Responses of Freshwater Invertebrates to Imhoff Tank Sewage Effluents: A Preliminary Study in Four Watercourses with Different Ecological Status (Abruzzo, Central Italy)

In many EU countries, a large proportion of domestic effluents is treated in small, decentralized wastewater treatment plants (WWTPs) which often lack appropriate treatment technologies. The low efficiency of these systems and their incorrect maintenance often contribute to environmental deterioration, with a large quantity of inadequately treated sewage dispersed in the soil or discharged into superficial watercourses. In the Abruzzo region (Central Italy), almost all rivers and streams receive wastewater from Imhoff tanks with only primary treatments. The ecological impacts of these effluents have rarely been investigated. This paper aimed to evaluate the response of freshwater invertebrates to Imhoff effluents in receiving watercourses with different ecological status. Our results showed that wastewater from Imhoff plants negatively affected the structure and composition of freshwater communities, with a reduction in the richness and abundance of more sensitive taxa and an increase in the most tolerant ones. These negative effects were more accentuated during low flow periods and in streams with moderate ecological status. To improve the ecological status of rivers and streams and to limit the diffusion of pathogens and micropollutants in freshwater, a more efficient (secondary treatments, possibly with nature-based solutions) and sustainable (water reuse or sewage sludge recycling) approach to wastewater treatment in decentralized WWTPs is urgently needed.