Ecological risk assessment of sewage sludge as soil amendment: lethal and sublethal effects in Eisenia fetida

Sewage sludge applications as soil amendment call for a proper ecological risk assessment due to unexpected delivery of toxic chemicals and materials. Standardized acute toxicity assays have proven to provide limited information in terms of potential hazard for soil organisms. Here, sublethal endpoints as physiological and tissue alterations were proposed as suitable tools for sewage sludge ecological risk assessment. Acute toxicity assay using earthworm Eisenia fetida (OECD n.207) was performed on sewage sludge samples from two wastewater treatment plants (A and B), and mortality, body weight, body morphology, and histology of intestinal cavity were evaluated at the end of the exposure period. Sewage sludge dilutions (1:50 and 1:100) from plant B caused a low percentage of mortality (5%), while none was observed in worms exposed to plant A. However, all specimens showed external body alterations as vesicles, swellings, constrictions, and discoloration. Histological sections highlight damages to epidermis, circular and longitudinal muscles, and chloragogenic tissue regardless of sewage sludge origin. Our findings confirm that standardized end points of acute toxicity are not sensitive enough in worms exposed to sewage sludges, while sublethal endpoints as body morphology and tissue histology are thus more suitable and appropriate endpoints for assessing potential detrimental outcomes and ecological risk for soil biota.