Drinking water treatment plays an important role in maintaining public health. Chlorine is the most often disinfectant used for microbiological protection of water. Required residual chlorine concentration must be in treated water when it leaves treatment plant, to comply with the regulations. The chlorine residual must be 0.5 mg/L at the entrance into the distribution system and 0.25 mg/l at consumers, according to the law. Any less and there is no guarantee that the water has adequate quality. The major problem that occurs as water flows between treatment plant and the consumer is water quality deterioration because of decreasing of residual chlorine concentration, especially for long residence times. This can be lead to high microbiological concentration in water downstream and it is necessary additional treatment with chlorine. This study presents a theoretical model based on dispersion equation, which was validated with experimental measurements in drinking water mains that supply Ramnicu-Valcea town, Romania.