Sophie Paule Canovas

Constructed wetlands (CWs) are nature-based solutions for wastewater treatment whose functioning relies on interactions between substrates, plants, microorganisms, and invertebrates. While most research has focused on microbial communities and vegetation, the role of macrofauna in CW functioning and biodiversity remains poorly understood. This study investigates the spontaneous colonization and ecological characteristics of macrofaunal communities in vertical subsurface-flow CWs. Macrofauna was sampled in six young small-sized CWs in the Lauragais region, near Toulouse, France, including five domestic wastewater treatment systems and the experimental filtrOCampus CW. Soil from lawns surrounding filtrOCampus was also sampled for comparison. Across the six CWs, 37 macrofaunal taxa were identified, with an average density of 3483 ind./m². Oligochaetes were the dominant group, representing approximately 64% of individuals and reaching a biomass of about 210 g/m² in filtrOCampus. Nematodes and springtails were also particularly abundant. Overall, the communities displayed moderate diversity and were largely composed of terrestrial, humidity-tolerant taxa. Their composition was strongly influenced by the surrounding environment and by the alternating wet and dry conditions characteristic of vertical subsurface-flow CWs. The results demonstrate that CW substrates are spontaneously colonized by relatively diverse invertebrate communities within only 2–5 years. Although diversity was generally lower than in surrounding soils, older CWs tended to show greater taxonomic diversity and evenness. With 77% detritivores and 23% predators, these communities reflect organic-matter-rich, detrital environments. CWs therefore appear to provide an additional biodiversity-supporting habitat and may contribute to ecological connectivity, particularly in urban and peri-urban areas.

Constructed wetlands (CWs) are nature-based solutions for wastewater treatment, yet their wide implementation remains limited due to constraints such as clogging. To mitigate it, ecological intensification through earthworm inoculation has been proposed, despite it could also promote hydraulic short-circuiting and ultimately impair treatment performance. This study investigated the effects of the inoculation of three earthworms’ species (Eisenia fetida, Eisenia andrei and Dendrobeana hortensis) on the hydrodynamics of CWs, based on the hypothesis that their burrowing activity could reduce clogging and promote a more homogeneous distribution of flow within the substrate. Outdoor experiments were conducted at mesocosm scale (0.2 m2), combining saturated hydraulic conductivity (Ks) monitoring and tracer tests. Complementary laboratory-scale experiments (0.01 m2) using artificial substrate and galleries were performed to compare the effects of different burrow structures. They revealed that only vertical percolating galleries significantly affected both Ks and hydraulic residence time distribution (RTD). In contrast at mesocosm-scale, the earthworms did not significantly change Ks or RTD curves but tended to increase the mean residence time and shift flow dynamics within the porous matrix towards more dispersive behavior, without generating preferential flow pathways. These findings are consistent with the ecological traits of these earthworm species, whose burrowing activity creates a network of multidirectional burrows near the substrate surface rather than preferential vertical flow pathways. Although no clogging occurred in these young mesocosms (< 2 years old), precluding conclusions on long-term effects, the selected earthworm species appeared promising for mitigating clogging in CWs while preserving their hydraulic functioning.

Wastewater discharge or reuse raises concerns about the dissemination of emerging contaminants such as pharmaceutical residues and other organic micropollutants, into the environment. Constructed wetlands (CWs) are nature-based solutions for wastewater treatment whose performance could potentially be enhanced through ecological intensification. This study investigated whether earthworms, as ecological engineers, can improve organic micropollutants removal, influence microbial communities and antibiotic resistance genes in CWs. Experiments were conducted on an experimental platform using mesocosm-scale (0.2 m²) CWs treating raw wastewater from a university campus building. EWs inoculation shifted substrate bacterial community structure and composition and slightly improved the removal of several pharmaceuticals and a human metabolite (e.g., atenolol, benzoylecgonine, citalopram, clarithromycin, diclofenac, irbesartan, metoprolol, sulfamethoxazole and trimethoprim). Earthworms also slightly reduced accumulation of spiked compounds (clarithromycin, diclofenac, metoprolol and trimethoprim), while substrate accumulation represented only a negligible fraction of their overall removal. A decrease in antibiotic resistance gene accumulation was also observed in the substrate. These effects coincided with an increased abundance of bacterial taxa involved in recalcitrant organic matter degradation. Overall, these findings highlight the potential of ecosystem engineers as a low-cost strategy for the ecological intensification of micropollutant removal in constructed wetlands, while further research is needed to confirm these effects with greater statistical power and assess their long-term persistence.

Wastewater discharge or reuse raises concerns about the dissemination of harmful substances, particularly emerging contaminants such as pharmaceutical residues and other organic micropollutants, which can pollute ecosystems and pose risks to human health. This study investigates earthworms-assisted constructed wetlands (EWs-assisted CWs) as a sustainable approach to remove pharmaceutical and other organic micropollutants residues from campus building wastewater. By leveraging earthworms (EWs), the aim is to improve the removal of targeted compounds, and reduce the spread of antibiotic resistance. Experiments were conducted on an experimental platform using mesocosm-scale (0.2 m²) and pilot-scale (5 m²) CWs treating raw wastewater from a university building. At the mesocosm scale, EWs inoculation led to shifts in bacterial community structure and composition and slightly improved removal of several pharmaceuticals and a human metabolite (e.g., atenolol, benzoylecgonine, clarithromycin, diclofenac, irbesartan, metoprolol, mycophenolic acid, and sulfamethoxazole). Additionally, earthworms slightly reduced accumulation of spiked compounds in the substrate. A decrease of antibiotic resistance gene accumulation was also observed in the substrate. These effects may be linked to an increased abundance of bacterial taxa involved in recalcitrant organic matter degradation. However at the pilot scale, no significant effects resulting from EWs inoculation were observed, likely due to unexpected spontaneous EWs colonization in control conditions and low worm biomass in EWs-assisted CWs. Therefore, results from the mesocosm experiment are still considered encouraging to keep investigating the full potential of EWs-assisted CWs.