Greenhouse gas emissions from many wastewater treatment plants may be more than twice as large as previously thought. This is shown in a new study from Linköping University, where the researchers used drones with specially manufactured sensors to measure methane and nitrous oxide emissions.

“We show that certain greenhouse gas emissions from wastewater treatment plants have been unknown. Now that we know more about these emissions, we also know more about how they can be reduced,” says Magnus Gålfalk, docent at Tema M - Environmental Change at Linköping University, who led the study published in the journal Environmental Science & Technology.

Wastewater treatment plants receiving sewage from households and industries account for approximately 5 per cent of human-induced methane and nitrous oxide emissions, according to the UN Intergovernmental Panel on Climate Change, IPCC.

To calculate this, the IPCC uses so-called emission factors that are linked to how many households are connected to the treatment plant. The calculation model then yields a number for the emissions from each wastewater treatment plant. This number is an estimate and not the result of actual measurements, which has turned out to be problematic.

According to the researchers, wastewater treatment plants continuously work to reduce the emissions. But with the current reporting system, the emissions remain on the same level, according to the IPCC model, regardless of whether actual emissions are decreasing or not.

“It would be better if the emissions reported were based on actual measurements. This would make it easier for municipalities to show the benefits of investments to mitigate the emissions,” says Magnus Gålfalk.

Together with Professor David Bastviken at LiU, he has used a specially built drone that measured emissions of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) at twelve Swedish treatment plants that use anaerobic digestion as a sludge treatment. The measurements showed that methane and nitrous oxide emissions are significantly higher – about 2.5 times – than the IPCC calculation models show.

The emissions occurred mainly after digestion when the sludge is stored to reduce the amount of potentially harmful micro-organisms before being used as, for example, fertilizer. The current study shows that the amount of methane released in storage has been underestimated. And the researchers discovered something else – the measurements also showed that large amounts of nitrous oxide were emitted.

Nitrous oxide is a very powerful but fairly unknown greenhouse gas – it has a climate impact almost 300 times higher than carbon dioxide per kilogram.

“We show that the climate impact from nitrous oxide emissions from sludge storage is as great as that from methane, and this wasn’t known before. So it’s a major extra source to keep an eye on,” says Magnus Gålfalk.

The study was mainly funded by the European Research Council, Horizon 2020, the Swedish Research Council, Formas and Svenskt Vatten Utveckling (SVU; part of the Swedish Water and Wastewater Association).