Electroosmotic Dewatering Research Articles

Evaluation of Suitable Material Properties of Sludge for Electroosmotic Dewatering

To estimate suitable material properties of sludge for electroosmotic dewatering, we experimentally measured the electrical and physical properties of a sludge bed. The use of an upper electrode that had a smaller area than the cross-sectional area of the bed was also investigated with the aim of reducing the electrical contact resistance between the electrode and the bed caused by dewatering. We found that sludge with a lower specific electrical resistance and a higher hydrodynamic compressibility in cake filtration provided greater electroosmotic dewatering performance. However, such a sludge was not necessarily available for using a smaller-area upper electrode.

Application of Electroosmosis for Sludge Dewatering—A Review

This work aims at providing an outline of engineering applications for electroosmosis, especially for sludge dewatering. A review of the theoretical analysis of electroosmotic dewatering is presented. The equations representing electroosmotic flow through porous media are initially discussed. Consolidation theory, which describes the progress of dewatering, is then described. In addition, various empirical research results addressing the effects of operational conditions and conditioning of sludge on both energy consumption and final water content of the sludge are reviewed. It is anticipated that this review will provide useful information to researchers pursuing solid–liquid separation of difficult-to-dewater materials such as sludge.

Constant-current electroosmotic dewatering of superabsorbent hydrogel

The electroosmotic dewatering (EOD) behaviors of gelatinous materials, such as superabsorbent hydrogels or swelling clay, are discussed. The apparent liquid velocity through the materials can be represented in terms of effective-osmotic, electroosmotic and swelling pressure gradients. Taking the creep deformation of the materials into consideration and assuming that the mechanical properties of the materials can be represented by the Terzaghi–Voigt combined model, the basic differential equation expressing EOD of gelatinous materials is solved. The progress of EOD is represented by an average consolidation ratio Uc as in mechanical expression. The agreement between calculated and experimental Uc is satisfactory when the creep deformation of the material is considered. The disagreement between theory and experiment in the latter part of EOD may be due to the change of the property of hydrogel caused by dissolution of Fe(III) from stainless steel electrode.

Influence of Chemical Conditioning on Electroosmotic Dewatering Behavior for Activated Sludge

Applying electroosmosis to sludge dewatering practically, methods for improving electroosmotic dewatering must be explored. The influence of initial pH, adding Al2 (SO4)3 dose and ash content in sludge on electroosmotic dewatering behavior was investigated. The results show that acid or alkaline environment is unfavorable for electroosmotic dewatering, especially initial pH of sludge closing to the isoelectric point, electroosmotic dewatering behaves worst. With increasing Al2(SO4)3 coagulant-aid dose, the decreasing rate of water content becomes slower gradually during electroosmotic dewatering. Besides, higher ash content in sludge has a better effect on vaccum filtration, but slightly impact on electroosmotic dewatering. When ash content increases to 69.9% from 52.4%, final water content reduces to 64.3% from 78.3% using vaccum filtration, and the decreasing amount of water content by electroosmotic dewatering ranges from 16.5% to 12.3%.

Pressurised electro-osmotic dewatering of activated and anaerobically digested sludges: Electrical variables analysis

Pressurised electro-osmotic dewatering (PEOD) of two sewage sludges (activated and anaerobically digested) was studied under constant electric current (C.C.) and constant voltage (C.V.) with a laboratory chamber simulating closely an industrial filter. The influence of sludge characteristics, process parameters, and electrode/filter cloth position was investigated. The next parameters were tested: 40 and 80A/m2, 20, 30, and 50V—for digested sludge dewatering; and 20, 40 and 80A/m2, 20, 30, and 50V—for activated sludge dewatering. Effects of filter cloth electric resistance and initial cake thickness were also investigated.The application of PEOD provides a gain of 12 points of dry solids content for the digested sludge (47.0% w/w) and for the activated sludge (31.7% w/w). In PEOD processed at C.C. or at C.V., the dewatering flow rate was similar for the same electric field intensity. In C.C. mode, both the electric resistance of cake and voltage increase, causing a temperature rise by ohmic effect. In C.V. mode, a current intensity peak was observed in the earlier dewatering period. Applying at first a constant current and later on a constant voltage, permitted to have better control of ohmic heating effect. The dewatering rate was not significantly affected by the presence of filter cloth on electrodes, but the use of a thin filter cloth reduced remarkably the energy consumption compared to a thicker one: 69% of reduction energy input at 45% w/w of dry solids content. The reduction of the initial cake thickness is advantageous to increase the final dry solids content.

Influence of Electric Field Application by Reducing Area of Electrode Compared with Cross-Sectional Area of Sludge Bed on Electro-Osmotic Dewatering Process

In the process of electro-osmotic dewatering of a semi-solid sludge put between two plate-type electrodes placed above and below a sludge bed, the water content in part of the bed near the upper electrode opposite to a lower drainage surface is considerably reduced, resulting in electrically poor contact between the upper electrode and the bed being dewatered, and then an excessive increase in electrical contact resistance between them. This obviously obstructs the efficiency of electro-osmotic dewatering throughout the entire bed, and consequently, the continuation of the dewatering process becomes increasingly difficult. From the viewpoint of lowering the influence of increasing electrical contact resistance, the usage of an upper plate-type electrode, in which the area of such electrode is smaller than the cross-sectional area of the sludge bed, is regarded to be practical in improving electro-osmotically the performance of sludge dewatering. In this study, electro-osmotic dewatering is experimentally investigated by reducing the area of not only the upper electrode, but both the upper electrode and the lower one on the drainage surface, and compared with the same-area electrode as the cross-section of the bed. Thus, the influence of such electrode arrangements in reducing the area of the electrode(s) on the dewatering process in terms of water removal and electrical energy consumption is examined using calcium carbonate as solid particles for sludge conditioning. As a result, it has been found that the amount of removed water and electrical energy efficiency for water removal can be increased compared with the same-area electrode as the bed, except for the case of an excessively small electrode area.

Electrochemical Modification of Physicochemical Soft Rock

Two mechanisms about electrochemical modification of clay minerals are summarized: one of electroosmotic dewatering and stabilization and the other of cation substitutions, structures and properties change, forming new minerals. The analyses of physicochemical soft rock (PCSR) indicate that physicochemical property of PCSR is dominated by physicochemical property of clay minerals. Therefore, it is possible to indurate PCSR in the coal-mine soft rock roadway with the electrochemical method.

Mechanism and Effect of Electrochemical Modification on Physicochemical Soft Rock

Two mechanisms about electrochemical modification of clay minerals are summarized: one of electroosmotic dewatering and stabilization and the other of cation substitutions, structures and properties change, forming new minerals. The analyses of physicochemical soft rock (PCSR) indicate that physicochemical property of PCSR is dominated by physicochemical property of clay minerals. Therefore, it is possible to indurate PCSR in the coal-mine soft rock roadway with the electrochemical method.

The effect of the addition of fly ash to municipal digested sludge on its electroosmotic dewatering

Effective handling of municipal digested sludge requires that the product cake have low water content. To this end, we investigated the change in sludge dewaterability after the addition of fly ash to municipal digested sludge, dewatering of which is difficult because of its high organic content. The performance of the dewatering is compared with that of electroosmotic dewatering (EDW) and conventional mechanical dewatering (CMD). Fly ash classified by sieving to the size of 25–75 μm and >75 μm is added to the municipal digested sludge by 10, 20, and 50 wt% by wet base. When adding fly ash particles to municipal digested sludge, dewatering efficiency improved with smaller fly ash particle size and with increase in the amount. When sludge was dewatered using an electroosmotic dewatering method, the dewatering efficiency is improved about 40% by adding fly ash of 25–75 μm particle size with 20 wt% when compared with conventional mechanical dewatering method without adding the fly ash. It is concluded that fly ash particles rich in inorganic material are helpful in the dewatering process when added to municipal digested sludge and EDW is more effective than CDW.

Mathematical model for electroosmotic dewatering of activated sludge

The mechanical dewatering of activated sludge is difficult due to its high compressibility, which can be improved by electroosmosis. In electroosmosis, direct electric field is applied to sludge cake. Based on the conductivity modes of different sludge beds, a model is presented in which sludge cake consists of two series parts in the circuit: a dewatered bed and an undewatered one. The dewatered bed called solid conductor is mainly made up of immovable water and sludge particles. The undewatered bed includes movable water and solid conductor, which are connected in parallel in the circuit. The model describes the variation of water content with time and electric power consumption as a function of water content in sludge cake, and interprets the reason for the variation of electroosmotic dewatering rate. Comparison with the experimental data for electroosmotic dewatering under constant voltage supports the validity of the model.

Electroosmotic Flows in Sludge at Dewatering

Electroosmotic dewatering of activated sludge is an important transport process playing a critical role in electroosmotic dewatering of sludge. In the present work, a comprehensive physical model considering both pressure-driven and electroosmosis-driven flows in an activated sludge floc was developed and numerically solved. Meshes were produced via scanning images based on real sludge flocs. The permeabilities of pressure and electroosmotic flows were evaluated, from which the total detwatering flow rate can be estimated.

Influence of Filter Cloth on the Cathode on the Electroosmotic Dewatering of Activated Sludge

The mechanical dewatering of activated sludge is troublesome due to its high compressibility of solids. The dewatering can be enhanced dramatically by the use of electroosmosis, in which an electric field is applied to the sludge cake. In this study, the influence of filter cloth on the cathode on the dewatering of activated sludge was investigated. It was found that thicker filter cloth led to lower water removal from the sludge cake, so a stainless steel cathode net with small pore size instead of filter cloth was applied, which improves the dewatering efficiency and reduces the electric power consumption. Moreover, water absorbent materials were helpful to remove the water from the sludge cake. For the electroosmotic dewatering at 7 kPa and 24 V·cm −1, the water content in the sludge cake decreased to 60% (by mass) with the average 0.075 kW·h·kg −1 of water removed by using the cathode net.

Structure Evolution of Wastewater Sludge During Electroosmotic Dewatering

An experimental investigation was conducted to explore the characteristics of electroosmotic dewatering of wastewater sludge. Inner pore structure images of sludge were obtained by microcomputed tomography (micro-CT) scanning; the 3D pore structures of sludge and its dynamical evolution were studied by means of pore size distribution (PSD), box-counting fractal dimension, and volume change of sludge matrix. Transport of water content inside sludge was also associated with interactions between local capillary stress and global structural force during electroosmotic dewatering; its impact on the change of pore structure was analyzed. As water content decreased, the local capillary force split the pore surface and created cracks, and the global structural force shrank the sludge matrix and constricted the pore surface. Inner water transport and the evolution of pore structure closely depended on the location between the anode and cathode of electroosmosis.

Analysis of Combined Electroosmotic Dewatering and Mechanical Expression Operation for Enhancement of Dewatering

This article presents an efficient approach for the analysis of combined electroosmotic dewatering (EOD) and mechanical expression (ME) using the Terzaghi-Voigt combined model by considering creep deformation of the material. The EOD-ME process enhances dewatering because it combines the advantage of electroosmosis and mechanical dewatering, resulting in reduced void ratio compared to individual operation. The basic differential equation based on the model is solved analytically by assuming that both an electroosmotic pressure gradient E pg and a modified consolidation coefficient C e of the material are constant, resulting in the equation of solid compressive pressure in the material as a function of time and position. The analysis also leads to the equation of the average consolidation ratio U c, which is a measure of the progress of dewatering; this equation can describe well the empirical results under various conditions. The proposed approach can clearly explain the relationship of the driving forces of individual mode of operation. The large water content of semisolid bentonite–water mixture can effectively be reduced by the EOD-ME combined operation analyzed in this research.

Electroosmotic flow in sludge flocs

Electroosmotic flow in sludge flocs is one of important transport processes playing a critical role in electroosmotic dewatering of sludge. In the present work both fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) are employed to obtain the interior structural information of flocs and the structural properties are incorporated into the computation of electroosmotic flow inside flocs. Some important parameters, including flowrate distribution, pressure drop and permeability, were specifically investigated to explore the flow characteristics. An investigation was also conducted to compare the electroosmotic flow with the corresponding pressure driven flow inside flocs.

Desalination of a brick by application of an electric DC field

Salts in masonry can cause various problems as decay of the masonry itself, lost adhesion of plaster and hygroscopic moisture. Chlorides are among the most common building salts and the present paper is focused on removal of chlorides from a brick in an applied electric field as a step towards developing an electrochemical desalination method for brick masonry. Experiments were conducted in laboratory scale with one type of bricks that were contaminated with either NaCl or KCl through submersion in salt solutions prior to application of current. It was seen that NaCl was slower supplied to the brick during submersion and slower removed in the applied electric field than KCl. This indicates that the removal rate of chloride depends on the associated cation and this must be taken into account when desiding the duration of full scale actions. The electrochemical desalination was very efficient and 99% removal of chloride was obtained. The final concentration in the brick after treatment was less than 10 mg Cl/kg and this concentration is unproblematic. When low salt concentrations were reached during the electrochemical treatment, electroosmotic dewatering of the brick started, showing that electroosmotic dewatering occurs at low ionic concentrations.

Effect of Operating Parameters on the Effectiveness of Electric Field-Enhanced Separations

The purpose of this work is to summarize our latest research results about constant-current electroosmotic dewatering (EOD). The Terzaghi-Voigt combined model is used for the analysis of EOD of various solid-liquid systems by considering the creep deformation of the material. Bentonite clay, KC-flock, zinc oxide, and some of their mixtures are used as experimental materials. It is assumed that the electroosmotic pressure gradient E pg and modified consolidation coefficient C e of the materials are constant. The relation between the ratio of creep deformation to total deformation B and the modified consolidation coefficient C e is determined and compared with previous observations for the case of mechanical expression. The effectiveness of dewatering is described depending on operating parameters such as preconsolidation pressure, material composition, electric current density, and total solid volume per unit cross-sectional area. The model is verified by comparing the calculations with experimental data.

Theoretical analysis of centrifugal dewatering of superabsorbent hydrogels using Terzaghi–Voigt combined model

This paper presents results of experimental work carried out to study the centrifugal dewatering behaviors of superabsorbent hydrogels. Taking the creep deformation of the hydrogels into consideration and assuming that the mechanical properties of gel network can be represented by Terzaghi–Voigt combined model, the basic differential equation expressing the centrifugal dewatering of superabsorbent hydrogels is solved. The progress of centrifugal dewatering is represented by an average consolidation ratio U c as in mechanical expression. The agreement between calculated and experimental U c is satisfactory when the creep deformation of the material is considered. Consistent with mechanical expression and electro-osmotic dewatering, as the driving force for centrifugal dewatering increases, the modified consolidation coefficient of the hydrogel network C e also increases. Effectiveness of centrifugal dewatering, expressed as a relative difference between the initial void ratio and final void ratio, increases with the rotational speed. For a given rotational speed, the final void ratio remains constant regardless of the initial weight of the gel. On the other hand, as the rotational speed of the centrifuge gets higher, the final void ratio decreases for all initial weights of the gel investigated in the current study. The magnitude of creep deformation B depends upon the amount of initial weight of the hydrogel. For a given rotational speed of the centrifuge, the magnitude of creep deformation becomes larger as the initial amount of hydrogel is increased.

Analysis of constant-current electro-osmotic dewatering of various solid–liquid systems by considering the creep deformation

A constant-current electro-osmotic dewatering (EOD) of various solid–liquid systems is analyzed using the Terzaghi–Voigt combined model by considering the creep deformation of the material. Electro-osmotic dewatering experiments under constant electric current density of bentonite clay, KC-flock, zinc oxide and some of their mixtures are carried out. The basic differential equation based on the model is solved analytically by assuming that both electro-osmotic pressure gradient E pg and modified consolidation coefficient C e of the material are constant. The solution can explain the time course of changes in solid compressive pressure distribution. The theory can also explain the final moisture distribution of the material. The progress of electro-osmotic dewatering can be represented by an average consolidation ratio U c as in mechanical expression. Both C e and E pg increase with an electric current density i. A larger C e leads to a higher dewatering rate; a larger E pg results in a higher solid compressive pressure distribution in the material. It is found that the amount of creep deformation depends upon the dewatering rate; the faster the dewatering rate, the larger the creep deformation, i.e., the creep deformation is more remarkable when a higher current density is applied to the material. The effectiveness of EOD can be represented by the relative difference of the initial and final void ratios. Variations of the effectiveness of dewatering with the material composition, electric current density i, preconsolidation pressure p s1 and total solid volume per unit cross-sectional area ω 0 are investigated. It is found that higher composition of charged material and higher current density show improved dewatering. For higher ω 0, slight increase in effectiveness of dewatering is observed. For significant decrease of preconsolidation pressure, a relatively small increase in the effectiveness is observed. Therefore, among these, the material composition and current density are the main EOD parameters that can be used to determine the feasibility of EOD as efficient solid–liquid separation process.

Dewatering of mine tailings using electrokinetic geosynthetics

Many mining operations produce tailings that dewater very slowly under self-weight consolidation. One way of reducing the water content of such tailings is by electroosmotic dewatering. Although the technique has been used with some success in civil engineering applications, it is still largely seen as a solution of last resort. This is probably due to the high energy costs reported in the literature, as well as problems of very rapid corrosion of metal electrodes. This paper describes a study using newly developed electrokinetic geosynthetics (EKGs) as electrodes for the in situ dewatering of mine tailings. Laboratory tests were undertaken on mineral sands tailings in both a purpose-built testing cell and a laboratory testing tank using EKGs, followed by an outdoor experiment in a tank containing approximately 9 m3of the tailings. This test was run for over 2 months. Energy consumption in the outdoor test was less than 1 kWh per dry tonne of material dewatered and there was no sign of electrode deterioration even after 2 months of usage. The results point to a potentially powerful technique for reducing the water content of tailings ponds in situ, thus increasing storage space, improving stability, and facilitating closure of these facilities.Key words: tailings, dewatering, electroosmosis, electrokinetic geotextiles, consolidation.