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

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

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

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

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

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

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

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

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

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

Detachment - Macroscopic View Mintz: Ginn et al.:

Particle Size Distribution Function

Variation inAcross a Water Treatment Plant

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

Alum Coagulation Diagram

Alum Coagulation Diagram

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

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

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

Sphere - Flat Plate Interactions (2)

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

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

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

Equations Describing Collapse-Pulsing for all Filter Beds

Total Interaction Force: Hydrophilic Clay Vs Hydrophobic Bacteria

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

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

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

Turbidity and Bacterial Removal During Backwashing

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

Pathogenic Protozoa Low infective doses Resistant to chlorine disinfection Analytical techniques

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

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

Cyst Removal vs Particle Removal Nieminski and Ongerth (1995)

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

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

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

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.

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.