Rivers naturally emit the greenhouse gases CO₂, methane and nitrous oxide (N₂O). Strictly speaking, water bodies are known to account for about 50 % of methane emissions. To analyse the specific effects of wastewater effluent discharge on river GHG emissions, microbiologist Ida Peterse and ecologist Lisanne Hendriks of Radboud University, NL, studied two lowland rivers both receiving wastewater effluent: river Linge and river Kromme Rijn. They found a peak of methane emissions in a distance of 2 km downstream of the discharge points, reaching a value five times higher than at the wastewater treatment plant and upstream of the discharge point.
Effects of six wastewater treatment plants
In total, the researchers analysed the GHG emissions around the discharge points of six wastewater treatment plants, one on the River Kromme Rijn and five on the River Linge.
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.“
Treated wastewater still contains nutrients
Methane is formed in water bodies by biologicyl degradation of organic materials in the sediment. If additional organic material together with phosphor and nitrogen coming from wastewater treatment plants enters the water body, this may cause algae growth.
“These eventually die and sink to the bottom, which in turn is an ideal situation for methane-producing micro-organisms”, explains Hendriks.
Since that process takes a while, the methane emissions are not elevated just after a discharge point, but about 2 km downstream.
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.”
