<p>Effective management of dissolved organic matter (DOM) is essential for protecting public health, as DOM directly controls drinking water treatability, disinfection by-product (DBP) formation, and biological stability in distribution systems. Subtropical surface waters, typically enriched in humic and high-molecular-weight DOM, present elevated risks for the formation of potentially carcinogenic DBPs when treated using conventional processes. This study evaluated a sequential, multi-barrier treatment strategy combining acidified ferric chloride (FeCl₃) coagulation, powdered activated carbon (PAC) adsorption, and ozonation for DOM removal and transformation in water from the Logan River (Queensland, Australia). Raw water exhibited moderate dissolved organic carbon (DOC; ~ 6,9 mg L⁻¹), elevated specific ultraviolet absorbance (SUVA; 3,2 L m⁻¹ mg⁻¹), and low biodegradable dissolved organic carbon (BDOC; ~ 1,4% of total organic carbon), indicating predominantly aromatic and refractory DOM with high DBP precursor potential. FeCl₃ coagulation at 90–120 mg L⁻¹, particularly at pH 5,0–5,5, removed up to 60% of humic substances and biopolymers, outperforming aluminum-based coagulation. PAC adsorption preferentially eliminated chromophoric and moderate-molecular-weight fractions, achieving approximately 54% total organic carbon reduction. Subsequent ozonation transformed residual aromatic structures into more hydrophilic compounds while, when applied after FeCl₃ coagulation, maintaining the lowest BDOC recorded concentrations (~ 43 µg L⁻¹) among all tested conditions, indicative of preserved biological stability. LC–OCD analysis confirmed substantial reductions in aromaticity and DBP precursor pools. Overall, this iron-based, multi-barrier strategy effectively mitigates health risks associated with DBP formation while ensuring biologically stable drinking water, offering a robust framework for optimizing treatment trains in subtropical surface waters.</p> Graphical abstract <p>Untreated subtropical surface water is dominated by aromatic, refractory dissolved organic matter (DOM), leading to high disinfection by-product (DBP) formation potential and poor biological stability. A sequential multi-barrier treatment combining acidified FeCl₃ coagulation, powdered activated carbon (PAC) adsorption, ozonation, and chlorination effectively controlled DOM reactivity. FeCl₃ coagulation removed up to 60% of humic substances and biopolymers, while PAC adsorption further reduced total organic carbon by ~ 54%. Ozonation transformed residual aromatic DOM into more hydrophilic compounds while maintaining low biodegradable dissolved organic carbon (BDOC, ~ 43 µg L⁻¹), and final chlorination ensured microbial safety with minimized DBP formation. Overall, the integrated treatment produced biologically stable drinking water suitable for safe distribution. </p>