05. November 2025, Topic: Aquatic Ecotoxicology Sediment Ecotoxicology

Sediment worms investigate the effect of wastewater on stream health

Sediment worms can be used to assess the impact of pollutants on watercourses. New studies show that treated wastewater contaminates sediments and, in some locations, can also infiltrate groundwater. A significant improvement was achieved by upgrading the facilities under investigation with an additional treatment stage.

Wastewater treatment plants (WWTPs) play an important role in protecting rivers and groundwater, but their effluent can pollute these ecosystems. Two new studies show how tiny sediment worms from the oligochaete group can help us understand the ecological effects of wastewater from WWTPs on watercourses and assess the success of WWTP upgrades. Streams and rivers are connected to groundwater by coarse sediments and the porous layer beneath them. This layer acts as a filter: it can store pollutants, but also contributes to the natural self-purification of water bodies. Polluted surface water can seep downwards and pollute the groundwater; cleaner groundwater flowing upwards can improve water quality in the river.

Complex dynamics in river sediments

Conventional chemical analyses provide snapshots and no information about the dynamics of pollution resulting from the introduction of pollutants and the hydrological exchange between surface water and groundwater. Oligochaete communities, on the other hand, reflect the condition of the water body over longer periods of time. They include species that are sensitive to pollutants as well as those that are highly resistant to them. While some of these species live only in surface sediments or only in groundwater, others occur in both habitats. The composition of the oligochaete community therefore varies depending on pollution and exchange between water bodies and groundwater. "By analysing them, we can assess not only sediment pollution but also the dynamics of vertical hydrological exchange," explains Régis Vivien.

To test the method and analyse the effects of WWTP effluent, the researchers examined the sediments above and below three wastewater treatment plants: Oberglatt, Muri and Vallorbe. In Oberglatt and Muri, they also compared the worm communities before and after the plant was upgraded with an additional activated carbon purification stage.

Significant impact of wastewater treatment plants and success of upgrades

The scientists found that all three WWTPs had a negative impact on the oligochaete communities. Even treated wastewater therefore alters the biology of watercourses. In some sections of the river, groundwater flowed upwards, helping to dilute the pollutants, while in other areas, river water seeped downwards and could thus contaminate the groundwater. After the WWTPs in Oberglatt and Muri were upgraded with activated carbon, the proportion of resistant oligochaete species decreased downstream of the WWTPs – an indication that sediment pollution had decreased.

The studies confirm that examining oligochaete communities is an effective method for detecting specific effects of wastewater and evaluating the success of WWTP upgrades ( ). It also provides information on the vulnerability of groundwater to surface water pollution and the self-purification capacity of watercourses – both of which are crucial for integrated water management.

Currently, the method requires expert knowledge to identify the species of animals, but DNA-based identification or simplified identification could soon make it more accessible for routine monitoring. For water managers and ecologists, the message is clear: tracking worms in sediments provides an integrative picture of the health of watercourses and shows whether costly upgrades to sewage treatment plants are delivering the expected ecological benefits.

Publications

Vivien, R., & Ferrari, B. J. D. (2025). New data on the use of oligochaete communities for assessing the impacts of wastewater treatment plant effluents on receiving streams. Water, 17(5), 724 (17 pp.). https://www.dora.lib4ri.ch/eawag/islandora/object/eawag:33977

Vivien, R., & Ferrari, B. J. D. (2025) Effet de rejets de STEP sur le milieu récepteur. Evaluation basée sur les oligochètes de la matrice poreuse – Cas de la STEP de Muri., Aqua & Gas 105, 11, 68-75 https://www.dora.lib4ri.ch/eawag/islandora/object/eawag:35730

Share via:

E-mail Facebook Twitter