General type of processes treating a water for its public use in Korea, are chemically aided sedimentation tanks followed by rapid sand filters. Settled sludge from a coagulation/flocculation and sedimentation process is easily drained into the sludge treatment train from the bottom of tanks but sludge floated along with numerous micro-bubbles on the surface of coagulation/flocculation tanks and head-part of sedimentation tanks, initially built and operated without a skimmer and barrier against moving floats, has caused serious aesthetic and operational problems. In this study, sources of the micro-bubbles combining with floc particles and causing floating sludge, and factors affecting formation of those bubbles were investigated. There are several factors which can produce gas bubbles in the water treatment plant. Pumps and conduits highly pressurized during water intake and transport to the water treatment plant might be one of the possibilities which can produce micro-bubbles due to the pressure differences. The amounts of nitrogen and carbon dioxide gases evolved during prechlorination and coagulation processes were calculated by using actual plant operational data. Changes of interfacial properties of coagulated water were examined as a possibility of enhancing micro-bubbles production. Among these three factors, it was found that changes of surface characteristics had a significant and direct effect on the formation of micro-bubbles at this plant.