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Presentation on theme: "FILTRATION AND BACKWASHING"— Presentation transcript:

A. Amirtharajah School of Civil and Environmental Engineering Georgia Institute of Technology Atlanta, GA


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 backwash recycle waste sludge

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

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 Viruses 0.01 -0.025 mm Bacteria 0.2 - 1 mm
dp Viruses mm Bacteria mm Cryptosporidium 3 - 5 mm Giardia mm 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 Backwash remnants Outlet TB above media in media TM Effluent Turbidity Function of influent Clean back-wash Media Strainer TU Filter breakthrough TU TM TB Time TR

15 Alum Coagulation Diagram

16 Alum Coagulation Diagram

17 Conceptual Model of Filtration
Attachment (+)  Filter coefficient () (-) 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 Amirtharajah, A., “Optimum Backwashing of Filters with Air Scour - A Review,” Water Sci. and Tech., Vol. 27, No. 10, , 1993. Ahmad, R. et al., “Effects of Backwashing on Biological Filters,” JAWWA, Vol. 90, No. 12, 62-73, Dec

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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