Biological treatment forms an integral part of wastewater treatment. Biological aerated filters (BAFs) are submerged attached growth bioreactors which provide biological treatment as well as filtration in a single unit. The packing media used in BAFs plays an important role in the system performance and determines the ability of the system to meet treatment objectives. The performance of upflow BAFs was compared using North American clay media and Severn Trent monomedia at low organic and hydraulic loads (0.18 kg tCOD/m3d – 0.6 kg tCOD/m3d and 0.1 m/hr – 0.38 m/hr, respectively). Two identical, two stage, bench scale, upflow BAFs were constructed using PVC pipes with an internal diameter of 0.11 m. The system was operated at the Peppers Ferry Wastewater Treatment facility for two months and was fed with effluent from the primary clarifier. Grab samples of influent and effluent from the BAFs were collected thrice a week to evaluate carbon oxidation, solids removal and nitrification. In order to evaluate system recovery when BAFs are operated intermittently, a drying cycle of eleven days was introduced. Both media performed satisfactorily with respect to carbon oxidation and nitrification. On average, total COD and total suspended solids (TSS) removal rates were, respectively greater than 80% and 55%. Conversion of ammonia to nitrate was greater than 90% throughout the study. It was concluded that additional factors like media properties and economic factors need to be considered in selection of the media. / Master of Science

Although the passive films that form on stainless steels have been extensively studied, the concentration depth profiles are not fully understood. Their thinness makes passive films hard to study, but angle-resolved X-ray photoelectron spectroscopy (ARXPS) is a non-destructive technique that can be used to obtain depth information. An iterative approach to deconvolute ARXPS measurements into depth profiles is discussed, and the chemistry of passive films on a molybdenum-containing 316L stainless steel is investigated. Bipolar electrochemistry, in which the sample is placed along an electric field created by two driving electrodes in an electrolyte, is investigated as a screening tool. It is shown that the method is useful to create corrosion gradients on 304 stainless steel, both under pitting and non-pitting conditions. Chromium carbide thin films were deposited by magnetron sputtering with a variety of deposition parameters on stainless steel, and subsequently analyzed. It is shown that these films present a promising material system for protective coatings to improve the corrosion resistance of stainless steels while also maintaining other useful properties, such as low interfacial contact resistance. Particular attention is given to the electrochemical evaluation of the films, whose high carbon concentrations necessitates different interpretations of the electrochemical results compared to for stainless steels.

In ariden Gebieten tritt Schwefelwasserstoff häufig im Grundwasser auf, wodurch dessen Nutzung für die Trinkwassergewinnung aufgrund des störenden Geruchs und Geschmacks stark beeinträchtigt wird. Die in der Praxis oft eingesetzte Belüftung erweist sich zumeist als ineffizient und führt zu Geruchsbelästigungen in der Umgebungsluft. Das Ziel der Arbeit, die im Rahmen eines deutsch-israelischen Forschungsprojekts angefertigt wurde, bestand darin, einen wirksamen und für den praktischen Einsatz bei der Wasseraufbereitung geeigneten Katalysator zur Entfernung von Schwefelwasserstoff zu entwickeln, hinsichtlich verschiedener Verfahrensparameter und relevanter Randbedingungen durch systematische Batch- und Säulenversuche zu erproben und zu optimieren. Wichtige Kriterien zur Charakterisierung der Katalysatoren waren die Kinetik des Sulfidumsatzes, die Langzeitstabilität und die Zusammensetzung der Reaktionsprodukte. Die Nachahmung natürlich vorkommender Strukturen (z. B. Häm-Gruppe) und deren Anpassung an eine katalytische Sulfidoxidation war der Grundgedanke am Anfang der Arbeit. Entsprechende Materialien, organische Metallkomplexe (Porphyrine) auf Kohlenstoffträgern (Acetylen Black), wurden bereits erfolgreich bei der Sauerstoffreduktion in Brennstoffzellen eingesetzt. Cobalttetraphenylporphyrin (CoTPP) zeigte von allen getesteten Materialien die beste katalytische Wirksamkeit zur Sulfidoxidation. Die Sulfidumsetzung lief sowohl bei höheren pH-Werten als auch mit zunehmender Temperatur schneller ab. Anhand von Untersuchungen zum Einfluss des pH-Werts konnte bestätigt werden, dass eine katalytische Wirksamkeit nur für die Oxidation der Sulfid-Spezies HS- und S2- besteht. Mit Aktivkohle konnte ebenfalls eine katalytische Sulfidumsetzung erzielt werden, jedoch lag die Aktivität hier im Vergleich zum CoTPP deutlich niedriger. Bei allen getesteten katalytisch wirksamen Materialien entstand als vorrangiges Reaktionsprodukt Schwefel, gebildete Schwefel-Sauerstoff-Verbindungen wie Sulfat und Thiosulfat waren von untergeordneter Bedeutung. Die Untersuchungen zeigten, dass Acetylen Black aufgrund der sehr geringen Teilchengröße technisch kaum einsetzbar ist, weshalb weitere Trägermaterialien erprobt wurden. Im Gegensatz zu Aktivkohle oder Anthrazit erwiesen sich Weichfilze, insbesondere der Sigratherm Kohlenstoff-Weichfilz (KFA-Filz), als sehr gut geeignete Trägermaterialien. Beim Einsatz von Aktivkohle lagerte sich der gebildete Schwefel in den Porenräumen ab, was zu einer erheblichen Verringerung der katalytischen Aktivität führte. Dagegen wurde unter Verwendung des modifizierten Filzmaterials auch bei sehr langen Laufzeiten (bis 3000 Stunden) keine Abnahme der katalytischen Wirksamkeit beobachtet. Durch diese Katalysatormatrix konnte somit die Deaktivierung des Katalysators durch elementaren Schwefel verhindert werden. Bei abschließenden Versuchen unter Verwendung einer kleintechnischen Versuchsanlage konnte gezeigt werden, dass der Katalysator für den großtechnischen Einsatz geeignet ist. Im Vergleich zur Aktivkohle sind zwar größere Investitionskosten notwendig, andererseits können mit dem CoTPP-Material deutlich längere Laufzeiten realisiert werden. Aufgrund seiner guten technischen Handhabbarkeit ist der modifizierte KFA-Filz sowohl in kleinen dezentralen Anlagen (ländliche Siedlungen) als auch in größeren Wasserwerken einsetzbar. Zusätzliche Chemikalien sind für den Betrieb eines solchen Filters nicht erforderlich. Eine weitere Verbesserung der Sulfidentfernung wird bei langen Filterlaufzeiten durch sulfidoxidierende Bakterien bewirkt. Das Ziel, einen effizienten, technisch einsetzbaren Katalysator zur oxidativen Sulfidentfernung aus Wässern zu entwickeln, wurde somit erreicht. / Hydrogen sulfide often occurs in groundwater of arid areas. Because of its malodour, H2S containing water cannot be used as drinking water without treatment. Aeration as the most common treatment technique is less effective and leads to nasty odour of ambient air. Catalytic oxidation could be an alternative. The aim of this work was to develop and to optimize a technically applicable oxidation catalyst as well as to test its applicability under practical conditions. Various N4-chelates (e. g. porphyrins), which are frequently used for the reduction of oxygen in fuel cells, were evaluated for catalytic oxidation of sulfide at selected boundary conditions and process parameters using batch and column experiments. The new catalysts should be characterized in comparison with other materials. The oxidation kinetics, the long-time stability of the catalyst and the composition of oxidation products were the main criteria used for catalyst assessment. Cobalt tetraphenylporphyrin (CoTPP) showed the highest catalytic activity of all tested materials. The rate of sulfide transformation increased significantly with increasing temperature and at pH values higher than 6. A catalyst suitable for technical use in fixed-bed reactors was obtained by coating of a supporting material (carbon felt KFA) with the active substance. For all investigated materials, sulfur was found to be the main reaction product of the sulfide oxidation. In contrast to activated carbon, which showed catalytic activity for sulfide oxidation too, modified KFA felt materials were not blocked and deactivated by formed sulfur, even after long-term use. The new catalyst is well qualified for a stable oxidation of sulfide in water. In comparison to activated carbon, higher investment costs are required, but the carbon felt supported porphyrin has a significant longer lifetime. Because of its easy use, modified KFA felt is applicable both in small local plants and in large waterworks. There is no necessity to add chemicals or to install complex control equipment. As a positive side-effect, further improvement of sulfide elimination caused by sulfide-oxidizing bacteria was found during long filter run times.

<p> This report examines the dynamic response of a 0. 5 metre pilot scale rotating biological contactor when operated under transient influent conditions. Experimental data are presented for two modes of operation: carbon oxidation and carbon oxidation plus nitrification. During the carbon oxidation experimental runs, a 2.0 metre RBC was operated in parallel with the 0.5 metre RBC, thus allowing comparisons in the performance of the units. </p> <p> When the 0. 5 metre RBC was operated under nitrifying conditions, it was found that the effluent filterable TKN responded positively to influent variations in filterable TKN loading, TKN concentration and hydraulic loading. Transfer f unction noise models were developed which successfully predicted the time varying effluent TKN response. The response in effluent filterable TKN was predicted most precisely by influent TKN loading. Models based on influent TKN concentration and flow were not as precise in predicting effluent response. The effluent response of the 0.5 metre RBC was found to be greater than the response of activated sludge pilot units when operated at similar levels of removal. </p> <p> When the 0. 5 metre RBC was operated in the carbon oxidation 'mode, significant effluent responses were observed for carbon loading and concentration. Little correlation was found between influent flow and effluent carbon concentration. Operating under identical conditions, the 2.0 m RBC showed significant responses to carbon loading, carbon concentration and flow. </p> / Thesis / Master of Engineering (MEngr)

Ce travail de thèse s’est intéressé aux mécanismes de dégradation de nanoparticules de Pt supportées sur carbone utilisées pour catalyser les réactions électrochimiques dans une pile à combustible à membrane échangeuse de protons (PEMFC) et à leur conséquences d’un point de vue cinétique. Nous avons mis en évidence les différents mécanismes (maturation d’Ostwald 3D, corrosion du support carboné, migration/agrégation des cristallites métalliques) conduisant à une perte de surface active électrochimiquement et avons trouvé des conditions permettent d’isoler chacun de ces mécanismes. En premier lieu, nous avons montré que les nanoparticules de Pt supportées sur carbone ne sont pas immobiles mais agrègent en conditions réactionnelles notamment en présence de molécules réductrices. La vitesse de ce processus varie dans l'ordre CO > CH3OH > H2 et a été reliée à (i) la baisse du travail d’adhésion engendrée par la chimisorption de ces molécules et (ii) la réduction des groupements oxygénés présents sur le support carboné natif.Nous nous sommes également intéressés au mécanisme d’électrooxydation électrochimique du Vulcan XC72, un noir de carbone classiquement utilisé dans les couches catalytiques de PEMFC. Des mesures par spectroscopie Raman ont montré que les domaines désordonnés du Vulcan XC72 (non-graphitiques, hybridation sp3) sont corrodés de façon préférentielle dans des conditions expérimentales proches de celles d’une cathode de PEMFC. Les domaines ordonnés du support carboné (carbone graphitique, hybridation sp2) sont également corrodés, la vitesse de ce processus étant largement inférieure à ce qui est observé sur les domaines désordonnés. En conséquence, les nanoparticules de Pt se détachent ou agglomèrent comme le révèlent des expériences de microscopie électronique en transmission couplées à l’électrochimie. L’ensemble de ces mécanismes de dégradation conduit à un abaissement de la densité du nombre de particules métalliques et augmente la distance entre ces dernières. Dans le chapitre IV, nous montrons que des électrocatalyseurs Pt/Sibunit electrocatalysts possédant (i) un faible chargement massique en Pt, et (ii) de grandes distances inter-particules présentant une faible activité pour la réduction du dioxygène de l’air. Le nombre moyen d’électrons transférés par molécule de dioxygène décroît bien sous la valeur théorique de 4 lorsque l’épaisseur de la couche catalytique ou le chargement massique diminue. Nous avons relié cela à un transport et à une ré-adsorption plus difficiles des intermédiaires réactionnels notamment le péroxyde d’hydrogène. Une diminution du nombre de sites catalytiques peut également engendrer une limitation des cinétiques réactionnelles par l’adsorption de l’oxygène. Au vu de l’ensemble des résultats précédents, nous avons conclu que des cristallites de plus grande taille permettraient d’améliorer la durabilité des matériaux contenus dans les couches catalytiques de PEMFC. Des nano-fils de Pt (NWs) avec une taille moyenne de cristallite de 2,1 ± 0,2 nm ont été synthétisés. Nous avons montré que la morphologie du matériau joue un rôle conséquent à la fois en termes d’activité électrocatalytique et de durabilité : les matériaux Pt NWs/C permettent une réduction de prêt de 170 mV de la surtension d’oxydation d’une monocouche de monoxyde de carbone et possèdent une activité catalytique élevée et stable pour l’électrooxydation du méthanol. Cette dernière a été attribuée à (i) l’augmentation de la masse des cristallites de Pt résultant de l’augmentation en taille (nanoparticules à nano-fils) et (ii) une surface de contact élevée avec le support carboné. Ces matériaux possèdent un potentiel intéressant pour résoudre les problèmes de durabilité rencontrés avec les matériaux 0D utilisés de façon conventionnelle. / State-of-the-art catalytic layers of proton-exchange membrane fuel cells (PEMFCs) utilize ionomer-bonded Pt-based nanocrystallites supported on a high surface area carbon support to accelerate the rate of the hydrogen oxidation reaction at the anode, and of the oxygen reduction reaction at the cathode. Post-mortem analysis of PEMFC catalytic layers reveal that four degradation mechanisms yield Pt surface area losses (and hence decreased PEMFC performance): (i) aggregation and/or detachment of the metal nanoparticles, (ii) corrosion of the carbon support, (iii) Ostwald ripening (dissolution/redeposition of the Pt-based crystallites) yielding the formation of ionic species and (iv) chemical reduction of the Ptz+ species in ion conductors, yielding the formation of electrically disconnected Pt crystallites. A major concern of the Ph.D. was to be able to isolate the migration of the Pt nanocrystallites. For that purpose, commercial Pt/Vulcan XC 72 electrocatalysts were aged in “mild” conditions, for which the corrosion of both the Pt nanoparticles and the carbon support could not be considered dominant. Evidences were provided that the Pt/C nanoparticles are not immobile but prone to agglomerate in the presence of H2, CH3OH, and CO, three molecules of interest for PEMFCs. The migration rate of the Pt crystallites was the largest in CO-containing solution and decreased in the order CO > CH3OH > H2. We postulate that the morphological changes of the Pt/C nanoparticles may be caused by (i) a change of the work of adhesion between the metal phase and the carbon support or (ii) the reduction of the oxygen-bearing surface groups strongly interacting with the Pt nanocrystallites. We also investigated the mechanism of the electrochemical oxidation of Vulcan XC72, a carbon black conventionally used in PEMFCs. Raman spectroscopy measurements evidenced that the disordered domains of the Vulcan XC72 support (non-graphitic, sp3-hybridized) are preferentially oxidized. The ordered domains (graphitic carbon, sp2-hybridized) of the Vulcan XC72 support are also oxidized but at a much smaller rate than that observed on the non-graphitic domains. A major consequence of the oxidation of the high-surface area carbon support is the aggregation and the detachment of the supported Pt nanoparticles. In Chapter V, we used model Pt/Sibunit electrocatalysts to show that aged catalytic layers with (i) low Pt to C weight fraction, (ii) large inter-particle distance, feature bad performance for the oxygen reduction reaction (ORR). The average number of transferred electrons produced during the ORR decreases below 4 with the decrease of the catalyst layer thickness or the Pt loading. This was rationalized by considering the lower probability for H2O2 molecules (the major reaction intermediate) to be re-adsorbed and further reduced into water in the catalytic layer when the inter-particle distance decreases. Finally, Pt nanowires (NWs) with 2.1 ± 0.2 nm crystallite size were synthesized by a soft template method. Evidences were provided that the morphology of the Pt material plays a pivotal role both in terms of electrocatalytic activity/stability: the 1D Pt NWs/C demonstrate a reduction by ca. 170 mV of the CO oxidation overpotential and feature high and stable MOR activity with respect to a conventional Pt/C 20 wt. % catalyst. The enhanced durability of Pt NWs/C was rationalized by considering (i) the increase in weight of the Pt nanomaterials resulting from the increase in size (from nanoparticles to nanowires) and (ii) the enhanced contact surface area between the Pt NWs and the carbon support. The development of 1D Pt nanostructures, such as Pt NWs, hold promises to solve the durability issues faced with the 0D materials currently used in PEMFCs.Keywords: proton exchange membrane fuel cell, durability of PEMFC materials, electrochemical carbon oxidation, crystallite migration, Pt nanowires.