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CEE - Georgia Tech FILTRATION AND BACKWASHING A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA.

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Presentation on theme: "CEE - Georgia Tech FILTRATION AND BACKWASHING A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA."— Presentation transcript:

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


3 CEE - Georgia Tech Particle Removal ä Improve taste, appearance ä Sorbed metals and pesticides ä Pathogens: bacteria, viruses, protozoa

4 CEE - Georgia Tech Organic Removal in Biofiltration ä Prevent biofouling of distribution system ä Remove DBP precursors

5 CEE - Georgia Tech Multiple-Barrier Concept watershed protection chemical addition screen waste sludge coagulation flocculation sedimentation filtration backwash recycle waste sludge disinfection distribution system direct filtration raw water

6 CEE - Georgia Tech Fundamental and Microscopic View 1.Filtration: ä Attachment ä Detachment 2.Backwashing: ä Detachment

7 CEE - Georgia Tech Mechanisms of Filtration transport attachment detachment fluid streamline collector, d c particle, d p

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

9 CEE - Georgia Tech Trajectory Theory dcdc dcdc dcdc dpdp Diffusion d p < 1  m Sedimentation d p > 1  m Interception Viruses  m Bacteria  m Cryptosporidium  m Giardia  m

10 CEE - Georgia Tech History of Filtration Theory (2) Trajectory Analysis - Microscopic View

11 CEE - Georgia Tech Detachment - Macroscopic View Mintz: Ginn et al.:

12 CEE - Georgia Tech Particle Size Distribution Function

13 CEE - Georgia Tech Variation in  Across a Water Treatment Plant

14 CEE - Georgia Tech Time Filter Ripening Effluent Turbidity Clean back- wash Backwash remnants Function of influent Filter breakthrough TUTU TMTM TBTB TRTR TBTB TMTM TUTU Outlet Media Strainer Filter Effluent Quality above media in media

15 CEE - Georgia Tech Alum Coagulation Diagram

16 CEE - Georgia Tech Alum Coagulation Diagram

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

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

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

20 CEE - Georgia Tech Sphere - Flat Plate Interactions (2)

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

22 CEE - Georgia Tech Backwashing Filters ä Weakness of fluidization backwash ä Improvement due to surface wash ä Collapse-pulsing air scour The best for cleaning

23 CEE - Georgia Tech Theory for Collapse-Pulsing a, b = coefficients for a given media Q a = air flow rate Percentage of minimum fluidization water flow

24 CEE - Georgia Tech Equations Describing Collapse- Pulsing for all Filter Beds

25 CEE - Georgia Tech Total Interaction Force: Hydrophilic Clay Vs Hydrophobic Bacteria

26 CEE - Georgia Tech Biofiltration ä Ozonation ä Microbial counts in effluent ä Head loss ä Effect of biocides ä Particle removal

27 CEE - Georgia Tech Biological Filtration and Backwashing ä Precursor Removal ä Minimize DBP’s ä Effect of Hydrophobicity

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

29 CEE - Georgia Tech Turbidity and Bacterial Removal During Backwashing

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

31 CEE - Georgia Tech Pathogenic Protozoa ä Low infective doses ä Resistant to chlorine disinfection ä Analytical techniques

32 CEE - Georgia Tech Outbreaks of Cryptosporidiosis ä Surface and groundwater sources ä Runoff ä Sewage spills ä Coagulation ä Filtration ä rate changes ä Backwash recycle ä Contaminated distribution system

33 CEE - Georgia Tech Particle Counts ä Continuous on-line monitoring ä Low operating costs ä High sensitivity ä Detachment of aggregates

34 CEE - Georgia Tech Cyst Removal vs Particle Removal Nieminski and Ongerth (1995)

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

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

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

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

39 CEE - Georgia Tech 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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