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News article15 February 2023Directorate-General for Environment

Are waste-water treatment plants failing to protect the ecological health of European streams?

Issue 595: Less than half of the European rivers and streams are in good ecological health due to the multiple pressures from society.

Are waste-water treatment plants failing to protect the ecological health of European streams?
Photo by M-Production, Shutterstock

One pressure is the release of untreated and insufficiently treated urban wastewater, which can occur when the EU legislation on urban wastewater discharges is poorly implemented by the Member States.1

Multiple stressors affect freshwater ecosystems, but urban wastewater is the most significant anthropogenic point source pressure affecting European freshwater ecosystems, exceeding the pressures arising from other point sources such as industry and intensive farming, mines and waste disposal. This study investigates how wastewater from households and small industry is presenting an increasing threat to stream ecosystems and provides strategies for future management of UWWTPs.

Despite physical and biological treatment of urban wastewater, the researchers say that urban wastewater discharges are a pressure on water bodies, with only 42% of the 1.7 million kilometres of European rivers and streams have a good ecological status (most water bodies do not meet good ecological status because they are affected by multiple pressures) 2. The publication of systematic continental-scale monitoring data of the ecological status of aquatic ecosystems in Europe, combined with European WWTP infrastructure data has resulted in several large-scale studies of the impact of stressors on freshwater habitats3.

The 26 523 waste-water treatment plants in Europe process wastewater from 447 million inhabitants and from small industries that discharge into public sewers4. This wastewater includes pharmaceutical residues, pesticides, nutrients, organic matter, microplastics and hazardous substances. The researchers suggest that there is a high priority to identify why, after decades of regulatory intervention, the ecological status of European streams is not meeting the requirements of the European Water Framework Directive (WFD).

Furthermore, the study seeks to identify a percentage threshold of waste-water discharge to water flow, above which ecological harm can occur. The researchers suggest this may help inform more targeted future regulatory guidance to improve the ecological health of Europe’s important waterways in line with targets in Europe’s Zero Pollution Action Plan for air, water and soil.

The researchers set up a database from a number of European datasets containing ecological status, modelled river discharge (water flow) and the estimated amount of WWTP effluent discharge from the 26 523 European WWTPS to the rivers and streams. Using this data, they then calculated an Urban Discharge Fraction (UDF), an urban waste-water discharge share of stream flow.

They compared the UDF spatial data for WWTPs to an existing pan-European study of impacts on rivers of seven other known stressors of aquatic ecosystems5, and found no correlations between UDF and these stressors. This reinforced their hypothesis that UDF could act as an additional, and independent stressor, for the intensity of wastewater-related ecological impacts. The researchers analysed stream-size-dependent ecological impacts of UDF across Europe, adjusted to stream segments, catchments and river types. Using this data, ecologically-safe operating spaces for UDF were calculated – within which streams would be protected.

The analysis showed that the ecological status of European streams (and small rivers) declined consistently with a higher urban waste-water discharge fraction of streamflow. The researchers found that the critical value of waste-water discharge fraction for streamflow was 6.5% – above which ecological damage would occur. This means that the flow of a stream that a WWTP is discharging into needs to be monitored to ensure that UDF does not exceed 6.5%. However, the researchers found that one third of the WWTPs in Europe analysed exceeded this critical threshold. They noted that, based on their model and their threshold, implementing this maximum level across European WWTPs would more than double the number of streams with a good ecological status.

The researchers suggest that waste-water treatment fails to protect stream ecosystems in Europe for two main reasons:

  • Firstly, across ecoregions and catchments, entry points of treated wastewater are not always in the best position for dilution, because the flow is too low compared to the amount of wastewater being introduced. This problem is worse under low-flow conditions.

  • Secondly, a system-oriented approach for the entire waste-water sector, considering the quantity and locations of waste-water generation, treatment, reuse and discharge throughout the river network, is missing.

Furthermore, the researchers suggest the UDF threshold to be considered as an upper limit for a safe operating space regarding UDF in streams. New strategies for waste-water management could then be introduced that are relevant for smaller streams, combining an integrated view of waste-water generation, treatment, and waste-water reuse, with direct consideration of the flow of the receiving water body, they say.


  1. Urban wastewater collection and treatment has been regulated in the EU since 1991 by the Urban Waste Water Treatment Directive. In its 2019 Evaluation of the Directive, the European Commission identified a number of shortcomings in this old Directive and therefore launched a revision process. In October 2022, the Commission adopted a proposal for a revised, modern Urban Wastewater Treatment Directive addressing these shortcomings and aligning the Directive with ambitions of the European Green Deal.
  2. EEA (2018) European waters - assessment of status and pressures 2018. Available from
  3. For example, Vigiak, O., Udias, A., Pistocchi, A., Zanni, M., Aloe, A., Grizzetti, B., 2021. Probability maps of anthropogenic impacts affecting ecological status in European rivers. Ecol. Indic 126, 107684. Available from:
  4. Certain industries can discharge into public sewers. They usually hold a permit to do so. Larger industrial plants are covered by the Industrial Emissions Directive and mostly treat their wastewater onsite or in specific industrial wastewater plants
  5. Lemm, J.U., Venohr, M., Globevnik, L., Stefanidis, K., Panagopoulos, Y., van Gils, J., Posthuma, L., Kristensen, P., Feld, C.K., Mahnkopf, J., Hering, D. and Birk, S. (2021) Multiple stressors determine river ecological status at the European scale: Towards an integrated understanding of river status deterioration, Global Change Biology, 27(9), 1962-1975. Available from:


Büttner, O., Jawitz, J.W., Birk, S. and Borchardt, D., (2022). Why wastewater treatment fails to protect stream ecosystems in Europe. Water Research, 217: 118382. Available from:

To cite this article/service:

Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.


Publication date
15 February 2023
Directorate-General for Environment

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