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Simple oxygen tweaks could curb 'laughing gas' emissions from wastewater systems

Laughing gas (nitrous oxide or N₂O) is primarily released by the microorganisms 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.

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, Dr. Nina Roothans applied advanced techniques—including DNA and protein analyses—to study, in detail, how individual microorganisms 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, in Nature Water, revealed how environmental and operational factors—such as temperature and oxygen levels—impact nitrous oxide emissions.

Oxygen impacts emissions

One of Roothans' key 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 Ph.D. 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 Roothans' work are also expected to benefit the agricultural sector, where microbial nitrous oxide emissions pose an even greater challenge.