Electro-Osmotic Dewatering (EOD) of Bio-Materials

  • Arun S. MujumdarEmail author
  • Hiroshi Yoshida
Part of the Food Engineering Series book series (FSES)


In practical application of electro-osmotic dewatering (EOD), it is very important to increase the dewatering rate, and to decrease the final water content and the electric power consumption for water removal. In a batch apparatus for dewatering operation of colloidal suspensions or sludges, electric power applications, such as alternating current (AC) electric field and interrupted or intermittent electric field, and also arrangements and configurations of the electrode in contact with the suspension or sludge can be available for improving the performance of electro-osmotic dewatering. The effects of these electric field applications and the electrode arrangements and configurations on the dewatering processes are shown, and the usage of electro-osmotic dewatering is focused to biomaterials such as sewage/activated sludge, waterworks sludge, food processing products and wastes, and biomass sludge.


Sewage Sludge Alternate Current Water Removal Electric Power Consumption Specific Electric Conductivity 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

1 Introduction

Electro-osmotic dewatering (EOD) is typically performed by applying an external electric field under direct current (DC) condition to a semisolid material placed between two electrodes. In the process of EOD for a bed of semisolid material of which the initial water content is uniform throughout the bed, as shown in Fig. 1, EOD proceeds downwards and the water content in part of the material near the upper electrode opposite to the drainage surface is locally reduced, resulting in an increase of electrical contact resistance between the upper electrode and the material being dewatered (Yoshida and Yukawa 1991; Yoshida and Yukawa 1992; Yoshida 1993). It is supposed that gas produced by electrolysis occurring at the electrode increases the electrical contact resistance, and such increase of electric resistance has a negative effect on the distribution of electric field strength throughout the bed in the dewatering process. Consequently such a circumstance for EOD hinders continuation of the dewatering, and the efficiency of EOD is reduced excessively.
Fig. 1

Proceeding of a bed of material dewatered by EOD

For applying EOD practically to various kinds of materials, it is important essentially to increase the dewatering rate, and to decrease the final water content and the electric power consumption for water removal, as much as possible. From these points of view, various applications of electric field, which are different from continuous DC condition, have been attempted to improve the performance of EOD. In order to reduce the negative effect for dewatering, which is characteristic of EOD, such as the increase of electrical contact resistance mentioned above, many investigations have been carried out for better and higher performance of EOD.

The mechanism of EOD is different from that of such widely used dewatering processes as mechanical methods using fluid pressure, compressive and centrifugal forces. EOD has some advantages compared with mechanical dewatering methods, and it can be more effective for solid–liquid mixtures consisting of colloidal particles and gelatinous and biological materials that are not successfully dewatered by mechanical methods. EOD has been applied to numerous materials, mainly inorganic materials such as peat, clay, soil, coal, cement, concrete, metal and hydroxides (Rampacek 1966). EOD for clay slurries and washed mineral sludges has been examined for over a century. Referring to biomaterials, EOD has been used for dewatering of waterworks and sewage sludges, food processing wastes, biological wastes, and biomass sludges.

R&D on EOD carried out in the last two decades is described in this chapter. This overview includes various published applications of electric field for processing EOD which are expected to be available for enhancing the performance of EOD are shown in terms of water removal and energy consumption.

2 Theoretical Backgrounds

2.1 Recent Theoretical Developments

Recent development of theories of EOD have principally been made by Iwata et al. (Iwata et al. 1991a; Iwata 2000; Iwata et al. 2004).

In EOD of a compressible semisolid material such as a sludge consisting of liquid and solid particles, both electric field and hydraulic pressure gradients are concurrently generated in a bed of the compressible material, and not only the liquid but also the solid particles in the material move with the progress of dewatering. In EOD combined with mechanical pressure, the effects of the strength of electric field (E) and the hydraulic pressure (p L ) distributions produced in the bed must be taken into account in a theoretical consideration of the dewatering process. Since the solid particles as well as the liquid migrate in the bed with dewatering, it is convenient to use the mass of particles accumulated above per unit area of the drainage surface, ω, as a variable for representing an arbitrary position in the direction of the height of the bed. In other words, ω is defined a moving material coordinate system as the mass of solids per unit cross-sectional area of the bed measured from the lower electrode. Then the superficial linear velocity q of liquid flow in such a compressible bed of dewatered material is expressed as follows (Iwata et al. 1991a):
$$q = \frac{1}{{\mu \alpha _C }}\left( {\frac{{\sigma _S E}}{{\rho _p \varepsilon }} + \frac{{\partial p_L }}{{\partial \omega }}} \right)$$
where μ is the viscosity of liquid; σ S , the effective charge on the solid surface per unit volume of particles; ε p , the density of solid particles; ɛ, the porosity of the bed; and α c , the specific hydrodynamic resistance of the material dewatered. Equation 1 takes into account the tortuousity and size of liquid flowing path, and the first term of the right hand side in Equation (1) represents the electro-osmotic flow, while the second term, the hydraulic pressure flow in the material.
Using the electric current density, I, passing through the cross-section of the bed, the specific electric conductivity of the material λ, the volumetric specific surface area of the particles S v , and the Kozeny constant k, E and α c in Equation (1) are respectively given by
$$ E = \frac{I}{\lambda } ,\quad \alpha _C = \frac{{kS_V^2 (1 - \varepsilon )}}{{\rho _p \varepsilon ^3 }}$$
In expression (mechanical pressure) under an electric field, the relation between p L and the solid compressive pressure p S in the material bed is given by the same formula as the following one for pure expression.
$$\frac{{\partial\! p_L }}{{\partial \omega }} + \frac{{\partial\! p_S }}{{\partial \omega }} = 0$$
Using Equations (23), Equation (1) can be rewritten as
$$q = \frac{1}{{\mu \alpha _C }}\left( {\frac{{\sigma _S I}}{{\rho _p \varepsilon \lambda }} - \frac{{\partial p_S }}{{\partial \omega }}} \right)$$
If the void ratio e defined by ɛ/(1-ɛ) is used, the material balance of liquid with respect to an infinitesimal element in the bed leads to the continuity equation relating the change in q to the change in e as follows:
$$\frac{{\partial e}}{{\partial t}} = \rho _p \frac{{\partial q}}{{\partial \omega }}$$
where t is the dewatering time. Substitution of Equation (4) into Equation (5) gives
$$\frac{{\partial e}}{{\partial t}} = \frac{\partial }{{\partial \omega }}\left\{ {\frac{{\rho _p }}{{\mu \alpha _C }}\left( {\frac{{\sigma _S I}}{{\rho _p \varepsilon \lambda }} - \frac{{\partial p_S }}{{\partial \omega }}} \right)} \right\}$$
Equation (6) is the basic equation representing for the process of EOD, and is reduced to the consolidation equation if I \(\,=\,\)0. The basic differential Equation (6) can be numerically solved using the empirical correlations of e versus p S , σ S versus e, and λ versus e, respectively, and the initial and boundary conditions given appropriately. Then one can estimate all of the e- and the p S -distributions in the material bed in the dewatering process, as shown in Fig. 2, for example.
Fig. 2

Estimated time changes of e-distributions under pure EOD and EOD combined with expression

As \(\partial e/\partial t\) in Equation (6) should be 0 at the end of dewatering, Equation (6) gives the following equation:
$$\frac{{\partial p_S }}{{\partial \omega }} = \frac{{\sigma {}_SI}}{{\rho _p \varepsilon \lambda }}$$

Equation (7) suggests that the e- and the p S -distributions are not uniform numerically at the final stage of dewatering, and that e at the drainage surface does not change; that is, the initial value of e is maintained during dewatering and p s becomes a maximum value near the upper electrode opposite to the drainage surface.

Based on Equation (6), another theoretical method was proposed recently (Iwata et al. 2004). Here, the term on the left-hand side in Equation (5) is expressed as follows:
$$\frac{{\partial e}}{{\partial t}} = \frac{{\partial e}}{{\partial p_S }} \cdot \frac{{\partial p_S }}{{\partial t}}$$
As in the mechanical consolidation process, the modified consolidation coefficient C e defined by the following equation is introduced (Shirato et al. 1967).
$$C_e = \frac{{\rho _p }}{{\mu \alpha _C \left( { - \frac{{de}}{{dp_S }}} \right)}}$$
On the assumption that C e is constant during dewatering, Equation (6) is rewritten as follows:
$$\frac{{\partial\! p_S }}{{\partial t}} = C_e \frac{\partial }{{\partial \omega }}\left( {\frac{{\partial p_S }}{{\partial \omega }} - E_{pg} } \right),E_{pg} = \frac{{\sigma {}_SI}}{{\rho _p \varepsilon \lambda }}$$
where E pg is a driving force for electro-osmotic flow and may be called “electro-osmotic pressure gradient" physically in a sense. Assuming E pg to be also constant during dewatering, Equation (6) can be reduced eventually to the following formula representing the mechanical consolidation process. Accordingly EOD may be recognized as a kind of consolidation.
$$\frac{{\partial p_S }}{{\partial t}} = C_e \frac{{\partial ^2 p_S }}{{\partial \omega ^2 }}$$
Equation (11) can be solved by using the proper initial and boundary conditions giving the following equation:
$$\begin{aligned}p_S (\omega ,t) = & p_{S,i} + E_{pg} \omega - \sum\limits_{n = 1}^\infty {\left\{ {\frac{{8\omega _0 E_{pg} ( - 1)^{n - 1} }}{{(2n - 1)^2 \pi ^2 }}} \right\}} \sin \left\{ {\frac{{(2n - 1)\pi }}{2} \cdot \frac{\omega }{{\omega _0 }}} \right\}\\ &\exp \left\{ { - \frac{{(2n - 1)^2 \pi ^2 }}{4} \cdot \frac{{C_e t}}{{\omega _0 ^2 }}} \right\}\end{aligned}$$

In this equation, p S,i is the pre-consolidation pressure, and ω 0 , the total mass of solid particles per unit cross-sectional area of the bed.

Besides the above equation, the average consolidation coefficient U C, which represents the progress of dewatering can be expressed from Equations (4), (12) as follows:
$$U_C = \frac{{L_i - L}}{{L_i - L_f }} = 1 - \frac{{32}}{{\pi ^3 }}\sum\limits_{n = 1}^\infty {\frac{{( - 1)^{n - 1} }}{{(2n - 1)^3 }}} \exp \left\{ { - \frac{{(2n - 1)^2 \pi ^2 }}{4} \cdot \frac{{C_e t}}{{\omega _0 ^2 }}} \right\}$$

Here L is the thickness of the bed in the dewatering process, and L i and L f , the initial and the final thicknesses of the bed, respectively. C e and E pg in Equations (910) are determined based on experimental data. Thus, for the process of EOD under constant electric current, the time evolutions of L, the p S - and the e-distributions in the material are estimated from Equations (1213).

2.2 Principles for High Performance EOD

As described previously in the Section 1, the water content near the upper electrode opposite to the drainage surface is locally reduced during EOD, resulting in increase of electrical contact resistance between the electrode and the material being dewatered. The specific electric conductivity, λ?, of the material is ordinarily decreased with decreasing water content, and then such a circumstance enhances the increase of the electrical contact resistance. Consequently the efficiency of EOD is reduced markedly.

For high-performance EOD, it is advisable to maintain low electrical contact resistance and to make the strength of electric field throughout the material bed as uniform and large as possible in the dewatering process. As it is difficult to estimate local strength of electric field acting effectively for dewatering in the material, the average strength of electric field, E av , applied to all over the bed in the dewatering process is discussed as follows.

The electric resistance between two electrodes, namely the electric resistance, R E , of the material bed, is expressed by
$$R_E = \frac{L}{{\lambda _{av} A}}$$
where λ av is an equivalent average specific electric conductivity of the material. L and A are the thickness and the cross-sectional area of the bed, respectively, and these may be regarded as the distance between two electrodes and the area of the electrode, respectively. R E in Equation (14) is also given by Equation (15) using the voltage, V, applied to the bed and the electric current,i, passing through the cross-section of the bed.
$$R_E = \frac{V}{i} = \frac{V}{{AI}}$$
where I is the electric current density. From Equations (1415), E av is expressed by
$$E_{av} = \frac{V}{L} = \frac{i}{{A\lambda _{av} }} = \frac{I}{{\lambda _{av} }}$$

If L and λ av are assumed to be nearly constant, V and I should be increased to intensify Eav. L is actually decreased and λ av is also usually decreased with dewatering, as described above.

Incidentally, EOD can be operated under constant voltage or constant current condition. Accordingly, when an electric field is applied at a constant voltage operation (V \(\,=\,\)constant), E av in Equation (16) should be increased theoretically because of the decrease of L. However, a net value of E av applied to the material being dewatered is just reduced due to the occurrence of a large electrical contact resistance enhanced with the progress of dewatering. While in the operation at constant current (i \(\,=\,\)constant), net E av in Equation (16) could be increased with decreasing λ av in spite of the enhancement of electrical contact resistance. In this operation, however, the electric power would be poorly utilized; in other words, the applied voltage required to maintain a constant current condition would be too high due to the large electrical contact resistance. Equation (16) also indicates that E av is increased if A is smaller.

From the viewpoints mentioned above, various applications for better and higher performance of EOD can be devised in principle to improve the dewatering rate, the final water content, and the efficiency of electric power consumption for water removal.

2.3 Characteristics of Materials Suitable for EOD

The electro-osmotic velocity of liquid flow u is given by the following equation derived on the basis of an electrostatic capacitor model, for example.
$$u = \frac{{\zeta D}}{{4\pi \mu }}E = \frac{{\zeta D}}{{4\pi \mu }} \cdot \frac{I}{\lambda }$$

where ζ is the zeta potential of the solid particle and D is the dielectric constant of liquid. E, μ, I, and λ are as before. The electro-osmotic flow is a phenomenon that is caused by the ζ-potential in an electric double layer at the interface between different phases. In the derivation of Equation (17), it is assumed that the thickness of the electric double layer is quite small compared with the diameter of the capillary or pore size. As the electric double layer thickness is usually smaller than the capillary diameter, Equation (17) is applicable to most cases, and u is independent of the capillary structure; in other words, the electro-osmotic flow is not affected by the diameter of the particles contained in the dewatered material. Hence, EOD can be particularly effective for the material including very fine particles (e.g., colloids).

Equation (17) also indicates that u is proportional to the electrical properties such as the ζ-potential of solid particles and the dielectric constant of liquid D, and that is inversely proportional to the viscosity μ and the specific electric conductivity λ of the liquid. Therefore, large ζ and small μ and λ of the original material can be appropriate for EOD, and it is advantageous that ζ changes to the larger and λ changes to the smaller value in the dewatering process.

EOD has so far been known to be effective for dewatering of colloidal, gelatinous, and biological materials, which are not performed successfully by conventional mechanical dewatering methods. And the aspects of biomaterials that can be considered for EOD from the viewpoints described above may be summarized as follows:
  1. 1.

    As the electro-osmotic flow is theoretically considered to be independent of the capillary structure, EOD could be effective for materials that have minute capillary structures like fibrous plants.

  2. 2.

    Sewage sludge or activated sludge can hardly be dewatered mechanically, because such sludge disposed biologically contains very fine particles such as microorganisms and organic decomposed products.

  3. 3.

    The suspended particles in typical sewage sludge generally have negative ζ-potential. However, in the case that many kinds of colloidal particles are contained not as the suspended particles in the liquid, the viscosity μ of the liquid becomes high, resulting in a disadvantage for EOD.

  4. 4.

    Coagulated or agglomerated sludge produced in wastewater and sewage treatments is very compressible, collapsible and transformable, so that such a sludge is extremely difficult to dewater by mechanical methods such as vacuum dewatering, pressure dewatering or expression.

  5. 5.

    Gelatinous materials are often found in food processing products, and those are also hardly removed by mechanical dewatering methods.

  6. 6.

    Proteins, polymers made of amino acids, exist as both cationic and anionic electrolytes in aqueous solution and have a net charged surface with positive or negative ions environmentally depending on pH of the solution. Then the proteinaceous macromolecules have positive or negative ζ-potential in the solution, and the proteins may be regarded as the particulates in solid-liquid separation for bio-materials.

  7. 7.

    If corrosion of the electrode material is caused by electrolysis, the bio-materials dewatered electro-osmotically are contaminated, and also they may be denatured by ohmic heating.


3 Applications of Electric Field for High-Performance EOD

As explained briefly in Introduction, the characteristics of EOD have been examined in many experimental studies. Combined operations of electrical and mechanical dewatering fields were investigated and used in practice as a method for improvement of EOD. The combined fields for high-performance EOD were applied by the operations of EOD combined with vacuum dewatering, with hydraulic-pressure dewatering, and with mechanical expression (Lockhart 1992; Yoshida 1993; Wakeman and Tarleton 1999). Figure 3 shows schematically the process of EOD combined with expression, for example. In the case of EOD only, the water content in a bed of the material is reduced near the upper part of the bed, but much water still remains in the lower part at the final stage of the dewatering, as shown later. On the other hand, if the top surface of the bed is impermeable, mechanical expression proceeds with dewatering from the lower part of the bed. Thus, EOD and mechanical expression are complementary; they remove water from both the upper and the lower sides. Consequently, a combination of these dewatering operations can be expected to be a useful means for improvement of EOD (Yoshida 1993). The process of EOD combined with mechanical expression was discussed theoretically and experimentally by taking into account the hydraulic pressure distribution in a bed of the dewatered material (Iwata et al. 1991b).
Fig. 3

Schematic diagram of combined dewatering of electro-osmosis coupled with expression

A batch-type experimental apparatus used for EOD is illustrated in Fig. 4. A semisolid material to be dewatered is put between two filter media sets in contact with each of the upper and the lower electrodes made of perforated plate or wire netting in the vertical direction, and subsequently an electric field for EOD is typically applied and investigated under a continuous DC condition. The polarity of both electrodes is determined in consideration of the polarity of ζ-potential of solid particles so as to remove water downwards from the material.
Fig. 4

Typical batch-type experimental apparatus for EOD

In EOD under continuous DC operation for the semisolid material (e.g. sludge) of which the initial water content is uniform throughout the sludge bed, a final distribution of water content in the direction of height of the bed at the end of dewatering is shown in Fig. 5. The ordinate in this figure represents a dimensionless parameter that is obtained by normalizing the distance from the bottom of the bed by the whole height of the bed at the end of dewatering, because the whole bed height actually decreases with the progress of dewatering. Similar results are obtained experimentally in the combined process of EOD and vacuum dewatering (Yoshida 1993; Vijh 1999a).
Fig. 5

Final distribution of water content in sludge bed dewatered using electro-osmosis

As shown in Fig. 5, EOD under the condition of DC power supply is successively performed downwards and the water content is reduced considerably in the upper part of the bed opposite to the drainage surface. The reduction of the water content near the upper electrode enhances the increase of electrical contact resistance between the upper electrode and the top surface of the sludge bed shown in Fig. 4, resulting in a large expenditure of the voltage applied to the bed. In such circumstances, the applied electric field does not act effectively in the lower part of the bed, and then continuation of EOD becomes increasingly difficult.

Different applications of electric field, besides continuous DC, have recently been investigated in an effort to improve the performance of EOD. New developments for high-performance EOD are described in the next section.

3.1 Electric Field Application with Electrode Polarity Reversals

An improvement for the performance of EOD under continuous DC condition had been originally tested by Gray and Somogyi (1977). They investigated an electro-osmotic treatment with the electrode polarity reversals of every 30 min for red mud sludge and suggested that reversals of electrode polarity or electric current direction improved consolidation of the sludge and more efficient dewatering.

The electric field application of alternating current (AC) with periodic reversals of the electrode polarity has been also investigated from the viewpoint of lowering an increase of the electrical contact resistance between the upper electrode and the sludge being processed (Yoshida et al. 1999). By applying a very-low-frequency AC electric field combined with mechanical expression to the sludge bed, the direction of liquid flow in the bed by electro-osmosis can be reversed periodically with time, and this EOD process was expected to be useful for sludge dewatering from both sides of the bed between the electrodes.

The AC operation for an efficient EOD process was investigated in the region of low frequency below 1 hertz (Hz) for the electrode polarity reversals. An AC electric field was experimentally applied to the sludge bed actually by using an electric power supply system consisting of function generator, power amplifier, and oscilloscope, and both rectangular and sine waves can be mostly used as the wave form of AC voltage, as shown in Fig. 6. An effective value (root mean square-value: RMS-value) of the voltage applied under the condition of AC was used for a comparison between the AC and DC operations, and the effective applied voltage was constant in each wave form and equal to the DC voltage applied constant during the dewatering process.
Fig. 6

Schematic diagram of wave forms of an AC electric field

The effects of the wave form and the wave frequency on final dewatered amount per unit cross-sectional area of the sludge bed for a clay material are shown in Fig. 7. On a semi-log plot as shown in this figure, the value of final dewatered amount Q t approximately in the low region below 1 Hz of the wave frequency f increases with deceasing f in both the rectangular and sine waves, and then in the range below about 0.01 Hz Q t becomes larger than the DC operation. From these results, it is found that the AC operation at a certain low frequency can improve the performance of EOD compared with the DC operation. This fact suggests that the process of EOD with electrode polarity reversals can be effective not only for reducing the water content uniformly throughout the whole sludge bed but also reducing the excessive increase of the electrical contact resistance caused by DC operation. However, it was also reported that the efficiency of electric power consumption for the amount of removed water under AC was lower than that under DC except near the end of dewatering (Yoshida et al. 1999).
Fig. 7

Relation between Q t and f in both rectangular and sine waves

3.2 Interrupted or Intermittent Electric Field Application

Lockhart (1986), Lockhart and Hart (1988) studied many of the variables in terms of EOD, and one of the variables was interrupted electric power application; in other words, on–off power application. According to their studies, the experimental results of on–off alternations with equal periods up to 5 min suggested that better results than continuous DC power application were not always appeared, and that the results depended on the material dewatered and the apparatus used in the experiments. It was also noted that the interrupted power applications did not offer any inherent advantage or power saving capability, but the on–off power regimes were not suggested to be not useful for practical value from power interruptions.

Therefore, interrupted EOD of a clay sludge was demonstrated for the first time as a new method of interrupting power operation (Rabie et al. 1994; Gopalakrishnan et al. 1999b). In this pioneering method shown in Fig. 8, a constant voltage or current is applied for t on , the on-time, followed by a period of t off , the off-time when the power is turned off. This pattern is then repeated. Almost the same experimental apparatus as shown in Fig. 4 was used, and the electrodes were short-circuited through a shunt resister with extremely small resistance during the periods of power interruption. In the interrupted regimes, the off-time ranged from 0.5 to 20 s with the on-time fixed 30 s. Figure 9 gives an example of efficient interruption with a short circuit and shows the volume of water removed as a function of the cumulative on-time for both continuous DC and the interrupted processes with three off-times of 0.5, 3, and 20 s. It is seen that the largest volume of water was removed for the shortest off-time of 0.5 s, and that the interrupted EOD process with the off-time of 0.5 s removed finally about 40% more water than continuous DC.
Fig. 8

Schematic diagram of time variation of applied DC voltage or current

Fig. 9

Relation between water removal and cumulative on-time

Figure 10 shows the relation between the volume of water removed and the cumulative electric power consumption for continuous DC, and the interrupted processes with a short circuit of the off-times of 0.5 and 20 s. For a given electric power consumption, the interrupted EOD process with a short circuit of the 0.5 s off-time removed much water more than the DC process, whereas the interrupted process with the 20 s off-time removed less than the DC process. From these results, a certain operation in the interrupted mode has a beneficial effect if the electrodes are short-circuited during the off-time, and the dewatering rates and the final volume of water removed can be larger than those by continuous DC mode for equal energy consumption. For the interrupted EOD process enhanced by short-circuiting the electrodes, it was also reported that an experimental optimum off-time was 0.1 s for an on-time of 30 s for Hydrocol clay suspensions (Gopalakrishnan et al. 1996a).
Fig. 10

Relation between water removal and energy consumption

For efficient performance of EOD, an intermittent electric field, abbreviated to IEF, made by rectifying an AC electric field can be used to reduce the excessive increase of the electrical contact resistance with proceeding of the dewatering. The rectified intermittent electric field was constituted of half waves as shown in Fig. 11, in which both rectangular and sine waves were used as the wave form of AC, and the magnitude of AC voltage applied could be divided by two conditions of both the same peak-value voltage (PV) and the same effective RMS-value voltage (EV) as the voltage applied under DC and AC electric fields. Figure 11 gives an example of both PV and EV with 40 V under the IEF in each wave form.
Fig. 11

Schematic diagram of intermittent electric field (IEF) obtained by half-wave rectification of AC electric field with rectangular and sine waves

In the rectified half-wave IEF, a diode was added to the AC circuit described before for half-wave rectification of AC, and on-time of the electric power supply was equal to off-time, and the on-off time was set in the region of more than 50 s, namely, very low frequency below 0.01 Hz of AC. Then the process of EOD under the IEF was investigated experimentally, under both conditions of the same PV and the same EV as the constant voltage applied under DC or AC condition (Yoshida 2000).

Figure 12 shows the time evolutions of the amount of removed water W under IEF of which AC with sine wave form was rectified to half waves at a very low frequency of 0.001 Hz (on–off time\(\,=\,\)500 s). Figure 13 shows the relation between W and the electric power consumption P in the case of rectangular wave form with f of 0.001 Hz. These figures show a comparison among the IEF, DC and AC, where their EVs are constant and the same, respectively. It can be seen from Figs. 12 to 13 that both the dewatering rate dW/dt and W under the IEF are increased compared with those under the DC and AC, and that the electric power efficiency W/P under the IEF is remarkably higher than those under the DC and AC. These results are considered to be due to the aforementioned electrical contact resistance which is reduced to some extent during the dewatering process under the IEF.
Fig. 12

Time evolutions of W under IEF, DC and AC

Fig. 13

Relations between W and P under IEF, DC and AC

The final amount of removed water W f under the rectified half-wave IEF with rectangular wave AC was investigated by varying the cycle time, corresponding to the period of one on and off state, and the relation between W f and the cycle time was shown in Fig. 14, where the voltages applied under the IEFs were in both cases of PV\(\,=\,\)40 V and EV\(\,=\,\)40 V, and the cycle time was varied from 0.5 or 1.0 to 10 min. From this figure, W f can be found to become maximal at around 5 min in both cases, and even in the case of PV, W f in a certain range of the cycle times is larger than that under the DC condition (Yoshida et al. 2001).
Fig. 14

Relation between total water removal W f and cycle time under IEF

An IEF under pulsed DC was also investigated by varying the ratio of on/off time R defined as:
The relation between W f and R in this pulsed IEF was obtained as shown in Fig. 15. In this figure, the on-time is 4 min in all the pulsed DC IEF; therefore R \(\,=\,\)1.0 represents the half-wave IEF and the off-time is 4 min. Figure 16 also shows the relation between the electric power efficiency for the final amount of removed water W f /P f and R under the pulsed DC IEF. These results indicate that there is an optimum value of R in terms of both W f and W f /P f that are respectively highest at approximately R \(\,=\,\)0.75 (off-time\(\,=\,\)3 min) for all IEFs (Yoshida et al. 2001).
Fig. 15

W f under pulsed DC IEF

Fig. 16

Efficiency of electric power con-sumed for W f under pulsed DC IEF

The effect of the half-wave IEF under pulsed DC was investigated by using a cross-flow continuous-type experimental apparatus which consisted of two vertical cylindrical electrodes. In such an apparatus used for electrokinetic dewatering of slurry, an electric field was applied to the slurry flowing downwards continuously between the vertical cylindrical electrodes. Thus the slurry can be dewatered electro-osmotically toward both sides of the flowing path across the vertical electrodes. The half-wave IEF was also available for the cross-flow continuous dewatering apparatus, and the intermittent power application could be useful for improving the performance of EOD not only in the batch experiment but also in the continuous one (Yoshida 2001).

3.3 Electric Field Generated by Combination of Constant Current and Constant Voltage Conditions

The process of EOD has been principally operated and investigated under conditions of either constant voltage, abbreviated to CV, or constant current, abbreviated to CC here. If the electric resistance between two electrodes R E , namely the electric resistance of the dewatered material is increased with proceeding of the dewatering, the EOD process operated at CV goes to an end because the electric current passing through the material bed gradually decreases, while the operation at CC has to be stopped forcibly because the voltage required to maintain the condition of CC is too high. The increase or decrease of R E depends on the physical and electrical properties of the material to be dewatered. Therefore, a combination of CV and CC could be determined according to the properties of the material so as to make the average strength of electric field, E av , applied to the whole bed of the material as large as possible in the dewatering process, as described previously in the Section 2.2.

From this point of view, the operation of CV combined with CC was investigated as a means to improve the EOD process, and such a combined operation was shown to be effective in terms of the final amount of water removed W f and the efficiency of electric power consumption for its removal W f /P f , as an example shown in Fig. 17 (Yoshida et al. 2004).
Fig. 17

Effect of combination of CV with CC

3.4 Electrode Arrangements and Configurations

Electrode arrangements for electrodewatering in relation to impoundments had been investigated by a group of the Commonwealth Scientific and Research Organization (CSIRO, Australia) (Lockhart 1986). Both horizontal and vertical electrode arrangements were examined for in situ dewatering of sand and coal washery tailings, and their electrodes were respectively set with mesh arrangement with considerable open area in each impoundment. The field trials for dewatering were carried out using horizontal electrode arrangements at a coal washery tailing pond, and the findings were attractive, especially considering the high cost of chemical flocculants needed for centrifugal dewatering practice. The vertical electrode arrangements were found to be more appropriate at a sand washery tailing pond.

As described in the initial part of the Section 3, using EOD, the water content in the sludge bed being processed cannot usually be reduced in the lower part near the drainage surface. That is, it is difficult electro-osmotically to dewater throughout the whole sludge bed. In view of this, an EOD system with multistage upper electrodes was proposed, and it is schematically shown in Fig. 18 (Yoshida 1993). In this figure, the experimental apparatus for the system is a case of three-stage perforated upper electrodes as the anode is arranged vertically at regular intervals within the sludge bed. It can be seen that the bed is divided into three parts by their upper electrodes in the vertical direction. At the beginning, an electric field is applied between the highest upper electrode (anode) and the lower electrode (cathode) on the drainage surface. Thereafter, when the excessive increase of the electrical contact resistance between the highest electrode and the sludge is observed; in other words, a drop of the applied voltage is observed to be considerably large near the highest upper electrode, the upper electrode can be changed from the highest to the middle one by use of the rotary switch. Thus, applying an electric field to the sludge bed alternatively by switching in turn from the higher upper electrode downward to the lower one, the water content can be reduced throughout the whole sludge bed. Using a bentonite clay sludge and stainless steel for the electrode material, consequently higher-performance of EOD could be realized using the multistage upper electrode method (Yoshida 1993). However, it may be difficult to remove the upper electrodes from inside of sludge bed after completing the dewatering process.
Fig. 18

Multistage upper electrode-type EOD apparatus (e.g. three-stage)

Another method using a third electrode called the “gate" electrode placed between the upper and lower electrodes was experimented with to enhance the performance of EOD, as illustrated in Fig. 19 (Yamada et al. 2001). According to the experimental results reported, it was possible to control both the flow of electric current and the migration of particles due to electrophoresis caused by the voltage applied to the third gate electrode, and as the result a greater liquid flow could be achieved than ordinary EOD operation. Yamada et al. (2001) also explained their good results obtained under certain conditions in terms of a model of the field effect transistor (FET) that involved electronic charge carriers that could be modulated by choosing a voltage of the gate electrode. Vijh (2002) pointed out that the FET model is not applicable to the phenomena involved in the gate-electrode system, and suggested that the gate-electrode EOD is not essentially much different from the multistage electrode EOD method mentioned above, but the gate electrode system possibly provides a method for improving the EOD process under certain conditions.
Fig. 19

Experimental arrangement for EOD involving a gate electrode

Using a rotating upper electrode replaced with a stationary or fixed electrode was experimentally investigated for an improvement of EOD (Ho and Chen 2001). In this case, a sludge of bentonite was used and excess sludge was loaded beyond the upper electrode to keep good contact between the electrode and the dewatered sludge. The perforated plate upper electrode used for anode was rotated with the speed from 0 to 300 rpm. This method promoted significantly an increase in the rate of dewatering with the rotational speed, and the final amount of water removed from the sludge was demonstrated to reach maximum at about 240 rpm, as shown in Fig. 20. It was discussed that such increase as maximum water removal was due to the combined effect of both the “falling-off" of the dewatered sludge from the rotating anode electrode and sufficient supply of fresh sludge on the anode, namely the refreshment of the portion near the anode with wet fresh sludge. The increase of water removal with rotating speed was associated with an electric current increased by anode rotation. For these reasons, both effects would improve the performance of EOD, and the upper electrode rotation also would be useful for making the water content distribution along the sludge bed more uniform and higher solid concentration of the sludge dewatered.
Fig. 20

Increase of dewatering with anode rotational speed

In order to improve the performance of EOD from the viewpoint of increasing electrical contact resistance, it is also possible to insert the upper electrode into the material as dewatering proceeds, as shown in Fig. 21. Then a porous plate generally used for the upper electrode could be replaced with several rod-type electrodes which are inserted into the sludge bed. In using the rod-type electrodes, if the area of the rod-type electrode available for dewatering is regarded as the area of the bottom surface of each rod, the total area of the upper electrodes which are in contact with the sludge effectively for the dewatering is supposed to become smaller than the area of a porous plate upper electrode used ordinarily.
Fig. 21

Rod-type upper electrode inserted into sludge bed with EOD

From this point of view, electric field application was experimentally investigated by decreasing the area of the plate-type upper electrode opposite to the drainage surface compared with the cross-sectional area of the sludge bed, and the influence of such electric field application as decreasing the area of the upper electrode on the dewatering process was discussed in terms of total amount of water removed and electric power consumption (Yoshida and Okada 2006).

Half of the cross-sectional area of the sludge bed was employed for the upper electrode area and the electric field application was operated by the CC condition, and then the experimental results were compared with the upper electrode with the same area as the cross-sectional area of the bed. Figure 22 shows the variations of E av with the lapse of time for the upper electrode with half area of the cross-section of the sludge bed and with the same area as that. The value of E av was calculated using Equation (16) using the changes of V and L with time. At the beginning, E av in the upper electrode with half area of the cross-section of the bed is larger than the same-area upper electrode as the bed, as shown in this Fig. 22. This can be ascribed to the increase of R E or V in the half-area electrode. It is also suggested that an increase in the electrical contact resistance in the half-area electrode is reduced compared with the same area electrode. Figure 23 compares the total amount of water removed, W f , and the final efficiency of electric power consumption for water removal, W f /P f , in both operating conditions. It is found that both W f and W f /P f in the half-area electrode are larger than the same-area electrode of the bed. As mentioned above, a decrease of the area of the upper electrode can be expected for the purpose of improving EOD performance.
Fig. 22

Time variations of E av in upper electrode with half area of sludge bed

Fig. 23

Effect of half area of upper electrode on W f and W f /P f

As shown in Fig. 24, an essentially different type of experimental apparatus for EOD from the experimental one shown in Fig. 4 was proposed, in which an electric field was applied horizontally to facilitate the removal of gases produced at electrodes and to keep the anode soaked in water during the dewatering process (Zhou et al. 2001). This experimental apparatus was investigated for dewatering of waste activated sludge taken from a wastewater treatment plant. The activated sludge can be related to the Section 4. Figure 25 illustrates the time evolutions of water removal with the electric field strength as a parameter. It is found that the water removal can be increased by increasing the applied voltage using the horizontal electric field. It was also reported that, compared to the operation of EOD applied in a vertical electric field, the apparatus using an application of the horizontal electric field had advantages in terms of high efficiency, simple structure, and ease of operation.
Fig. 24

Experimental apparatus for horizontal electric field

Fig. 25

Time evolutions of water removal with applied voltage

4 EOD of Bio-Materials

As described earlier in the Section 2.3, EOD has the potential for dewatering of biomaterials and has some advantages compared with conventional mechanical dewatering methods. It has so far been used in practice mainly for dewatering of biomaterials or biosludges produced from bioindustry and the related industrial fields: for example, sewage/activated sludge, waterworks sludge, food processing products and wastes, and biomass sludge.

4.1 Sewage/Activated Sludge and Biomass Sludge

Many installations for disposal of biosludge produced by wastewater treatment process are used widely, so that a large amount of excess sewage/activated sludge gets discharged from the process. Such sludges that are colloidal in nature with extremely small particle size are very difficult to dewater purely mechanically.

Figure 26 gives an example of the effectiveness of EOD on sewage sludge dewatering (Yukawa et al. 1986). Figure 27 shows that the water content in the discharged cake of sewage sludge can be remarkably reduced by EOD, reaching about 50%, which is a value that cannot be attained by mechanical expression (Kondoh and Hiraoka 1990). Accordingly, a few equipments for EOD were developed in Japan, and their practical operations are shown in Figs. 2829, for example. As shown in Fig. 28, one of them consisted of a rotary drum in which surface of the drum was used as one electrode and a seamless moving belt as the other electrode, and the sludge fed into the space between the drum and moving belt was dewatered electro-osmotically under the combined field of electric field and expression (Yamaguchi et al. 1986). Another industrial EOD device is filter press type as shown in Fig. 29 (Kondoh and Hiraoka 1990). This figure illustrates schematically the sectional view of the filter chamber of the equipment. Each filter chamber consists of filter cloths, filter plates, membrane for expression, and electrodes. After the filter chamber is filled with sludge, the sludge is dewatered by pressure filtration at first and then expressed by inflation of the membrane, and finally dewatered electro-osmotically by an electric field. However, it seems that these equipments are not widely used at this time.
Fig. 26

Effect of EOD on activated sludge

Fig. 27

Effect of combined field dewatering on sewage sludge

Fig. 28

Rotating drum type EOD machine

Fig. 29

Sectional view of filter-press-type EOD machine

CSIRO in Australia investigated the applicability of EOD to aerobic wastewater treatment sludges which were particularly difficult to dewater using conventional mechanical equipment (Barton et al. 1999). It was reported that the bench-scale dewatering experiments produced cakes with solid contents of 42–46% in weight using EOD, compared with 24–30 wt% using pressure dewatering alone. Thus, EOD showed a substantial increase in the cake solid contents from sewage sludges produced by biological wastewater treatment processes. Barton et al. (1999) discussed a mathematical model for EOD described in the Section 2.1; the basic differential Equation (6), which represents the process of EOD, for the adaptability of application to sewage treatment sludges, and consequently preliminary results provided meaningfully a reasonable simulation of the modeling procedure for the EOD process of sewage sludges as well.

In order to identify a relationship between water removal limits by EOD and the forms of water in sewage sludge, the freezing–thawing technique was also examined using an analysis of proton nuclear resonance (NMR) spectroscopy. And the NMR analysis showed to provide a good way of quantifying the degree of association of water with the solids in thickened sewage sludge, and the dewatering limits by EOD was identified by estimating the forms of water removed by mechanical dewatering (Barton et al. 1999).

For the activated sewage sludge in wastewater treatment, the combined field techniques of both EOD and pressure filtration or expression have been investigated on laboratory and pilot-plant scales by many researchers to find the dewaterability of the sludge and the technical possibility for it. And these techniques were confirmed to be promising for the dewatering of the biological sludges in terms of attainable solid contents (to more than 60 wt%) and very low power consumption compared to that needed by thermal drying techniques (Smollen and Kafaar 1994; Saveyn et al. 2001, 2005, 2006).

Biomass sludge resulting from the concentration of waste slurry from biological wastewater treatment and enzyme production are rich in nutrients, and the sludge can be used as fertilizer for agricultural soils. For the purpose of this utilization, it has to be dewatered previously from around 3–5 wt% to 30 wt% in dry solid contents, which could be easily demonstrated by the operation of EOD experimentally, and low power consumption could be obtained when keeping the strength of electric field to a low level, since the electrical conductivity was quite high for the biomass sludge (Hansen et al. 2003).

4.2 Food Processing Products and Wastes

The operation of EOD combined with mechanical pressure (expression), which was called the combined field dewatering, has so far been applied to food processing industry, and several studies have been carried out on EOD of food materials.

To produce sardine powder for a gelatinous product such as “kamaboko", a continuous screw-press-type bench scale dewatering machine that is applicable to avoide protein denaturation of fish meat was developed by applying combined field of electro-osmotic and mechanical methods. In this EOD machine, the screw is used as one electrode and the strainer as the other electrode. Final water content of sardine minced meat soaked with water was only 60% by the conventional mechanical method, but it was reduced to less than 40% in only 15–18 min by applying the combined field (Suzuki et al. 1990).

The process of the combined field dewatering, involving electro-osmosis and expression, was studied experimentally for seaweed (Lightfoot and Raghavan 1994, 1995). They observed that the addition of electro-osmosis to expression significantly improved the performance of dewatering of seaweed. They also found that electro-osmosis reduces the energy consumed for producing kelp meal for animal consumption if dewatering is followed by thermal drying to produce the final product dried.

The potential benefits of combined field dewatering were also investigated for brewer's grain, apple pomace, and vegetable waste (Orsat et al. 1996). They conducted that their solid concentrations with the addition of an electric field to expression can be increased respectively from 30% to 48% for brewer's spent grain, from 23% to 53% for apple pomace, and from 11% to 67% for vegetable waste. As mentioned above, the combined field dewatering of kelp (seaweed) before drying process had been shown to be useful for reducing energy requirements for drying (Lightfoot and Raghavan 1994). This preliminary investigation led to the development of a pilot-scale operation for the production of kelp meal, and the kelp after treating chemically and draining was dewatered experimentally using the pilot-scale roller-press-type machine for the combined field dewatering (Orsat et al. 1999). The roller press consisted of seven rollers (four bottoms and three tops) and two idler rollers mounted on an aluminum truss frame, and the material to be dewatered was loaded onto a belt, which carried the material through the press rollers where the combined field was applied simultaneously. In this pilot-scale electro-osmotic roller press operation for kelp meal production, it was ensured that the combined field was effective at removing water from the drained kelp, in spite of remaining problems with capacity and reliability.

Vegetable wastes produced from the food industry are costly to handle because of their high moisture content. Effectiveness of the combined field dewatering for such vegetable waste sludges was studied experimentally on small scale (Chen et al. 1996). Chen et al. (1996) used fresh vegetable sludge samples made from one part by weight of cauliflower and two parts of cucumber as a model of vegetable waste. In the experiments with such a vegetable sludge, the final average water removal of 55% was achieved by the combined field dewatering, and this was twice as high as that of mechanical pressure alone and four times as high as that of EOD alone.

Figure 30 shows the water removal with different initial heights of the sludge bed for the combined field dewatering, in which the mechanical pressure was fixed at a constant value and the initial strength of electric field, defined as the applied voltage divided by the initial bed height, was also fixed at a constant value. It is observed in this figure that the water removed increases with increasing the initial height of sludge bed and also the increase in the water removal tends to be smaller with the initial bed height. Consequently, for the model vegetable sludges tested, it was found that the combined field dewatering is more effective than just EOD and mechanical pressure contributes significantly to the sludge dewatering, and that the increase in the initial bed height increases the amount of water removed.
Fig. 30

Effect of initial bed height for vegetable sludge

“Tofu" is a soy food generally regarded as one of the healthy foods and is very popular in east Asia. The production of tofu yields enormous waste stream known as “okara," which is the solid residue after separation of soymilk produced by squeezing or centrifugation of soybean mash. A large amount of okara discharged in tofu manufacturing process has to be dried and burned or sent to landfill, but the storage and transportation of it are very difficult because of its high moisture content on wet basis (around 80%). Hence, okara treatment and disposal have been a serious problem in food processing. To enhance the dewatering efficiency of okara, a twinscrew-press-type ceramic filter with electro-osmosis was designed and examined (Isobe et al. 1996; Li et al. 1999). It was reported that the water content of okara was decreased by the screw press from 85% to 74% of which the dewatering efficiency was 50%, and that was reduced considerably up to 40% in case of a batch compression process with the addition of electro-osmosis. The experiments by electro-osmosis were performed using DC and AC electric fields with the frequency from 0.2 to 0.5 Hz, and the low-frequency AC fields combined with mechanical pressure had the potential to be used in the dewatering of okara compared with DC operation (Isobe et al. 1996).

Making a sheet of tofu, mechanical pressure dewatering has been used commonly, but it has the disadvantage of low dewatering efficiency and poor quality control based on the experience of the manufacturers. The combined field of electro-osmosis and constant mechanical pressure was performed on the dewatering process of tofu sheet (Li et al. 2002; Xia et al. 2003). Li et al. (2002) examined the application of AC electric field with frequency ranging from 0.3 to 5 Hz for the dewatering of tofu sheet, and found that the dewatering time was shortened markedly and the AC frequency in the range 0.5–1 Hz provided good quality of tofu sheet dewatered in terms of strength, strain and toughness. Xia et al. (2003) investigated combined field dewatering for 10–30 min at applied voltage ranging from 30 to 50 V under various pulsed DC electric field as shown in Fig. 31, and the microstructure of tofu sheet was also observed by a scanning electron microscope (SEM). According to their results, the combined field dewatering was found to increase the dewatering rate for tofu sheet compared with that by mechanical pressure only. SEM micrographs showed that the network of tofu sheet near anode was significantly more compact and homogeneous, if the treatment of EOD was added to mechanical pressure dewatering.
Fig. 31

Application of different electric fields

Removal of water by EOD from tomato paste suspension was also used to concentrate the suspension (Al-Asheh et al. 2004; Jumah et al. 2005). For the purpose of this treatmet, an apparatus was designed in a vertical orientation, and laboratory tests demonstrated applying electric field under both DC and AC operations to the tomato paste suspension held between two electrodes. The effects of applied voltage, electric current, height of the bed, pH, and initial solid concentration of the suspension were examined in terms of water removal and energy consumption for dewatering. It is reported that significant amounts of water can be removed by EOD under the operating conditions used. This process saved 70% of energy compared with that needed to dry the same amount of water. The application of AC electric field provided more promising results than those by DC application.

4.3 Miscellaneous Topics

There are a few interesting papers which are related to EOD applications where dewatering per se is not objective (Vijh 1999a,b,c; 2004). Electrochemical treatment of cancerous tumors has been studied experimentally. Vijh (1999c) proposed a theoretical model to analyze the primary mechanism of electrochemical treatment of the tumors. The idea for the analysis was that the process of electrochemical treatment can be regarded as a case of EOD of the tumor tissue with the consequent changes in pH, with the concomitant role of reactions at the electrodes. That is, the electrochemical treatment of tumor causes a net flow of water from the anode to the cathode, causing EOD of the tissue, based on the electrically induced flow of water trapped between the tumor cells. All the main experimental observations—for example, water removal near the anode and water accumulation near the cathode as well as the associated pH changes and other factors considered to be involved in necrosis—were explained by the analysis proposed. Some suggestions were made based on the analysis to explain the enhancement of the efficacy of the process of electrochemical treatment of cancerous tumors.

Electrokinetic remediation of contaminated soils and dredged sediments isanother application of EOD, which has attracted major attention. However, this subject is outside of the scope of this chapter (e.g., Shaplro and Probstein 1993; Reddy et al. 2005).

5 Closing Remarks

Although EOD has been examined both theoretically and experimentally for decades and its advantages in dewatering are well documented, commercial applications are still few. Typically EOD is not as cost-effective as a stand-alone operation. It is necessary to use it in combination with pressure or vacuum dewatering. Thus the equipment capital cost is higher than that of conventional dewatering equipment. Future R&D will hopefully allow cost-effective development of EOD-enhanced equipment for sludge dewatering and contaminated soil remediation.


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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  1. 1.National University of Singapore, Mineral, Metal & Materials Technology CentreEngineering Science ProgrammeSingapore
  2. 2.Department of Materials Chemistry and BioengineeringOyama National College of TechnologyOyama

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