Nitrous oxide is known as the gas used in whipped cream cartridges or as an anaesthetic in hospitals. But it’s much more than that. This potent greenhouse gas is also released from biological wastewater treatment plants (WWTPs), posing a real threat to our climate. PhD candidate Nina Roothans (TU Delft), who recently graduated cum laude, has identified practical strategies to reduce nitrous oxide emissions. Her groundbreaking work has now been published in Nature Water .
Laughing gas (nitrous oxide or N2O) is primarily released by the micro-organisms responsible for cleaning our wastewater. These microbes live in complex communities, with each group performing a specific role. Emissions vary throughout the day and across seasons, and the intricate microbial processes behind them remain largely unknown - making it difficult to design effective strategies to reduce emissions.
Understanding the balance between micro-organisms
Motivated by the need to resolve this long-standing puzzle, Michele Laureni (assistant professor Bioprocess Engineering) and Mark van Loosdrecht (Professor Environmental Biotechnology) teamed up with the Dutch Water Authorities and STOWA. Under their supervision, the now Dr. Nina Roothans applied advanced techniques - including DNA and protein analyses - to study, in detail, how individual micro-organisms interact in WWTPs over a two-year period. The Amsterdam West WWTP from the Water Authority Amstel, Gooi and Vecht, operated by Waternet, was her model ecosystem. The research revealed how environmental and operational factors - such as temperature and oxygen levels - impact nitrous oxide emissions.
Oxygen impacts emissions
One of Roothanskey discoveries was that the accumulation of nitrite - a central intermediate in the breakdown of nitrogen compounds - is linked to an imbalance between two groups of bacteria: those that convert ammonia into nitrite, and those that convert nitrite into nitrate. Because nitrite is a precursor to nitrous oxide, this imbalance was identified as the primary cause of nitrous oxide or laughing gas emissions. Crucially, Roothans also identified the oxygen concentration - something operators have direct control upon - as the main factor inducing this imbalance.
And this, Laureni notes, is precisely where the opportunity lies: -The nice thing is that, in principle, the solution may not require major infrastructural changes. The idea is that gradually increasing oxygen, rather than doing so abruptly when winter approaches, may already significantly reduce emissions. Moreover, this work is a great example of how very fundamental research can directly lead to actionable engineering insights.-
Simple and cost-effective
These findings are especially relevant for the Water Authorities, as they not only point to potential ways of reducing nitrous oxide emissions but also suggest that the required interventions may be both simple and cost-effective. Whether the identified strategy proves successful in practice will become clear in the coming years. Two new PhD candidates are now continuing the work to further develop and validate the approach, in collaboration with the Water Authorities and Royal HaskoningDHV. The fundamental insights from Roothanswork are also expected to benefit the agricultural sector, where microbial nitrous oxide emissions pose an even greater challenge.
Read the paper in Nature Water
Long-term multi-meta-omics resolves the ecophysiological controls of seasonal N2O emissions.
Nina Roothans, Martin Pabst, Menno van Diemen, Claudia Herrera Mexicano, Marcel Zandvoort, Thomas Abeel, Mark van Loosdrecht and Michele Laureni.
All authors are affiliated with TUD, except Marcel Zandvoort with Waternet.
Ecophysiology of N2O-emitting microbial communities
Nina Roothans
If you would like to know more about Roothansgroundbreaking research, feel free to contact Bregt Timmerman, press officer for Climate & Energy at TU Delft. You can reach him at +31 6 38554542 or b.w.timmerman@tudelft.nl.
