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.

Humic substances (HSs) pose a significant challenge to safe drinking-water production due to their ubiquity, limited removal by conventional methods, and their role in forming toxic disinfection by-products, reinforcing the need for more efficient, energy-favorable, and scalable treatment technologies. This study developed and evaluated a compact hydrodynamic cavitation (HC) system that simultaneously induces coagulation and generates microbubbles for flotation-based HS removal. For the first time, HC is explored as a multifunctional unit capable of integrating rapid mixing, coagulant destabilization, and flotation within a single device. Optimal coagulation conditions were established at pH 5.0 and 9.5 mg L⁻¹ of ferric chloride. Process optimization using a Rotated Central Composite Design demonstrated that inlet pressure, flotation time, and initial HS concentration were the dominant operational factors, enabling the HC system to achieve a maximum removal efficiency of 81.9%. Five Venturi geometries with divergent angles of 4°, 8°, 11°, 14°, and 90° were investigated, with the 8° Venturi exhibiting superior performance due to stable microbubble formation and effective coagulant dispersion, as confirmed by CFD analyses. Comparative tests with a conventional Flotest unit showed that achieving similar efficiencies required at least 30% saturated water. In contrast, the HC system delivered equivalent removal in continuous flow without external air saturation. These findings demonstrate the potential of HC as an integrated coagulation–flotation core and highlight its promise as a compact, energy-efficient, and scalable technology for natural organic matter removal in water treatment.