Research conducted by microbiologist Ida Peterse and ecologist Lisanne Hendriks from Radboud University highlights a concerning effect of treated wastewater on methane emissions in rivers. Their study, published in the journal Science of The Total Environment, reveals that a stretch of river receiving treated wastewater releases five times more methane than a similar stretch without this discharge. This discovery underscores the impact that even treated water can have on river ecosystems, particularly in relation to greenhouse gas emissions.
The researchers focused on methane, a potent greenhouse gas, and its levels in the Linge and Kromme Rijn rivers, which are subject to treated wastewater discharges. Peterse and Hendriks used floating chambers to capture gases rising from the water’s surface and analyzed them to determine methane concentrations. Along with this, they investigated the water and soil for key nutrients. Their measurements were taken at various points along the rivers: 500 meters before the discharge of treated wastewater, at the discharge point itself, and at intervals up to two kilometers downstream.
The findings were striking. While methane emissions were slightly elevated near the discharge point, they peaked two kilometers downstream, with levels five times higher than at the discharge site. This suggests that the process triggered by the discharge of treated wastewater—though it meets local standards for cleanliness—induces a chain reaction in the river ecosystem that leads to significantly higher methane emissions further along the watercourse.
The researchers identified several factors that contribute to this phenomenon. Treated wastewater, while considered safe for discharge according to Dutch water treatment standards, still contains nitrogen, phosphate, and carbon. These nutrients are not only vital for aquatic life but also stimulate the growth of algae. When the algae die and sink to the riverbed, they create an ideal environment for methane-producing microorganisms to thrive, resulting in higher methane emissions as the organic material decays.
Although the effect on methane levels is not immediately noticeable just after the discharge point, the increased emissions become apparent as the water travels downstream. This delayed effect highlights how the impact of treated wastewater on the environment can be more complex than previously understood.
Peterse and Hendriks emphasize the importance of recognizing the role that wastewater discharge plays in methane emissions, even if the water meets regulatory standards for cleanliness. Rivers, which are responsible for around 50% of global methane emissions, could be a significant target for mitigation strategies aimed at reducing greenhouse gas emissions. By better understanding the relationship between wastewater and methane production, there may be potential to address this issue in river systems, contributing to broader efforts to tackle climate change.
This study contributes to growing evidence that the environmental impact of wastewater treatment extends beyond water quality and into broader ecological and climate considerations, particularly in terms of methane emissions. It suggests that even treated water, which is often viewed as environmentally neutral, can have unintended and significant consequences for greenhouse gas dynamics in aquatic ecosystems.
Source: Radboud University