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

This article discusses different approaches to aeration for acid mine drainage (AMD) treatment, focusing on the use of activated iron solids (AIS) and iron oxide technologies. It also explores the importance of pH, carbon dioxide removal, and bubble characteristics in aeration effectiveness.

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

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  1. Aeration Approaches and Activated Iron Solids (AIS) for AMD Treatment By Jon Dietz, Ph.D. Environmental Engineering & Science Iron Oxide Technologies, LLC dietzetal@adelphia.netwww.DGengr.com

  2. Iron Removal from Acid Mine DrainageA Two Step Process Ferrous Iron (Fe2+) Oxidation to Ferric Iron (Fe3+) – the rate limiting step in ALLtreatment technologies Precipitation of Ferric Iron (Fe2+) to a hydroxide solid – very fast but the conditions (e.g., pH) determine solids quality

  3. Oxidation & Hydrolysis(overall equations) Fe2+ + ¼O2 + H+ => Fe3+ + ½H2O Fe3+ + 3H2O => Fe(OH)3 + 3H+ 1 mg/L of D.O. = 7 mg/L Fe2+ 1.8 mg/L as CaCO3 = 1 mg/L Fe2+

  4. Ferrous Iron Oxidation Processes In AMD Treatment Homogeneous Ferrous Iron Oxidation A 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 Oxidation A 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.

  5. Aeration In AMD Treatment

  6. Homogenous Ferrous Iron Oxidation Solution-based Oxidation & Precipitation

  7. Minutes Hours Days Months Years Homogeneous Reaction RateImportance of pH At pH greater than 8 the oxidation rate slows because of ferrous hydroxide (“green rust”) precipitation Between pH 5 and 8 the oxidation rate doubles for every 0.15 pH increase At [O2] = 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.

  8. Simplified Calculation of pH or CO2 Acidity CO2 Acidity (mg/L CaCO3)= Alkalinity (mg/L CaCO3) 2  10-pH  10-6.4 or pH = 6.4 – Log [CO2 Acidity (mg/L CaCO3)  (2Alkalinity (mg/L CaCO3))]

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

  10. Natural Pond Aeration Air Nitrogen N2 Gas = 80% Oxygen O2 Gas = 19% Carbon Dioxide CO2 Gas = 0.003% All Other < 1% Natural Aeration occurs at the air/water interface through mass transport processes Water D.O. (Sat) =10 mg/L = 0.001% H2CO3 = 10 – 500 mg/L = 0.001 to 0.05% Depth ~ 5 feet

  11. Summary of Important Factors For Aeration Effectiveness • The time the water is in contact with Air increases amount of gas transport • Air:Water Interface duration • The amount of water surface area in contact with Air increases gas transport • Air:Water Interface Amount

  12. What is a Bubble? a pocket of air suspended in water. Aeration occurs at the air/water interface WATER Air in Bubble Nitrogen 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 The contact between and a bubble and water is the same as the contact layer between AIR and WATER

  13. Gas Transport from and to Air Bubbles Air Equilibrium Water Conditions D.O. = 10 mg/L H2CO3 = 1.5 mg/L Anoxic AMD Water Conditions D.O. = 0 mg/L H2CO3 = 300 – 500 mg/L Air Nitrogen N2 Gas = 80% Oxygen O2 Gas = 19% Carbon Dioxide CO2 Gas = 0.03% All Other < 1% CO2 O2 Henrys Law Bubble Rise

  14. Bubble Geometry Sphere Coarse Bubble Diameter ~ 1 cm Fine Bubble Diameter ~ 0.1 cm Not-to-scale diameter Surface Area = 4r2 0.0314 cm2 3.14 cm2 Volume = 4/3r3 0.523 cm3 0.000523 cm3 6 60 Surface Area: Volume Ratio An EQUAL volume of fine bubbles has 10 times the surface area as coarse bubbles  10 times the gas transport

  15. Bubble Rise Through Water Not-to-scale Ub = 22.3 cm/sec Coarse Bubble Diameter ~ 1 cm Ub = 7.0 cm/sec Fine Bubble Diameter ~ 0.1 cm Small single bubble Large bubble swarm Bubble Rise Velocity (Stokes Law) = Fine Bubbles rise at less than one-third the rise of coarse bubbles  Greater than 3 times the gas transport

  16. 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.

  17. 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.

  18. Pre-Aeration Tank Design for mine drainage treatment Partition Baffle Air Feed Line From Blower Full Grating AMD Inflow Outlet From Blower 12 feet Flow Flow Full Cover Grating X feet Membrane Diffuser Drop Out Air Feed Line (6 psi) Not-to-Scale 12 feet Detachable Drop-out 12 feet Membrane Diffuser

  19. Example of a Tank Pre-Aeration System

  20. In-Situ Pond Aeration with Lasaire Aeration System? Aeration increases dissolved oxygen and increases pH to increase iron oxidation and removal Underwater Fine Bubble Air Lines Blower Depth ~ 6-8 feet

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

  22. Upper Latrobe Passive Treatment SystemPreliminary 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: Fe2+ decreased from 55 to 0.5 mg/L in Aeration Zone Iron Removal: Complete in 2nd settling zone Total Iron ~ 3 mg/L Treatment Area Potentially Reduced By A Factor of 10

  23. AIS In AMD Treatment

  24. Solid/Solution Interface STEP 1 Solid Fe(OH)3 Solid Fe(OH)3 OH- Fe2+ Fe2+ OH- + OH- OH- Solid Fe(OH)3 STEP 2 New Iron Oxide + Solid Fe(OH)3 Fe2+ OH- Fe2+ OH- Fe2+ OH- + + 9 O O OH- OH- OH- OH- OH- OH- Fe2+ Heterogeneous Ferrous Iron Oxidation Surface-based Oxidation & Precipitation

  25. Affect Bench Test comparing Passive Treatment Oxidation to AIS Oxidation Passive Treatment Oxidation AIS Treatment Oxidation Passive Treatment Oxidation with Pre-aeration

  26. AMD Treatment in a Two-Stage Flow-Through AIS System(PATENT PENDING) Stage 1 Reactor Alkaline Material Doser Clarification System Stage 2 Reactor Inflow Treated Effluent AIS CSTR size varies AIS CSTR size varies Mixer/Aeration Mixer/Aeration AIS Recirculation Waste AIS To Thickener Not-to-Scale Tank Volume Varies

  27. 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

  28. 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).

  29. Preliminary Design for the Phillips AMD AIS Treatment System

  30. AIS Treatment Pilot TestingShamokin 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

  31. Initial Results from Shamokin Pilot Study

  32. 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.

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