)+ & Fe(OH)20) react with

dissolved oxygen to form ferric iron (Fe3+). Existing active

(e.g., lime) and passive treatment oxidation process.

Heterogeneous Ferrous Iron OxidationA solid/solution interface oxidation process whereby Ferrous Iron

(Fe2+) is sorbed to the surface of iron oxide (or other oxide surfaces) and in the presence of dissolved oxygen is catalytically oxidized to ferric iron (Fe3+). New active treatment known as AIS treatment utilizes this oxidation process.

Aeration In AMD Treatment

Homogenous Ferrous Iron OxidationSolution-based Oxidation & Precipitation

Fe2+OH-

OH-+ Fe2+

-

OH-STEP 1

Fe2+

-

OH-

O+ O Fe2+

-

OH-

O O

STEP 2

Fe3+

-

OH-

+ O O+ -

STEP 3

O O -+Fe2+3 + H+4 Fe3+3 + 2 H2O

Fe2+

-

OH-

OH-+ Fe2+

OH-

OH-

OH- SolidFe3+3 + 410

STEP 4

Fe2+OH-

OH-+ Fe2+

-

OH-STEP 1

Fe2+OH-

OH-+ Fe2+

-

OH-STEP 1

Fe2+

-

OH-

O+ O Fe2+

-

OH-

O O

STEP 2

Fe3+

-

OH-

+ O O+ -Fe2+

-

OH-

O+ O Fe2+

-

OH-

O O

STEP 2

Fe3+

-

OH-

+ O O+ -

STEP 3

O O -+Fe2+3 + H+4 Fe3+3 + 2 H2O

STEP 3

O O -+Fe2+3 + H+4 Fe3+3 + 2 H2O

Fe2+

-

OH-

OH-+ Fe2+

OH-

OH-

OH- SolidFe3+3 + 410

STEP 4

Fe2+

-

OH-

OH-+ Fe2+

OH-

OH-

OH- SolidFe3+3 + 410

STEP 4

Homogeneous Reaction Rate Importance of pH

Fe2+Fe(OH)20

Fe(OH)1+

At [OAt [O22] = 1.26 mM and 25] = 1.26 mM and 25C (portions of figure C (portions of figure

reproduced from Wehrli 1990). Open circles reproduced from Wehrli 1990). Open circles (o) are from Singer & Stumm (1970), and (o) are from Singer & Stumm (1970), and solid circles (solid circles () are from Millero ) are from Millero et alet al. (1987).. (1987).

Dashed lines are estimated rates for the Dashed lines are estimated rates for the various dissolved Fe(II) species.various dissolved Fe(II) species.

Minutes

Hours

Days

Months

Years

Between pH 5 and 8 the oxidation rate

doubles for every 0.15 pH increase

At pH greater than 8 the oxidation rate slows because of ferrous hydroxide (“green

rust”) precipitation

Simplified Calculation of pH or CO2 Acidity

CO2 Acidity (mg/L CaCO3) = Alkalinity (mg/L CaCO3) 2 10-pH 10-

6.4

pH = 6.4 – Log [CO2 Acidity (mg/L CaCO3) (2Alkalinity (mg/L CaCO3))]

or

Effect of Carbon Dioxide Acidity on pH

0

50

100

150

200

250

300

6 6.25 6.5 6.75 7 7.25 7.5

pH

CO

2 A

cid

ity

(m

g/L

Ca

CO 3

)

Approaching Equillibrium with the Atmosphere

Importance of Carbon Dioxide and its Removal on Iron Oxidation

Alkalinity = 100 mg/L

Depth ~ 5 feet

Natural Pond Aeration

AirNitrogen N2 Gas = 80%Oxygen O2 Gas = 19%

Carbon Dioxide CO2 Gas = 0.003%All Other < 1%

WaterD.O. (Sat) =10 mg/L = 0.001%

H2CO3 = 10 – 500 mg/L = 0.001 to 0.05%

Natural Aeration occurs at the

air/water interface through mass

transport processes

Summary of Important Factors For Aeration Effectiveness

1. The time the water is in contact with Air increases amount of gas transport

• Air:Water Interface duration2. The amount of water surface area in

contact with Air increases gas transport

• Air:Water Interface Amount

What is a Bubble? a pocket of air suspended in water.

Air in BubbleNitrogen N2 Gas = 80%Oxygen O2 Gas = 19%

Carbon Dioxide CO2 Gas = 0.03%All Other < 1%

The gas inside a bubble is the

same as in the AIR

WATER

The contact between and a

bubble and water is the same as the contact layer between AIR and WATER

Aeration occurs at the air/water

interface

Gas Transport from and to Air Bubbles

AirNitrogen N2 Gas = 80%Oxygen O2 Gas = 19%

Carbon Dioxide CO2 Gas = 0.03%All Other < 1%

Anoxic AMD Water ConditionsD.O. = 0 mg/L

H2CO3 = 300 – 500 mg/L

Bubble Rise

O2

CO2

Air Equilibrium Water ConditionsD.O. = 10 mg/L

H2CO3 = 1.5 mg/L

Henrys Law

xx

xeq C

PKH

Bubble Geometry Sphere

diameter

Coarse BubbleDiameter ~ 1 cm

Fine BubbleDiameter ~ 0.1 cm

Surface Area = 4r2

Volume = 4/3r3

Surface Area: Volume Ratio

3.14 cm2

0.523 cm30.0314 cm2

0.000523 cm3

6 60

Not-to-scale

An EQUAL volume of fine bubbles has 10 times the surface area as coarse bubbles

10 times the gas transport

Bubble Rise Through Water

Reactor Depth (ft)

Average Travel Time (sec)

Coarse Fine

2 2.7 13.7

10 8.6 43.3

Coarse BubbleDiameter ~ 1 cm

Fine BubbleDiameter ~ 0.1 cm

Not-to-scale

Bubble Rise Velocity (Stokes Law) =

Small single bubble

Ub = 22.3 cm/sec

Ub = 7.0 cm/sec

22 5.319

)(2bb

ww

awb RR

pv

ppgU

Large bubble swarm

Fine Bubbles rise at less than one-third the rise of coarse bubbles Greater than 3 times the gas transport

Summary of Aeration Principles

Fine bubbles have much greater surface area to volume ratio than coarse bubbles. An equal volume of fine bubbles will have 10

times the air to water interface. Fine Bubbles rise much more slowly than

coarse bubbles and have more time to react with water (greater than times longer. Fine bubbles will be in the aeration tank more

than 3 times longer than coarse bubbles.

How does this affect Aeration Systems?

Fine Bubble Aeration requires less air volume and reactor size than coarse bubble aeration to achieve the same or greater gas transport to (dissolved oxygen) and from (carbon dioxide acidity) water.

Coarse Bubble Aeration will require greater volumes of air (and power consumption) as well as tank volumes (capital costs) to achieve the same aeration.

Pre-Aeration Tank Designfor mine drainage treatment

AMD InflowOutlet

FlowFlow

Air Feed Line From Blower

PartitionBaffle

Drop OutMembrane Diffuser

From Blower

Not-to-Scale

X feet

12

fe

et

Full Grating

Membrane Diffuser

Detachable Drop-out

Air Feed Line (6 psi)

12 feet

12 feet

Full Cover Grating

Example of a Tank Pre-Aeration System

Depth ~ 6-8 feet

In-Situ Pond Aeration with Lasaire Aeration System?

Blower Underwater Fine Bubble Air Lines

Aeration increases dissolved oxygen and increases pH to increase iron oxidation and

removal

Upper Latrobe Passive Treatment System1st Application of Lasaire Aeration in AMD Treatment

Upper Latrobe Passive Treatment SystemPreliminary Results

Flow =350 gpm

Aeration Changes:pH Increase from 6.1 to 6.8DO Increase from 0 to 9.7 mg/L

Iron Oxidation:Fe2+ decreased from 55 to 0.5 mg/L in Aeration Zone

Iron Removal: Complete in 2nd settling zoneTotal Iron ~ 3 mg/L

Treatment Area Potentially Reduced By A Factor of 10

AIS In AMD Treatment

Heterogeneous Ferrous Iron OxidationHeterogeneous Ferrous Iron OxidationSurface-based Oxidation & Precipitation

Solid/SolutionInterfaceSTEP 1

Solid Fe(OH)3

Fe2+OH-

+OH-

Solid Fe(OH)3

Fe2+OH -

OH-

OH-

STEP 2

OO+Solid Fe(OH)3

Fe

2+O

H-

OH

-

Fe

2+O

H-

OH

-

Fe2+OH -

OH-

Fe2+

OH-

OH -

+ +9Solid

Fe(OH)3

New Iron Oxide

Affect Bench Test comparing Passive Treatment Oxidation to AIS Oxidation

SW Borehole Batch Test 4

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200

Time (min)

Fer

rou

s Ir

on

(m

g/L

)

Passive Test Modeled Passive

Pre-Aeration Test Modeled Pre-Aeration

SW Borehole AIS Batch Tests

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30

Time (min)

Fer

rou

s Ir

on

(m

g/L

)

1 g/L Test Modeled 1 g/L 2 g/L Test Modeled 2 g/L

Passive Treatment Oxidation

Passive Treatment Oxidation with Pre-

aeration

AIS Treatment Oxidation

AMD Treatment in a Two-Stage Flow-Through AIS System(PATENT PENDING)

AIS CSTR

size varies

Alkaline Material

Doser

Inflow

Mixer/Aeration

Not-to-ScaleTank Volume Varies

Treated EffluentAIS

CSTRsize varies

Mixer/Aeration

Stage 1 Reactor

ClarificationSystem

Stage 2 Reactor

Waste AISTo Thickener

AIS Recirculation

AIS Treatment Pilot Testing Phillips AMD AIS Study

Phillips Deep Mine DischargepH = 6.1 Ferrous Iron = 50 mg/L, Flow = 6 MGD

Phillips AIS Treatment StudyGenerator, Fuel Tank, Pilot System, Field Lab

Results from Phillips Pilot StudyAnalytical results from Phillips AIS pilot testing (Test AIS6) at

AMD Flow 80 gpm (Detention Time = 25 minutes/reactor)

Location pH

Dissolved Oxygen

mg/L Temperature

°C Total Iron

mg/L

Dissolved Iron mg/L

Raw 6.10 0.2 15.0 47.4 47.6 React 1 6.30 5.8 15.2 2,600 0.04 React 2 6.51 8.4 15.3 2,300 0.03 Clarifier 6.47 8.4 15.4 1.05 0.01

AIS Treatment effectively oxidizes ferrous iron in short detention

times needed to meet effluent objectives for the Phillips AMD. Observed oxidation rates by the AIS solids are greater than

predicted using the heterogeneous ferrous iron oxidation model. The 9 MGD Phillips treatment system will have a capital cost of

$2,790,000 with an annual operating cost between $50,000 and $270,000 (depending on inclusion of labor and solids reuse).

The treatment costs for the Phillips discharge range between of $0.025 and $0.18 per 1,000 gallons of treated water (depending on

inclusion of various operating costs and reflection of capital costs in the estimate).

Preliminary Design for the Phillips AMD AIS Treatment System

AIS Treatment Pilot Testing Shamokin Scotts Tunnel Pilot Study

Scotts Tunnel AIS Treatment StudyReactors, Floc Tank, Clarifier, Gyro Doser

Scotts Deep Mine DischargepH = 5.75 Ferrous Iron = 25 mg/L, Flow = 10 MGD

Initial Results from Shamokin Pilot Study

Preliminary results from the Shamokin AIS pilot testing

Location pH

Dissolved Oxygen mg/L

Temperature °C

Total Iron mg/L

Dissolved Iron mg/L

Raw 5.75 6.0 12.0 22.5 22.5 React 1 6.60 9.8 16.8 420 0.10 React 2 6.80 10.0 17.4 360 0.01 Clarifier 6.60 -- -- 3.4 0.00

Summary

Aeration is important in AMD treatment to add dissolved oxygen and remove carbon dioxide (for pH control).

Aeration can reduce the size of passive aerobic ponds by a factor of 10 (where land area is a limiting factor).

AIS Treatment is an effective AMD treatment method lowering treatment footprint to a fraction of the land area required for passive treatment.

AIS Treatment can substantially lower costs compared to conventional chemical treatment and be comparable to passive treatment.