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Water Research 39 (2005) 1047–1060

www.elsevier.com/locate/watres

Capture of water-borne colloids in granular beds using externalelectric fields: improving removal of Cryptosporidium parvum

Pramod Kulkarnia, Gabriel Dutarib, David Weingeista, Avner Adinb,c,Roy Haughtd, Pratim Biswasa,�

aEnvironmental Engineering Science Program, Washington University in Saint Louis, Campus Box: 1180, Saint Louis, MO 63130, USAbEnvironmental Engineering and Science Division, Department of Civil and Environmental Engineering, University of Cincinnati,

Cincinnati, OH 45221, USAcDepartment of Soil and Water Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel

dUnited States Environmental Protection Agency, 26 West M.L. King Drive, Cincinnati, OH 45268, USA

Received 20 October 2003; received in revised form 12 October 2004; accepted 21 December 2004

Abstract

Suboptimal coagulation in water treatment plants often results in reduced removal efficiency of Cryptosporidium

parvum oocysts by several orders of magnitude (J. AWWA 94(6) (2002) 97, J. AWWA 93(12) (2001) 64). The effect of

external electric field on removal of C. parvum oocysts in packed granular beds was studied experimentally. A

cylindrical configuration of electrodes, with granular media in the annular space was used. A negative DC potential was

applied to the central electrode. No coagulants or flocculants were used and filtration was performed with and without

application of an electric field to obtain improvement in removal efficiency. Results indicate that removal of C. parvum

increased from 10% to 70% due to application of field in fine sand media and from 30% to 96% in MAGCHEMTM

media. All other test particles (Kaolin and polystyrene latex microspheres) used in the study also exhibited increased

removal in the presence of an electric field. Single collector efficiencies were also computed using approximate trajectory

analysis, modified to account for the applied external electric field. The results of these calculations were used to

qualitatively explain the trends in the experimental observations.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Cryptosporidium parvum; Granular filtration; Electrostatic filtration; Granular filter models

1. Introduction

Cryptosporidium parvum is a known water-borne

pathogen, and its outbreaks have been well documented

in the United States occurring in communities served by

filtered surface water (Hayes et al., 1989). The C. parvum

e front matter r 2005 Elsevier Ltd. All rights reserve

atres.2004.12.026

ing author. Tel.: +1 314 935 5482;

5464.

ess: pratim.biswas@seas.wustl.edu (P. Biswas).

oocyst is resistant to conventional disinfection. An

important aspect of Cryptosporidium risk management

is to achieve high removal of oocysts using the filtration

systems (Frey et al., 1995). Of many factors that are

responsible for the outbreaks of C. parvum in water

treatment plants, operational failure of filtration units

has been recognized as one of the important parameters

(Fox and Lytle, 1996). Suboptimal coagulation pretreat-

ment has been found to decrease the C. parvum removal

by orders of magnitude (Dugan et al., 2001; Huck et al.,

2002). Huck et al. (2002) have reported that under

d.

ARTICLE IN PRESS

Nomenclature

a particle radius

ac radius of spherical collector

A, B same as A+ and B+, respectively, defined in

Rajagopalan and Tien (1976)

CS coarse sand media

Cin concentration of particles at the inlet of the

filter column

Cout concentration of particles at the outlet of the

filter column

D same as D+ defined in Rajagopalan and

Tien (1976)

ER electric field strength

f tr; f m

r drag correction factors defined in Table 1 of

Rajagopalan and Tien (1976)

FS fine sand

H Hamakar constant

L depth of filter bed

q total electric charge on the particle

qv space charge density in the granular medium

in the annular space

MgO magnesium oxide media

NDL electric double layer group 3, dimensionless,

NDL ¼ ka

NE1electric double layer group 1, dimensionless,

NE1¼ �Dkðz

2p þ z2

cÞ=12pmU inf

NE2electric double layer group 2, dimensionless,

NE2¼ 2ðzpzcÞ=ðz

2p þ z2

cÞ

NEF non-dimensional external electric field force,

NEF ¼qE

6pmaU infNG non-dimensional gravity force, NG ¼

2a2ðrp � rwÞg=9mU inf

NLO non-dimensional van der Waals force,

NLO ¼ H9pma2U inf

NR dimensionless particle size, NR ¼ a=as

r radial coordinate for particle position

around the collector in the Happel cell

R radial distance from the central electrode

RC radius of outer electrode

R0 radius of central electrode

s1, s2, s3 same as s1, s2, and s3 defined in Rajagopalan

and Tien (1976)

uE electrophoretic velocity of particle

Uinf approach velocity of the liquid

Greek letters

aFSther theoretical enhancement factor in fine sand

bed

d surface-to-surface separation between the

particle and the collector non-dimensiona-

lized by a

e porosity of granular bed

eD dielectric permittivity of water

ZFS;ONther theoretical single collector efficiency in fine

sand bed, with electric field

ZFS;ONexp experimental single collector efficiency in

fine sand bed, with electric field

ZPSL=FS;ONther theoretical single collector efficiency of

PSL in fine sand bed, with electric field

Ztotal overall removal efficiency of filter bed

k Debye inverse length

m dynamic viscosity of water

ylefts y corresponding to intersection of left

critical trajectory with Happel cell’s bound-

ary

yrights y corresponding to intersection of right

critical trajectory with Happel cell’s bound-

ary

y angular coordinate for particle position in

Happel cell

rp particle density

rw density of water

C stream function

zp electrokinetic potential of particle

zc electrokinetic potential of collector surface

P. Kulkarni et al. / Water Research 39 (2005) 1047–10601048

optimal coagulation conditions the C. parvum removal

was about 3-Log10 and decreased to 10–20% in the

absence of coagulation. Fox and Lytle (1996) recom-

mended optimal turbidity reduction through stringent

controls on coagulants and flocculent dosage. While

conventional filtration can yield a high degree of

removal, there are number of operational parameters

that need to be optimized for successful operation of

filtration systems, in particular, the coagulation system.

Of the technologies available to the drinking water

industry, membrane processes provide the most satis-

factory removal of C. parvum, however have been

prohibitively expensive for large public supply systems

(Frey et al., 1995). Conventional treatment practices are

generally capable of meeting 2–3-log10 removals in most

of the cases subject to optimal pretreatment. Alternative

technologies such as diatomaceous earth filtration

(Ongerth and Hutton, 2001), dissolved air flotation

(Plummer et al., 1995), and slow sand filtration (Fogel et

al., 1993) seem capable of achieving comparable, or even

greater, levels of Cryptosporidium removal. The poten-

tial of electrokinetic transport of biological colloids

under externally applied electric fields, however, has not

yet been utilized to improve removal of C. parvum. Most

biological colloids in nature possess a small surface

charge that may enable them to acquire small drift

velocities when placed in electric fields. This additional

migration velocity could be utilized to improve their

ARTICLE IN PRESSP. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1049

probability of deposition on collector surfaces in the

filter media, leading to better separation.

Influence of electrical force has been widely exploited

in aerosol filtration (Kraemer and Johnstone, 1955;

Nielsen and Hill, 1976; Shapiro et al., 1983) and

industrial gas cleaning devices such as electrostatic

precipitators (Flagan and Seinfield, 1988) to achieve

better separation. The application of electric field in

solid–liquid separation has mainly been restricted to

cake filtration and sludge dewatering (Moulik, 1971;

Lockhart, 1983a, b; Ptasinki and Kerkhof, 1992; Bollin-

ger and Adams, 1984). In electrofiltration, the electric

field is applied such that particles move in the opposite

direction of the liquid flow, leading to increased porosity

of the filter cake. Whereas in the sludge dewatering

electroosmotic flows are induced in the suspension using

the electrical field and the interstitial liquid is separated

from the solid phase. Other studies have utilized

electrical forces to capture non-biological particles in

deep bed liquid filters. The earliest studies used bi-

component metallic filters (Fowkes et al., 1970; Liber-

man et al., 1974). These studies postulated that droplets

(in emulsions) or particles can be removed by electro-

phoretic deposition, resulting from the self-generated

electric fields developed in the narrow interstices

between adjacent dissimilar collectors, which serve as

electrode pairs (Fowkes et al., 1970). Judd and Solt

(1989) have used electrokinetic transport of particles to

enhance filterability of aqueous suspension in fiber

filters. Zhang et al. (2000) have recently demonstrated

effectiveness of applied electric field in conducting and

non-conducting granular bed to increase the capture of

aqueous suspensions.

The objective of this study was to investigate the

effectiveness of an external electric field to improve

removal of C. parvum oocysts from aqueous suspension

in granular packed columns. Granular media was

packed in the annular space between the two concentric

cylindrical metal electrodes and the suspension contain-

ing test particles and oocysts were passed through the

bed, similar to that in conventional filtration. A low

strength DC field was applied across the two concentric

electrodes during the filtration process. Experiments

were performed at different voltage levels and media

types.

2. Experimental

2.1. Apparatus

A schematic diagram of the experimental setup used

for the filtration experiment is shown in Fig. 1(a). A

constant level overhead tank was used to supply water

to the filter column under gravity, so that the pressure

head at the inlet of the filter remained same at all times

(�2 m of water). Particle free (PF) water was used in all

experiments and was obtained by filtering the tap water

with a 0:2mm filter (Gelman Sciences, #12112). The

details of the filter column, along with electrode

configuration, are shown in Fig. 1(b). The stainless-steel

electrodes were arranged in a cylindrical configuration

with the outer electrode placed concentrically, along the

walls of the column (Fig. 1(b)). A constant DC voltage

was applied across the electrodes, using a constant-

voltage power supply (Hewlett Packard, HPE 3630A).

Due to the cylindrical configuration, a non-uniform

electrical field was obtained in the annular space

between the two electrodes. Other studies have used

parallel plate configuration that yielded a uniform

electrical field between the plates (Judd and Solt, 1989,

1990; Zhang et al., 2000). Granular media was placed to

fill the annular space between the electrodes completely

to a depth of 19 cm. The column had various ports for

sampling influent/effluent, pressure measurements, and

entry and exit of the particle suspension. A degassing

port was provided at the top of the column to vent gases,

if any, formed during electrolysis at the electrodes. A

constant outflow of suspension was maintained through

this port to ensure pressurized column free of gas

bubbles. Inlet and outlet pressure heads were monitored

using the piezometers. Flow meters were used to

monitor the flows (Cole Parmer, Model P-03227-30).

Subject particles were injected at the inlet using a syringe

infusion pump (Harvard Apparatus, Model-22). An in-

line static mixer (24 mixing elements, 15 cm in length)

was used to thoroughly mix the injected particles with

the mainstream flow.

2.2. Materials and methods

2.2.1. Suspension medium

All filtration experiments were performed using PF

tap water. Table 1 summarizes the properties of the

water used. PF water was obtained by filtering the tap

water through a series of filters: 25mm (Cole Parmer #

01509-35), 0:45mm (Cole Parmer #29830-10) and 0:2mm

(Gelman Sciences, #12112). PF water had very few

background particles with a mean diameter of 2mm and

a concentration of 10–50 #/mL.

2.2.2. Filter media

Three different filter bed media types were used in the

experiments. Fine sand (FS) media (Parry Co., OH) was

0.43–0.60 mm in diameter with a geometric mean

diameter of 0.51 mm and uniformity coefficient of 1.32.

Coarse sand (CS) media was (Parry Co., OH)

1.18–1.68 mm in diameter with a geometric mean

diameter of 1.41 mm and uniformity coefficient of 1.45.

The sand was cleaned and washed with de-ionized (DI)

water, soaked in 0.05N HCl solution for 24 hours and

dried at 110 oC followed by another thorough cleaning

ARTICLE IN PRESS

Fig. 1. (a) Schematic diagram of the experimental setup used in the filtration experiments. (b) Details of the filter column used in this

study.

P. Kulkarni et al. / Water Research 39 (2005) 1047–10601050

with DI water. The third media type used was dead-

burned, milled, technical grade magnesium oxide

(MAGCHEMTM

P-98, Martin Marietta Magnesia Spe-

cialties Inc.) (MgO) and was 0.60–1.18 mm in diameter

with a mean of 0.85 mm and uniformity coefficient of

1.48. In-situ media porosity was determined for all the

media (by volumetric measurements) and was 0.43, 0.46

and 0.41 for FS, CS and MgO, respectively. An estimate

of zeta potential for the media types used, were obtained

by pulverizing the large grains into particles with

diameter smaller than 30 mm and subsequently perform-

ing electrophoretic mobility measurements (Malvern

Zetasizer II). The measured zeta potential for the three

media, viz., FS, CS and MgO are reported in Table 2(a)

and were –20.13, �39.89, and +16.2 mV, respectively.

2.2.3. Particles

Three different types of particles were used: (i) Kaolin

clay (ii) Polystyrene latex (PSL) microspheres and (iii) C.

parvum oocyst. All particle suspensions were prepared

using PF water. C. parvum oocyst were obtained from

six week-old immuno-suppressed female rat species,

ARTICLE IN PRESSP. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1051

following a modified protocol by Yang et al. (1996) and

the procedure is described in detail by Dutari (2000).

The samples were purified by cesium chloride solution,

resulting in 99% purity. The final oocyst samples were

then suspended in phosphate buffer saline solution, pH

7.4 with antibiotics/antimycotic and stored at 4 1C for

further usage. C. parvum in suspension with concentra-

tions �108 #/mL were obtained and were further diluted

as required, such that an influent oocyst concentration

Table 1

Physical and chemical characteristics of water medium used to

prepare the suspensions

Parameter CWWa

pH 8.270.1

Total hardness as CaCO3 15471.20

Total alkalinity as CaCO3 67.670.89

Ca (as Ca) 38.170.7

Mg (as Mg) 14.170.4

Chloride 49.471.1

Temperature (1C) 22

Fluoride 0.9860.02

Nitrate as NaNO3 4.570.3

Sulfate 110.471.1

Sodium 2171.1

TOC 0.5170.05

Calculated ionic strength (mM) 7.26

Background particle

concentration (#/mL)

o50 (after filtration)

aCincinnati Water Works Annual Report (1998) (Miller

Plant).

Table 2

Media Size (mm)

(a) Filter media characteristics

Fine sand (FS) 0.51

Coarse sand (CS) 1.41

Magnesium oxide (MgO) 0.85

Particle Mean size (mm)

(b) Characteristics of colloidal particle used in this study

Kaolin 0.77870.315d

PSL 5.170.06f

Cryptosporidium parvum 4.0–6.0

aMalvern Zetasizer II, average of 10 measurements.bMeasurements performed at pH ¼ 8, in tap water.cMeasurements performed at pH ¼ 8, in tap water, using MalverndMalvern Autosizer II.eAldrich Chemicals, 1332-58-7.fPS06N/001264 Bangs Laboratories Inc.g(Medema et al., 1998).

of 5� 103–104 #/mL was obtained. Kaolin

(�Al2Si2O5(OH)4; Aldrich Chemical Co., 22883-4)

suspensions were prepared by mixing the Kaolin clay

with the PF water in required amounts and mixing with

a commercial grade kitchen grinder for 2 min. This

resulted in a uniform clay suspension with a mean

diameter of 0.78 7 0.32mm. An inlet turbidity of �10

NTU was used in all experiments. Monodisperse, 5:1mm

PSL particles, with SO¼4 surface-active group, were

obtained from Bangs Laboratory (#PS06N). Stock

suspensions were prepared by dispersing the PSL

particles in PF water and sonicating for 10 min. Zeta

potential of all particles was measured (Malvern

Zetasizer II) in PF water at pH 8 and was –15.65 mV

for Kaolin, �22.02 mV for PSL and –10.33 mV for C.

parvum particles. Table 2(b) summarizes the character-

istics of all the particles used in this study.

2.2.4. Sample analysis and characterization

Clay suspensions were characterized by measuring the

turbidity of the suspension using a turbidimeter (HACH

2100AN). PSL and C. parvum suspensions were

characterized by particle number concentrations. Parti-

cle counting and sizing was done with optical particle

counters (HIAC-Royco, HR-LD150 and Particle Mea-

suring Systems, AAPS 200). C. parvum counting was

performed under controlled conditions as per the

procedure developed by Dutari (2000). C. parvum

oocysts could be counted with less than 10% accuracy

(compared to enumeration by Immuno fluorescent assay

followed by hemacytometer method) and showed up as

particles with geometric mean diameter of 2:1mm

(Dutari, 2000).

Zeta potential (mV)a,b Porosity of bed

�20.1371.66 0.43

�20.0971.31 0.46

�00.0371.39 0.41

Density (g/cc) Zeta potentialc (mV)

2.6e�15.6571.33

1.064f�22.0271.05

1.045g�10.3371.51

Zetasizer II.

ARTICLE IN PRESS

Table 3

Summary of deposition experiments performeda

Set Particles Media Voltage (V) Objective

I Clay FS, MgO 0, 5, 10, 20 � Study enhancement in capture efficiency of clay particles due to applied field

� Influence of strength of applied field

II PSL FS, CS, MgO 0, 20 � Use PSL as surrogate for C. parvum and study enhancement due to applied field

� Influence of filter media type

III C. parvum FS, MgO 0, 20 � Study enhancements in capture efficiency of C. parvum due to applied field

aFor all experiments: Flow ¼ 4.8 L/h, Central electrode—negative; FS ¼ Fine sand; CS ¼ Coarse sand.

P. Kulkarni et al. / Water Research 39 (2005) 1047–10601052

2.3. Experimental procedure

All experiments were direct filtration runs and no

coagulants or flocculants were used prior to filtration.

Before starting the filtration experiment, the filter media

was degassed and backwashed (20% bed expansion)

with PF water for 30 min. All the electrical connections

between electrodes and power supply were completed.

Negative (or positive) potential was applied to the

central electrode and the outer electrode was grounded.

Before start of the experiment, suspension and influent

flow rates were set and the system was allowed to

stabilize. At time t ¼ 0; the particle injection was started

and simultaneously voltage was applied across the

electrodes. The inlet and outlet pressure head and flow

were monitored with time. A constant flow of 4.8 L/h

was maintained through the column in all experiments.

Samples were collected at predetermined intervals at the

inlet, outlet and at the degassing (or gas exit) port.

During filtration, in the presence of electrical field, some

gas formation was observed at the electrodes (in the

form of fine bubbles), however it did not disturb the

packed bed or the filtration process. The gas bubbles

periodically escaped through the gas exit port.

2.4. Experimental plan

To understand the influence of voltage level on the

improvement in collection efficiency, experiments were

first performed with Kaolin particles in FS at different

voltage levels. Systematic experiments were then per-

formed with Kaolin and PSL particles in three media

types (FS, CS and MgO) in the presence and absence of

electric field. PSL particles, due to their physical

resemblance with the C. parvum oocysts, were used as

surrogate particles to investigate influence of electric

field. Finally, to obtain enhancement in capture effi-

ciency, experiments were performed with actual C.

parvum oocysts in FS and MgO columns. Table 3

summarizes all experiments performed, along with the

experimental conditions used in this study.

2.4.1. Performance measures

Inlet (Cin) and outlet (Cout) particle concentration

(turbidity for Kaolin) were monitored with time to

obtain particle breakthrough curves (Cout=Cin vs. t).

Overall removal efficiency of the bed was defined as:

Ztotal ¼ 1 �Cout

Cin

� �. (1)

Clean-bed single collector efficiency (Zexp), was obtained

from the initial quasi-steady portion of the experimental

particle breakthrough curve using the following relation

(Yao et al., 1971):

Zexp ¼�4

3

ac

ð1 � �ÞLlog

Cout

Cin

� �. (2)

An experimental enhancement factor (aexp) was defined

as:

aexp ¼

Zexp

� �with field

Zexp

� �without field

. (3)

The enhancement factor indicated the degree of

enhancement in collection efficiency when an electrical

field was applied. aexp values of 1 indicated that there is

no improvement in collector efficiency due to electrical

field, whereas values above 1 indicated increase in

collection efficiency.

3. Theory

A charged colloidal particle, in the presence of an

electric field experiences a force proportional to its

charge and the electric field. This additional migration

velocity, adds a component of motion relative to the

ARTICLE IN PRESSP. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1053

fluid streamlines, resulting in increased probability of

deposition. The transport of charged colloidal particles

under the influence of the external electric field can be

modeled by applying a force balance on the particle

moving adjacent to a collector surface. While detailed

multiscale stochastic models, incorporating influence of

morphology on deposition flux, have been developed

(Kulkarni et al., 2003, 2005), trajectory approach was

used to obtain estimates of clean-bed collector efficien-

dydr

¼1

NRþ 1 þ d

� �

�

1f t

r

�Að1 þ dÞ2f mr � NG cos y� NEF sin yþ NE1

NE2� expð�NDLdÞ

� ��

expð�NDLdÞ1�expð�2NDLdÞ

� ��

NLOasp

d2ð2þdÞ2

264

375

1s1

Bs2 þ Dð1 þ dÞs3 þ NG sin yþ NEF cos y½

0BBBBBBBB@

1CCCCCCCCA

, ð6Þ

cies in this study. The approach outlined by Rajagopa-

lan and Tien (1976) was modified to account for the

presence of an external electric field. The approach

entails applying a force and torque balance on the

colloidal particle moving near a spherical collector.

Fluid flow around the collector was modeled using a

Happel’s cell around the spherical collector. It was

further assumed that the spherical collector, along with

the Happel cell is placed in an electric field of uniform

strength. Electroosmosis at the collector surface was

neglected and only electrophoretic motion of the particle

was considered. Particle diffusivity was neglected, this

being justified as C. parvum is a relatively large particle.

The particles were assumed to have a constant surface

potential (zeta potential), density and size. The forces

considered include—viscous drag (FD), electric double

layer force (FDL), London—van der Waals force (FLO),

gravitational force (FG), and the external electric field

force (FEF), respectively. The electric field force was

given by

FEF ¼ q ER, (4)

where q was total charge on the particle (determined

from its zeta potential) and ER the external electric field

strength. An estimate of electric field strength ER at

distance R in the annular space between the electrodes

was obtained using the following equation (Flagan and

Seinfield, 1988):

ER ¼qv R

2�Dþ

V � qvðR2c � R2

0Þ=4�D

R ln Rc=R0

� � , (5)

where qv is the space charge density, Rc is radius of outer

electrode, and R0 is radius of central electrode. The field

was computed assuming only water in the annular space

(no granular media) with an effective dielectric constant

of 80. An average value of ER (based on the maximum

and minimum value of ER at electrode surfaces) was

used in trajectory calculations. Other forces were

evaluated as outlined by Rajagopalan and Tien (1976).

Neglecting particle inertia, the equation for particle

trajectory is then obtained from the force balance and is

given by

where y and r are angular and radial coordinates for

particle position in the Happel cell, respectively. A, B, D,

f tr; f m

r ; s1; s2; and s3 are parameters describing flow and

drag correction as defined in Rajagopalan and Tien

(1976). NG; NE1; NE2

; NDL; NLO are dimensionless

groups characterizing different forces. NEF is non-

dimensional external electrical field force given by

NEF ¼qER

6pmaU inf. (7)

The limiting or critical trajectory was obtained by

numerically integrating equation (6) backwards with

the initial condition:

r ¼1

NRþ 1 at y ¼ p, (8)

where NR is size parameter and is equal to the ratio of

radius of particle to that of collector. The single

collector efficiency was then obtained from the limiting

trajectory, and was equal to the ratio of total volume of

fluid enclosed by the limiting trajectories to that entering

the Happel cell.

Of the various non-dimensional numbers discussed

above, NR, NG, and NEF influence the single collector

efficiency (Zther) most and have a wide variation as

shown in Fig. 2(a). The figure shows range of NR and

NG values for particle-media pairs used in this study.

The range of values was computed based on variability

in particle and collector size and particle density for each

pair. Average values of NG for particles used in this

study were—4.06� 10�4 for Kaolin, 4.59� 10�4 for

PSL, and 4.41� 10�4 for C. parvum. C. parvum oocysts

cover a wide range of NG values due to large variation in

oocyst density. The range of NG values reported in

ARTICLE IN PRESSP. Kulkarni et al. / Water Research 39 (2005) 1047–10601054

Fig. 2(a) is based on oocysts density measured by

Medema et al. (1998). They reported the oocysts density

in the range of 1.005–1.10 gm/cc with a geometric mean

of 1.045 gm/cc. Kulkarni et al. (2004) have recently

measured settling velocity of oocysts in water to be

0.029mm/s (NG ¼ 2:09 � 10�5). The minimum and

maximum values of NEF (corresponding to minimum

and maximum ER) were 0.057 and 0.22 for Kaolin, 0.486

and 1.87 for PSL, and 0.228 and 0.877 for C. parvum,

respectively.

Fig. 2(b) shows a typical critical trajectory around the

spherical collector in presence of field (acting from left to

right in this case) obtained from the solution of

trajectory equation (6). The trajectories are asymmetric

due to action of the electric field. It should be noted that

in the absence of the field, the critical trajectories on

both sides would be symmetrical (with direction of

gravity and flow coinciding). Due to the electrical force,

NG

NR

10–3

10–4

10–5

10–4 10–3

Kaolin/CS

Kaolin/MgOKaolin/FS

C. parvum/FS

C. parvum/MgOC. parvum/CS

PSL/FS

PSL/MgOPSL/CS

Flow Direction

Critical Trajectories

Happel cell

Spherical collector

Ext

erna

l ele

ctri

cfi

eld

(E)

10–2

(b)

(a)

Fig. 2. (a) Range of NG and NR for particle-media pairs used in

this study. (b) Limiting (or critical) trajectories around a

spherical collector in the presence of electrical field, obtained

using approximate trajectory calculations (NG ¼ 5 � 10�3;NR ¼ 1 � 10�3; NE1

¼ 1:77 � 10�5; NE2¼ 1; NDL ¼ 794:6;

NLO ¼ 2:97 � 10�6; NEF ¼ 0:5).

the particle has an additional force toward the collector

and as a result trajectories of most particles end on

the collector surface. The left critical trajectory shifts

to the left, whereas, on the right side of the collector

the particles move away from the collector. The total

collector efficiency is proportional to the volume

of suspension, enclosed between the two limiting

trajectories.

4. Results and discussion

The influence of gas bubble formation at the

central electrode on the filtration efficiency of the

particles was first examined. The particle removal

could also take place due to entrapment of particles

by the rising gas bubbles—a mechanism somewhat

similar to air floatation. To investigate role of this

mechanism, particle concentration was monitored

at the inlet, gas exit port, and outlet of the column

during filtration experiments (in the presence of electric

field). In all experiments the particle concentration at the

inlet to the filter column was same as that at the gas exit

port, indicating that particle removal by ‘floatation’

mechanism is negligible. Thus removal efficiency was

entirely attributed to deposition on the filter collector

medium.

4.1. Effect of applied voltage on capture efficiency

Residual concentration at the inlet and outlet of the

filter column was measured with time, in the presence

and absence of electric field, to obtain the degree of

improvement in collection. Polarity of the electrodes did

not have any significant influence on the removal

efficiency. In the absence of electric field, head loss in

the column was insignificant as particles were collected

with a low efficiency. In the presence of an electric field,

however, the head loss increased by approximately 50%

due to enhanced collection of particles.

The influence of strength of the external field on the

removal efficiency of Kaolin particles was first

experimentally studied by varying the applied voltage.

Fig. 3(a) shows residual concentration (averaged over

the first 60 min) of Kaolin particles in the outlet of FS

column at various voltages. The residual concentration

decreases with increasing voltage as expected. The inset

plot in Fig. 3(a) shows experimental single collector

efficiency (Zexp) as a function of dimensionless electro-

phoretic velocity (uE=U inf ) of Kaolin particles. Electro-

phoretic velocities (uE) are based on the measured zeta

potential of the particle reported in Table 2(b). Zexp were

computed from particle breakthrough curves, using

Eq. (2). The inset plot shows that Zexp increases linearly

with uE=U inf initially and then reaches a saturation

value at high uE=U inf : The initial slope of curve

ARTICLE IN PRESS

Fig. 3. (a) Residual concentration of Kaolin particles (Cout/Cin) at the outlet of FS at different applied voltages. Also shown in the inlet

is a plot of single collector efficiency as a function of dimensionless electrophoretic mobility. (b) Variation in single collector efficiency

(Z) as a function of orientation angle (f, relative to direction of flow) and electric field group (NEF). The following parameters were

used: NE1¼ 1:77 � 10�5; NE2

¼ 1; NLO ¼ 2:97 � 10�6; NDL ¼ 794:6; NR ¼ 0:001; � ¼ 0:4; a ¼ 2:5mm; U inf ¼ 1:175 � 10�3 m=s: (c)

Variation in single collector efficiency (Zther) as a function of gravity group (NG) and electric field group (NEF). Same parameters as in

(b) were used.

P. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1055

(i.e.,dZexp

dðuE=U inf Þ) is about 1.72. Judd and Solt (1989)

observed a linear increase in Zexp with uE=U inf over a

wide range of electrophoretic mobility. They reported

this slope to be in the range of 0.23–0.28 for negative

polarity (electric field and flow acting in opposite

directions) and 0.36–0.44 for positive polarity (electric

field and flow acting in the same direction). Parallel plate

configuration of electrodes with a fibrous filter media in

between was used in their work. The higher value ofdZexp

d uE=U infð Þin this study is possibly due to the orientation of

the electrical field perpendicular to the flow direction.

The orientation of electric field, relative to macroscopic

flow direction, plays an important role. For instance,

Judd and Solt (1989) observed that collector efficiencies

are about 67% greater when the electric field was

oriented along the flow, compared to that when it was

acting in the opposite direction. Other studies have

mostly used external field direction parallel flow direc-

tion and gravity (Solt and Judd, 1989, 1991; Zhang et

al., 2000). Theoretical calculations also show that

maximum improvement can be observed when the field

is oriented perpendicular to the flow direction. Fig. 3(b)

shows enhancement factor calculated using the trajec-

tory approach described earlier as a function of angle of

orientation of electric field (relative to macroscopic flow

ARTICLE IN PRESS

Time (min)0 50 100 150 200 250

C/C

0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8FS, w/o Electric field

MgO, w/ Electric field

MgO, w/o Electric field

FS, w/o Electric field

Kaolin particles

Fig. 4. Normalized residual concentration (Cout/Cin) of Clay

particles at the outlet of filter column with time in FS and MgO

beds. Removal clearly increases due to application of electrical

field.

P. Kulkarni et al. / Water Research 39 (2005) 1047–10601056

velocity). Theoretical enhancement factor ather was

defined, similar to aexp, as the ratio of theoretical

collector efficiencies with and without electric field. At

high field strength, maximum enhancement is observed

when the field is oriented perpendicular to the main-

stream flow direction. Due to the electrode configuration

used in this study, macroscopic field direction (in the

absence of media) is inherently perpendicular to the flow

direction. The presence of granular media may further

lead to redistribution of the local field in pore interstices.

The rate of decrease in residual concentration

decreases at higher voltage in Fig. 3(a). The smaller

improvement in removal at higher fields could possibly

be due to interfering electrokinetic processes in the filter

bed (such as electroosmosis at the collector surface,

electrolysis at the electrode surface, etc.). For instance,

as the field strength increases, the rate of electrolysis at

the electrode surface also increases. This further ‘shields’

the electrodes and thus the effective field in the interior

of the bed decreases (Judd and Solt, 1989).

In order to explore the influence of electrical field

(NEF) on the capture efficiency, calculations were

performed using the trajectory approach. Enhancement

factors were obtained as a function of NEF at different

values of NG. It should be pointed out that, low value of

a does not imply low collector efficiency and is only an

indicator of degree of enhancement sought by electric

field for a given particle and collector properties. Fig.

3(c) shows a plot of variation in enhancement factor

(ather) as a function of electrical force (NEF) at different

values of gravitational force (NG). Also shown in the

inset is a plot of variation of theoretical single collector

efficiency (Zther) as a function of NEF and NG. Fig. 3(c)

shows that at any given NG, the enhancement factor

(ather) increases with increasing electrical field strength

and reaches a saturation value at sufficiently high NEF

(�10). The rate of increase in ather is maximum between

the NEFE0.01 and 1. Also, at any given value of NEF,

ather increases with decreasing NG. The electrical forces

are thus most effective when other mechanisms of

particle capture (e.g., inertial) are less dominant.

4.2. Experimental capture efficiencies

Fig. 4 shows a plot of residual concentration of

Kaolin at the outlet of FS and magnesium oxide (MgO)

columns, in the presence and absence of electric field. A

negative potential of 20 V was applied to the central

electrode. In all cases, there is considerable improvement

in removal efficiency due to applied electric field.

Deposition rate of Kaolin in FS in the absence

of electric field decreases with time (residual concentra-

tion increases with time) due to unfavorable

particle–particle and particle–surface interactions (par-

ticle and collector surface are both negatively charged).

However, in the presence of the external field in FS,

residual concentration of Kaolin particles rapidly

decreases. Removal efficiency (averaged over the first

60 min) increases by a factor of E1.5. In case of the

MgO column, residual concentration decreases from

�30% to about 4% in the presence of the electric field, a

factor of 1.4 increases in removal efficiency. The overall

removal efficiency of Kaolin decreased in the following

order—ZMgO;ONtot 4ZMgO;OFF

tot 4ZFS;ONtot 4ZFS;OFF

tot :Experimental (Zexp), and theoretical (Zther) collector

efficiencies for Kaolin particles are listed in Table 4. In

the absence of field (0 V, conventional filter operation),

predicted efficiencies (Zther) differ by approximately

35–40%, compared to Zexp in both FS and MgO

columns. However, qualitative trends are predicted

well by the trajectory model, indicating that

ZMgO;OFF4ZFS;OFF:In the presence of the electric field, collector

efficiencies are substantially overpredicted by the trajec-

tory calculations. Zther values are an order of magnitude

higher compared to corresponding Zexp values. The Zther

values were computed based on estimated values of

electric field strength in the annular space (as described

earlier) and were possibly overestimated. The field was

computed assuming an effective dielectric constant of

E80 and presence of granular media was neglected. In

an actual system, however, the field distribution

could be complicated by the electric double layer around

the electrodes. The electrolyte ions form an electric

double layer around the electrode surface, which short-

ens the range of potential distribution around the

electrode. This may further lead to weak electrical fields

in the interior of granular medium. Judd and Solt (1989)

have also noted that capture due to electric field was

lower by a factor of 2–3 compared to that predicted by

the theory.

ARTICLE IN PRESS

Table 4

Experimental and theoretical single collector efficiencies for Kaolin, PSL and Cryptosporidium parvum particles

Particles Column k Without electric field With electric field

Experimental Theoretical Experimental Theoretical

Kaolin FS 1.38� 10�3 1.41� 10�3 2.36� 10�3 2.84� 10�2

MgO 5.59� 10�3 1.55� 10�3 8.47� 10�3 2.92� 10�2

PSL FS 2.86� 10�3 6.38� 10�3 5.36� 10�3 7.57� 10�2

CS 1.23� 10�3 2.60� 10�3 1.79� 10�2 1.16� 10�1

MgO 2.14� 10�3 4.05� 10�3 1.65� 10�2 8.85� 10�2

C. parvum FS 1.13� 10�4 6.39� 10�3 2.81� 10�3 3.06� 10�2

MgO 1.95� 10�3 4.22� 10�3 7.64� 10�3 4.60� 10�2

Time (min)0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8PSL particles

Fine Sand, Without fieldFine Sand, With field

MgO, Without fieldMgO, With field

C/C

0

Fig. 5. Normalized residual concentration (Cout/Cin) of PSL

particles at the outlet of FS and MgO columns with time.

P. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1057

All experimental trends could be qualitatively well

explained by the trajectory calculations. For Kaolin

particles, experimental efficiencies were greater in MgO

column, compared to FS (ZMgO;ONexp 4ZFS;ON

exp ), in the

absence and presence of the electrical field. The

corresponding theoretical efficiencies also exhibit the

same trend, with greater efficiencies in the MgO column.

Interestingly, experimental enhancement factor was

greater in FS compared to that in MgO (aFSexp4aMgO

exp ).

Fig. 5 shows variation in residual concentration of

PSL particles with time for FS and MgO columns in the

presence and absence of field. Again, residual concen-

tration is higher in the absence of field, and rapidly

decreases in the presence of field. Removal efficiency

increases from 60% to 81% in FS and from 35% to

96%, a factor of 2.7 increase, in MgO column.

Enhancement was higher in MgO compared to that in

FS column. Table 4 also lists Zexp and Zther for PSL

particles, in the presence and absence of electric field.

Efficiencies are substantially overestimated by the

trajectory calculation in the presence of field, possibly

due to overestimation of electric field in the annular

space. However, the experimental trends are well-

explained qualitatively. In the absence of electrical

field, experimental efficiencies increase as—ZFS;OFFexp 4

ZMgO;OFFexp 4ZCS;OFF

exp ; in the order of increasing NR.

Corresponding theoretical values follow the same

trend (i.e., ZFS;OFFther 4ZMgO;OFF

ther 4ZCS;OFFther ). Interestingly,

in the presence of electric field, efficiencies follow a

reverse trend—ZCS;ONexp 4ZMgO;ON

exp 4ZFS;ONexp : A similar

trend is reflected in the corresponding theoretical

efficiencies (i.e., ZCS;ONther 4ZMgO;ON

ther 4ZFS;ONther ). The ob-

served trends in enhancement across media type are

also qualitatively well explained by the trajectory

model. Experimentally observed enhancement factors

decrease from 14.5 in CS, to 7.7 in MgO to 1.87 in FS

(aCSexp4aMgO

exp 4aFSexp). Theoretical enhancement factors,

though higher in magnitude, follow the same trend

(i.e.,aCSther4aMgO

ther 4aFSther).

Fig. 6 shows the residual concentration of C. parvum

oocysts at the outlet of FS and MgO columns. In the

absence of electric field, removal efficiency in FS is

approximately 10% (averaged over the first 60 min) and

increases to about 70% when an electric field is applied,

an increase by a factor of E7. In the MgO column, the

removal efficiency increases from E30% to 90% when

the field is applied. Table 5 is a comparison of removal

efficiencies of C. parvum from different studies. Huck et

al. (2002) have a reported a removal efficiency of

10–20% in pilot plant study when no coagulant was

used. Dai and Hozalski (2002) have reported a C.

parvum removal of about 14% under similar experi-

mental conditions. Removal efficiencies in the absence

of field in this study are comparable to these values.

Also, removal in MgO (in the absence of field) was

greater; possibly due to favorable surface conditions

(collector and particle are oppositely charged).

ARTICLE IN PRESS

20

0-3

P. Kulkarni et al. / Water Research 39 (2005) 1047–10601058

Also listed in Table 4 are experimental and theoretical

single collector efficiencies for C. parvum. In the absence

of field, predicted efficiencies (Zther) are higher by a

factor of 56 in FS and 2.1 in MgO column, compared to

corresponding Zexp in both FS and MgO. However, in

the presence of electric field, collector efficiencies were

greater in MgO (i.e., ZMgO;ONexp 4ZFS;ON

exp ) with similar

trend predicted by the model, ZMgO;ONther 4ZFS;ON

ther :Removal of PSL particles can be compared with that

of C. parvum oocysts, under identical conditions, to

assess their suitability as surrogates. Fig. 7 presents

experimental single collector efficiencies of PSL and C.

parvum particles. Each data point in the Figure

represents experimental single collector efficiency of

PSL particle (on the x-axis) and C. parvum oocysts (on

y-axis), under identical operating conditions. Good

Time (min)

0 50 100 150 200

Res

idua

l con

cent

ratio

n, C

/C0

0.0

0.2

0.4

0.6

0.8

1.0C. parvum

Without Field, FS

With Field, FS

Without Field, MgO

With Field, MgO

Fig. 6. Normalized residual concentration (Cout/Cin) of C.

parvum particles at the outlet of filter FS and MgO columns.

Table 5

Comparison of removal efficiencies of Cryptosporidium parvum from

Reference Ztotal (%) Re

Huck et al. (2002)a E10–20 Co

(dc

L ¼

Dai and Hozalski (2002)b E14 Dir

L ¼

This studyb E10 (0 V) Dir

7:2E70 (20 V)

This studyb E30 (0 V) Dir

I ¼

E90 (20 V)

aPilot scale.bBench scale; V ¼ filtration velocity, L ¼ Depth of filter media, e ¼

correlation between PSL and C. parvum removal was

observed (R2495%). The slope of 1 would imply that

PSL particles are good surrogates for C. parvum in the

range of parameters studied here. The slope of best fit in

this study was about 0.8, indicating that PSL particles

exhibit greater removal compared to that of C. parvum.

This was possibly due to higher density and zeta

potential of PSL particles (high NG and NEF). Nieminski

and Ongerth (1995) have reported good correlation

between removal of C. parvum and similar sized

surrogate particles.

different studies

marks

nventional dual media filtration, Media: Anthracite

¼ 1–1.1 mm, L ¼ 50:8 cm)+Sand (dc ¼ 0.43–0.5 mm,

20:3 cm), V ¼ 9:8 m=h; No coagulation used

ect filtration, Media: Glass beads (dc ¼ 0:5 mm), V ¼ 5 m=h;25 cm; � ¼ 0:4; 5 ppm NOM, 10 mM Ca++ (coagulant).

ect filtration, Media: FS, V ¼ 4:8m=h; L ¼ 19 cm; � ¼ 0:43; I ¼

mM; no coagulant used

ect filtration, Media: MgO, V ¼ 4:8 m=h; L ¼ 19 cm; � ¼ 0:41;7:2 mM; no coagulant used

media porosity, dc ¼ media grain diameter.

Single collectore efficiency of PSL, x 10-3

0 5 10 15 200

5

10

15

Sing

le c

olle

ctor

eff

icie

ncy

of C

. par

vum

, x

1

m=0.8

Fig. 7. Comparison of experimental collector efficiencies of

PSL and C. parvum particles.

ARTICLE IN PRESSP. Kulkarni et al. / Water Research 39 (2005) 1047–1060 1059

5. Conclusions

An external DC electric field resulted in significant

improvement in the removal of test particles (Kaolin and

PSL) and the Cryptosporidium oocysts in three different

types of media-FS, CS, and MgO. C. parvum removal

increased from 10% to 70% due to application of field

in FS media and from 30% to 90% in a MgO column.

The MgO column seems to be a better choice of

media due its high removal capacity with and without

electric field. In the absence of electric field

the experimental removal decreased in the following

order: ZFS=C:parvum;OFFexp 4ZMgO=PSL;OFF

exp 4ZMgO=C:parvum;OFFexp 4

ZCS=PSL;OFFexp 4ZFS=PSL;OFF

exp :Whereas, in the presence of

field, it decreased in the following order:

ZCS=PSL;ONexp 4ZMgO=PSL;ON

exp 4ZMgO=C:parvum;ONexp 4ZFS=PSL;ON

exp 4

ZFS=C:parvum;ONexp :

Trajectory calculations qualitatively

explained the experimental trends.

The method offers advantage over conventional

filtration in that the removal efficiency in the presence

of an electric field is relatively insensitive to variation in

particle size and concentration. As a result, particles of

various size, including C. parvum, can be removed with

relatively high efficiency. The method can be used as a

good augmenting treatment method in water treatment

plants. Also, it can be particularly appropriate for

groundwaters, where chemical coagulation-based sys-

tems would be impractical and undesirable. Also the

method could have a potential application in rural

package treatment units and can be operated economic-

ally using solar or wind energy sources. On the other

hand, the technique is restricted in its application to low

to medium conductivity suspensions, if it is to be an

energy efficient method. Variability of zeta potential of

particles could also be an issue.

Acknowledgements

The work was done at Washington University and

was supported by a contract from USEPA, Grant#2C-

R135-NAEX-Washington University.

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