Toxicity of percolates from geotechnical applications with water treatment sludge incorporation

Although Water Treatment Plant Sludge (WTPS) is viable for geotechnical reuse, its environmental safety regarding realistic percolate generation remains underexplored. This study investigated the ecotoxicity of field-condition percolates using pilot-scale simulations with neat WTPS and a 50% clayey soil-sludge mixture. Over 12 months, tropical rainfall regimes were simulated to evaluate seasonality and two WTPS dewatering pre-treatments: centrifugation and natural drying. Acute ecotoxicity was assessed using the microalgae Raphidocelis subcapitata. Results revealed the dewatering method as a critical toxicity determinant: while neat centrifuged sludge percolate caused 94% growth inhibition, its 50% incorporation into the clayey soil matrix reduced this to 3%. Conversely, naturally dried sludge and its 50% mixture maintained high toxicity (>72% inhibition), with seasonality showing no significant effect. These findings demonstrate that WTPS reuse requires careful environmental management, as toxicity is driven by dewatering methods and soil matrix interactions. Crucially, ecotoxicity testing is not mandatory under Brazilian standard NBR 10004, which classifies WTPS as non-hazardous based solely on physical–chemical parameters. Incorporating ecotoxicological evaluation into waste regulation is therefore essential for safe WTPS integration into circular economy models, as percolate monitoring reveals clear environmental risks.