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Assessing the Effects of Wastewater Treatment Plant Effluents on the Ecological Quality Status in a Mediterranean River Basin

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Many Mediterranean rivers are characterized by an inter-annual climatic variability, additionally intensified by climate change. Moreover, these rivers have limited extensions and a reduced dilution capacity which make them vulnerable to human pressures, especially to water pollution. During the 1970s, the Besòs river basin had historically suffered the continuous discharge of untreated residual water, leading to be considered as the most polluted river in Europe. During the 1990s and 2000s, environmental laws such as the Catalan Treatment Plan and the Water Framework Directive (2000/60/EC) were implemented to resolve the problem. Consequently, the number of wastewater treatment plants (WWTPs) and long-term monitoring programs to assess the ecological status increased in the territory. In order to evaluate these changes, this study assesses the biological quality status (IBMWP, QBR and IPS) in the Besòs river water at three spatial scales (headwater, middle and lower sections). Our results clearly show that water quality across the basin has been improved. This improvement has become more significant after the implementation of new treatment systems. The trends in ammonia concentrations (NH4+, in mg/L) and conductivity (EC, in μS/cm) discharged from 13 selected WWTPs during the period 2000–2018 displays a clear decreasing tendency. Finally, we have applied a thorough statistical analysis to the IBMWP dataset to demonstrate the positive influence of the treatment systems on the ecological quality status across the river basin. We conclude that the implementation and enforcement of environmental laws are valid and effective tools to improve water quality in Mediterranean rivers.


• Wastewater treatment plant upgrading has positive effects on biological quality status.

• High NH4+ and conductivity levels provoke alterations on biological quality status.

• Correct quality status (IBMWP) is reached when NH4+ < 1 mg/L and EC is within 500–2000 μS/cm.

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Data available in a public (institutional, general or subject specific) repository that does not issue datasets with DOIs (non-mandated deposition).


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We especially express our gratitude to Manel Isnard, Fluvial Systems responsible of the Consorci Besòs Tordera, Josep Mas-Pla, University of Girona, and Maeve O’Reilly for sharing their expertise during the course of this research. Datasets of NH4+ and EC discharged by the WWTPs of Besòs river basin were provided by the Consorci Besòs Tordera, which data repositories are openly available at Consorci Besòs Tordera (2019; The biological (IBMWP index (macroinvertebrates) and IPS index (diatom algae) and hydromorphological (QBR, riparian vegetation) quality status data of the Besòs river basin from 1997 to 2017 were extracted from Sànchez-Mateo et al. (2017) ( and Fortuño et al. (2018) ( R-code that supports the findings of this study are available in the Supplementary Material II.


This research has been supported by the research project of Observatori Rivus (UAB-ICTA) funded by the Fundació Rivus, Consorci Besòs Tordera, the city councils of Arbúcies, Hostalric, Malgrat de Mar, Sant Celoni and Santa Maria de Palautordera.

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All authors contributed to the study conception and design. The conceptual framework of the paper was performed by Eduard Pla and Sònia Sànchez-Mateo. Material preparation and the data compilation were performed by Antoni Mas-Ponce and the analysis were performed by Roberto Molowny-Horas. The first draft of the manuscript was written by Antoni Mas-Ponce and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Correspondence to Antoni Mas-Ponce.

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Mas-Ponce, A., Molowny-Horas, R., Pla, E. et al. Assessing the Effects of Wastewater Treatment Plant Effluents on the Ecological Quality Status in a Mediterranean River Basin. Environ. Process. 8, 533–551 (2021).

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