Aeration Approaches and Activated Iron Solids (AIS) for AMD Treatment By Jon Dietz, Ph.D. Environmental Engineering & Science Iron Oxide Technologies, LLC

Iron Removal from Acid Mine Drainage A Two Step Process 1. Ferrous Iron (Fe 2+ ) Oxidation to Ferric Iron (Fe 3+ ) – the rate limiting step in ALL treatment technologies 2. Precipitation of Ferric Iron (Fe 2+ ) to a hydroxide solid – very fast but the conditions (e.g., pH) determine solids quality

Oxidation & Hydrolysis (overall equations) Fe 2+ + ¼O 2 + H + => Fe 3+ + ½H 2 O Fe H 2 O => Fe(OH) 3 + 3H + 1 mg/L of D.O. = 7 mg/L Fe mg/L as CaCO 3 = 1 mg/L Fe 2+

Ferrous Iron Oxidation Processes In AMD Treatment Homogeneous Ferrous Iron Oxidation A solution-based oxidation process whereby Ferrous Ions and hydroxide complexes (Fe 2+, Fe(OH) + & Fe(OH) 2 0 ) react with dissolved oxygen to form ferric iron (Fe 3+ ). Existing active (e.g., lime) and passive treatment oxidation process. Heterogeneous Ferrous Iron Oxidation A solid/solution interface oxidation process whereby Ferrous Iron (Fe 2+ ) 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 (Fe 3+ ). New active treatment known as AIS treatment utilizes this oxidation process.

Aeration In AMD Treatment

Homogenous Ferrous Iron Oxidation Solution-based Oxidation & Precipitation

Homogeneous Reaction Rate Importance of pH At [O 2 ] = 1.26 mM and 25  C (portions of figure reproduced from Wehrli 1990). Open circles (o) are from Singer & Stumm (1970), and solid circles (  ) are from Millero et al. (1987). Dashed lines are estimated rates for the 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 CO 2 Acidity CO 2 Acidity (mg/L CaCO 3 ) = Alkalinity (mg/L CaCO 3 )  2  10 -pH  pH = 6.4 – Log [ CO 2 Acidity (mg/L CaCO 3 )  (2  Alkalinity (mg/L CaCO 3 ) ) ] or

Importance of Carbon Dioxide and its Removal on Iron Oxidation Alkalinity = 100 mg/L

Depth ~ 5 feet Natural Pond Aeration Air Nitrogen N 2 Gas = 80% Oxygen O 2 Gas = 19% Carbon Dioxide CO 2 Gas = 0.003% All Other < 1% Water D.O. (Sat) =10 mg/L = 0.001% H 2 CO 3 = 10 – 500 mg/L = 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 duration 2. 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 Bubble Nitrogen N 2 Gas = 80% Oxygen O 2 Gas = 19% Carbon Dioxide CO 2 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 Air Nitrogen N 2 Gas = 80% Oxygen O 2 Gas = 19% Carbon Dioxide CO 2 Gas = 0.03% All Other < 1% Anoxic AMD Water Conditions D.O. = 0 mg/L H 2 CO 3 = 300 – 500 mg/L Bubble Rise O2O2 CO 2 Air Equilibrium Water Conditions D.O. = 10 mg/L H 2 CO 3 = 1.5 mg/L Henrys Law

Bubble Geometry Sphere diameter Coarse Bubble Diameter ~ 1 cm Fine Bubble Diameter ~ 0.1 cm Surface Area = 4  r 2 Volume = 4 / 3  r 3 Surface Area: Volume Ratio 3.14 cm cm cm cm 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) CoarseFine Coarse Bubble Diameter ~ 1 cm Fine Bubble Diameter ~ 0.1 cm Not-to-scale Bubble Rise Velocity (Stokes Law) = Small single bubble U b = 22.3 cm/sec U b = 7.0 cm/sec 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 Design for mine drainage treatment AMD Inflow Outlet Flow Air Feed Line From Blower Partition Baffle Drop Out Membrane Diffuser From Blower Not-to-Scale X feet 12 feet Full Grating Membrane Diffuser Detachable Drop-out Air Feed Line (6 psi) 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 System 1 st Application of Lasaire Aeration in AMD Treatment

Upper Latrobe Passive Treatment System Preliminary Results Flow =350 gpm Aeration Changes: pH Increase from 6.1 to 6.8 DO Increase from 0 to 9.7 mg/L Iron Oxidation: Fe 2+ decreased from 55 to 0.5 mg/L in Aeration Zone Iron Removal: Complete in 2 nd settling zone Total Iron ~ 3 mg/L Treatment Area Potentially Reduced By A Factor of 10

AIS In AMD Treatment

Heterogeneous Ferrous Iron Oxidation Surface-based Oxidation & Precipitation Solid/Solution Interface STEP 1 Solid Fe(OH) 3 Fe 2+ OH - + Solid Fe(OH) 3 Fe 2+ OH - STEP 2 OO + Solid Fe(OH) 3 Fe 2+ OH - Fe 2+ OH - Fe 2+ OH - Fe 2+ OH Solid Fe(OH) 3 New Iron Oxide

Affect Bench Test comparing Passive Treatment Oxidation to AIS Oxidation 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-Scale Tank Volume Varies Treated Effluent AIS CSTR size varies Mixer/Aeration Stage 1 Reactor Clarification System Stage 2 Reactor Waste AIS To Thickener AIS Recirculation

AIS Treatment Pilot Testing Phillips AMD AIS Study Phillips Deep Mine Discharge pH = 6.1 Ferrous Iron = 50 mg/L, Flow = 6 MGD Phillips AIS Treatment Study Generator, Fuel Tank, Pilot System, Field Lab

Results from Phillips Pilot Study  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 Study Reactors, Floc Tank, Clarifier, Gyro Doser Scotts Deep Mine Discharge pH = 5.75 Ferrous Iron = 25 mg/L, Flow = 10 MGD

Initial Results from Shamokin Pilot Study

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.