# Presentation: 511684

## 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

**watershed protection**

**chemical addition**

**screen**

**waste sludge**

**coagulation**

**flocculation**

**sedimentation**

**filtration**

**backwash** **recycle**

**waste sludge**

**disinfection**

**distribution system**

**direct filtration**

**raw water**

## Fundamental and Microscopic View

- 	1.	Filtration:
**Attachment**
**Detachment**
- 	2.	Backwashing:
**Detachment**

## Mechanisms of Filtration

**transport**

**attachment**

**detachment**

**fluid streamline**

**collector, d****c**

**particle, d****p**

## History of Filtration Theory(1)

- Phenomenological - Macroscopic View
- Basic Equations:
- Ives:

## Trajectory Theory

**d****c**

**d****c**

**d****c**

**d****p**

- Diffusion
- dp < 1 m

- Sedimentation
- dp > 1 m

- Interception

- Viruses
- 0.01 -0.025 m
- Bacteria
- 0.2 - 1 m
*Cryptosporidium*
- 3 - 5 m
*Giardia*
- 6 - 10 m

## 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

**Time**

**Filter Ripening**

**Effluent Turbidity**

**Clean back-wash**

**Backwash remnants**

**Function**
**of influent**

**Filter breakthrough**

**T****U**

**T****M**

**T****B**

**T****R**

**T****B**

**T****M**

**T****U**

**Outlet**

**Media**

**Strainer**

**above**
**media**

**in **
**media**

## Alum Coagulation Diagram

## Alum Coagulation Diagram

## Conceptual Model of Filtration

- Filter coefficient ()

******(-) Detachment**

**Attachment**** ****(+) ******

- 0

- Time

- Filter
- Ripening

- Effective
- Filtration

- Turbidity
- Breakthrough

- Wormhole
- Flow

## Question:

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

- 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)

**a**

**z**

- Van der Waals Force:

- 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**

**Secondary**

**minimum**

**Primary minimum**

**Distance**

**Release of     **

**    ****extracellular**

**polymeric substances at**

**secondary minimum**

**Potential Energy of Interaction**

**Repulsion**

**Attraction**

## 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.