In order to comply with current and anticipated water quality regulations and to control the rising levels of micropollutants in river water, a number of water treatment systems employ ozonation and granular activated carbon (GAC) processes. However, ozonation produces undesirable and harmful disinfection by-products (DBPs) such as aldehydes. In this study, batch and continuous experiments were conducted to investigate the factors influencing the distribution and formation of aldehydes in river water and to investigate the behavior of total aldehydes (TA) in water treatment system processes. The mean influent concentrations of dissolved organic carbon (DOC) and TA were 3.1±0.8 mg/L and 62.6±21.3 ㎍/L, respectively, while TA and bromide levels in the river water increased with a decrease in the water flow rate and temperature. In pre-and post-ozonation processes, the TA concentration increased with a higher ozone dosage and pH. In particular, the concentration of TA with both pre-ozonation and during winter was higher than with post-ozonation and during summer. In the coagulation and sedimentation processes, polyaluminum sulphate organic magnesium (PSOM) coagulation resulted in slightly lower TA removal than polyaluminum silicate sulphate (PASS) coagulation. Both the high removal of turbidity and low pH with different coagulants were achieved in the order of PSOM>PASS>PACl. TA levels were significantly lower at the top of the sand and biological activated-carbon beds.