In situ determination of solidosity profiles during activated sludge electrodewatering

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In situ determination of solidosity profiles during activated sludge electrodewatering Hans Saveyna,, Daan Curversa, Leo Pelb, Pieter De Bondtc, Paul Van der Meerena a

Ghent University, Particle and Interfacial Technology Group, Coupure Links 653, 9000 Ghent, Belgium Eindhoven University of Technology, Department of Applied Physics, Transport in Permeable Media, P.O. Box 513, 5600 MB Eindhoven, The Netherlands c University Hospital Ghent, Department of Nuclear Medicine, De Pintelaan 185, 9000 Ghent, Belgium b

art i cle info

A B S T R A C T

Article history:

Two non-invasive techniques were evaluated for the on-line measurement of sludge

Received 22 November 2005

solidosity profiles during both pressure and electrodewatering operations.

Received in revised form

In a first approach, a radioactive tracer adsorbed onto the sludge solids was monitored

3 April 2006

by a gamma camera. Although this technique appeared very flexible in use, the lack of

Accepted 4 April 2006

resolution highly limited its usefulness for (electro)dewatering experiments. Improvement in gamma camera resolution by the development of new detectors might, however,

Keywords: Radiotracer

increase the future applicability of this technique. In a second technique, nuclear magnetic resonance measurements on a specially

Magnetic resonance imaging

designed electrodewatering unit were made. Hereby, reliable on-line measurements of the

Solid–liquid separation

solidosity profiles of activated sludge during electrodewatering could be made, with a

Filtration

resolution of less than 1 mm. Thus, the mechanisms of electroosmotic- and pressure-

Electroosmosis

driven cake dewatering could be illustrated. Given the measurement time required for measuring one sludge profile, both techniques appeared mainly suited for slowly varying processes, such as activated sludge expression, and not for fast changing processes, such as the initial phases of sludge filtration. & 2006 Elsevier Ltd. All rights reserved.

1.

Introduction

Sewage sludge is known to be a poorly dewaterable material (Sorensen and Hansen, 1993; Vaxelaire and Cezac, 2004). In order to improve the dewatering performance, new techniques have been explored in recent years. A frequently studied technique is electrodewatering, in which the pressure-driven dewatering operation is assisted by an electric field, thus yielding faster dewatering kinetics and improved final cake dry matter contents by means of electroosmotic water transport. Whereas most studies have been focussing on the practical dewatering performance of this technique on Corresponding author. Tel. +32 9 264 60 25; fax +32 9 264 62 42.

E-mail address: [email protected] (H. Saveyn). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.04.003

bench models or in pilot installations, only a few studies are known that try to gain a better understanding of the electrodewatering process by mathematical modelling of the spatial variability of the porosity or solidosity of the sludge cake as a function of time (Barton et al., 1999; Iwata et al., 1991). In order to validate existing modelling work or to construct new models for (electro)dewatering, techniques are needed that allow the determination of the porosity or solidosity distribution in the suspension/cake system inside the dewatering unit as a function of time. The simplest and most straightforward technique is the determination of the dry matter content following mechanical

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slicing of a cake by means of a knife or microtome (Meeten, 1993). Advantages of this technique are its low cost and simplicity, as well as the fact that no calibration is required. However, this technique is time-consuming and destructive: for every moisture profile in time to be determined, a new cake has to be made. Slight differences in the process for consecutive dewatering runs can make it difficult to bundle the data obtained from the different cakes for a general overview. Furthermore, many cakes, such as these obtained from activated sludge, are known to be subject to a rebound after pressure release, which makes that the structure of the unpressurized cake differs from the pressurized one (Lee et al., 2003, Saveyn et al., 2005b). Finally, being an invasive technique, the sludge structure may be altered by manual handling. Non-destructive techniques are far more preferable for determining moisture profiles. Tarleton (1999) described a technique based on the moisture content dependent electrical resistance of a filter cake, where diametrically opposite electrodes are used around a cylindrical filter cell, allowing the determination of one-dimensional porosity profiles with a resolution of 1 mm.The fact that these electrodes are installed in the filter chamber may however disturb the dewatering process. Moreover, when applying this technique for electrodewatering, the external electric field imposed may interfere with the electrode measurements and cause undesired electrochemical reactions at the electrodes. Examples of non-destructive, non-invasive techniques known for monitoring solid–liquid separation processes are X-ray scattering, which is excluded for visualization of electrodewatering because of signal artefacts generated by the presence of the metallic electrodes (Gielen et al., 2003) or neutron scattering, which is very accurate, but extremely expensive (Cabane et al., 2002; Pignon et al., 2003). Proton nuclear magnetic resonance (1H-NMR) is a technique that combines relatively fast measurements with a fair level of resolution and has been shown successful in measuring moisture profiles in sludge pressure dewatering experiments (LaHeij et al., 1996). Alternatively, radio-active molecules are frequently used as tracers in medical applications (Jurisson et al., 1993) and radiotracer measurements have been used succesfully for monitoring sludge profile determination in settling experiments, as reported in a recent study by De Clercq et al. (2005). The purpose of this paper is to evaluate the feasibility of radiotracer and proton NMR techniques for measurement of the cake formation and expression process during activated sludge electrodewatering.

2.

Materials and methods

2.1.

Materials

Thickened activated sludge was sampled from the Ossemeersen Waste Water Treatment Plant (Aquafin, Ghent, Belgium). After sampling, sludge was stored at 4 1C for a maximum of 3 days, to reduce the effect of biochemical composition change. Ciba polyelectrolyte Zetag 7878FS40 was used to condition the sludge; it is a copolymer of polyacrylamide and quaternized

dimethylaminoethyl acrylate (CAS Number 69418-26-4) (Dentel, 2001). Aqueous polyelectrolyte solutions were prepared at a 2 g/l active polyelectrolyte concentration (0.2%), at least 24 h prior to application, in order to allow the polyelectrolyte chains to completely unfold for optimized contact efficiency. For every (electro)dewatering run, a new piece of Nordifa Lainyl MC4/S5/6 polypropylene filter medium was taken.

2.2.

Methods

2.2.1.

Radio-active tracer measurement

In a first experiment, a radioactive labelling test was evaluated for the determination of the concentration of activated sludge. A sludge sample (2.66% mass/mass dry matter) was conditioned at a dose of 6 g polyelectrolyte kg 1 dry matter, leading to a positive floc surface charge: polyelectrolyte addition increased the sludge’s electrophoretic mobility from 1.0370.02 to 2.2270.12 E 8 m2 s 1 V 1. One liter of conditioned sludge was mixed with 10 ml NaTcO4 solution with an initial activity of 4.7 mCurie (173.9 MBq, 9.0 pmol), containing the metastable isotope Tc-99 m as a gamma radiation emitter with a half life of 6.01 h. A sample containing 950 ml of the labelled sludge was dewatered at 4 bar (400 kPa) pressure for 1 h in a labscale dewatering unit, which has been described extensively in Saveyn et al. (2005a), during which time gammascans of the filter column were taken with a Picker Prism 1500xP 185 gamma camera (resolution of 256  256 pixels with 16 bit pixel values). The scans were taken with increasing time intervals: 120 scans of 0.5 s from 0 to 60 s, 60 scans of 1 s from 60 to 120 s, 48 scans of 10 s from 120 to 600 s and 100 scans of 30 s from 600 s to 3600s. The images obtained during the dewatering process were treated with Xmedcon software to retrieve the numerical intensity values for every pixel. The dewatering kinetics were also followed by measuring the piston height (by a Norgren position sensor) and filtrate mass (by a digital Ohaus Navigator precision balance) as a function of time.

2.2.2.

Proton nuclear magnetic resonance measurement

The setup and procedure used for determining the sludge profile during electroosmotic dewatering runs of conditioned activated sludge by proton NMR is based on LaHeij et al. (1996). A new cylindrical electrodewatering unit was constructed from a polyethylene terephthalate (Ertalyte TX) tube with 20 mm internal diameter. The tube was closed at both sides by a PVC screw cap, containing filtrate drainage channels at the bottom and an inlet for pressurized air at the top. Inside the tube, sludge was dewatered by a moving piston consisting of a polyethylene terephthalate disc. Both the bottom screw cap and the piston were equipped with starshaped electrodes made of radially positioned platinum wires (0.25 mm diameter, 10 mm length, 16 wires at the piston and 20 wires at the bottom screw cap) connected to a central conductor. A flexible connection with the piston electrode was made by means of a spirally wound copper wire. An adjustable constant current source was used for applying the electric field (Philips, maximum 100 mA and 72 V), where the electrode at the bottom screw cap functioned as cathode and the electrode at the piston functioned as anode. Samples of 50 ml conditioned sludge were dewatered at a constant

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Fig. 1 – (a) Schematic representation of the electrodewatering unit constructed for the NMR measurements of sludge moisture content and (b) electrode configuration on the bottom screw cap and piston.

dewatering pressure of 3 bar (300 kPa). A schematic diagram of the electrodewatering unit and electrode configuration is given in Fig. 1. The capillary suction time (CST) of (conditioned) sludge was measured with a Triton Electronics 304 M CST meter using Triton CST paper. An NMR scanner was used with a conventional electromagnet, generating a field of 0.7 T (30.9 MHz). It has the possibility to accommodate cylindrical samples with a diameter of 30 mm. Measurements were based on the spinecho sequence method with a long delay time of 1–3 s and a spin-echo time of 1 ms. A relative signal intensity was calculated by dividing the measured signals by the recorded intensity of a standard 0.1 M CuSO4 solution, as such yielding a standardized value. For obtaining the one-dimensional profiles, the electrodewatering unit was lowered through the measuring channel stepwise (0.75 mm stepsize) by an externally programmed stepping motor.

3.

Results and discussion

3.1.

Radio-active tracer test

Virtually complete adsorption of the tracer molecules onto the conditioned sludge was ascertained by scanning both the filtrate and cake at the end of the dewatering test, showing less than 0.15% of the total radioactivity to be present in the filtrate. It is assumed that this was due to the electrostatic interaction forces between the negatively charged TcO4

marker molecule and the net positively charged conditioned sludge system. Although not tested in this study, for suboptimally conditioned sludges, i.e. where a negative surface charge is measured, the cationic Tc-99 m Sestamibi marker could be used, which was shown to adsorb on sludge systems with a negative surface charge (De Clercq et al., 2005). Based on the gravimetrically determined sludge mass in the filter column, a calibration was performed between gamma counts and dry matter content. Given the fact that the tracer was almost completely associated with the sludge solids, the radio-active labelling tests basically yielded mass-based solids distributions, which can be transformed into volumebased solidosity or porosity profiles. Fig. 2 shows the sludge solids distribution inside the filter press at different moments during a sludge pressure dewatering run. Before the start of the dewatering, the sludge flocs were homogeneously distributed in the filter column, indicated by the overall constant solids concentration of the suspension. During the initial dewatering stage, the solids concentration is slightly elevated towards the filter medium, as seen in the profile at 120 s, suggesting some very porous cake build-up. Farther away from the filter medium, the sludge solids concentration still equals that of the initial suspension. Although, the collimator grid in the gamma camera ought to filter out all radiation that is not perpendicular to the plane of the detector, it was observed that the intensity level on every pixel was influenced by neighboring gamma emitters, causing signal distortion and peak broadening. As a result of this, the boundary between cake and suspension is blurred.

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Solids concentration (% m/m).

14 12

3600 s

10 8

1800 s

6 600 s

4

Before start of dewatering operation

2 120 s 0 0

50

100 150 200 Distance from filter medium (mm)

250

300

Fig. 2 – Solids distribution inside the filter column as a function of dewatering time, measured by a radioactive tracer test. During the dewatering process, the piston moved from a distance of 246.5 mm (at 0 s) to 44.2 mm (at 3600 s) from the filter medium.

Despite the fact that for extremely compressible materials the initially formed cake layers may already be subject to compression before the cake formation has finished (Sorensen et al., 1996), the nominal transition moment from filtration to expression was determined from the evolution of the filtrate mass as a function of time as proposed by Shirato et al. (1970), and appeared to be approximately 600 s. It is seen that in the expression phase, from 600 s onwards, the fully formed cake was compressed and the cake solids content therefore increased, but the piston movement and therefore the water removal was decelerated considerably. As the dewatering of similar conditioned sludge samples was shown to evolve quickly in the early filtration stage and markedly slower during the expression phase, it had been decided to make scans of short duration in the beginning of the dewatering run, followed by longer scans after a few minutes, to enable optimal recording of the dewatering kinetics. A disadavantage of this strategy is the low accuracy of the initial scans. As can be seen from Fig. 2, the profiles at 120 and 600 s are more noisy than the ones at 1800 and 3600 s due to the lower acquisition times: the former profiles were recorded with acquisition times of 1 and 10 s, respectively, whereas the latter profiles were recorded with an acquisition time of 30 s. The profile before the start of the dewatering operation was recorded with an acquisition time of 200 s. Due to the stochastic nature of the radio-active radiation emission—it is widely known that the standard deviation of a counting measurement equals the square root of the count number—higher acquisition times led to more reliable results. Moreover, gamma ray count numbers can also be influenced by the tracer concentration and also by local solids concentration, such that at low solids concentrations, it is more difficult to obtain accurate results than at high solids concentrations. Therefore, irrespective of the rate of filtration, the accuracy of the less concentrated domains will always be less reliable and profiles recorded at the beginning of the dewatering run, with low local solids concentrations, will be less reliable than profiles recorded at the end of the

dewatering run, where the solids are more densely packed. These drawbacks make this technique less suited for monitoring the initial fast filtration phase, but more practical for monitoring the slow sludge expression phase (Novak et al., 1999). Despite its limited resolution of about 5 mm (Wasserman, 1998), this non-invasive and on-line technique offers potential for measuring the solids content distribution as a function of dewatering time. It is also very practical in use, since the camera position can be set as desired and the existing (electro)dewatering equipment can be used without any modification. The presence of the metallic electrodes does not pose any problem to the measurement neither. Although the technique requires the use of a sophisticated and expensive gamma camera, it is simple to perform and very flexible in use, since it works almost as a standard video camera, but one that registers tracer molecules, which are associated with the sludge solids. However, improvement of gamma camera detector resolution is clearly needed in order to allow its use for dewatering modelling purposes. Therefore, this technique was not considered for any further sludge (electro)dewatering tests up to now.

3.2.

Nuclear magnetic resonance test

Given that the nuclear magnetic resonance signal intensity is correlated to the amount of water, it should be noted that NMR measurements primarily yield moisture distributions, which can be transformed into solidosity profiles.

3.2.1.

Electrode design

The work of LaHeij et al. (1996) had already demonstrated that NMR is a useful tool for monitoring sludge moisture profiles during pressure dewatering of sewage sludge. A major obstacle in applying the NMR technique for measuring moisture patterns during electrodewatering is the necessary presence of electrodes for establishing the electric field. These electrodes and the currents transported through the system

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the cake weights before and after drying at 1051C. This calibration curve is shown in Fig. 4. For very wet samples (moisture content of more than 85%), the calibration was difficult to perform, which was due to the fact that the long delay time of 3 s appeared not sufficient to recover the full signal intensity and a longer delay time would be necessary, which would therefore increase the measurement time. This indicates that the NMR measurement is not so well suited for measuring very wet samples, such as activated sludge in the early filtration phase, but rather for relatively dry samples, such as activated sludge in the expression phase.

may interact with the electromagnetic fields of the NMR measurement and cause signal distortion. Therefore, these should be designed in order to allow a uniform electric field, but with a minimum impact on the measurement mechanism. In preliminary tests with the miniature electrodewatering cell, the electrodes were formed by a sputtered layer of gold on the piston and bottom screw cap. This approach caused two major problems. Firstly, due to Eddy currents occurring in the gold layer, the NMR signal was distorted and no clear difference between wet and dry zones, e.g. near the filter medium, could be seen. This effect could be largely reduced by scratching away some parts of the gold layer, leaving a star-shaped electrode surface. A second issue was the fragility of the electrode surface: after each dewatering test, the electrode from the piston stuck to the sludge cake and a new electrode had to be sputtered. Therefore, it was decided to construct both electrodes as a set of star-shaped platinum wires connected to a central conductor. Moreover, the connection wires to the electric power source were equipped with ferrite cores in order to filter out high-frequency distortion signals, e.g. from external radio transmitters. As a consequence, the NMR signal was less corrupted by highfrequency noise components, and the Fourier transform could be performed more reliably using fewer averages and therefore shorter sampling periods. This methodology allowed the measurement of sludge moisture profiles with high-resolution, as can be seen from the sharp signal transition obtained between a dry and a wet zone could (Fig. 3). The possibility of recording high resolution profiles in relatively short time frames thus enabled to better track the kinetics of the dewatering process.

3.2.2.

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3.2.3.

Solidosity profiles during electrodewatering

Fig. 5 shows an example of the spatio-temporal solidosity profiles that were obtained with this technique for 4 different constant current operations: 0 mA (i.e. mere pressure dewatering), 15, 30 and 45 mA constant currents. The currents were applied after 45 min (2700 s) of pressure dewatering, and the applied pressure was maintained until the end of the dewatering run. In the case of 45 mA constant current (Fig. 5d), the experiment had to be ended prematurely as the power source could not deliver the required current anymore because of the fast increasing resistance due to the quick dewatering of the cake. In all experiments, the sludge was well conditioned, as indicated by measured capillary suction times of less than 10 s, and it was noted that the filtration phase was already ended during recording of the first profile, so that the majority of the dewatering run consisted of expression. The time required to register one solidosity profile varied between 3 min (dry compact cake, by the end of the dewatering run) and more than 10 min (wet thick cake/suspension mixture, at the beginning of the dewatering run). Moreover, given the limitations of the calibration, which only yields reliable solidosity values above 15%, it is concluded that this technique is also less suited for monitoring the initial fast

Calibration

With the given setup, a calibration was performed by measuring the signal profiles of different cakes of which the moisture content was determined afterwards by measuring

Massive Au electrode

Starshaped Pt electrode

0.50

Relative signal intensity ()

0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Position (mm) Fig. 3 – NMR signal at the boundary between a dry zone and a 0.1 M CuSO4 solution (located at position 5 mm) for two electrode configurations. Dashed line: massive gold electrode leading to signal distortion by ring currents; full line: starshaped platinum electrode giving a sharp signal transition.

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1.0

Relative signal intensity ()

0.9

Measured data

Calibration curve

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.50

0.55

0.60

0.65 0.70 0.75 Moisture content (% v/v)

0.80

0.85

0.90

Fig. 4 – Calibration curve linking the relative NMR signal to the moisture content of the sludge cake.

Fig. 5 – Spatio-temporal sludge solidosity profiles during (electro)dewatering, as determined by the NMR technique, for 4 electric currents (0, 15, 30 and 45 mA). The currents were applied after 2700 s of pressure dewatering, as indicated by the thick black lines in the center parts of the subplots. The colorbar represents the solidosity color code used in all subfigures.

filtration phase, but very practical for monitoring the slow expression phase. The resolution of this technique appears very good: at the transition from cake to piston (top side of the profile), it is seen that the resolution of the technique is less than 1 mm, which is much better than the resolution obtained with the radio-active labelling technique. In the mere pressure dewatering run (Fig. 5a), the expression slows down quickly, resulting in a profile with moderate solidosity at the filter medium and lower but rather uniform solidosity above the filter medium. This experimental finding is in line with the occurrence of a so-called dense skin layer

near the filter medium, as described in other dewatering studies with very compressible materials (Bierck and Dick, 1990; Sorensen and Hansen, 1993; Sorensen et al., 1996; Tiller and Green, 1973). Contrary to the experimental observations of LaHeij et al. (1996), no sudden change in cake structure was noticed during the dewatering operation. Based on control experiments on the dewatering equipment as used by LaHeij et al. (1996), it is most probable that this sudden change was caused by air displacement of liquid from the filter cake, since it was noticed that pressurized air leaked at the sides of the piston in their filter press. Moreover, on the new dewatering equipment, the sudden change in cake moisture content was

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also noticed when sludge samples were dewatered by pressurized air without a piston. Therefore, the use of a piston seemed to be the only acceptable way to apply a moving electrode to the sludge system and to simultaneously avoid the formation of an unsaturated cake because of water displacement by pressurized air. It should be noted that although the piston O-rings had been lubricated with silicone grease, some friction remained at the piston/wall interface, such that the piston only moved when a threshold pressure of ca. 0.5 bar (50 kPa) was applied. However, according to Sorensen et al. (1996) the expression of highly compressible materials such as biological sludge is hardly influenced by the applied pressure, once a certain (small) threshold pressure is surpassed. This is due to the fact that it is a thin skin layer at the cake/medium interface and not the resistance of the whole porous structure that controls deliquoring during expression. Given that the skin layer is further compressed by an increase in applied pressure, the decreased permeability of it will prevent an increase in dewatering rate. It is therefore expected that the pressure loss due to friction at the piston had only a minor effect on the dewatering kinetics. In the cases where an electric current is applied from 45 min onwards (Figs. 5b–d,), it is seen that near the end of the pressure dewatering stage the profiles seem to slow down significantly, because of the limitations of the pressure dewatering operation. Once the electric current is applied, at 2700 s, the profiles show a quick decrease again in cake thickness and an overall increase in solidosity. The speed of the decrease in cake thickness and increase in solidosity is clearly linked to the applied current as can be seen from a comparison of the profile evolution for 15, 30 and 45 mA. The measured profiles also clearly demonstrate the electroosmotic transport mechanism, by which water is displaced from the anode zone towards the cathode zone. This is most clearly visible in Fig. 5c, where a region of increasing solidosity at the top of the cake is noticed, and a region of decreasing solidosity near the cathode upon application of the electric field. Obviously the electroosmosis effect not only caused an internal rearrangement of moisture distribution, but liquid is also removed from the cake, due to the application of the pressure field. These experimental results demonstrate the power of NMR measurements for determining cake solidosity profiles from electrodewatering experiments of activated sludge, during the actual dewatering process. The profiles clearly indicate that the sludge cake consists of different zones of solidosity, especially during electrodewatering, and thus yields much more valuable information than a general dry matter value, as can be determined by desiccation of the cake after the (electro)dewatering run has been finished. Moreover, the technique can be applied on-line during the (electro)dewatering process, which has clear advantages compared to off-line techniques, as it is known that a sludge cake undergoes a certain rebound and as such deformation upon release of the mechanical force applied for the dewatering operation. The results obtained may allow a better practical understanding of the electroosmotic dewatering process and can help in the development of new mathematical models for electrodewatering.

4.

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Conclusion

Two non-invasive techniques for on-line measurement of sludge solidosity profiles during electrodewatering were evaluated. Given the measurement time required for measuring one sludge profile, both techniques appeared mainly suited for slowly varying processes, such as sludge expression, and not for fast changing processes, such as sludge filtration. Although the radio-active tracer technique appeared very flexible in use, it has little value so far for (electro)dewatering experiments due to a lack of resolution of the currently available gamma cameras. However, the development of new detectors with increased resolution might render this technique very practical and reliable for future work. Alternatively, nuclear magnetic resonance measurements on a newly constructed electrodewatering unit were made. By designing a special electrode configuration, reliable on-line measurements of the solidosity profiles of sludge subject to electrodewatering could be made, with a resolution of less than 1 mm, showing the mechanisms of electroosmotic- and pressure-driven cake dewatering. The latter technique seems therefore very promising for evaluating new developments in electroosmotic dewatering and for validating new or existing mathematical models for electroosmotic dewatering of highly compressible materials, which can then be used to improve the process efficiency and thus boost the development of the electrodewatering technology.

Acknowledgements Hans Dalderop and the whole technical staff of the TU Eindhoven, Physics Department, are gratefully acknowledged for their helpful assistance in the design and construction of the filter press. R E F E R E N C E S

Barton, W.A., Miller, S.A., Veal, C.J., 1999. The electrodewatering of sewage sludges. Dry. Technol. 17 (3), 497–522. Bierck, B.R., Dick, R.I., 1990. Insitu examination of effects of pressure differential on compressible cake filtration. Water Sci. Technol. 22 (12), 125–134. Cabane, B., Meireles, M., Aimar, P., 2002. Cake collapse in frontal filtration of colloidal aggregates: mechanisms and consequences. Desalination 146 (1–3), 155–161. De Clercq, J., Jacobs, F., Kinnear, D.J., Nopens, I., Dierckx, R.A., Defrancq, J., Vanrolleghem, P.A., 2005. Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer. Water Res. 39 (10), 2125–2135. Dentel, S.K., 2001. Conditioning. In: Spinosa, L., Vesilind, P.A. (Eds.), Sludge into Biosolids. IWA Publishing, London, UK, pp. 278–314. Gielen, I., Van Caelenberg, A., van Bree, H., 2003. Computed tomography (CT) in small animals—Part 1: technical aspects. Vlaams Diergen. Tijds. 72 (3), 158–167. Iwata, M., Igami, H., Murase, T., Yoshida, H., 1991. Analysis of electroosmotic dewatering. J. Chem. Eng. Jpn. 24 (1), 45–50.

ARTICLE IN PRESS 2142

WAT E R R E S E A R C H

40 (2006) 2135– 2142

Jurisson, S., Berning, D., Jia, W., Ma, D.S., 1993. Coordination-compounds in nuclear-medicine. Chem. Rev. 93 (3), 1137–1156. LaHeij, E.J., Kerkhof, P., Kopinga, K., Pel, L., 1996. Determining porosity profiles during filtration and expression of sewage sludge by NMR imaging. AICHE J 42 (4), 953–959. Lee, S.J., Chu, C.P., Tan, R.B.H., Wang, C.H., Lee, D.J., 2003. Consolidation dewatering and centrifugal sedimentation of flocculated activated sludge. Chem. Eng. Sci. 58 (9), 1687–1701. Meeten, G.H., 1993. A dissection method for analyzing filter cakes. Chem. Eng. Sci. 48 (13), 2391–2398. Novak, J.T., Agerbaek, M.L., Sorensen, B.L., Hansen, J.A., 1999. Conditioning, filtering, and expressing waste activated sludge. J. Environ. Eng.-ASCE 125 (9), 816–824. Pignon, F., Alemdar, A., Magnin, A., Narayanan, T., 2003. Smallangle X-ray scattering studies of Fe-montmorillonite deposits during ultrafiltration in a magnetic field. Langmuir 19 (21), 8638–8645. Saveyn, H., Pauwels, G., Timmerman, R., Van der Meeren, P., 2005a. Effect of polyelectrolyte conditioning on the enhanced dewatering of activated sludge by application of an electric field during the expression phase. Water Res 39 (13), 3012–3020.

Saveyn, H., Meersseman, S., Thas, O., Van der Meeren, P., 2005b. Influence of polyelectrolyte characteristics on pressure-driven activated sludge dewatering. Colloid Surf. A—Physicochem. Eng. Asp. 262 (1-3), 40–51. Shirato, M., Murase, T., Kato, H., Fukaya, S., 1970. Fundamental analysis for expression under constant pressure. Filtr. Sep. 7, 277–282. Sorensen, P.B., Hansen, J.A., 1993. Extreme solid compressibility in biological sludge dewatering. Water Sci. Technol. 28 (1), 133–143. Sorensen, P.B., Moldrup, P., Hansen, J.A.A., 1996. Filtration and expression of compressible cakes. Chem. Eng. Sci. 51 (6), 967–979. Tarleton, E.S., 1999. The use of electrode probes in determinations of filter cake formation and batch filter scale-up. Miner. Eng. 12 (10), 1263–1274. Tiller, F.M., Green, T.C., 1973. Role of porosity in filtration ix skin effect with highly compressible materials. AIChE J 19 (6), 1266–1269. Vaxelaire, J., Cezac, P., 2004. Moisture distribution in activated sludges: a review. Water Res 38 (9), 2215–2230. Wasserman, H.J., 1998. Quantification of gamma camera spatial resolution by means of bar phantom images. Nucl. Med. Commun. 19 (11), 1089–1097.