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

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 it 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 medium

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 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 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 Particulates 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 attachment (not limited by transport) – Proof coming up!

Mechanisms of Particle Transport Gravity Diffusion Interception These are dimensionless groups Transport by mechanism x nondimensionalized by dividing by advective transport If P term is 1 then transport toward a surface is as fast as transport through the filter

Model Parameters m viscosity 1.00E-03 Ns/m2 rp Particle density 1040 kg/m3 rw Fluid density 1000 dp Particle diameter 1.00E-06 g Acceleration due to gravity 9.8 m/s2 kB Boltzman constant 1.38E-23 J/°K r Pore radius 3.0E-05 T Absolute temperature 293 °K v Filter approach velocity m/s z Depth of filter 1 f Constant for diffusion transport 4.04

Estimate Dimensionless Transport for a Bacteria Cell by Diffusion viscosity 1.00E-03 Ns/m2 dp Particle diameter 1.00E-06 kB Boltzman constant 1.38E-23 J/°K r Pore radius 3.00E-05 T Absolute temperature 293 °K v Filter approach velocity 3.0E-05 m/s f Constant for diffusion transport 4.04 Advection is 40x greater than diffusion

Fraction Remaining

How deep must filter be for diffusion to remove 99% of bacteria? Assume attachment efficiency is 1 aT is ____ f is ____ z is _____ What does this mean? 1 0.01 3 mm

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!

Cayuga Lake Seston Extract Concentrate particles from Cayuga Lake Acidify with 1 N HCl Centrifuge Centrate contains polymer Neutralize to form flocs

CLSE Characterization Hypothesis: The organic fraction is most important How much CLSE should be added to a filter?

Organic Carbon Accumulation in Filters Fed Cayuga Lake Water

Organic Carbon Accumulation Rate Approximately 100 ppb (mg/L) in Cayuga Lake 230 mg TOC /m2/day accumulated in filters fed Cayuga Lake Water 620 mg to 15,000 mg CLSE as TSS /m2/day fed to filters Total organic carbon Total Suspended Solids

E. coli Removal as a Function of Time and CLSE Application Rate Horizontal bars indicate when CLSE feed was operational for each filter. Log remaining is proportional to accumulated mass of polymer in filter

Head Loss Produced by CLSE

What do we know about this Polymer? Soluble at very low (<1) and at very high (>13) pH Forms flocs readily at neutral pH Contains protein (amino acids) In acid solution amino acids are protonated and exist as cations In basic solution amino acids are deprotonated and exist as anions

Dipolar Structure of Amino Acids H—N —CH—C—O—H H R O .. Carboxyl group Amino group In acid solution In base solution H—N —CH—C—O—H H R O + H—N —CH—C—O H R O .. cation anion

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 Characterize the polymer Develop a better source of the polymer (algae culture, bacteria culture, or synthetic?) 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 coagulant?

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 polymer in filter attachment Predation Sticky polymer

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?