Design Framework for Electrokinetically Enhanced Dewatering of Sludge

Design Framework for Electrokinetically EnhancedDewatering of Sludge

S. Glendinning1; C. K. Mok2; D. Kalumba3; C. D. F. Rogers4; and D. V. L. Hunt5

Abstract: Electrokinetic �EK� dewatering involves the application of a �direct current� voltage across sewage sludge, driving water underan electrical gradient from the positive electrode �anode� to the negative electrode �cathode�. Researchers have shown the technique to beefficient means of driving off water from the sludge, thus improving strength and reducing volume. This paper presents an integratingframework for EK dewatering under constant voltage and constant current conditions, founded on the mathematics of simple electricalcircuits and demonstrated by laboratory experimentation. The derived equations and experimental results showed that electroosmotic flowrate decreases with time when dewatering with constant voltage and is constant when constant current conditions are maintained. Havinga linear relationship between flow and time, EK dewatering with constant current not only enhances the sludge dewatering efficiency, butalso has the advantage of simplifying design procedures.

DOI: 10.1061/�ASCE�EE.1943-7870.0000159

CE Database subject headings: Design; Dewatering; Sludge; Slurries; Waste management; Water discharge; Electrokinetics.

Author keywords: Design; Dewatering; Electrokinetics; Electroosmosis; Sludge; Slurries; Waste management; Water discharge.

Background

The treatment of sewage inevitably involves the creation of sew-age sludge and the disposal of this sludge is one of the mostproblematical issues affecting wastewater treatment �Metcalf andEddy 1991�. A medium size sewage treatment works with a popu-lation equivalent of say 100,000 produces approximately 50,000 tof liquid per year, or 137 t/day. In Europe in 2005, the annualproduction of wastewater sludge was more than 10 million t �drysolids� and this was expected to rise significantly due to construc-tion of new wastewater treatment works �Paul and Debellefon-taine 2007�. Sludge production and therefore the problems ofdisposal are not restricted to Europe; for direct comparisonCanada produced 0.67 million t in the same time period �Renzetti2005�.

1Reader in Environmental Geotechnics, Drummond Building, Schoolof Civil Engineering and Geosciences, Newcastle Univ., Newcastle uponTyne, NE 1 7RU, U.K. E-mail: [email protected]

2Researcher, Drummond Building, School of Civil Engineering andGeosciences, Newcastle Univ., Newcastle upon Tyne, NE 1 7RU, U.K.E-mail: [email protected]

3Research Associate in Geotechnical Engineering, Drummond Build-ing, School of Civil Engineering and Geosciences, Newcastle Univ.,Newcastle upon Tyne, NE 1 7RU, U.K. E-mail: [email protected]

4Professor, Geotechnical Engineering, School of Civil Engineering,College of Engineering and Physical Sciences, Univ. of Birmingham,Edgbaston, Birmingham, B15 2TT, U.K. E-mail: [email protected]

5Postdoctoral Research Fellow, School of Civil Engineering, Collegeof Engineering and Physical Sciences, Univ. of Birmingham, Edgbaston,Birmingham, B15 2TT, U.K. E-mail: [email protected]

Note. This manuscript was submitted on August 13, 2009; approvedon September 11, 2009; published online on September 14, 2009. Dis-cussion period open until September 1, 2010; separate discussions mustbe submitted for individual papers. This paper is part of the Journal ofEnvironmental Engineering, Vol. 136, No. 4, April 1, 2010. ©ASCE,
ISSN 0733-9372/2010/4-417–426/$25.00.
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In Europe, traditional outlets for sewage sludge have includedspreading it on agricultural land �46%� forming a cake for depositto landfill �38%� or incineration �17%� �Ginestet 2007�. However,the cheapest route has been to dig a lagoon on the site of thesewage works and pump the liquid sludge into it. Environmentalconcerns have rendered the practice of permanently depositingsludge produced at a sewage works in a lagoon on the site of suchworks universally unacceptable, evidenced for example by a re-cent E.U. Landfill Directive to stop the practice in Europe. SinceMay 30, 2007, nonhazardous waste �e.g., sewage sludge� shouldbe treated �dewatering of primary sludge constitutes treatment�before it can be landfilled �Environment Agency �EA� 2007�.There is also growing pressure for restrictions placed on spread-ing sludge to agricultural land, and again some countries haveoutlawed the practice. Thus, increasingly more emphasis will beplaced on creating sludge cake for depositing in landfill or incin-eration, or other means of disposal. Indeed, whatever means isadopted for dealing with the problem, solid wastes are consideredto be environmentally less harmful or challenging to deal withthan liquid wastes.

For sludge disposal to landfill or incineration, the amount ofwater associated with the sludge is critical to the economics of thesolution: transportation of water represents wasted energy in con-sumed fuel; incineration requires a greater energy input to initiatethe process. Therefore, there is an increasing worldwide demandfor a more effective means of dewatering sewage sludge.

To create a sludge cake, water must be removed �and subse-quently treated� and the solids content increased. This has beentraditionally carried out by belt or filter presses, or centrifuges�Novac 2006�. The problem with many of these devices is that thewater that can be extracted from the sludge by hydraulic means islimited. This is because of the way in which water is bound to thesludge particles or flocs. Several researchers have shown thatelectrokinetic �EK� dewatering of sludge is more efficient thanconventional hydraulically driven methods such as belt and filterpresses �Glendinning et al. 2007�. Water flow within these mate-
rials is induced by the migration of ions and is independent of the
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ASCE license or copyright; see http://pubs.asce.org/copyright

hydraulic conductivity of sludge which is very low. Barton et al.�1999� demonstrated that at bench scale conventional pressurefiltration could be enhanced by the application of an electric fieldand proved its potential to improve dewatering of difficult mate-rials such as sewage sludge cakes. They produced sludge cakeswith solids contents of 35–46% �by weight� using electrokineti-cally enhanced dewatering, compared with 24–30% using pres-sure filtration alone. Gingerich et al. �1999� showed that finalcake solids could be increased to as much as 50% with the appli-cation of 60 V direct current �DC� to wastewater sludge.

The effectiveness of the EK dewatering process is controlledby a number of treatment variables including treatment time, po-tential difference, and sample thickness. Yuan and Weng �2003�investigated the effects of processing time and potential gradient�potential difference divided by sample thickness� on the waterremoval by the EK process and the results showed that increasingeither the potential gradient or the processing time would enhancethe sludge dewatering effectiveness �in terms of percentage ofsolids content achieved�.

Thus while the concept has been demonstrated and indicationsof the influencing parameters have been reported in the literature,there is no specific direction to the engineer wishing to makepractical use of the process either to modify existing machinery orto maximize the efficiency via the choice of appropriate operatingconditions such as voltage or current levels, sample thickness, andtreatment time. This paper presents a framework for the design ofelectrokinetically enhanced dewatering of sludge using processesbased on parallel plates, as used in belt presses or plate presses,and demonstrates its applicability through an experimental pro-gram which examines the above parameters together with the typeof electrode used and the level of applied pressure. It also dis-cusses the implications of the results on the operating parametersof a practical sludge dewatering device.

Theoretical Basis for EK Dewatering

When a DC is applied to saturated fine-grained soils, the electro-potential developed between the electrodes induces fluid flowthrough the soil mass in a process known as electroosmosis. TheHelmholtz-Smoluchowski theory is the earliest and most widelyused �Mitchell 1991� to describe this phenomenon. It states thatthe electroosmotic flow of water through a fine-grained materialcan be written as

qe = ke � ie � A �1�

where qe=electroosmotic flow rate; ie=potential gradient=�V /�L �where V is voltage and L is distance between twoelectrodes�; A=cross-sectional area; and ke=coefficient of elec-troosmotic permeability. This is very similar in form to Darcy’sequation for hydraulic flow through a soil

qh = kh � ih � A �2�

where qh=water flow rate; ih=hydraulic gradient=�H /�L�where H is hydraulic head�; A=cross-sectional area; and kh is thehydraulic permeability, or coefficient of hydraulic conductivity, ofthe soil. There is a fundamental difference in the two permeabili-ties, ke and kh; whereas ke is independent of the size of the indi-vidual grains or pores and kh is very strongly influenced by poresize �Casagrande 1949�.

To simplify the analysis of the EK dewatering process, ke isassumed to be a constant in most mathematical models �Esrig
1968�. From Eq. �1�, if ke is assumed to be constant, then the
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graph for the cumulative volume of extracted water versus ap-plied voltage should be linear when the cross-sectional area of thecell and the voltage gradient are maintained at a constant. How-ever, in reality ke decreases with increasing treatment time and theelectroosmotic flow rate slows down due to electrochemical reac-tions and desiccation at the anode �positive electrode� causing anincrease in electrical resistance, i.e., in practice conditions changeand this must be recognized. Fig. 1�a� shows a typical graph ofcumulative volume of collected water against treatment time plot-ted from experimental data for a sample of constant cross-sectional area with a constant potential gradient. Slopes OA andOB represent the global flow rate at times A and B, respectively,after the start of the experiment. Given that the gradient of OA isgreater than that of OB, it is clear that ke decreases with time asthe flow rate decreases with time. Therefore, the assumption thatke is constant for design cannot be valid since a constant flow rateis unattainable under constant voltage conditions. In Table 1, thefactors affecting the EK dewatering process are summarized. It isapparent from the table that many factors have to be considered inthe design of EK dewatering systems. Therefore, further refine-ment of the Helmholtz-Smoluchowski equation to produce a de-sign framework is fraught with difficulty and alternative solutionsare required.

Deriving Design Equations for EK Dewatering

The equations presented herein are based upon modeling the en-tire dewatering system as an electric circuit and using supportingmathematics to produce an integrating framework for design. Thefollowing assumptions are used in the derivation: �1� the sludge ishomogeneous and fully saturated; �2� there is no movement ofsolid particles by electrophoretic migration; �3� the water in thesludge moves electroosmotically in the one-dimensional directionbetween electrodes; �4� ke is constant with time; �5� Darcy’s lawand Ohm’s law are applicable during EK dewatering; and �6� theelectrodes and sludge are considered to be in a static system.

Modeling the EK System as an Electrical Circuit

In an EK dewatering system, two conducting electrode platesconnected across a voltage source are placed sandwiching a massof sludge. In physics, this would be regarded as an electricalcircuit connecting a capacitor. A capacitor consists of two con-ducting plates separated by an insulating layer called a dielectric.When a capacitor is connected in a circuit across a voltage source,the voltage forces electrons onto the surface of one plate and pullselectrons off the surface of the other plate resulting in a potentialdifference between the plates. In modeling an EK dewatering sys-tem, the sludge would represent the insulating layer separating

(a) (b)

Fig. 1. Typical experimental EK dewatering under �a� constant volt-age conditions; �b� constant current conditions

two conducting electrodes. Using this analogy, EK dewatering

0


may be regarded as a process similar to charging a capacitor. AnEK dewatering system can, therefore, be considered as havingthree elements in series: a power supply, a capacitor �C�, and aconstant resistance �RD� �Fig. 2�. RD on its own represents theprocess of dewatering sludge with constant ke and without anyelectrochemical reaction effect. The potential difference acrossthe constant resistance RD�VD� is the potential difference neededto generate the electroosmotic flow. The potential differenceacross the capacitor C�VC� varies with time and represents thevariation in ke with time brought about by electrochemical effects.At time=0, VC is assumed to be 0 and VC is assumed to increasewith time. When time tends to infinity ��, VC is assumed to beequal to VO and VD is 0. At that time, current ceases to flow,

Table 1. Factors Affecting the EK Dewatering Process �after Mok �2006

Factor

Soil mineral electric conductivity When soil electric conductimedium. The electrical resithermal energy, resulting in

Pore fluid electric conductivity Efficiency increases as pore

Moisture content Efficiency depends on moissupplied continuously at an

pH During the EK process, pHcathode due to OH− generaincrease the zeta potential fmay reverse. A drop in pH

Zeta potential Zeta potential is affected byincreases with a higher zetaused to assess the effective

Ionic strength Excessive increases in ionicthe pore water is too high,decreases. Reasonably lowrequired for high energy ef

Electric field density Increasing electric field den

Current density High current will generatecurrent used. Applying thethe successful dewatering p

Gas generation Gas generation reduces the�additional energy is expenevolution of hydrogen gasresult of electrochemical re

Salinity A moderate salt concentratireduce power consumption

Heat generation Heat generation is an unwaprocess. The effect of inter

(a) (b)

Fig. 2. Schematic of closed circuit of EK dewatering process with �a�constant voltage; �b� constant current

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which means there is no voltage across the dewatering device attime equal to � when electroosmotic flow has ceased.

There are two potential options for the control of EK dewater-ing: constant voltage with variable current or constant currentwith variable voltage. Both approaches are discussed furtherherein.

EK Dewatering with Constant Voltage

In Fig. 2�a�, C is connected in series with RD to a power supply ofconstant voltage VO. It is shown that the voltages across RD and Care, respectively, VD and VC with the current, I�t� being a functionof time. Since the capacitance �C� of a capacitor is defined as thecharge �Q� stored per unit voltage �V� applied across it, C canmathematically then be expressed as

C = Q/V �3�

Current �I� is defined as the movement of charge �Q� per unittime �t�, expressed as

I = Q/t �4�

Equating total charge Q, stored in the capacitor, to the totalamount of water that can be removed from the sludge specimen,accordingly the EK capacitance �Ce� of an EK system can bedefined as the amount of water removed per unit voltage applied

Description

creases, the process efficiency decreases as currents bypass the poreof the bulk material being treated converts electrical energy intoperature rise of the suspension or filter cake �Weber and Stahl 2002�.

electric conductivity increases.

ntent. The EK process will eventually shut down when no water is

at the anode due to H+ generated by electrolysis and is high at thehe efficiency is significantly dependent on the pH value. Low pH willegative to less negative or positive such that the electroosmotic flowsludge due to dewatering will cause a decrease in ke.

oncentration, pH and sludge conductivity. The electroosmotic flow ratetial �more negative�. The zeta potential is proportional to ke and can beelectroosmosis �Shang 1997�.

gth will lead to increases in specific conductivity. If ionic strength oflead reduce the zeta potential so that electroosmotic flow rateerate supernatant or electrolytic ionic strength �e.g., 10−4–10−2 M� is

y.

creases the electroosmotic flow rate.

cid and increase the transport rate of ions but is expensive in terms oft density judiciously throughout the entire treatment zone is crucial to�Shang and Lo 1997�.

content of the sludge but is inefficient in terms of energy usecreating gas relative to the movement of the water�. Unwantedcur at the cathode in tandem with heating and electrode corrosion as aat high salinity.

uld allow for a better dewatering effect at a lower voltage and thereforeand Lo 1997�.

ffect since it uses energy without significantly enhancing the dewateringt will change electrical properties �e.g., ke� of the sludge.

��

vity instancea tem

fluid

ture coode.

is lowtion. Trom nof the

ion cpoten

ness of

strenit willto modficienc

sity in

more acurrenrocess

waterded inmay ocactions

on wo�Shang

nted enal hea

�Eq. �5��



Ce = Q/V �5�

When the power supply is switched on, the initial current I0 isindependent of Ce whose potential difference is 0. Thus

IO = VO/RD �6�

Because I decreases with t, the potential difference across �RD�can be written as

VD = I�t�RD �7�

The potential difference across Ce is

VC = Q/Ce �8�

dVC

dt=

I�t�Ce

�9�

At any instant, the voltage from power supply V0 is the sum of VD

and VC. Thus

VO = VD + VC �10�

and

dVO

dt=

dVD

dt=

dVC

dt

Substituting from Eqs. �7� and �9�

0 = RD

dI�t�dt

+I�t�Ce

From first order nonhomogenous differential equation

adf�x�

dx+ bf�x� + c = 0

f�x� = Be−bx/a +c

b�e−bx/a − 1�, where f�x� = B at x = 0

Therefore

I�t� =VO

RDe−t/RDCe �11�

I�t� = IOe−t/RDCe �12�

Substituting Eq. �12� into Eq. �7�, the potential difference acrossthe dewatering device is

VD�t� = RDIOe−t/RDCe �13�

Substituting Eq. �13� into Eq. �10�, the potential difference acrossthe capacitor is:

VC�t� = VO�1 − e−t/RDCe� �14�

It is clear from Eq. �13� that the voltage across the dewateringdevice is a function of time and decreases exponentially. There-fore, the potential gradient �V /�L also is a function of time anddecreases exponentially. The cumulative volume of the effluentcan be given by

Vol�t� =�0

t

ke � A �VD�t�

Ldt �15�

Assuming L remains constant throughout EK treatment, and sub-stituting Eq. �13� into Eq. �15�, the cumulative volume of col-
lected water by EK can be expressed as


Vol�t� =ke � A � RD � IO

L�

0

t

e−t/RDCedt

Vol�t� =ke � A � VO

L�1 − e−t/RDCe� �16�

Eq. �16� shows that EK flow rate decreases with time eventhough ke is assumed to be constant. According to the Eq. �16� therelationship between the cumulative volume and time, duringsludge dewatering at constant voltage, could be represented as inFig. 1�a�.

EK Dewatering with Constant Current

Under constant current conditions �Fig. 2�b��, since the resistanceof the capacitor increases periodically, the voltage from the powersupply has to increase in order maintain the constant current.However, because I and RD are constant, VD remains constant,i.e.,

VD = IRD �17�

When the power supply is switched on, the initial voltage V0 isindependent of variable C. Thus

VO = IRD �18�

At any instant, the voltage from the power supply V�t� is the sumof VD and VC. Thus,

V�t� = VD + VC �19�

dV�t�dt

=dVD

dt+

dVC�t�dt

�20�

During the EK dewatering process, while VC varies with time andVD is a constant. Thus

dV�t�dt

=dVC�t�

dt�21�

Therefore, the rate of change of voltage from the power supply isidentical to that of VC.

Assuming L is constant, and substituting Eq. �17� into Eq.�15�, the cumulative volume of collected water can be expressedas

Vol�t� =keAIRD

L�

0

t

dt �22�

Vol�t� =keAIRDt

L�23�

Eqs. �23� shows that the EK flow rate is proportional to the cur-rent when the sludge is electrokinetically treated with constantcurrent �Fig. 1�b��. The slope of the graph in Fig. 1�b� representsthe electroosmotic flow rate qe�m3 /s� which can be expressed as

qe = kejeARO �24�

where je represents current gradient=�I /�L. In Eq. �24�, ke isstill used to define an intrinsic material property, but the units arein m2 /s-A-� to account for the way it changes with increasingresistance �which increases with treatment time�.

While these theories provide a solid basis for understandingthe phenomena and providing a means of design, there are prac-
tical limitations that will temper any outcomes. For example, the
0


necessary throughput volumes will influence the operating param-eters in a belt or plate press. Similarly the volume of effluentunder constant voltage �Fig. 1�a�� is limited and this in turn limitsthe time over which the process is effective, while under constantcurrent the process is limited by drying, occurring around theanode or a practical limitation on voltage as it increases to keepthe current constant as the resistance increases. The type of sludgewill also control the efficacy of dewatering. It is appreciated thatthe equations developed appear more suitable for a batch �static�process where there is a clearly defined start and finishing point tothe treatment time. However, even in a dynamic system, wherethere is a continuous feed of sludge, these equations are consid-ered to be useful when determining the average belt separationand sludge residency time, and therefore in the control of thethroughput rate. Using constant current in this situation is alsoconsidered useful for managing the differences in material andchemical properties of the input feed.

Experimental Program

To demonstrate the applicability of the derived design equations,a series of experiments was conducted investigating the cumula-tive volumes of effluent water from treated sludge under bothconstant voltage and constant current conditions. The overall ef-ficiency of EK treatment based on the dewatering rate, final solidscontent and power consumption were also examined. The practi-cal situation modeled is EK enhancement of the belt press, al-though the same principles apply to batch processing in a platepress assisted by EK technology. This testing was done using acombination of different levels of applied stress and EK usingdifferent operating parameters. This, in fact, is essential sinceelectrode contact must be maintained as dewatering occurs, so aminimum applied pressure of 25 kPa was adopted. The maximumlikely length of treatment in a belt press is 1 h, so this length ofexperiment was adopted with frequent readings up to this time.While a plate press would typically operate over a longer periodof time, if EK enhancement were to be contemplated then a con-siderable reduction in treatment time would be required and thustreatments up to 1 h are consistent with this philosophy. The EKprocess results in heat generation, though not to a level sufficientto significantly affect effluent volumes �the value of ke mightvary, but no water loss due to evaporation would occur�. In addi-tion it is likely that the heat generated in the closed laboratory cellwould be greater than that in an operational belt press. Gas gen-eration due to hydrolysis, on the other hand, would occur equallyin the laboratory or in practice and the results are therefore di-rectly applicable.

Experiments were conducted in the laboratory-scale dewater-ing cell shown in Fig. 3, which consisted of a cylinder fabricatedfrom an acrylic plastic, specimen compartment �236 mm long and143 mm in diameter� and a piston for application of a back pres-sure. In this way the cell, whose original design was introducedby Banerjee and Vitayasupakorn �1984� but later modified byHamir �1997�, allowed for combined EK and mechanical pressurefiltration dewatering conditions. On the top of the cell, where a141-mm diameter fixed electrode was located, was a 3-mm open-ing for the escape of effluent water and gases. A second electrode,also 141 mm in diameter, was mounted onto the top of the piston.The whole piston assemblage was free to move inside the cellupon the application of back pressure, thereby ensuring that the
two electrodes remained in good contact with the test specimen
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for the entire testing period. The two electrodes were perforatedto permit effluent drainage and gas dissipation.

A Thandar TS30225 30 V-2 A Precision Direct Current powersupply was selected to provide a potential difference across thesludge sample. The top electrode was connected to the negativeterminal of the power supply, while the other electrode was con-nected to the positive terminal. This ensured that the bottom-upmovement of water in the cell, in opposition to the gravitationalflow, was mainly due to the EK process.

The sewage sludge used in the study was sourced from Esholtsewage treatment works near Bradford, West Yorkshire, U.K. Ithad been primary settled and then secondary filtered on large filterbeds and had initially been mechanically dewatered using a beltpress to an initial solid content and density of 16% and1.07 Mg /m3, respectively. The sludge was selected for this studyas it is typical of the type produced by the U.K. water industry.The results produced and the conclusions drawn are also typicalfor a range of other materials �containing particles with net nega-tive surface charge� that the authors have tested �e.g., Glendinninget al. 2007� and therefore should not be regarded as wholly ma-terial specific.

Experimental Series 1: EK Dewatering with ConstantVoltage

Perforated copper electrodes, 1 mm thick and 141 mm in diam-eter, were used. For each test run, 500 g of sludge were pouredinto the cell, making an initial sample thickness of 30 mm. Thisthickness was considered to be the approximate average height ofthe sludge in a belt press operation. A pressure of 25 kPa wasapplied while simultaneously switching on the power supply,which had previously been set to provide a nominal constant volt-age of 10 V across the specimen. Experiments adopted a process-ing time of 1 h. In each case, the electric current and volume ofeffluent water were monitored during treatment. The final solidscontent of the sludge specimen was determined at the end ofexperiment. The experiments were repeated using an increasedconstant voltage of 15 V.

Experimental Series 2: EK Dewatering with ConstantCurrent

Using the same procedure, the quantity of sludge and test durationas in Experimental Series 1, these experiments were undertakenby keeping the current constant at 1 and 2 A. Readings were taken

Electrical connection tocathode Effluent to measuring cylinder

and gas vented to air

Sludge Specimen

Piston

Electrode (anode)

Air from Pressure systemElectrical connectionto anode

Electrode (cathode)

Acrylic Plastic Cylinder

Fig. 3. Schematic diagram of the EK cell

of the cumulative volume of extracted water and voltage, until the



voltage reached a maximum of 30 V set by the rating of thepower supply. At this point the current was allowed to vary ac-cordingly.

Experimental Series 3: Investigation of TreatmentVariables

Tests were conducted following the same procedure as for Experi-mental Series 2, but investigated the effect of applied pressure byincreasing the mechanical pressure to 50 and 75 kPa, the effect ofelectrode conductivity by replacing the copper electrodes withcarbon electrodes of the same dimensions, and the effect ofsample size by halving the sample thickness to 15 mm. In all thedifferent test scenarios, parallel control tests in which no voltagewas applied across the specimen, were conducted.

Results and Discussion

Constant Voltage versus Constant Current

Figs. 4 and 5 show the results from the tests conducted underconstant voltage. The results demonstrate that current decreasedapproximately exponentially with time �Fig. 4�, while the efflu-ent volume increased with time, but at an ever decreasing rate�Fig. 5�. During the EK treatment cell resistance increased overtime. This was attributed to electrochemical reactions within thesludge sample as well as the drying of the anode due to the loss ofpore solution as treatment progressed. Consequently, the currentacross the sludge sample decreased gradually resulting in the pro-gressive decrease of dewatering flow rate as predicted in Eq. �16�.Based on the analogy of charging a capacitor under a constantvoltage, the variation of the current and the cumulative volume of

Fig. 4. Variation of electrical current at constant voltage: Experimen-tal Series 1

Fig. 5. Cumulative effluent volume at constant voltage: ExperimentalSeries 1



extracted water would be expected to decay exponentially withtime. These experiment results concur with this hypothesis.

The relationship of cumulative effluent volume against timefor experiments run at constant current conditions is presented inFig. 6. It is observed that the results could be separated into fourdistinct stages �Fig. 7�. The initial stage �A� is characterized by noeffluent coming from the cell. This period is regarded as the timerequired for electroosmotic water to fill the voids at the top of thecell and the tube leading to the collecting vessel. In practicalapplications, with a constant throughput of sludge, this is thoughtto be of no consequence.

During the second stage �B�, the cumulative volume of efflu-ent has a linear relationship with time. This relationship remainsuntil the voltage reaches the maximum voltage output of thepower supply. This linear relationship is mathematically repre-sented by Eq. �23�. In any practical application, this relationshipcould be maintained by placing no limits on the power supply �acostly solution in terms of power demand and with limited returnsfor very long treatment times�, or by limiting the treatment time.

During the third stage �C�, the power supply cannot maintainconstant current, as the resistance of the sludge is too high. Ac-cordingly, the rate of the water extraction decreases as the currentdrops. Under these conditions, the dewatering process is similarto that under constant voltage when the cumulative volume ofextracted water against time is governed by Eq. �16�. ThroughoutStage C, the current is decreasing gradually until the resistance istoo high for dewatering to continue. By Stage D, the EK processhas effectively stopped and therefore no further dewatering oc-curs. Transition into Stage D could be avoided by limiting thetreatment time, as discussed above.

Fig. 6. Cumulative effluent volume at constant current: ExperimentalSeries 2

Time

A B C D

Cumulative Volume

Fig. 7. Four stages of the experimental results with EK dewatering atconstant current

0


Effect of Applied Pressure on the Dewatering Process

The experimental results are shown in Figs. 8 and 9. The periodrequired to maintain constant current �tb� for a 15-mm-thicksample subjected to applied pressures of 25, 50, and 75 kPa was20, 24, and 20 min, respectively �Fig. 8�. For a 30-mm-thicksample under the same applied pressures, the respective tb was20, 45, and 40 min �Fig. 9�. The results illustrated the significantincrease in tb when the applied pressure was doubled. This wasattributed to the higher pressure compressing the sludge particlesand removing air voids within the sludge samples, resulting in alower rate of increase in resistance. However, compared with 50kPa, tb was shortened when the pressure was increased to 75 kPa.The larger hydraulic gradient created by the higher pressure �of75 kPa� could have provided the dominant influence here andcaused the water to be extracted more rapidly. As a result, thisdecreased the moisture content of the sample more rapidly, caus-ing an increase in resistance. Even though the resistance was de-creased due to the reduction of air volume by applying high

1: tb is increased significantly due to low rate of increase in resistance caused by compaction2: tb is decreased slightly due to increase in resistance, caused by increase in solid content

22 24

1

2

Constant current 2 A,15 mm Sample, usingCopper electrodes

Fig. 8. Effect of applied pressure on cumulative effluent volume�15-mm-thick sample�

1

2

45

1

2

Constant current 2 A,30 mm Sample, usingCopper electrodes

Fig. 9. Effect of applied pressure on cumulative effluent volume�30-mm-thick sample�

24 45

Constant current 2 A,Pressure of 50 kPa,Copper electrodes

Fig. 10. Effect of sample thickness on cumulative effluent volume�50 kPa applied pressure�

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pressure, the effect of reducing water content was more signifi-cant, resulting in an overall increase in resistance.

Overall, an increase in pressure resulted in limited improve-ment in the volume of water obtained. In practice, this wouldindicate that modifications to existing pressure dewatering sys-tems �belt or filter presses� to have an additional EK functionwould be of significant benefit.

Effect of Sample Thickness on Dewatering Process

The results of the experiments investigating the effect of samplethickness on the dewatering process are shown in Figs. 10 and 11and Table 2. It may be seen that doubling the sample thicknessdoubles the time to reach the same solid content and may bepredicted from Eq. �23� where L equates to the sample thicknessand the volume of extracted water is proportional to the resultingsolids content. It may also be seen that less power is consumed toobtain same solid content in the sludge when the sludge is 15 mm,compared to the sludge with thickness of 30 mm.

On the other hand, as shown in Figs. 10 and 11, the electroos-motic flow rate qe �slope of the graphs� in Stage B was almost thesame for both sample thicknesses when dewatering with constantcurrent and pressure. An argument that EK flow rate is not af-fected by the thickness of the sample when current is kept con-stant may be predicted by rewriting Eq. �24� as

qe =ke � I � A

L�

� � L

A�25�

where �L /A=Ro=resistance of the sludge. Rearranging

Table 2. Effect of Sample Thickness, Pressure, and Treatment Time onPower Consumption

Appliedpressure�kPa�

Thickness�mm�

Treatmenttime�min�

Volume ofcollectedeffluent�mL�

Solidcontent

�%�

Electricalpower

consumption�kWh /m3�

50 30 20 92 19.6 28.1

15 10 47 19.7 24.3

30 30 136 22.0 41.9

15 15 68.5 22.0 33.8

75 30 20 113 20.7 29.2

15 10 55 20.5 24.9

30 30 161 23.6 44.6

15 15 80 23.5 36.1

22

Constant current 2 A,Pressure of 75 kPa,Copper electrodes

Fig. 11. Effect of sample thickness on cumulative effluent volume�75 kPa applied pressure�



qe = ke � I � � �26�

Therefore, it may be shown that electroosmotic flow rate qe is afunction of ke, current I, and initial resistivity �. It is independentof cross-sectional area A and sample thickness L. The dimensionof the sludge affects its resistance only. Therefore, in the design ofan EK dewatering system, the sample thickness should be kept asthin as possible to minimize the resistance, provided that constantcurrent can be maintained for dewatering the sludge to a requiredfinal solid concentration.

Effect of Varying Current

The results of the experiments investigating the effect of currenton the dewatering process are shown in Figs. 12 and 13. Theresults show that the overall flow rate �electroosmotic plus hy-draulic� increases significantly with increase in the current from 1to 2 A. The improvement in the amount of overall dewatering byincreasing current is substantially more than that achieved by in-creasing pressure. Therefore, in the EK dewatering system, elec-troosmosis is the main mechanism of water transport andhydraulic flow due to pressure is secondary.

However, higher current resulted in a shorter time to maintaina constant current. This is thought to be because the higher cur-rent results in higher solid content more rapidly and more elec-trochemical reaction taking place, both increasing the resistancemore rapidly with time. Therefore, a high current for a short pe-riod would achieve the greatest amount of dewatering if this ispossible with the electrical setup.

Electrode Materials

The results of the experiments investigating the effect of differentelectrode materials on the dewatering process are shown in Figs.14 and 15. The results show that the resistance increased slowlywith time when using carbon electrodes whereas the resistanceincreased more rapidly with time when using copper electrodes.

Fig. 12. Temporal variation of cumulative effluent volume at differ-ent current and pressure �linear relation in Stage B�

Fig. 13. Effect of current on overall flow rate



This is because carbon electrodes require 4e− for the oxidation ofeach carbon atom and copper electrodes require only 2e−. There-fore, the rate of electrochemical reaction with carbon electrodes isslower than that with copper electrodes.

The time to maintain constant current using copper electrodeswas 20 min, as compared to 32 min using carbon electrodes. Thiswas due to the more rapid rate of electrochemical reaction whichincreased the electrical resistance. This resulted in wasting elec-trical energy for high electrical resistance. Therefore, in practiceusing lower conductivity electrodes reduces the amount of powerconsumed to achieve the same amount of dewatering, and in-creases the overall life expectance of the electrodes.

Power Consumption and Dewatering Rates

Table 3 shows results for: cumulative volume of extracted water;initial and final solids; and power consumed after 60 min whenelectroosmotically dewatering 500 g of humic sludge �initialheight of 30 mm�. The results relate to three experimental condi-tions: �1� constant voltage �10 and 15 V�; �2� constant current �1and 2 A�; and �3� no electricity applied �this condition being theexperimental control�.

To evaluate the results against engineering process alternativesfor electroosmotic sludge dewatering, data relating to power con-sumption versus percentage of solids in filter cakes may be re-quired. To better compare experimental results �i.e., dewateringrates� with engineering design options, data relating to percentageof solids in filter cake with time would be more useful. Thisinformation is considered to form a vital contribution to the de-sign framework and therefore data relating to Experimental Con-dition �1� above have been plotted in Fig. 16 and those relating to�2� have been plotted in Figs. 17 and 18.

It can be seen in Figs. 16–18 that power consumption is di-rectly related to applied current and voltage �when operatingunder constant pressure conditions�. This is best illustrated by the50% increase in power consumption that occurred when the op-

32

Sample 30 mm thick,Current 2 A, Pressure 25 kPa

Fig. 14. Effect of electrode material on cumulative volume of efflu-ent

32

Sample 30 mm thick,Current 2 A, Pressure 25 kPa

Fig. 15. Effect of electrode material on cell resistance

0


erational voltage was increased from 10 to 15 V �using an appliedpressure of 25 kPa�. Very similar increases in power consumptionwere reported by Yang et al. �2005� while dewatering oily sludgeat operational voltages of 20 and 30 V, respectively. Increasingthe current from 1 to 2 A at constant pressure �i.e., 25, 50, and 75kPa� resulted in a power consumption increase per unit volume of74, 58, and 54%, respectively �Table 3�.

Increasing the voltage �while maintaining a constant pressure�significantly improved the dewatering rate, for example at a pres-sure 25 kPa and a constant voltage of 15 V, a total of 92 mL ofwater �solids increased from 16.4 to 20.2%� was extracted ascompared to 65 mL at 10 V �solids increased from 15.3 to 17.6%�and no water at 0 V �Table 3�. Table 3 highlights the significantincrease in solids content achieved through electrokinetically en-hanced dewatering as compared to using pressure filtration alone.

Increasing the current from 1 to 2 A, resulted in a 10 mL��10%� increase in extracted water when a pressure of 25 kPawas applied �Table 3 and Figs. 17 and 18�. However increasingthe pressure to 50 kPa resulted in �100% more water being ex-tracted when doubling the current. When comparing the results

Table 3. Effect of Pressure, Current, and Voltage on Power Consumptionafter 60 Min

Appliedpressure�kPa�

Constantvoltage

�V�

Volume ofextracted

water�mL�

Initialsolids

content�%�

Finalsolids

content�%�

Electricalpower

consumption�kWh/kg�

�a� Electroosmotic dewatering with constant voltage

25 15 92 16.4 20.2 0.203

10 65 15.3 17.6 0.133

�b� Electroosmotic dewatering with constant current

25 2 93 16.4 20.1 0.361

25 1 83 16.5 19.8 0.185

50 2 223 15.4 27.8 0.169

50 1 96 16.2 20.0 0.099

75 2 235 15.5 29.2 0.158

75 1 118 16.1 21.1 0.085

�c� Electroosmotic dewatering without electricity �control�

25 0 0 16.1 16.1 —

50 0 13 16.2 16.6 —

75 0 16 16.1 16.6 —

Fig. 16. Solid in filter cake and power consumed versus time �con-stant voltages—10 and 15 V�

JOURNA


for a current of 2 A with those of 1 A. Increasing the pressure to75 kPa and using a current of 2 A resulted in the largest volume ofwater being extracted �235 mL�. The value was almost 100%larger than that obtained when using a current of 1 A. Similarfindings were reported by Larue et al. �2001�. In all cases thedewatering rate can be seen to decrease with time due to theelectrochemical reaction taking place.

The power consumption values under varying conditions�applied pressure, current, and voltage� ranged from 0.085–0.361 kWh/kg of water extracted �Table 3�. The importance ofthe complete range of values reported here are broadly similarto those reported elsewhere when using similar sludges; forexample, Zhou et al. �2001� reported values from 0.013–0.119 kWh/kg for waste activated sludge and Gazbar �1994� re-ported values from 0.05–0.2 kWh/kg for anaerobically digesteddomestic sludge. In all cases these power requirements are sig-nificantly less than those required for thermal drying, reported tobe 1.2 kWh/kg.

Conclusions

There is no doubt that the disposal of sewage sludge is a problemfacing the whole of the industrial world. Increasing environmentaland economic pressures mean that there is a demand for more

Fig. 17. Solid in filter cake and power consumed versus time �con-stant current—1 A�

Fig. 18. Solid in filter cake and power consumed versus time �con-stant current—2 A�



effective means of dewatering these materials. Several researchershave demonstrated that EK enhancement of traditional pressesprovides the potential for such a solution. However, to maximizeits efficiency, a robust design methodology needs to be developed.

This paper has set out the framework for such a design meth-odology, founded on the mathematics of simple electrical circuitsand demonstrated by laboratory experimentation adopting the par-allel plate approach. It concludes that the most effective means ofdewatering is effected by maintaining a constant current throughthe sludge while applying a modest constant pressure. This couldbe achieved in practice by making modifications to a pressuredewatering system. From the results of the experiments presentedherein, the following conclusions may be drawn about the designof such a combined system:1. When applying the concept of EK dewatering, the electroos-

motic flow rate is independent of sludge cross-sectional areaand sample thickness if constant current is maintained. Thismeans that keeping current constant in an EK dewateringsystem allows for better control over final solids content,especially if input rates and material characteristics fluctuate.

2. The time required to obtain a desired final solids concentra-tion is longer when the thickness of the sludge is increased.Also less power is consumed to obtain the same solid contentin the sludge when the sample thickness of sludge is reduced.Therefore reducing the thickness of the sludge is advanta-geous. In this case, an initial sludge thickness of 15 mm �asopposed to 30 mm� produced significantly improved results.

3. The electroosmotic flow rate increases with an increase in thecurrent. Therefore maximizing electroosmotic flow rate bymaximizing current has advantages over constant voltage op-erating systems �where current reduces with time�.

4. An increase in pressure resulted in improvement in the vol-ume of water obtained �increases in hydraulic gradient andremoving the possibility of air void formation results in theability to maintain constant current for longer periods oftime�. However, this had a limited effect compared to in-creasing the applied current. Modest pressure �typical ofpressures applied in belt presses� is all that is required toachieve effective dewatering.

5. The length of time over which it is possible to maintainconstant current increases with the maximum voltage outputavailable, but the voltage can be expected to rise to relativelyhigh levels as the treatment time becomes lengthy. Thereforereducing the treatment time reduces the power consumed andpermits better control of the dry solids output.

6. Using less electrochemically reactive electrodes �such as thecarbon electrodes used here� requires less energy for thesame effect and is therefore more economic in terms ofpower consumption than, for example, using copper elec-trodes. Therefore, reducing the reactivity of the conductingelements within the dewatering device will enhance the over-all efficiency of the process as well as the longevity of theelectrodes.

Overall, the EK dewatering process is directly affected byvariation of the treatment variables, particularly treatment time,sample thickness and applied current. Additionally, design willinclude consideration of the material being treated, power supply,applied pressure and type of electrodes, which all affect the de-watering process indirectly. It appears that the most efficient pro-
cess would be one that keeps a high current between plate


electrodes of low reactivity across a relatively thin sample for ashort period of time. These variables could be produced by a beltpress, modified with and EK function operating at a high through-put and high current. However, the dynamics of such a systemwould need further investigation, preferably at full scale.

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Design Framework for Electrokinetically Enhanced Dewatering of Sludge

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