A stretch of river into which treated wastewater has been discharged emits five times more methane than a stretch of river without that wastewater. This is according to research by Ida Peterse and Lisanne Hendriks of Radboud University.
For the study, microbiologist Peterse and ecologist Hendriks measured methane emissions at different points of the Linge and Kromme Rijn rivers. Peterse: ‘Using a floating chamber, we caught the gases rising from the river on the water surface and analysed them. We also investigated which nutrients were in the water and in the soil.' The researchers did this 500 metres before a water treatment plant, at the discharge point of the treatment plant itself, 500 metres after it and so on, up to two kilometres after it. ‘Two kilometres after the discharge point of a treatment plant, we saw a peak of methane emissions, as much as five times higher than at the discharge point itself.’
Effects on the river
The researchers thus show that the treated water, although clean according to Dutch standards, has effects on a river. Hendriks: ‘The treated water also contains nitrogen, phosphate and carbon. All those nutrients in the water cause more algae to grow, for example. These eventually die and sink to the bottom, which in turn is an ideal situation for methane-producing micro-organisms.' Since that process takes a while, methane emissions are not much higher just after a discharge point. A bit further on, however, they are.
Peterse: ‘Although water discharged into rivers meets Dutch standards, it is important to realise that it still contributes to higher methane emissions. Since water systems like rivers are 50% responsible for methane emissions, this is something we might be able to address.'
