What’s New in Water Treatment? How do Slow Sand Filters remove Particles? Coagulants and Filter Aids Sticky Particles and Sticky Media

Filter Performance Models Iwasaki (1937) developed relationships describing the performance of deep bed filters. Iwasaki, T. (1937). "Some Notes on Sand Filtration." Journal American Water Works Association 29: 1591. C is the particle concentration [number/L3] l0 is the initial filter coefficient [1/L] z is the media depth [L] The particle’s chances of being caught are the same at all depths in the filter; pC* is proportional to depth

Filtration Performance: Dimensional Analysis What is the parameter we are interested in measuring? _________________ How could we make performance dimensionless? ____________ What are the important forces? Effluent concentration C/C0 or pC* Inertia London van der Waals Electrostatic Viscous Gravitational Thermal Need to create dimensionless force ratios!

Choose viscosity! In Fluid Mechanics inertia is a significant “force” for most problems In porous media filtration viscosity is more important that inertia. We will use viscosity as the repeating parameter and get a different set of dimensionless force ratios Inertia Gravitational Viscous Thermal Viscous

Gravity velocities forces v pore Gravity only helps when the streamline has a _________ component. horizontal Use this equation

Diffusion (Brownian Motion) v pore Diffusion velocity is high when the particle diameter is ________. kB=1.38 x 10-23 J/°K T = absolute temperature small dc is diameter of the collector The exponent was obtained from an analytical model

Geometric Parameters What are the length scales that are related to particle capture by a filter? ______________ __________________________ Porosity (void volume/filter volume) (e) Create dimensionless groups Choose the repeating length ________ Filter depth (z) Collector diameter (media size) (dc) Particle diameter (dp) (dc) Number of collectors!

Write the functional relationship Length ratios Force ratios If we double depth of filter what does pC* do? ___________ doubles How do we get more detail on this functional relationship? Empirical measurements Numerical models

Total removal (SSF conditions) Number of contact opportunities contacts per attachment Transport velocity/advection Tufenkji, N. and M. Elimelech (2004). "Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media." Environmental-Science-and-Technology 38(2): 529-536. With my corrections to eliminate London dependency

How deep must a filter (SSF) be for diffusion to remove 99% of bacteria? Assume a is 1 and dc is 0.2 mm, V0 = 10 cm/hr NR is ____ pC* is ____ z is _____ What does this mean? 1/200 2 5.4 cm If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!

Total removal (RSF conditions) dc=0.5 mm Approach velocity is 5 m/hr

How deep a Rapid Sand Filter) will remove 90% of cryptosporidium? dp is 4 mm Assume a is 1 and dc is 0.5 mm, V0 = 5 m/hr NR is ____ pC* is ____ z is _____ 4/500 1 0.18 m If the attachment efficiency were 1, then we could get great particle capture in a 1 m deep filter!

Slow Sand Filtration First filters to be used on a widespread basis Fine sand with an effective size of 0.2 mm Low flow rates (10 - 40 cm/hr) Schmutzdecke (_____ ____) forms on top of the filter causes high head loss must be removed periodically Used without coagulation/flocculation! filter cake

Typical Performance of SSF Fed Cayuga Lake Water 1 Fraction of influent E. coli remaining in the effluent 0.1 0.05 1 2 3 4 5 Time (days) (Daily samples) Filter performance doesn’t improve if the filter only receives distilled water

How do Slow Sand Filters Remove Particles? How do slow sand filters remove particles including bacteria, Giardia cysts, and Cryptosporidium oocysts from water? Why does filter performance improve with time? Why don’t SSF always remove Cryptosporidium oocysts? Is it a biological or a physical/chemical mechanism? Would it be possible to improve the performance of slow sand filters if we understood the mechanism?

Slow Sand Filtration Research Apparatus Manometer/surge tube Cayuga Lake water (99% or 99.5% of the flow) Manifold/valve block Peristaltic pumps Sampling Chamber Auxiliary feeds (each 0.5% of the flow) Sampling tube Lower to collect sample To waste 1 liter sodium azide 1 liter E. coli feed Filter cell with 18 cm of glass beads

Biological and Physical/Chemical Filter Ripening Continuously mixed Cayuga Lake water 0.05 Quiescent Cayuga Lake water 0.1 1 2 4 6 8 10 Time (days) Control Sodium azide (3 mM) 1 Physical/chemical Fraction of influent E. coli remaining in the effluent Gradual growth of _______ or ________ 0.1 biofilm predator 0.05 1 2 3 4 5 Time (days) What would happen with a short pulse of poison?

Biological Poison Biofilms? Abiotic? predator 0.08 0.1 1 2 3 4 5 6 Time—h Control Sodium azide pulse Sodium chloride pulse Biofilms? Abiotic? q Fraction of influent E. coli remaining in the effluent predator Conclusion? _________ is removing bacteria predator

Chrysophyte long flagellum used for locomotion and to provide feeding current short flagellum 1 µm stalk used to attach to substrate (not actually seen in present study)

Particle Removal by Size 1 control 3 mM azide 0.1 Recall quiescent vs. mixed? Fraction of influent particles remaining in the effluent Effect of the Chrysophyte 0.01 What is the physical-chemical mechanism? 0.001 0.8 1 Particle diameter (µm) 10

Role of Natural Particles in SSF Could be removal by straining But SSF are removing particles 1 mm in diameter! To remove such small particles by straining the pores would have to be close to 1 mm and the head loss would be excessive Removal must be by attachment to the sticky particles!

Particle Capture Efficiency Sand filters are inefficient capturers of particles Particles come into contact with filter media surfaces many times, yet it is common for filters to only remove 90% - 99% of the particles. Failure to capture more particles is due to ineffective __________ Remember the diffusion surprise? attachment

Techniques to Increase Particle Attachment Efficiency Make the particles stickier The technique used in conventional water treatment plants Control coagulant dose and other coagulant aids (cationic polymers) Make the filter media stickier Potato starch in rapid sand filters? Biofilms in slow sand filters? Mystery sticky agent imported into slow sand filters?

Mystery Sticky Agent Serendipity! Head loss through a clogged filter decreases if you add acid Maybe the sticky agent is acid soluble Maybe the sticky agent will become sticky again if the acid is neutralized Eureka!

Attachment Mediating Polymer (AMP) Concentrate particles from Cayuga Lake Acidify with 1 N HCl Centrifuge Centrate contains polymer Neutralize to form flocs

AMP Characterization Alum! Did I discover alum?

Which part of AMP is the important actor? What causes the particle removal? Alum Iron the organic matter (the volatile solids) or a Combination of Al and organic matter

The dilution delay the organic matter was significant Students compared filters treated with AMP, aluminum, and iron They used the amount of aluminum and iron that was in the AMP Found that AMP was far superior We concluded _______________________________ 4 years later we discovered that they had made a dilution error and hadn’t actually applied nearly as much aluminum and iron as was present in the AMP Further experimentation revealed that alum improves filter performance just like AMP the organic matter was significant

E. coli Removal as a Function of Time and Al Application Rate No E. coli detected Log remaining is proportional to accumulated mass of Al in filter

Head Loss Produced by Al

Aluminum feed methods Alum must be dissolved until it is blended with the main filter feed above the filter column Alum flocs are ineffective at enhancing filter performance The diffusion dilemma (alum will diffuse efficiently and be removed at the top of the filter)

Performance Deterioration after Al feed stops? Hypotheses Decays with time Sites are used up Washes out of filter Research results Deterioration rate increases with higher particle loads

Sticky Media vs. Sticky Particles Potentially treat filter media at the beginning of each filter run No need to add coagulants to water for low turbidity waters Filter will capture particles much more efficiently Sticky Particles Easier to add coagulant to water than to coat the filter media

Future Work Develop application techniques to optimize filter performance How can we coat all of the media? Will the media remain sticky through a backwash? Will it be possible to remove particles from the media with a normal backwash? What are the best ways to use this new filter aid?

Conclusions Filters could remove particles more efficiently if the _________ efficiency increased SSF remove particles by two mechanisms ____________ _______________________ Log remaining is proportional to accumulated mass of alum in filter attachment Predation Naturally occurring aluminum

Polymer in a void between glass beads

Polymer in a void between glass beads

Polymer on and bridging between glass beads

Polymer Bridge between Glass Beads

How can we make filter media sticky? Why do slow sand filters work? Slow sand filters don’t use any coagulants, yet their performance improves with time Their improved performance is due to natural particulate matter that is captured by the filter What is it about this particulate matter that makes the filters work better?