Wastewater treatment plants (WTP) place a greater burden on the climate than was previously thought. They generate greenhouse gases at various processing stages and, in total, account for over one per cent of all greenhouse gas emissions in Switzerland. In the case of N2O (nitrous oxide), which is particularly harmful to the climate and the ozone layer, they account for a massive 20 per cent or so of total emissions. “Wastewater treatment plants are significant N2O emitters, not only in Switzerland but also worldwide,” according to Wenzel Gruber, a postdoc at Eawag’s Process Engineering Department, who says the global significance of emissions from WTPs has so far been “seriously underestimated”. This is due to the lack of measurement data with sufficient temporal and spatial resolution.
Broad data set thanks to long-term monitoring
Under the umbrella of the N2Oara project at Eawag, researchers conducted 14 long-term observation series on various types of treatment plants in Switzerland. This enabled them to produce a broad data set for WTP emissions, and pave the way for a deeper understanding of emissions drivers. The results of the research project were recently presented. in the journal AQUA & GAS. Emissions of N2O from biological wastewater treatment make up the greatest proportion of greenhouse gas emissions from the treatment process as a whole. Biological treatment centres around the processes of nitrification and denitrification. The findings show that, if these process steps could be optimised, overall greenhouse gas emissions from a WTP could be reduced by up to 75 per cent.
No impact on the quality of treated wastewater
According to the researchers, optimisation could be achieved by boosting nitrogen elimination and preventing the accumulation of nitrite. “As our study shows, it’s possible to achieve a huge reduction in N2O emissions without reducing the quality of the treated water,” explains Gruber. In fact, the measures described here actually improved the effluent quality. However, before robust optimisation measures can be recommended, there is a need for greater insights into the mechanisms involved – and it is with this objective in mind that Eawag has launched two new projects. Eawag has been addressing the production of nitrous oxide in WTPs for a long time: the first dissertation on N2O from WTPs was authored in 1996 and was followed by another doctoral thesis in 2013. Nitrous oxide research was then presented to a wider audience at the Info Day 2018 on Wastewater as a Resource.
Increasing nitrogen elimination by Swiss wastewater treatment plants is also an issue at the political level. In December 2020, the National Council adopted a motion to this effect (20.4261). An amendment to the Water Protection Ordinance with a statutory limit value for nitrite is now in preparation. If this is implemented, a significant reduction in nitrous oxide emissions from wastewater treatment plants can be expected in the future.