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Electrochimica Acta xxx (2006) xxx–xxx

Electrokinetic remediation of bottom ash frommunicipal solid waste incinerator

Giombattista Traina a,∗, Luciano Morselli b, Giuseppe Persano Adorno a

a Istituto Giordano S.p.A., Rimini, Italyb Department of Industrial Chemistry and Materials, University of Bologna, Italy

Received 27 September 2005; received in revised form 3 April 2006; accepted 23 May 2006

bstract

The electrokinetic remediation was studied to verify the possibility to reclaim the bottom ash from municipal solid waste incineration (MSWI).n Italy, a production of 1 million tons per year of this kind of residue has been estimated, 90% of which is still landfilled. This work shows theesults of four electrokinetic remediation tests for the removal of Pb, Cu, Zn, Cd and chlorides, using an open cell with graphite electrodes andithout enhancing agents. The four tests have, respectively, been performed at a constant current density of 0.89, 1.67, 2.04 and 2.48 mA cm−2,ith duration of 42, 68, 47 and 40 h. Heavy metals occur in ashes in various forms, such as exchangeable, adsorbed, precipitated, organically

omplexed and residual phases. In order to determine the nature of any given system, in terms of specific chemical species and pertaining mobilities,equential extraction analyses have been performed. The release of pollutants was investigated for treated and untreated ash. After treatment, the

oncentration of pollutants in the leachate was reduced by 31–83%, better results being obtained for chlorides. Both the low amount of heavyetal extracted and the increase of ash pH during the electrokinetic tests, suggest to use enhancing agents or a cation exchange membrane at the

athode, to prevent the precipitation of metals as hydroxides.2006 Elsevier Ltd. All rights reserved.

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eywords: Electrokinetic remediation; Bottom ash; Electroreclamation; Munic

. Introduction

Municipal solid waste incineration is one of the most populareans of dealing with non-recyclable solid waste. Although the

olume of waste to be disposed in landfills could be reducedo 90% by incineration, considerable amounts of ash are dis-harged through this process. The solid phase generated fromhe MSWI can be divided into two distinct parts: the bottom ash,hat is the inert and incombustible residue from the combustionrocess, and the fly ash, which derives from the cleaning processf the flue gas. Bottom ash is the most important by-productf the incineration process; therefore, the improvement of ashuality seems to be a very important R&D task for sustainable

Please cite this article in press as: G. Traina et al., Electrochim. Acta (200

aste management. The use of incinerator bottom ash (IBA) foroad construction is widespread in a large number of Europeanountries with consequent environmental benefits in reduced

∗ Corresponding author. Tel.: +39 338 8006175; fax: +39 541 345540.E-mail address: ing traina@yahoo.it (G. Traina).

(vMcpco

013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2006.05.067

olid waste incineration; Heavy metals

rimary aggregate consumption [1]. About 1 million tons perear of IBA are currently produced in Italy, 90% of which istill landfilled. The obstacles of recycling IBA are due to theeaching of some pollutants, such as Cu, Cr, Pb, chlorides andulphates, that often exceeds the limits established by Italianaws (according to the D.M. 5/2/98 and EN 12457-4), and there-ore expensive treatments of the ash are required before bottomsh re-use. The most common treatment, used to improve thenvironmental characteristics of this non-hazardous waste iniew of recycling, is encapsulation (stabilization/solidification).he encapsulation of ash from incineration is typically carriedut by mixing the ash with cement or other pozzolanic materialso form a monolithic material that effectively excludes moisturephysical encapsulation). Sometimes, this process needsarious additives, which can increase the cost of the treatment.oreover, the matrix may not be effective at binding some

6), doi:10.1016/j.electacta.2006.05.067

ations and the leaching of chloride salts can lead to loss ofhysical strength and durability. In addition, the final productan fail the leaching test. This work is aimed at the evaluationf electrokinetic remediation (EKR) of the bottom ash from

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ARTICLEA-12017; No. of Pages 6

G. Traina et al. / Electroch

SWI, for the removal of both heavy metals and salts (chlo-ides). The paper presents four lab tests carried out with a basicystem, using two graphite electrodes in direct contact withhe ash and without using enhancing agents or ion exchange

embranes.

.1. History and principles of electrokinetic remediation

Electrokinetic soil processing is also referred to as elec-rokinetic remediation, electroreclamation and electrochemicalecontamination. The process is substantially based on a low-evel direct current of the order of some mA cm−2, which crosseshe area comprised between electrodes to remove contaminants.he low-level direct current results in physicochemical andydrological changes in the soil mass, and leads to speciesransport by coupled mechanisms. Electrolysis of water pro-uces hydrogen ions in the anodic compartment, which causesn acidic front to migrate through the soil/ash cell. This, inurn, causes desorption of contaminants from the surface ofoil/ash particles, and results in their electromigration (the trans-ort of ions under the influence of the electric field). If theoil pore is idealized as a capillary, the mobile cations formconcentric shell within the capillary. Under an applied elec-

rical potential, this space charge, generally of cationic nature,oves toward the cathode, dragging the pore fluid and resulting

n electro-osmosis (the hydraulic flow induced by an electriceld) [2]. Electromigration and electro-osmosis are the tworincipal mechanisms involved in the electrokinetic remedia-ion technique. This technology has recently made significantdvances and has been tested for commercial application inhe United States and The Netherlands [3]. Geokinetics (Theetherlands) and Electrokinetics Inc. (Baton Rouge, LA) have

ompleted several large-scale pilot studies for the removal ofeavy metals from clay soils. In addition, new applicationsave been investigated in the last few years, and the electro-ialytic remediation (EDR) method, developed at the Tech-ical University of Denmark [4], surely deserves to be men-ioned. This approach has already been applied with successo the treatment of wood waste, biomass ash, contaminatedarbour sediments and fly ash from MSWI [5–9]. However,he applied technique is quite different from the typical elec-rokinetic remediation: in the EDR, electrodes are placed ineparate compartments where electrolyte solutions are circu-ated and where the heavy metals are concentrated at the endf the remediation. The electrolytes solutions also ensure thatgood contact between the electrode surface and the sur-

oundings is maintained, and that gases, due to electrode reac-ions, are transported away from the electrodes. Ion exchange

embranes separate the soil/ash from the electrolyte solu-ions. An anion exchange membrane (AEM) that allows onlyhe passage of anions is placed between the soil/ash and thelectrolyte solution at the anode compartment, while a cationxchange membrane (CEM) is placed between the soil/ash

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nd the electrolyte solution at the cathode side. The CEM issed, specially in soil remediation, to prevent OH− ions, pro-uced by the electrolysis of water at the cathode, enrich theoil.

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PRESSActa xxx (2006) xxx–xxx

Since the technique has not been considered for the removalf heavy metals from MSWI bottom ash, yet, the present paperims to report preliminary results on this possible application.

. Materials and methods

.1. Bottom ash: characteristics and sample preparation

The IBA studied in this work derives from a grate furnacencinerator in the north of Italy. This incinerator of municipalolid waste has a nominal capacity of 18 tons/h and includesheat recovery system to produce electricity. For each ton of

ncinerated waste, 300 kg of bottom ash and 30 kg of fly ash areroduced. According to the Italian regulations, the former is aon-hazardous waste whereas the latter is hazardous, and it issually landfilled with or without inertization. After the com-ustion, the bottom ash is usually quenched in water and thenonveyed to the recycling process. However, not many plantsave a complete recycling treatment and only the ferrous andoarse fraction separation is generally carried out. The coarseraction, specially the incomplete incineration materials, returnack to the grate furnace, whereas the residual 80% of bot-om ash, with a typical particle size of less than 35–45 mm, istill landfilled, because of its high content of heavy metals andhlorides. This ash fraction is suitable of reuse as a lightweightggregate, particularly in road base construction, but improve-ents in both environmental and mechanical characteristics are

equired. For carrying out this research, samples of ash were col-ected weekly during October and November 2004, from a singlencinerator plant. Only the raw material was treated, to avoid theging that usually causes changes in pH and other physicochem-cal characteristics. Hence, the ash was sieved at 20 mm, and theesidual ferrous fraction was extracted by a hand magnet. Afterhat the ash was inserted into the cell and compacted to obtainhe desired density. No water was added before or during therocess, the water content of the ash depending thus only on theuenching process. The ash was analysed using methods estab-ished by Italian standards and regulations in force. The averagehysicochemical properties of considered samples of ash areollected in Table 1.

The electrokinetic remediation technique is not able toemove the heavy metals firmly bound to the soil/ash matrix.eavy metals occur in ashes in various forms, such as exchange-

ble, adsorbed, precipitated, organically complexed and resid-al phases. These determine their environmental mobility andioavailability, and finally their potentiality to contaminate thenvironment [10]. Heavy metals existing as loosely bound frac-ions, such as exchangeable or adsorbed forms on the clay/sandurface, or associated with organic matter and oxides with weakonding strength, tend to be easily moved and dispersed. Inrder to determine the nature of any given system, in termsf the chemical species present and their relative mobilities,equential extraction analyses have been performed [11]. This

6), doi:10.1016/j.electacta.2006.05.067

pproach consists of several steps, which allow the determina-ion of the form in which the polluting metals are present, andesults obtained are helpful for the assessment of the risk ofong-term contamination.

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Table 1Physicochemical characteristics of bottom ash from MSWI

Value Value

SiO2 (%) 36.81 ± 0.7 CEC (meq 100 g−1) 6.7CaO (%) 34.96 ± 0.71 Electrical conductivity (mS cm−1) 8.29Fe2O3 (%) 5.39 ± 0.34 pH 12.78Al2O3 (%) 4.51 ± 0.28 Hydraulic permeability (cm s−1) 2.5 × 10−5

Zn (mg kg−1) 2790 ± 63 LOI 550◦ (%) 2.40Pb (mg kg−1) 2820 ± 95 LOI 970◦ (%) 5.01Cu (mg kg−1) 2810 ± 12 Gravel (%) 54.41Cr (mg kg−1) 350 ± 51 Sand (%) 42.49Cd (mg kg−1) 38 ± 5Chlorides (mg kg−1) 13811 ± 92Sulfates (mg kg−1) 9887 ± 21

Table 2Chemicals used in the sequential extraction

Exchangeable 1 M MgCl2 (pH 7)Weak acid soluble fraction 1 M CH3COONa + CH3COOH (pH 5)Reducible fraction 0.04 M NH2OH·HCl in 25% (v/v)

CH3COOH at 100 ◦COxidizable fraction 0.02 M HNO3 + H2O2 (30%) + 3.2 M

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The five extraction steps are described in Table 2. Leaching ofetals from the ash was determined using the EN 12457-4 test,hich provides information on leaching of granular waste underxed conditions (deionized water as leachant, 10 l/kg as liquid

o solid ratio and agitating for 24 h). The resulting eluate, afterltration, was analysed for metal and chloride content, by atomicbsorption spectrophotometry (AAS) and ion chromatography,espectively.

.2. Electrokinetic remediation experiments

The experimental rectangular cells (6 cm × 14 cm × 16 cm)sed in this study were made of glass; graphite electrodes weresed with dimensions of 5.8 cm × 9 cm × 0.5 cm. Two sheetsf filter paper were placed at the interfaces between the ash

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ed and the electrodes (Fig. 1); no enhancing agents were useduring the experiments, and the electrolyte was constituted bynly the ash water, arising from the quenching process. A dc

Fig. 1. Schematic diagram of experimental apparatus.

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ower supply (60 V and 3 A) and a PC, to control the currentensity and to measure the energy consumption, completed thelectrokinetic apparatus (Fig. 1). Both anode and cathode com-artments were open at the top to vent the electrolysis gases.our experiments were carried out, using four different currentensities but maintaining the same distance between electrodes;xperimental details are collected in Table 3. After each reme-iation test, the ash bed was divided into five slices, whichave been oven-dried at 105 ◦C for 24 h to evaluate the moistureontent.

.3. Analytical method

Heavy metal concentrations were measured by carefully mix-ng 50 mg of ash with 300 mg of anhydrous lithium metaboratento a clean dry platinum crucible, fusing the mixture for 30 mint 800–900 ◦C, hence suddenly cooling the mixture and thenissolving it in a beaker with 25 ml of hydrochloric acid (25%,/w). The obtained solution was analysed by AAS. Ash pHas measured by mixing 10 g of dry ash and 25 ml of dis-

illed water. After 1 h of contact time, the solution pH waseasured using a radiometer pH electrode. Chloride and sul-

ate concentrations were measured by mixing 10 g of dry ashnd 250 ml of distilled water, and heating at 120 ◦C for 2 h; the

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iquid phase was vacuum-filtered with a 0.45 �m filter, to sep-rate the solid particles, diluted to 250 ml and analysed by ionhromatography.

able 3xperimental details for the four remediation tests

Test

I II III IV

uration (h) 42 68 47 40urrent density (mA cm−2) 0.90 1.67 2.04 2.48

nitial voltage (V cm−1) 0.27 0.46 0.56 0.63inal voltage (V cm−1) 0.43 1.06 1.25 2.12rea of ash bed (cm2) 44 30 29 28olume of ash bed (cm3) 464 306 299 282ength of ash bed (cm) 10.5 10.2 10.2 10ensity (g cm−1) 1.88 1.82 1.76 1.7

nitial water content (%) 23 26 27.5 28

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. Results and discussion

.1. Heavy metals speciation

The composition of used IBA, as obtained by particle sizenalysis, was as follows: 54.41% gravel, 42.49% sand and 3%ilt/clay. The initial concentrations of metal contaminants inottom ash are also collected in Table 1, whereas their chem-cal forms, determined by the sequential extraction method,re shown in Fig. 2. The sequential extraction procedure gaveow concentrations of exchangeable metals (or carbonate-bound

etals), Cd and Pb being the only exceptions. Cu is predomi-antly bound to organic matter, while Zn is adsorbed onto then and Fe oxides; Cu leaching is predominantly due to the for-ation of highly soluble organo-copper complexes. Typically,

ottom ash has a TOC of 1–4%. Although the exact speciationf organic material and complexes has not been identified inottom ash from MSWI, still the high organic carbon content isorrelated with high Cu leaching [12].

.2. Electrokinetic remediation results

The mass balance for heavy metals, performed after eachxperiment and based on initial and final amounts, gave no real-stic removal values (i.e., −2 to 30% for Zn, −50 to 15% forr, 13 to 49% for Cu), since it was influenced by the high non-omogeneity of the ash; in contrast, the removal efficiency forhlorides (60%) seems to be a correct estimation (Fig. 5a). As aatter of fact, the behavior of pollutant concentrations (Fig. 3)

n the ash bed after remediation could be a more representativeesult, as well as the comparison of pollutants release before andfter the remediation treatment. In order to evaluate how the elec-roreclamation influences the leaching of pollutants, the releasef contaminants from the ash treated in the second test was com-ared to the release of an untreated specimen, according to theN 12457-4 test method. Results, collected in Table 4, show

Please cite this article in press as: G. Traina et al., Electrochim. Acta (2006), doi:10.1016/j.electacta.2006.05.067

ignificant improvements for the environmental characteristicsf the ash, as due to the electroreclamation process, speciallyor copper and chlorides, for which the leached concentrationsere found to be approximately four (Cu) and six times lower

Fig. 3. Heavy metal concentration in the five slices, after each remediation test(1–5, from anode to cathode). The increase in Pb, Zn and Cu concentrations maybe related to the presence of a non-ferrous screw, hidden in the ash (in slice 3),during test II.

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Table 4Leaching results of pollutants for an untreated sample and for a sample treatedby electroreclamation (according to EN 12457-4)

Pollutants UBAa TBAb % Reduction

Pb (ppm) 0.16 0.11 31.25Zn (ppm) 0.689 0.324 52.98Cu (ppm) 1.204 0.277 76.99Cr (ppm) 0.19 0.095 50.00Cd (ppm) 0.181 0.101 44.20C −

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Cl−), from the treated specimen. A low Pb-extraction was rec-gnized in all four experiments, probably due to its precipitations hydroxide.

In the second experiment, which was carried out using a cur-ent density of 1.667 mA cm−2, a coarse non-ferrous screw, notxtracted by the hand magnet, was found in the middle of the ashell, after the electrokinetic treatment. In this part of the ash (sliceII), a strange intensification of the Pb, Cu and Zn concentrationsas found, as shown in Fig. 3, which might be due to the pres-

nce of the non-ferrous object. Screws or other extraneous metalomponents were found only once, during the whole experimen-al investigation, so no conclusions can be drawn about the rolef the screw (brass) or how cation migration was influenced by itn run II. However, metal components might act like a false earth,nd extra precipitation of heavy metals around the screw could

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ave occurred in test II. When an electric field passes throughconductor, electrode reactions can take place: the object can

e reduced or oxidised [9,13], and the heavy metal ions can belectrodeposited on it. In Fig. 4, electric field lines for situations

ig. 4. Electric field lines in the case of: (a) a soil without coarse objects oronductors and (b) with a conductor (e.g., a screw) [13].

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ithout and with coarse objects or conductors buried inside thesh bed are represented [9].

At the end of each test, and particularly at the end of theecond one, blue salts (probably copper hydroxyl carbonate orther copper minerals) were found on both electrode surfaces.or that reason, electrodes were scratched and analysed to eval-ate which metal deposited on anode and cathode, respectively.his analysis showed Cu-migration toward both electrodes, butredominantly toward the anode; the same pattern was verifiedor Zn, while Cd and Pb moved toward the cathode. However,uring all experiments, only small amounts of Pb were depositedn the electrodes. Conversely, high chloride mobility was found,ven if chlorides, moving toward the anode (and thus againsthe electro-osmotic flow), need at least 60 h to get out of all ashed. In the third experiment, carried out for 68 h, 60% of chlo-ides were removed, and increasing the current density, chloridesigration has further been enhanced (Fig. 5). As above antici-

ated, the electro-osmotic flow moved toward the cathode andts rate increased by increasing the current density. The patternf moisture content in the ash (Fig. 5), after tests I and IV, clearlyhows this experimental evidence.

.2.1. Variation of pH and ohmic drop in the ash beduring the electrokinetic removal experiments

After the electrokinetic removal experiments, the pH of thesh was measured for all slices (Fig. 6). The electrolysis of watern the anode and cathode compartments generated H+ and OH−

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ons, which moved toward the electrode of opposite charge.hile, in soil remediation, the hydrogen action usually leads

o a decrease of soil pH, in these experiments the overall pH ofhe ash increased. An explanation can be found in the high pH

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6 G. Traina et al. / Electrochimica

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nd high buffering capability of the ash, which decreases theissolution and desorption rates of adsorbed and/or complexedpecies [14]. Starting from an initial value of 12.78, an increasef pH was found also in the first slice, where the role of protonshould have been more evident.

Fig. 6 also shows the overall cell voltage trends; the initiallyinear pattern changed to an exponential development, possiblyelated to cation precipitation as hydroxides. This in fact resultedn an increasing resistance and voltage drop within the ash bed.he change in pattern happens sooner by increasing the currentensity. This detail reveals that if no enhancing agents are used,he process is rapidly arrested by heavy metal precipitation (inorm of hydroxides, when cations meet the OH− front that movesrom the cathode), a phenomenon that takes place very quicklyhile using current densities as high as 2.5–3 mA cm−2.

. Conclusions

In this study, four electrokinetic remediation tests were car-

Please cite this article in press as: G. Traina et al., Electrochim. Acta (200

ied out, without using any enhancing agent, on bottom ash fromSWI. After test II, a metallic, non-ferrous screw was found in

he middle of the ash bed, and an abrupt increase of Pb, Cund Zn concentrations was found in the ash surrounding the

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PRESSActa xxx (2006) xxx–xxx

etallic object. Experiments showed an increase of ash pH, alsoearby the anode, which depended on applied current density.t 2.5–3 mA cm−2, the precipitation of heavy metal as hydrox-

des, especially nearby the cathode, rapidly occurred, leadingo an increase of bed resistivity and thus in a cell voltage. Pre-ipitation of metals nearby the cathode seems to be the mostignificant limit of the electrokinetic remediation of bottom ash,hich suffers from the high pH and buffer capacity of that mate-

ial. Further investigations are planned to verify the efficiencyf electroreclamation of bottom ash from MSWI, acidifying thesh or using enhancing agents and a cation exchange membraneo prevent OH− to enrich the ash.

cknowledgement

This research was jointly supported by Consortium Spinner,milia Romagna Region, Ministry of Labour and Social Policiesnd European Social Fund.

eferences

[1] J. Abbott, P. Coleman, L. Howlett, P. Wheeler, Environmental and HealthRisks Associated with the Use of Processed Incinerator Bottom Ash inRoad Construction, 2003, report on: www.breweb.org.uk.

[2] Y.B. Acar, A.N. Alshawabkeh, Environ. Sci. Technol. 46 (1993) 2638.[3] R. Lageman, R.L. Clarke, W. Pool, Eng. Geol. 77 (2005) 191.[4] L.M. Ottosen, H.K. Hansen, Electrokinetic Cleaning of Heavy Metal Pol-

luted Soil, Internal Report, Fysisk-Kemisk Institut & Institut for Geologiog Geoteknik, Technical University of Denmark, Denmark, 1992.

[5] A.B. Ribeiro, E.P. Mateus, L.M. Ottosen, G. Bech-Nielsen, Environ. Sci.Technol. 34 (2000) 784.

[6] A.J. Pedersen, Biomass Bioenergy 25 (2003) 447.[7] A.J. Pedersen, J. Hazard. Mater. B95 (2002) 185.[8] A.J. Pedersen, L.M. Ottosen, A. Villumsen, J. Hazard. Mater. B100 (2003)

65.[9] L.M. Ottosen, H.K. Hansen, A.B. Ribeiro, Influence of a stone or a screw in

the soil in electrodialytic remediation, in: Heavy Metals in the Environmentand Electromigration Applied to Soil Remediation (Proceedings) SecondSymposium, Technical University of Denmark, 1999, p. 107.

10] M. McBride, Environmental Chemistry of Soils, Oxford University Press,Oxford, 1994, p. 308.

11] A. Tessier, P.G.C. Campbell, M. Bisson, Anal. Chem. 51 (1979) 844.12] T. Van Gerven, K. Imbrechts, E. Van Keer, M. Jaspers, G. Wauters, C.

Vandecasteele, Investigation of washing, heating and carbonation as treat-ment techniques for the improvement of environmental characteristics ofMSWI-bottom ash, in view of recycling, Waste Manage. Environ. II (2004)3.

13] G.M. Nystrøm, Investigations of Soil Solution during Enhanced Electrodi-alytic Soil Remediation, Internal report, Department of Physical Chemistry,

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Technical University of Denmark, 2001.14] A.J. Pedersen, L.M. Ottosen, A. Villumsen, Electrodialytic removal of

heavy metals from MSWI fly ashes, in: K., Czurda, R., Haus, C., Kappeler,R., Zorn, (Eds.), Proc. EREM 2001, Third Symposium on ElectrokineticRemediation, 18–20 April 2001. Karlsruhe.(Germany).