During ETH Day on 16 November 2024, environmental engineer Valentin Faust was awarded the Otto-Jaag Water Protection Prize for his doctoral thesis. His work provides important insights about the production of fertiliser from human urine.
The ETH Zurich awards the Otto-Jaag Water Protection Prize for outstanding master’s and doctoral theses in the field of water protection and hydrology. This year, Valentin Faust received the award during the ETH Day on 16 November for his dissertation on the topic “Effects of pH on urine nitrification: from microbial selection to process performance.”
His work was part of the European Space Agency's MELiSSA Space Research Programme. MELiSSA stands for “Micro Ecological Life Support System Alternative” and aims to develop systems that will enable long-term manned space missions, for example to Mars. This requires regenerative systems that produce food, water and oxygen from waste in the form of closed cycles. The fertiliser used to produce food should to be obtained from urine.
Increasing process stability for use in space
Eawag has been working for a long time on the processes required to enable the recovery of resources from wastewater and to facilitate self-sufficient sanitation systems for places without sewage systems and water connections. The multi-stage process for recovering nitrogen, phosphorus and other nutrients from urine must function particularly reliably and smoothly for use in space. Valentin Faust’s work as a doctoral student in the Process Engineering Department at Eawag therefore aimed to increase the process stability of urine treatment and also to reduce the process’s carbon footprint. In particular, he took a close look at the process step on nitrification, in which bacteria convert the ammonium ions contained in urine into nitrate. This reaction is very sensitive to changes in the pH value. Faust investigated how the pH affects the composition of the microorganisms and the formation of undesirable reaction products such as nitrite and the greenhouse gas nitrous oxide.
To do this, he worked with nitrification reactors on a laboratory and pilot scale, among other things, and modelled the nitrification process in order to be able to predict under which conditions the process comes to a standstill and which operating strategies enable stable and climate-friendly nitrification.
Valuable findings for use on Earth, too
“Valentin Faust’s results provide important insights for the operation of nitrification reactors for the production of fertilisers from human urine,” explains Kai Udert, group leader in the Process Engineering Department at Eawag, who supervised Faust’s doctoral thesis. “This is very valuable for the further development of resource-oriented urban water management with the aim of closing nutrient cycles and protecting water resources."
Valentin Faust is delighted about the award. “I really enjoyed working together with an international team on the challenges of a closed-loop system – for use either on Earth or in space.” He doesn’t know yet what he will do with the prize money. “I’m certainly not buying a flight ticket to the moon.” Following a brief postdoc at Eawag, Faust is now working as a project leader at the University of Applied Sciences of Eastern Switzerland (OST) in the Applied Chemistry research group in the field of wastewater, water and odours.
Cover picture: ETH Rector Günther Dissertori presents Valentin Faust with the 2024 Otto-Jaag Water Protection Prize (Photo: ETH, Alessandro Della Bella).
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title => protected'Ammonia oxidation by novel "<em>Candidatus </em>Nitrosacidococcus urinae" is sensitive to process disturbances at low pH and to iron limitation at neutr al pH' (157 chars)
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categories => protected'nitrification; acidophilic AOB; source separation; chemical nitrite oxidatio n; human urine; life support system' (111 chars)
description => protected'Acid-tolerant ammonia-oxidizing bacteria (AOB) can open the door to new appl ications, such as partial nitritation at low pH. However, they can also be p roblematic because chemical nitrite oxidation occurs at low pH, leading to t he 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 tec hnical feasibility of ammonia oxidation under acidic conditions for source-s eparated urine with total nitrogen concentrations up to 3.5 g-N L<sup>-1</ sup> was investigated. On the other hand, the abundance and growth of acid-t olerant AOB at more neutral pH was explored. Under acidic conditions (pH of 5), ammonia oxidation rates of 500 mg-N L<sup>-1</sup> d<sup>-1</sup> an d 10 g-N g-VSS<sup>-1</sup> d<sup>-1</sup> were observed, despite high con centrations of 15 mg-N L<sup>-1</sup> of the AOB-inhibiting compound nitro us acid and low concentration of 0.04 mg-N L<sup>-1</sup> of the substrate ammonia. However, ammonia oxidation under acidic conditions was very sensit ive to process disturbances. Even short periods of less than 12 h without ox ygen or without influent resulted in a complete cessation of ammonia oxidati on with a recovery time of up to two months, which is a problem for low main tenance applications such as decentralized treatment. Furthermore, undesirab le nitrogen losses of about 10% were observed. Under acidic conditions, a no vel AOB strain was enriched with a relative abundance of up to 80%, for whic h the name “<em>Candidatus (Ca.)</em> Nitrosacidococcus urinae” is propo sed. While <em>Nitrosacidococcus</em> members were present only to a small e xtent (0.004%) in urine nitrification reactors operated at pH values between 5.8 and 7, acid-tolerant AOB were always enriched during long periods witho ut 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 “<em>C a.</em> Nitrosacidococcu...' (2324 chars)
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authors => protected'Faust, V.; Gruber, W.; Ganigué, R.; Vlaeminck, S. E.; Udert, K. M.' (102 chars)
title => protected'Nitrous oxide emissions and carbon footprint of decentralized urine fertiliz er production by nitrification and distillation' (123 chars)
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categories => protected'greenhouse gas emissions; resource recovery; MELiSSA; nitrite sensor; digest er supernatant' (90 chars)
description => protected'Combining partial nitrification, granular activated carbon (GAC) filtration, and distillation is a well-studied approach to convert urine into a fertili zer. To evaluate the environmental sustainability of a technology, the opera tional carbon footprint and therefore nitrous oxide (N<sub>2</sub>O) emissio ns should be known, but N<sub>2</sub>O emissions from urine nitrification ha ve not been assessed yet. Therefore, N<sub>2</sub>O emissions of a decentral ized urine nitrification reactor were monitored for 1 month. During nitrific ation, 0.4-1.2% of the total nitrogen load was emitted as N<sub>2</sub>O-N w ith an average N<sub>2</sub>O emission factor (EF<sub>N<sub>2</sub>O</sub>) of 0.7%. Additional N<sub>2</sub>O was produced during anoxic storage betwee n nitrification and GAC filtration with an estimated EF<sub>N<sub>2</sub>O</ sub> of 0.8%, resulting in an EF<sub>N<sub>2</sub>O</sub> of 1.5% for the tr eatment chain. N<sub>2</sub>O emissions during nitrification can be mitigate d by 60% by avoiding low dissolved oxygen or anoxic conditions and nitrite c oncentrations above 5 mg-N L<sup>-1</sup>. Minimizing the hydraulic retentio n time between nitrification and GAC filtration can reduce N<sub>2</sub>O fo rmation during intermediate storage by 100%. Overall, the N<sub>2</sub>O emi ssions accounted for 45% of the operational carbon footprint of 14 kg-CO<sub >2,equiv</sub> kg-N<sup>-1</sup> for urine fertilizer production. Using elec tricity from renewable sources and applying the proposed N<sub>2</sub>O miti gation strategies could potentially lower the carbon footprint by 85%.' (1590 chars)
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authors => protected'Faust, V.; Boon, N.; Ganigué, R.; Vlaeminck, S. E. ; Udert, K. M.' (100 chars)
title => protected'Optimizing control strategies for urine nitrification: narrow pH control ban d enhances process stability and reduces nitrous oxide emissions' (140 chars)
journal => protected'Frontiers in Environmental Science' (34 chars)
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categories => protected'resource recovery; decentralized treatment; microbial diversity; source sepa ration; robustness; MELiSSA; process stability' (122 chars)
description => protected'Nitrification is well-suited for urine stabilization. No base dosage is requ ired if the pH is controlled within an appropriate operating range by urine feeding, producing an ammonium-nitrate fertilizer. However, the process is h ighly 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 i n decentralized applications, two different strategies were tested: operatin g 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 i n the chemical speciation of ammonia and nitrite and, as shown, the microbia l production of nitrite. It was hypothesized that the higher fluctuations wo uld result in greater microbial diversity and, thus, a more robust process. The diversity of nitrifiers was higher in the wide-pH reactor, while the div ersity of the entire microbiome was similar in both systems. However, the wi de-pH reactor was more susceptible to tested process disturbances caused by increasing pH or temperature, decreasing dissolved oxygen, or an influent st op. In addition, with an emission factor of 0.47%, the nitrous oxide (N<sub> 2</sub>O) emissions from the wide-pH reactor were twice as high as the N<sub >2</sub>O emissions from the narrow-pH reactor, most likely due to the nitri te fluctuations. Based on these results, a narrow control band is recommende d for pH control in urine nitrification.' (1636 chars)
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title => protected'Influence of pH on urine nitrification: community shifts of ammonia-oxidizin g bacteria and inhibition of nitrite-oxidizing bacteria' (131 chars)
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categories => protected'source separation; resource recovery; nutrient recovery; decentralized treat ment; fertilizer production' (103 chars)
description => protected'Urine nitrification is pH-sensitive due to limited alkalinity and high resid ual ammonium concentrations. This study aimed to investigate how the pH affe cts nitrogen conversion and the microbial community of urine nitrification w ith a pH-based feeding strategy. First, kinetic parameters for NH<sub>3</sub >, HNO<sub>2</sub>, and NO<sub>2</sub><sup>-</sup> limitation and inhibition were determined for nitrifiers from a urine nitrification reactor. The turn ing point for ammonia-oxidizing bacteria (AOB), i.e., the substrate concentr ation at which a further increase would lead to a decrease in activity due t o inhibitory effects, was at an NH<sub>3</sub> concentration of 12 mg-N L<su p>-1</sup>, which was reached only at pH values above 7. The total nitrite t urning point for nitrite-oxidizing bacteria (NOB) was pH-dependent, e.g., 18 mg-N L<sup>-1</sup> at pH 6.3. Second, four years of data from two 120 L re actors 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<sup>-1</sup> and pH values between 2.5 and 8.5. At pH 6, the AOB Nitros omonas 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 HNO<sub>2</sub> accumulation. At pH 8.5, the AOB Nitr osomonas stercoris became dominant, and NH<sub>3</sub> inhibited NOB. Withou t 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 selecti on and kinetics of nitrifiers.' (1778 chars)
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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
