Reduction of sewage sludge toxicity through chemical stabilization and natural attenuation

The use of sewage sludge (SS) as a soil amendment requires effective toxicity mitigation strategies. This study aimed to evaluate and compare the effectiveness of chemical stabilization (CS) with hydrated lime versus natural attenuation (NA) in reducing the ecotoxicity of SS from a Brazilian Wastewater Treatment Plant (WWTP). Ecotoxicological effects were assessed using bioassays with Lactuca sativa (phytotoxicity) and Allium cepa (cytogenotoxicity) on elutriates collected from microcosms at time zero (T0) and after 90 days of NA (T90). The treatments included raw SS and chemically stabilized sludge (SSS), both mixed with soil. At T0, elutriates from both SSS and SS mixtures exhibited significant phytotoxicity, inhibiting L. sativa radicle growth compared to controls (p < 0.05), and induced significantly high frequencies of chromosomal aberrations (CA) and micronuclei (MN) in A. cepa (p < 0.05), indicating that CS alone was insufficient to eliminate genotoxicity and mutagenicity. However, after 90 days of NA, these toxic effects were no longer observed; radicle growth was significantly stimulated across all treatments (p < 0.05), and CA and MN frequencies returned to levels statistically indistinguishable from the negative control (p > 0.05). These findings demonstrate that, a 90-day period of NA was a more effective strategy for detoxifying the studied SS than CS alone, presenting a promising, low-cost approach for safer agricultural application.