Bioprocesses for the Removal of Volatile Sulfur Compounds from Gas Streams
Janssen, A.J.H. ; bosch, P.L.M. van den; Leerdam, R.C. van; Graaff, C.M. de - \ 2013
In: Air Pollution Prevention and Control: Bioreactors and Bioenergy / Kennes, C., Veiga, M.C., John Wiley and Sons - p. 249 - 266.
This chapter describes the biological removal of sulphur compounds from gas streams. First, an overview is given of the toxicity of sulphur compounds to animals and humans whereafter biological and industrial formation routes for (organic) sulphur compounds are given. Microbial degradation routes of volatile organic sulphur compounds under both aerobic and anaerobic conditions are presented. Finally, the most commonly applied processes for sulphur removal from gaseous streams are discussed and an overview is given of operating experiences for biological gas treatment systems. The chapter concludes with some remarks on future developments.
Reactions between Methanethiol and Biologically Produced Sulfur
Leerdam, R.C. van; Bosch, P.L.F. van den; Lens, P. ; Janssen, A.J.H. - \ 2011
Environmental Science and Technology 45 (2011)4. - ISSN 0013-936X - p. 1320 - 1326.
dissolved sodium sulfide - equilibrium distribution - inorganic polysulfides - desulfurization - mechanism - ions
Recently, new biotechnological processes have been developed to enable the sustainable removal of organic and inorganic sulfur compounds from liquid and gaseous hydrocarbon streams. In comparison to existing technologies (e.g., caustic scrubbing or iron based redox technologies) far less chemicals are consumed, while reusable elemental sulfur is formed as the main end-product. This research shows that in these processes a number of consecutive reactions occur between methanethiol (MT) from the hydrocarbon stream and the formed biosulfur particles, leading to the formation of (dimethyl) polysulfides. This is an important feature of this family of new bioprocesses as it improves the MT removal efficiency. The reaction kinetics depend on the MT and biosulfur concentration, temperature, and the nature of the biosulfur particles. The first reaction step involves a S(8) ring-opening by nucleophilic attack of MT molecules to form CH(3)S(9)(-). This work shows that CH(3)S(9)(-) reacts to polysulfides (S(3)(2-), S(4)(2-), S(5)(2-)), dimethyl polysulfides [(CH(3))(2)S(2), (CH(3))(2)S(3)], and dissociated H(2)S, while also some longer-chain dimethyl polysulfides [(CH(3))(2)S(4)-(7)] are formed at µM levels. Control experiments using orthorhombic sulfur flower (S(8)) did not reveal these reactions.
DROPLET to calculate concentrations at drinking water abstraction points : user manual for evaluation of agricultural use of plant protection products for drinking water production from surface waters in the Netherlands
Leerdam, R.C. van; Adriaanse, P.I. ; Horst, M.M.S. ter; Roller, J.A. te - \ 2010
Wageningen [etc.] : Alterra [etc.] (Alterra-rapport 2020) - 156
watervoorziening - drinkwater - waterkwaliteit - oppervlaktewater - pesticiden - handleidingen - oppervlaktewaterkwaliteit - waterwinning - water supply - drinking water - water quality - surface water - pesticides - guide books - surface water quality - water catchment
The user-friendly shell DROPLET, acronym for DRinkwater uit OPpervlaktewater- Landbouwkundig gebruik Evaluatie Tool, assists the Dutch Board for the Authorisation of Plant Protection Products and Biocides (Ctgb) in evaluating whether pesticides may exceed the 0.1 μg/L standard in one of the Dutch surface water abstraction points for drinking water production. It operationalises the methodology developed by a Dutch expert group described in Adriaanse et al (2008). This manual explains how to use (i) SWASH to enter compound properties and application pattern, (ii) to run MACRO to calculate the drainage fluxes, (iii) to enter the deposition according to the Dutch Drift Table in TOXSWA, next (iv) to run TOXSWA to obtain an edge-of-field concentration in the FOCUS D3 ditch and finally (v) to run DROPLET to obtain the concentrations in the nine Dutch abstraction points plus the Bommelerwaard. DROPLET maintains a central database (in addition to the SWASH database) and combines the peak concentration of the FOCUS D3 ditch with intake area and compound specific factors, such as crop areas and compound degradation to calculate concentrations in the abstraction points.
Inhibition of microbiological sulfide oxidation at natronophilic conditions by methanethiol and methylated polysulfides
Bosch, P.L.F. van den; Graaff, C.M. de; Fortuny-Picornell, M. ; Leerdam, R.C. van; Janssen, A.J.H. - \ 2009
Applied Microbiology and Biotechnology 83 (2009)3. - ISSN 0175-7598 - p. 579 - 587.
sulfur-oxidizing bacteria - hydrogen-sulfide - biological oxidation - aqueous-solution - soda lakes - removal - degradation - disulfide - kinetics - sludge
To avoid problems related to the discharge of sulfidic spent caustics, a biotechnological process is developed for the treatment of gases containing both hydrogen sulfide and methanethiol. The process operates at natron-alkaline conditions (>1 mol L-1 of sodium- and potassium carbonates and a pH of 8.5–10) to enable the treatment of gases with a high partial CO2 pressure. In the process, methanethiol reacts with biologically produced sulfur particles to form a complex mixture predominantly consisting of inorganic polysulfides, dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS). The effect of these organic sulfur compounds on the biological oxidation of sulfide to elemental sulfur was studied with natron-alkaliphilic bacteria belonging to the genus Thioalkalivibrio. Biological oxidation rates were reduced by 50% at 0.05 mM methanethiol, while for DMDS and DMTS, this was estimated to occur at 1.5 and 1.0 mM, respectively. The inhibiting effect of methanethiol on biological sulfide oxidation diminished due to its reaction with biologically produced sulfur particles. This reaction increases the feasibility of biotechnological treatment of gases containing both hydrogen sulfide and methanethiol at natron-alkaline conditions
Exposure in surface waters for the aquatic risk assessment at EU level
Adriaanse, P.I. ; Leerdam, R.C. van; Boesten, J.J.T.I. - \ 2009
pesticiden - afvloeiingswater - oppervlaktewater - waterverontreiniging - uitspoelen - toxische stoffen - risicoschatting - milieu - scenario-analyse - pesticides - runoff water - surface water - water pollution - leaching - toxic substances - risk assessment - environment - scenario analysis
Poster presentation. In the FOCUS Surface Water Scenarios, an important entry route for plant protection products (PPP) into the water course is run-off from the adjacent field. Not much is known on the effect of run-off on the concentration in the FOCUS stream
Anaerobic methanethiol degradation and methanogenic community analysis in an alkaline (pH 10) biological process for liquefied petroleum gas desulfurization
Leerdam, R.C. van; Bonilla-Salinas, M. ; Bok, F.A.M. de; Bruning, H. ; Lens, P.N.L. ; Stams, A.J.M. ; Janssen, A.J.H. - \ 2008
Biotechnology and Bioengineering 101 (2008)4. - ISSN 0006-3592 - p. 691 - 701.
organic sulfur-compounds - sludge-blanket reactor - methylotrophic methanogen - oxidizing bacteria - sulfide oxidation - soda lakes - sp-nov - sediments - dimethylsulfide - bioreactor
Anaerobic methanethiol (MT) degradation by mesophilic (30 degrees C) alkaliphilic (pH 10) communities was studied in a lab-scale Upflow Anaerobic Sludge Bed (UASB) reactor inoculated with a mixture of sediments from the Wadden Sea (The Netherlands), Soap Lake (Central Washington), and Russian soda lakes. MT degradation started after 32 days of incubation. During the first 252 days, complete degradation was achieved till a volumetric loading rate of 7.5 mmol MT/L/day, and sulfide, methane, and carbon dioxide were the main reaction products. Temporary inhibition of MT degradation occurred after MT peak loads and in the presence of dimethyl disulfide (DMDS), which is the autooxidation product of MT. From day 252 onwards, methanol was dosed to the reactor as co-substrate at a loading rate of 3-6 mmol/L/day to stimulate growth of methylotrophic methanogens. Methanol was completely degraded and also a complete MT degradation was achieved till a volumetric loading rate of 13 mmol MT/L/day (0.77 mmol MT/gVSS/day). However, from day 354 till the end of the experimental run (day 365), acetate was formed and MT was not completely degraded anymore, indicating that methanol-degrading homoacetogenic bacteria had partially outcompeted the methanogenic MT-degrading archea. The archeal community in the reactor sludge was analyzed by DGGE and sequencing of 16S rRNA genes. The methanogenic archea responsible for the degradation of MT in the reactor were related to Methanolobus oregonensis. A pure culture, named strain SODA, was obtained by serial dilutions in medium containing both trimethyl amine and dimethyl sulfide (DMS). Strain SODA degraded MT, DMS, trimethyl amine, and methanol. Flow sheet simulations revealed that for sufficient MT removal from liquefied petroleum gas, the extraction and biological degradation process should be operated above pH 9.
Methanethiol degradation in anaerobic bioreactors at elevated pH (>8): Reactor performance and microbial community analysis
Leerdam, R.C. van; Bok, F.A.M. de; Bonilla-Salinas, M. ; Doesburg, W. van; Lomans, B.P. ; Lens, P.N.L. ; Stams, A.J.M. ; Janssen, A.J.H. - \ 2008
Bioresource Technology 99 (2008)18. - ISSN 0960-8524 - p. 8967 - 8973.
organic sulfur-compounds - sludge-blanket reactor - methylotrophic methanogen - sp-nov - methanosarcina-mazei - dimethyl sulfide - estuarine methanogen - hydrogen transfer - sediments - bacteria
The degradation of methanethiol (MT) at 30 °C under saline¿alkaline (pH 8¿10, 0.5 M Na+) conditions was studied in a lab-scale Upflow Anaerobic Sludge Blanket (UASB) reactor inoculated with estuarine sediment from the Wadden Sea (The Netherlands). At a sodium concentration of 0.5 M and a pH between 8 and 9 complete MT degradation to sulfide, methane and carbon dioxide was possible at a maximum loading rate of 22 mmol MT L¿1 day¿1 and a hydraulic retention time of 6 h. The presence of yeast extract (100 mg/L) in the medium was essential for complete MT degradation. 16S rRNA based DGGE and sequence analysis revealed that species related to the genera Methanolobus and Methanosarcina dominated the archaeal community in the reactor sludge. Their relative abundance fluctuated in time, possibly as a result of the changing operational conditions in the reactor. The most dominant MT-degrading archaeon was enriched from the reactor and obtained in pure culture. This strain WR1, which was most closely related to Methanolobus taylorii, degraded MT, dimethyl sulfide (DMS), methanol and trimethylamine. Its optimal growth conditions were 0.2 M NaCl, 30 °C and pH 8.4. In batch and reactor experiments operated at pH 10, MT was not degraded
|Development of a family of large-scale biotechnological processes to desulphurise industrial gasses
Janssen, A.J.H. ; Leerdam, R.C. van; Bosch, P.L.F. van den; Zessen, E. van; Heeringen, G.J. van; Buisman, C.J.N. - \ 2007
In: Proceedings of the II International Congress in Biotechniques for Air Pollution Control, 3 - 10 October, 2007, La Coruna, Spain. - La Coruna, Spain : Universidade da Coruna - p. 167 - 183.
Anaerobic methanethiol degradation in upflow anaerobic sludge bed reactors at high salinity (> 0.5 M Na+)
Leerdam, R.C. van; Bok, F.A.M. de; Lens, P.N.L. ; Stams, A.J.M. ; Janssen, A.J.H. - \ 2007
Biotechnology and Bioengineering 98 (2007)1. - ISSN 0006-3592 - p. 91 - 100.
mill waste-water - sulfur-compounds - methylotrophic methanogen - estuarine methanogen - blanket reactor - sp-nov - sediments - sulfide - sulfate - adaptation
The feasibility of anaerobic methanethiol (MT) degradation at elevated sodium concentrations was investigated in a mesophilic (30°C) lab-scale upflow anaerobic sludge bed (UASB) reactor, inoculated with estuarine sediment originating from the Wadden Sea (The Netherlands). MT was almost completely degraded (>95%) to sulfide, methane and carbon dioxide at volumetric loading rates up to 37 mmol MT·L-1·day-1, 0.5 M sodium (NaCl or NaHCO3) and between pH 7.3 and 8.4. Batch experiments revealed that inhibition of MT degradation started at sodium (both NaCl and NaHCO3) concentrations exceeding 0.8 M. Sulfide inhibited MT degradation already around 3 mM (pH 8.3).
Anaerobic degradation of methanethiol in a process for Liquefied Petroleum Gas (LPG) biodesulfurization
Leerdam, R.C. van - \ 2007
Wageningen University. Promotor(en): Albert Janssen; Fons Stams, co-promotor(en): Piet Lens. - [S.l.] : S.n. - ISBN 9789085047872 - 198
ontzwaveling - vloeibaar aardoliegas - anaërobe afbraak - desulfurization - liquid petroleum gas - anaerobic digestion
Due to increasingly stringent environmental legislation car fuels have to be desulfurized to levels below 10 ppm in order to minimize negative effects on the environment as sulfur-containing emissions contribute to acid deposition (‘acid rain’) and to reduce the amount of particulates formed during the burning of the fuel. Moreover, low sulfur specifications are also needed to lengthen the lifetime of car exhaust catalysts. The research presented in this thesis focuses on the biological desulfurization of Liquefied Petrol Gas (LPG). Currently, LPG is mainly desulfurized by physical-chemical methods that absorb volatile sulfur compounds present (mainly hydrogen sulfide and thiols) into a strong caustic solution, whereafter the thiols are partially oxidized to disulfides whilst the dissolved hydrogen sulfide is discharged as a ‘spent sulfidic caustic’. Disadvantages of this physical-chemical method are the relatively high energy and caustic consumption and the production of a hazardous waste stream. As an alternative, a new three-step biotechnological LPG desulfurization technology has been studied, that produces elemental sulfur as an end-product from the bio-conversion of hydrogen sulfide (H2S) and methanethiol (MT). The new process involves: (i) extraction of the sulfur compounds from the LPG phase into a (bi)carbonate-containing solution; (ii) anaerobic degradation of MT to H2S, CO2 and CH4 and (iii) partial oxidation of H2S to elemental sulfur. The formed sulfur particles are removed from the system whilst the sulfur-free alkaline process water is re-used in the extraction process. The sulfur can be used for the production of sulfuric acid and hydrogen sulfide or for agricultural applications. In this research attention is paid to the feasibility of the second process step, i.e. the anaerobic treatment step as the first and third process step are already well described. Anaerobic degradation of MT appeared to be possible with a variety of anaerobic (reactor) sludges and sediments, both under methanogenic and sulfate-reducing conditions. The related compounds dimethyl disulfide and dimethyl sulfide were degraded as well, in contrast to ethanethiol and propanethiol, which were not degraded anaerobically. In the new LPG biodesulfurization process higher thiols are converted to their corresponding oily disulfides that have to be skimmed off from the reactor solution and can be sent for disposal, e.g. to an incinerator. The fifty percent inhibition concentration of MT, ethanethiol and propanethiol for methanogenic activity of anaerobic granular sludge on methanol and acetate was found between 6 and 10 mM (pH 7.2, 30°C). Hydrogen sulfide inhibited anaerobic MT degradation at concentrations below 10 mM, depending on the pH and the source of the inoculum. Dimethyl disulfide inhibited MT degradation already at concentrations below 2 mM. In a lab-scale upflow anaerobic sludge blanket (UASB) reactor that was inoculated with anaerobic granular sludge originating from a full-scale reactor treating paper mill wastewater, MT degradation was possible up to a volumetric loading rate of 17 mmol MT∙L-1∙day-1 (pH 7.0-7.5, 30°C, < 0.03 M total salts). MT degradation with this inoculum was inhibited by sodium concentrations exceeding 0.2 M. Initially, MT-degrading methanogenic archaea related to the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by methanogens belonging to the genus Methanomethylovorans, which were mainly present in small aggregates (10-100 μm) in between larger particles. Estuarine sediment from the Wadden Sea was used to inoculate an anaerobic reactor operated at Na+ concentrations of 0.5 M. The maximum volumetric degradation rate achieved amounted to 37 mmol MT∙L-1∙day-1 at pH 8.2-8.4 and 22 mmol MT∙L-1∙day-1 at pH 8.9-9.1 (30°C). MT degradation at pH 10 was not possible with this inoculum. In activity tests, no inhibition of MT degradation was observed till 0.8 M Na+. Initially, Methanosarcina mazei was the dominant MT-degrading methanogen, but after about 1.5 years of continuous reactor operation, methanogens related to Methanolobus taylorii became dominant, probably due to the pH shift to pH 9.0 in the reactor. In a UASB reactor inoculated with a mixture of estuarine and salt lake sediments from the Soap Lake (USA) and the Kalunda Steppe (Russia) it was possible to degrade MT at pH 10, at a maximum volumetric loading rate of 13 mmol MT∙L-1∙day-1 (30°C, 0.8 M Na+) in the presence of methanol as a co-substrate. The methanogenic archaea responsible for the degradation of MT were related to Methanolobus oregonensis. Thiols that are not degraded in the anaerobic reactor of the novel LPG desulfurization process are directed to the third process step, i.e. the aerobic bioreactor. Our research shows that here MT will react with biologically produced sulfur (both 1-16 mM; pH 8.7 and 10.3; 30-60ºC) to form poly-sulfur compounds, i.e. polysulfide ions and dimethyl polysulfides. The first reaction step is a S8 ring opening by nucleophilic attack to form CH3S9-. The reaction rate depends on the MT and bio-sulfur concentrations, pH and temperature. The activation energy of this reaction was determined to be 70 kJ·mol-1 at pH 8.7 and 16 kJ·mol-1 at pH 10.3. The CH3S9- ion is unstable and leads to shorter-chain sulfur compounds. The main end-products formed are polysulfides (S32-, S42-, S52-), dimethyl polysulfides [(CH3)2S2, (CH3)2S3] and H2S. Also long-chain dimethyl polysulfides [(CH3)2S4-7] are formed in trace amounts (μM level). Excess MT results in complete methylation of the initially formed inorganic polysulfides. An increased molar MT/S ratio results in the formation of relatively more (CH3)2S2 over (CH3)2S3. Flowsheet simulations of the new LPG desulfurization process reveal that for an acceptable degree of desulfurization (i.e. less than 10 ppm in the treated LPG product) the pH in the recycle stream to the extractor column must be higher than 9. This means that the used inocula (estuarine and salt lake sediments) provide good opportunities to be applied in the process.
Manual of PEARLNEQ v4
Boesten, J.J.T.I. ; Tiktak, A. ; Leerdam, R.C. van - \ 2007
Wageningen : WOT Natuur & Milieu (WOt-werkdocument 71) - 34
bodemtypen - pesticiden - bodemverontreiniging - computer software - handboeken - soil types - pesticides - soil pollution - computer software - handbooks
This document describes a PEARLNEQ-PEST combination, which can be used to estimate the parameters for long-term sorption kinetics in the PEARL model on the basis of an incubation experiment for a certain soil and a certain pesticide. The combination provides also the transformation half-life at reference temperature (when long-term sorption kinetics are included in PEARL, the definition of this half-life changes so it has to be recalculated
|Anaerobic degradation of methanethiol at high salinity in an upflow anaerobic sludge blanket reactor
Leerdam, R.C. van; Bok, F.A.M. de; Stams, A.J.M. ; Lens, P.N.L. ; Janssen, A.J.H. - \ 2006
Degradation of Methanethiol by Methylotrophic Methanogenic Archaea in a Lab-Scale Upflow Anaerobic Sludge Blanket Reactor
Bok, F.A.M. de; Leerdam, R.C. van; Lomans, B.P. ; Smidt, H. ; Lens, P.N.L. ; Janssen, A.J.H. ; Stams, A.J.M. - \ 2006
Applied and Environmental Microbiology 72 (2006)12. - ISSN 0099-2240 - p. 7540 - 7547.
dimethyl sulfide - sulfur-compounds - waste-water - sediments - communities - bioreactor - conversion - digestion - bacteria - sulfate
In a lab-scale upflow anaerobic sludge blanket reactor inoculated with granular sludge from a full-scale wastewater treatment plant treating paper mill wastewater, methanethiol (MT) was degraded at 30°C to H2S, CO2, and CH4. At a hydraulic retention time of 9 h, a maximum influent concentration of 6 mM MT was applied, corresponding to a volumetric loading rate of 16.5 mmol liter-1 day-1. The archaeal community within the reactor was characterized by anaerobic culturing and denaturing gradient gel electrophoresis analysis, cloning, and sequencing of 16S rRNA genes and quantitative PCR. Initially, MT-fermenting methanogenic archaea related to members of the genus Methanolobus were enriched in the reactor. Later, they were outcompeted by Methanomethylovorans hollandica, which was detected in aggregates but not inside the granules that originated from the inoculum, the microbial composition of which remained fairly unchanged. Possibly other species within the Methanosarcinacaea also contributed to the fermentation of MT, but they were not enriched by serial dilution in liquid media. The archaeal community within the granules, which was dominated by Methanobacterium beijingense, did not change substantially during the reactor operation. Some of the species related to Methanomethylovorans hollandica were enriched by serial dilutions, but their growth rates were very low. Interestingly, the enrichments could be sustained only in the presence of MT and did not utilize any of the other typical substrates for methylotrophic methanogens, such as methanol, methyl amine, or dimethylsulfide
Volatile organic sulfur compounds in anaerobic sludge and sediments: biodegradation and toxicity
Leerdam, R.C. van; Bok, F.A.M. de; Lomans, B.P. ; Stams, A.J.M. ; Lens, P.N.L. ; Janssen, A.J.H. - \ 2006
Environmental Toxicology and Chemistry 25 (2006)12. - ISSN 0730-7268 - p. 3101 - 3109.
microbiële afbraak - slib - sediment - anaërobe behandeling - afvalwaterbehandeling - sulfaten - thiolen - reductie - sulfaat - methanol - biodegradatie - microbial degradation - sludges - sediment - anaerobic treatment - waste water treatment - sulfates - thiols - reduction - sulfate - methanol - biodegradation - fresh-water sediments - dimethyl sulfide - membrane bioreactor - waste air - methanethiol - degradation - inhibition - removal - ph - methanogens
A variety of environmental samples was screened for anaerobic degradation of methanethiol, ethanethiol, propanethiol, dimethylsulfide, and dimethyldisulfide. All sludge and sediment samples degraded methanethiol, dimethylsulfide, and dimethyldisulfide anaerobically. In contrast, ethanethiol and propanethiol were not degraded by the samples investigated under any of the conditions tested. Methanethiol, dimethylsulfide, and dimethyldisulfide were mainly degraded by methanogenic archaea. In the presence of sulfate and the methanogenic inhibitor bromoethane sulfonate, degradation of these compounds coupled to sulfate reduction occurred as well, but at much lower rates. Besides their biodegradability, also the toxicity of methanethiol, ethanethiol, and propanethiol to methanogenesis with methanol, acetate, and H2/CO2 as the substrates was assessed. The 50% inhibition concentration of methanethiol on the methane production from these substrates ranged between 7 and 10 mM. The 50% inhibition concentration values of ethanethiol and propanethiol for the degradation of methanol and acetate were between 6 and 8 mM, whereas hydrogen consumers were less affected by ethanethiol and propanethiol, as indicated by their higher 50% inhibition concentration (14 mM). Sulfide inhibited methanethiol degradation already at relatively low concentrations: methanethiol degradation was almost completely inhibited at an initial sulfide concentration of 8 mM. These results define the operational limits of anaerobic technologies for the treatment of volatile organic sulfur compounds in sulfide-containing wastewater streams
|Degradation of methanethiol in an UASB reactor: reactor performance and salt tolerance
Leerdam, R.C. van; Bok, F.A.M. de; Lens, P.N.L. ; Janssen, A.J.H. - \ 2005
In: Proceedings of the International Congress Biotechniques for Air Pollution Control, A Coruna, Spain, 2005. - A Coruna, Spain : - p. 225 - 233.
|Anaerobic degradation of volatile organic sulfur compounds
Bok, F.A.M. de; Leerdam, R.C. van; Lomans, B.P. ; Janssen, A.J.H. ; Stams, A.J.M. - \ 2004
In: Book of Abstracts, Conference, Cancun, Mexico 2004. - Cancun, Mexico : ISME 10 - p. 20 - 20.
|Anaerobic biodegradation of mercaptans and effect of salt concentration
Leerdam, R.C. van; Bok, F.A.M. de; Lens, P.N.L. ; Janssen, A.J.H. - \ 2004
In: Proceedings ESEB, Oostende, 25-28 April 2004 Oostende, Belgium : - p. 903 - 903.
|Degradation of Methanethiol in an UASB-reactor
Leerdam, R.C. van; Bok, F.A.M. de; Lens, P.N.L. ; Janssen, A.J.H. - \ 2003