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Filtration and Backwashing: The Great Barrier to Particles, Parasites, and Organics

This article explores the importance of filtration and backwashing in removing particles, parasites, and organics from water. It discusses various filtration mechanisms, the history of filtration theory, biofiltration, and the effectiveness of collapse-pulsing air scour. The multiple-barrier concept of water treatment is emphasized.

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Filtration and Backwashing: The Great Barrier to Particles, Parasites, and Organics

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  1. FILTRATION AND BACKWASHING A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA 30332

  2. FILTRATION: THE GREAT BARRIER TO PARTICLES, PARASITES, AND ORGANICS

  3. Particle Removal • Improve taste, appearance • Sorbed metals and pesticides • Pathogens: bacteria, viruses, protozoa

  4. Organic Removal in Biofiltration • Prevent biofouling of distribution system • Remove DBP precursors

  5. Multiple-Barrier Concept chemical addition direct filtration filtration watershed protection sedimentation disinfection raw water distribution system screen coagulation flocculation waste sludge backwashrecycle waste sludge

  6. Fundamental and Microscopic View 1. Filtration: • Attachment • Detachment 2. Backwashing: • Detachment

  7. Mechanisms of Filtration particle, dp transport fluid streamline attachment collector, dc detachment

  8. History of Filtration Theory(1) Phenomenological - Macroscopic View Basic Equations: Ives:

  9. Trajectory Theory dp Viruses 0.01 -0.025 mm Bacteria 0.2 -1mm Cryptosporidium 3 - 5mm Giardia 6 - 10mm dc dc dc Diffusion dp < 1 mm Sedimentation dp > 1 mm Interception

  10. History of Filtration Theory (2) Trajectory Analysis - Microscopic View

  11. Detachment - Macroscopic View Mintz: Ginn et al.:

  12. Particle Size Distribution Function

  13. Variation inAcross a Water Treatment Plant

  14. Filter Effluent Quality Filter Ripening Outlet Backwash remnants TB above media in media TM Effluent Turbidity Function of influent Clean back-wash Media Strainer TU Filter breakthrough TU TM TB TR Time

  15. Alum Coagulation Diagram

  16. Alum Coagulation Diagram

  17. Conceptual Model of Filtration Attachment(+)  Filter coefficient () 0 (-) Detachment Filter Ripening Effective Filtration Turbidity Breakthrough Wormhole Flow Time

  18. Question: Why is it easier to remove alum or clay particles in contrast to polymer coated particles or micro-organisms during backwash?

  19. Sphere - Flat Plate Interactions (1) Van der Waals Force: a z Electrostatic Double Layer Force:

  20. Sphere - Flat Plate Interactions (2)

  21. Detachment During Backwashing Hydrodynamic Forces > Adhesive Forces 1. Spherical Particles - pH and Ionic Strength 2. Non-spherical Particles - Ionic Strength • Kaolinite Platelets

  22. Backwashing Filters • Weakness of fluidization backwash • Improvement due to surface wash • Collapse-pulsing air scour The best for cleaning

  23. Theory for Collapse-Pulsing a, b = coefficients for a given media Qa = air flow rate Percentage of minimum fluidization water flow

  24. Equations Describing Collapse-Pulsing for all Filter Beds

  25. Total Interaction Force: Hydrophilic Clay Vs Hydrophobic Bacteria

  26. Biofiltration • Ozonation • Microbial counts in effluent • Head loss • Effect of biocides • Particle removal

  27. Biological Filtration and Backwashing • Precursor Removal • Minimize DBP’s • Effect of Hydrophobicity

  28. Bacterial Adhesion Energy barrier Repulsion Potential Energy of Interaction Distance Secondary Attraction minimum Release of extracellular polymeric substances at secondary minimum Primary minimum

  29. Turbidity and Bacterial Removal During Backwashing

  30. Backwashing Biofilters • Collapse-pulsing air scour • Cleans better • No deleterious effect • Chlorinated backwash reduces TOC removal over time • Chloraminated backwash less than 2.0 mg/L may be used

  31. Pathogenic Protozoa • Low infective doses • Resistant to chlorine disinfection • Analytical techniques

  32. Outbreaks of Cryptosporidiosis • Surface and groundwater sources • Runoff • Sewage spills • Coagulation • Filtration • rate changes • Backwash recycle • Contaminated distribution system

  33. Particle Counts • Continuous on-line monitoring • Low operating costs • High sensitivity • Detachment of aggregates

  34. Cyst Removal vs Particle Removal Nieminski and Ongerth (1995)

  35. Minimizing Risk of Outbreaks • Optimal destabilization of particles • Filter-to-waste • Coagulants in backwash • Slow-start filtration • Minimizing flow rate changes in dirty filters • Treatment of backwash water • Filter effluent turbidity < 0.1 NTU

  36. Concluding Statement In the multiple-barrier concept, filtration is the “great” barrier to particles, parasites and organics.

  37. Summary and Conclusions • Importance of particle destabilization • Micromechanical force model • Biofiltration for organics removal • Effectiveness of collapse-pulsing air scour • Multiple-barrier concept

  38. References • Amirtharajah, A., “Some Theoretical and Conceptual Views of Filtration,” JAWWA, Vol. 80, No. 12, 36-46, Dec. 1988. • Amirtharajah, A., “Optimum Backwashing of Filters with Air Scour - A Review,” Water Sci. and Tech., Vol. 27, No. 10, 195-211, 1993. • Ahmad, R. et al., “Effects of Backwashing on Biological Filters,” JAWWA, Vol. 90, No. 12, 62-73, Dec. 1998.

  39. Acknowledgments This paper includes the work of several former students at Georgia Tech: M.S. students T.M. Ginn, L. Zeng and X. Wang and Ph.D students, Drs. P. Raveendran, R. Ahmad, K.E. Dennett and T. Mahmood. They were not only students but teachers too! Their work is acknowledged with gratitude.

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