Dutch study: Effluent increases greenhouse gas emissions from rivers

Rivers emit significantly more greenhouse gases, such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), after treated wastewater is discharged into them. Methane emissions are even five times higher two kilometers downstream of a Wastewater Treatment Plant (WWTP). This was recently revealed by research conducted by the Dutch Radboud University.

The study was published the 15^th of November in scientific journal Sciencedirect. It is conducted by Dutch researchers near six wastewater treatment plants along the rivers Linge and Kromme Rijn in the Netherlands. Using a floating measurement chamber, the researchers analysed emissions every 500 meters, starting from 500 meters upstream of the WWTP and extending to two kilometers downstream.

Methane emission peak

The researchers observed a methane emission peak two kilometers downstream of the discharge point. “The emission of methane was five times higher than at the discharge point itself”, the researchers stated on the Radboud University website. Methane is a far more potent greenhouse gas than CO₂ and reducing these gases can play an important in climate mitigation. According to research from the American Princeton University municipal wastewater treatment plants themselves also emit almost twice the amount of methane than scientists previously believed.

Ideal conditions for methane-producing microorganisms

The new research highlights that, while the effluent meets Dutch regulatory standards, it still has specific impacts on rivers. Researcher Lisanne Hendriks pointed out that treated wastewater continues to contain nitrogen, phosphate, and carbon. “All these nutrients encourage algae growth in the water. When the algae eventually die and sink to the riverbed, they create ideal conditions for methane-producing microorganisms. This process takes time, which is why methane emissions are not much higher immediately after the discharge point but increase further downstream.”

Improve wastewater treatment

Researcher Ida Peterse added: “We now know that effluent that meets standards still contributes to higher methane emissions. Since water systems like rivers are responsible for 50% of methane emissions, this is something we could potentially address. However, tackling this issue is not simple. A lot of research is being done to improve wastewater treatment processes, but it’s not just a matter of adding a few extra filters.”