Toxicity impacts of water treatment sludge disposal in rivers

Water treatment plants (WTPs) generate sludge rich in aluminum, iron, and suspended solids, which poses environmental risks when improperly disposed. We evaluated the impact of full-cycle WTP sludge discharge into the Uberaba River (Minas Gerais, Brazil), where a fish mortality event occurred downstream. Monitoring campaigns showed extremely high aluminum (11,076 mg/kg) and solid concentrations in the sludge, with downstream aluminum rising from 0.7 to 4.71 mg/L and peaking at 1661 mg/L during temporal sampling. A one-dimensional advection-dispersion model predicted aluminum concentrations up to 360 mg/L under low-flow conditions, exceeding values associated with fish mortality. Biotic ligand model results indicated toxic aluminum binding to gill receptors during peak discharge. Our findings provide evidence linking the sludge disposal to severe water-quality deterioration and the fish kill, while also demonstrating noncompliance with Brazilian environmental regulations. This study underscores the need for regulated sludge treatment and disposal practices to protect aquatic ecosystems.