Coagulation–Flocculation Combined with Ozonation for River Water Treatment: Physicochemical Mechanisms, Treatment Performance, and Environmental Implications

River water quality is increasingly threatened by organic matter, emerging contaminants, pathogens, and other pollutants, creating risks to aquatic ecosystems and human health. Coagulation/flocculation effectively removes suspended and colloidal contaminants, turbidity, and particulate-associated organic matter, but is generally less effective for dissolved organic matter and many emerging contaminants. This review synthesizes recent advances in combining coagulation/flocculation with ozonation for river-water treatment. Ozonation complements coagulation/flocculation by oxidizing dissolved organic compounds, transforming micropollutants, controlling color and odor, and contributing to microbial inactivation; however, ozone alone cannot provide comprehensive removal of particulate and colloidal matter, as well as certain dissolved and ozone-resistant contaminants. Treatment sequence is therefore important. Pre-ozonation can modify natural organic matter and enhance subsequent coagulation under appropriate conditions, whereas coagulation/flocculation followed by ozonation can be particularly advantageous when preliminary removal of suspended, colloidal, and coagulable organic matter reduces oxidant demand before oxidation of residual dissolved contaminants. Reported coagulant doses ranged from less than 1 to more than 100 mg/L, while ozone doses were generally approximately 0.5–2 mg/L, reflecting differences in water matrix, coagulant type, treatment configuration, and target contaminants. Economic performance depends on water quality, chemical consumption, ozone generation, treatment scale, and operating conditions. Environmentally, the combined process can reduce discharges of suspended matter, organic contaminants, micropollutants, and microorganisms, thereby lowering pollutant pressure on aquatic ecosystems. Overall, this integrated process represents a complementary treatment strategy for improving river-water quality and supporting more sustainable protection and management of freshwater resources.