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Arsenic Removal by Coagulation & Precipitation Processes

Arsenic Removal by Coagulation & Precipitation Processes. Presentation Prepared by: Joe Chwirka - Camp Dresser & McKee (chwirkajd@cdm.com) Bruce Thomson - University of New Mexico (bthomson@unm.edu) Presented by: YuJung Chang – HDR Engineering, Inc. Introduction.

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Arsenic Removal by Coagulation & Precipitation Processes

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  1. Arsenic Removal by Coagulation & Precipitation Processes Presentation Prepared by: Joe Chwirka - Camp Dresser & McKee (chwirkajd@cdm.com) Bruce Thomson - University of New Mexico (bthomson@unm.edu) Presented by: YuJung Chang – HDR Engineering, Inc. Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  3. Acid-Base Chemistry of As(V) Coagulation-Filtration - Chwirka & Thomson

  4. Acid-Base Chemistry of As(III) Coagulation-Filtration - Chwirka & Thomson

  5. Redox Chemistry of As Coagulation-Filtration - Chwirka & Thomson

  6. Solubility of As(III) Compounds Coagulation-Filtration - Chwirka & Thomson

  7. Solubility of As(V) Species Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  10. Solubility of Fe(OH)3 Coagulation-Filtration - Chwirka & Thomson

  11. Effect of pH on Surface Charge of Fe(OH)3 Coagulation-Filtration - Chwirka & Thomson

  12. Covalent Bond Formation(Grossl et al., 1997) Coagulation-Filtration - Chwirka & Thomson

  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) Coagulation-Filtration - Chwirka & Thomson

  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) Coagulation-Filtration - Chwirka & Thomson

  15. As Removal by Conventional Trt. - 2 Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  17. Arsenic Removal vs FeCl3 Dose, Albuquerque NM Coagulation-Filtration - Chwirka & Thomson

  18. Ambient pH: FeCl3 vs As Leakage, NAS Fallon Coagulation-Filtration - Chwirka & Thomson

  19. El Paso Jar Testing Coagulation-Filtration - Chwirka & Thomson

  20. pH Adjustment with CO2, NAS Fallon, NV Coagulation-Filtration - Chwirka & Thomson

  21. Silica Impacts Arsenic Removal at pH 7.0 and Above pH 8.5 pH 7.5 pH 6.5 FeCl3 Dose: 4 mg/L After Clifford and Ghurye Coagulation-Filtration - Chwirka & Thomson

  22. Silica Speciation with pH Coagulation-Filtration - Chwirka & Thomson

  23. Silica in US Water Supplies (NAOS) After Davis and Edwards Coagulation-Filtration - Chwirka & Thomson

  24. Polymeric Silica 50 mg/L 17 mg/L 5 mg/L After Davis and Edwards Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  26. Schematic of Coagulation/ Filtration FeCl3 Pressure Filter CO2 CO2 Treated Water Raw Water Aeration Rapid Mix Solids to Dewatering Solids to Landfill Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  28. Vertical Pressure Filter Coagulation-Filtration - Chwirka & Thomson

  29. Direct Filtration Performance(Based on 0.1 lbs Solids/sf) Coagulation-Filtration - Chwirka & Thomson

  30. Backwash Water as Percent of Production(Based on 250 gal/sf) Coagulation-Filtration - Chwirka & Thomson

  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. Coagulation-Filtration - Chwirka & Thomson

  32. Coagulation/ Pressure Filtration • Particle size • Particle breakthrough • backwash requirements • filter ripening • Backup Filters Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  35. What is C/MF? FeCl3 CO2 Microfiltration Unit CO2 Treated Water Raw Water Aeration Rapid Mix Solids to Dewatering Solids to Landfill Coagulation-Filtration - Chwirka & Thomson

  36. Pressure Driven Membranes 0.001µ 0.01µ 0. 1µ 1.0µ 10µ 100µ 1000µ Sand Dissolved Organics Bacteria Viruses Cysts Salts Colloids Media Filtration Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis Coagulation-Filtration - Chwirka & Thomson

  37. MF Process Operates in Direct Filtration Mode Filtrate Feed Feedwater (with contaminants) Filtrate Coagulation-Filtration - Chwirka & Thomson

  38. Solids are Removed from Module by an Air-Water Backwash Air Feedwater Air (100 psig) 1 2 3 Coagulation-Filtration - Chwirka & Thomson

  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 Coagulation-Filtration - Chwirka & Thomson

  40. What is C/MF? FeCl3 CO2 Microfiltration Unit CO2 Treated Water Raw Water Aeration Rapid Mix Solids to Dewatering Solids to Landfill Coagulation-Filtration - Chwirka & Thomson

  41. Pressure Driven Membranes 0.001µ 0.01µ 0. 1µ 1.0µ 10µ 100µ 1000µ Sand Dissolved Organics Bacteria Viruses Cysts Salts Colloids Media Filtration Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis Coagulation-Filtration - Chwirka & Thomson

  42. MF Process Operates in Direct Filtration Mode Filtrate Feed Feedwater (with contaminants) Filtrate Coagulation-Filtration - Chwirka & Thomson

  43. Solids are Removed from Module by an Air-Water Backwash Air Feedwater Air (100 psig) 1 2 3 Coagulation-Filtration - Chwirka & Thomson

  44. Pall Microfilter Coagulation-Filtration - Chwirka & Thomson

  45. Memcor PerformanceNAS Fallon, 15 mg/L FeCl3 32 GFD 27 GFD 27 GFD 38 GFD (No FeCl3) 25 GFD (No FeCl3) Coagulation-Filtration - Chwirka & Thomson

  46. Memcor Cleaning EfficiencyNAS Fallon, Citric Acid Coagulation-Filtration - Chwirka & Thomson

  47. Pall PerformanceNAS Fallon, FeCl3 45 mg/L Coagulation-Filtration - Chwirka & Thomson

  48. El Paso C/MF Pilot Studies Coagulation-Filtration - Chwirka & Thomson

  49. El Paso Pilot Studies • Only Pall MF tested • Ferric dose 10 mg/L • pH lowered to 6.8 with CO2 Coagulation-Filtration - Chwirka & Thomson

  50. El Paso Pall Performance Coagulation-Filtration - Chwirka & Thomson

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