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Le recteur de l’EPF, Günther Dissertori, remet à Valentin Faust le prix Otto Jaag pour la protection des eaux (Photo: EPF, Alessandro Della Bella).

Le prix Otto Jaag pour la protection des eaux 2024 revient à Valentin Faust

18 novembre 2024 | Claudia Carle

Lors de la Journée de l’EPF du 16 novembre, la thèse de Valentin Faust, ingénieur de l’environnement, a été récompensée du prix Otto Jaag pour la protection des eaux. Celle-ci fournit d’importantes connaissances pour la fabrication d’engrais à partir d’urine humaine.

L’EPF Zurich décerne le prix Otto Jaag pour la protection des eaux à des mémoires de master et de doctorat remarquables dans le domaine de la protection des eaux et des sciences aquatiques. Cette année, cette distinction a été décernée à Valentin Faust dans le cadre de la Journée de l’EPF du 16 novembre, pour sa thèse intitulée «Effects of pH on urine nitrification: from microbial selection to process performance».

Son travail faisait partie du programme de recherche spatiale MELiSSA de l’Agence spatiale européenne (ESA). MELiSSA est l’acronyme de «Micro Ecological Life Support System Alternative» et a pour objectif de développer des systèmes permettant à terme d’envoyer des missions spatiales habitées, par exemple sur Mars. À cette fin, il est essentiel de disposer de systèmes régénérateurs, qui produisent en circuits fermés alimentation, eau et oxygène à partir des déchets engendrés. L’engrais pour la production d’aliments serait produit à partir de l’urine.

Augmenter la stabilité des processus pour une utilisation dans l’espace

L’Eawag travaille depuis longtemps sur les processus nécessaires à la récupération de ressources dans les eaux usées ainsi que sur des systèmes sanitaires autarciques pour les lieux sans canalisations ni raccordement à l’eau. Pour pouvoir être utilisé dans l’espace, ce processus à plusieurs étapes de récupération de l’azote, du phosphore et d’autres nutriments dans l’urine doit être parfaitement fiable et fonctionner sans perturbations. L’objectif de Valentin Faust, doctorant au département Technologie des procédés de l’Eawag, consistait par conséquent à augmenter la stabilité du processus de traitement de l’urine tout en réduisant l’empreinte carbone du procédé. Dans ce but, il a notamment étudié en détail l’étape de nitrification, durant laquelle des bactéries transforment l’ammonium contenu dans l’urine en nitrate. Cette réaction est très sensible aux changements de la valeur du pH. Valentin Faust a étudié l’impact de cette dernière sur la composition des microorganismes ainsi que sur l’apparition de produits de réaction indésirables, tels que le nitrite ou le gaz hilarant, un polluant climatique.

À cet effet, il a travaillé notamment avec des réacteurs de nitrification en laboratoire et dans un environnement pilote, et modélisé le processus en question pour prédire les conditions dans lesquelles celui-ci s’arrête, et déterminer les stratégies d’exploitation qui permettent une nitrification stable aussi écologique que possible.

Des conclusions précieuses aussi pour une utilisation terrestre

«Les résultats de Valentin Faust fournissent d’importantes connaissances pour le fonctionnement des réacteurs de nitrification destinés à fabriquer de l’engrais à partir d’urine humaine», explique Kai Udert, directeur de thèse du doctorant et responsable de groupe au département Technologie des processus de l’Eawag. «C’est extrêmement précieux pour le développement ultérieur d’une gestion de l’eau axée sur la récupération des ressources en zones urbaines, avec pour objectif de fermer les cycles de nutriments et de protéger les eaux.»

Valentin Faust est ravi de cette distinction. «J’ai eu beaucoup de plaisir à travailler au sein d’une équipe internationale sur les défis posés par un système circulaire, qu’il soit employé sur terre ou dans l’espace». Il ignore encore ce qu’il fera de l’argent gagné avec ce prix. «En tout cas, je n’achèterai pas un vol pour la lune». Après un bref postdoc à l’Eawag, Valentin Faust travaille à présent comme responsable de projet à la Haute école spécialisée de Suisse orientale (OST) dans le groupe spécialisé de Chimie appliquée aux eaux usées, à l’eau et aux odeurs.
 

Photo de couverture: Le recteur de l’EPF, Günther Dissertori, remet à Valentin Faust le prix Otto Jaag pour la protection des eaux (Photo: EPF, Alessandro Della Bella).
 

Publications originales

Ammonia oxidation by novel "Candidatus Nitrosacidococcus urinae" is sensitive to process disturbances at low pH and to iron limitation at neutral pH

Acid-tolerant ammonia-oxidizing bacteria (AOB) can open the door to new applications, such as partial nitritation at low pH. However, they can also be problematic because chemical nitrite oxidation occurs at low pH, leading to the release of harmful nitrogen oxide gases. In this publication, the role of acid-tolerant AOB in urine treatment was explored. On the one hand, the technical feasibility of ammonia oxidation under acidic conditions for source-separated urine with total nitrogen concentrations up to 3.5 g-N L-1 was investigated. On the other hand, the abundance and growth of acid-tolerant AOB at more neutral pH was explored. Under acidic conditions (pH of 5), ammonia oxidation rates of 500 mg-N L-1 d-1 and 10 g-N g-VSS-1 d-1 were observed, despite high concentrations of 15 mg-N L-1 of the AOB-inhibiting compound nitrous acid and low concentration of 0.04 mg-N L-1 of the substrate ammonia. However, ammonia oxidation under acidic conditions was very sensitive to process disturbances. Even short periods of less than 12 h without oxygen or without influent resulted in a complete cessation of ammonia oxidation with a recovery time of up to two months, which is a problem for low maintenance applications such as decentralized treatment. Furthermore, undesirable nitrogen losses of about 10% were observed. Under acidic conditions, a novel AOB strain was enriched with a relative abundance of up to 80%, for which the name “Candidatus (Ca.) Nitrosacidococcus urinae” is proposed. While Nitrosacidococcus members were present only to a small extent (0.004%) in urine nitrification reactors operated at pH values between 5.8 and 7, acid-tolerant AOB were always enriched during long periods without influent, resulting in an uncontrolled drop in pH to as low as 2.5. Long-term experiments at different pH values showed that the activity of “Ca. Nitrosacidococcus urinae” decreased strongly at a pH of 7, where they were also outcompeted by the acid-sensitive AOB Nitrosomonas halophila. The experiment results showed that the decreased activity of “Ca. Nitrosacidococcus urinae” correlated with the limited availability of dissolved iron at neutral pH.
Faust, V.; van Alen, T. A.; Op den Camp, H. J. M.; Vlaeminck, S. E.; Ganigué, R.; Boon, N.; Udert, K. M. (2022) Ammonia oxidation by novel "Candidatus Nitrosacidococcus urinae" is sensitive to process disturbances at low pH and to iron limitation at neutral pH, Water Research X, 17, 100157 (11 pp.), doi:10.1016/j.wroa.2022.100157, Institutional Repository

Nitrous oxide emissions and carbon footprint of decentralized urine fertilizer production by nitrification and distillation

Combining partial nitrification, granular activated carbon (GAC) filtration, and distillation is a well-studied approach to convert urine into a fertilizer. To evaluate the environmental sustainability of a technology, the operational carbon footprint and therefore nitrous oxide (N2O) emissions should be known, but N2O emissions from urine nitrification have not been assessed yet. Therefore, N2O emissions of a decentralized urine nitrification reactor were monitored for 1 month. During nitrification, 0.4-1.2% of the total nitrogen load was emitted as N2O-N with an average N2O emission factor (EFN2O) of 0.7%. Additional N2O was produced during anoxic storage between nitrification and GAC filtration with an estimated EFN2O of 0.8%, resulting in an EFN2O of 1.5% for the treatment chain. N2O emissions during nitrification can be mitigated by 60% by avoiding low dissolved oxygen or anoxic conditions and nitrite concentrations above 5 mg-N L-1. Minimizing the hydraulic retention time between nitrification and GAC filtration can reduce N2O formation during intermediate storage by 100%. Overall, the N2O emissions accounted for 45% of the operational carbon footprint of 14 kg-CO2,equiv kg-N-1 for urine fertilizer production. Using electricity from renewable sources and applying the proposed N2O mitigation strategies could potentially lower the carbon footprint by 85%.
Faust, V.; Gruber, W.; Ganigué, R.; Vlaeminck, S. E.; Udert, K. M. (2022) Nitrous oxide emissions and carbon footprint of decentralized urine fertilizer production by nitrification and distillation, ACS ES&T Engineering, 2(9), 1745-1755, doi:10.1021/acsestengg.2c00082, Institutional Repository

Optimizing control strategies for urine nitrification: narrow pH control band enhances process stability and reduces nitrous oxide emissions

Nitrification is well-suited for urine stabilization. No base dosage is required if the pH is controlled within an appropriate operating range by urine feeding, producing an ammonium-nitrate fertilizer. However, the process is highly dependent on the selected pH set-points and is susceptible to process failures such as nitrite accumulation or the growth of acid-tolerant ammonia-oxidizing bacteria. To address the need for a robust and reliable process in decentralized applications, two different strategies were tested: operating a two-position pH controller (inflow on/off) with a narrow pH control band at 6.20/6.25 (∆pH = 0.05, narrow-pH) vs. a wider pH control band at 6.00/6.50 (∆pH = 0.50, wide-pH). These variations in pH also cause variations in the chemical speciation of ammonia and nitrite and, as shown, the microbial production of nitrite. It was hypothesized that the higher fluctuations would result in greater microbial diversity and, thus, a more robust process. The diversity of nitrifiers was higher in the wide-pH reactor, while the diversity of the entire microbiome was similar in both systems. However, the wide-pH reactor was more susceptible to tested process disturbances caused by increasing pH or temperature, decreasing dissolved oxygen, or an influent stop. In addition, with an emission factor of 0.47%, the nitrous oxide (N2O) emissions from the wide-pH reactor were twice as high as the N2O emissions from the narrow-pH reactor, most likely due to the nitrite fluctuations. Based on these results, a narrow control band is recommended for pH control in urine nitrification.
Faust, V.; Boon, N.; Ganigué, R.; Vlaeminck, S. E.; Udert, K. M. (2023) Optimizing control strategies for urine nitrification: narrow pH control band enhances process stability and reduces nitrous oxide emissions, Frontiers in Environmental Science, 11, 1275152 (14 pp.), doi:10.3389/fenvs.2023.1275152, Institutional Repository

Influence of pH on urine nitrification: community shifts of ammonia-oxidizing bacteria and inhibition of nitrite-oxidizing bacteria

Urine nitrification is pH-sensitive due to limited alkalinity and high residual ammonium concentrations. This study aimed to investigate how the pH affects nitrogen conversion and the microbial community of urine nitrification with a pH-based feeding strategy. First, kinetic parameters for NH3, HNO2, and NO2- limitation and inhibition were determined for nitrifiers from a urine nitrification reactor. The turning point for ammonia-oxidizing bacteria (AOB), i.e., the substrate concentration at which a further increase would lead to a decrease in activity due to inhibitory effects, was at an NH3 concentration of 12 mg-N L-1, which was reached only at pH values above 7. The total nitrite turning point for nitrite-oxidizing bacteria (NOB) was pH-dependent, e.g., 18 mg-N L-1 at pH 6.3. Second, four years of data from two 120 L reactors were analyzed, showing that stable nitrification with low nitrite was most likely between pH 5.8 and 6.7. And third, six 12 L urine nitrification reactors were operated at total nitrogen concentrations of 1300 and 3600 mg-N L-1 and pH values between 2.5 and 8.5. At pH 6, the AOB Nitrosomonas europaea was found, and the NOB belonged to the genus Nitrobacter. At pH 7, nitrite accumulated, and Nitrosomonas halophila was the dominant AOB. NOB were inhibited by HNO2 accumulation. At pH 8.5, the AOB Nitrosomonas stercoris became dominant, and NH3 inhibited NOB. Without influent, the pH dropped to 2.5 due to the growth of the acid-tolerant AOB “Candidatus Nitrosacidococcus urinae”. In conclusion, pH is a decisive process control parameter for urine nitrification by influencing the selection and kinetics of nitrifiers.
Faust, V.; Vlaeminck, S. E.; Ganigué, R.; Udert, K. M. (2024) Influence of pH on urine nitrification: community shifts of ammonia-oxidizing bacteria and inhibition of nitrite-oxidizing bacteria, ACS ES&T Engineering, 4(2), 342-353, doi:10.1021/acsestengg.3c00320, Institutional Repository

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