The Athens Water Supply and Sewerage Company (EYDAP SA) is the largest company operating in the Greek water market. EYDAP SA supplies approximately 4,000,000 inhabitants of Attica with potable water through an extensive network of a total length of 8078 km. The raw water is been treated and disinfected in four water treatment plants which are in the areas of Galatsi, Polydendri, Aharnon and Aspropyrgos. The company's laboratories are in charge to analyze treated water in order to ensure that the quality of the water reaching consumers complies with the guidelines set by the EEC. In total, thirty chemical parameters regarding the quality of treated water are checked whether they are within the limits mentioned in the 98/83/EEC Directive regarding The Quality of Water for Human Consumption. This study concerns the use of statistical tools for the improvement of quality control. The measurements of turbidity, residual chlorine and aluminium in the Polydendri tank for the year 2004 were of particular interest due to their potential fluctuations during the aforementioned time interval. Results are presented by means of thorough descriptive statistics and diagrams and statistical process control techniques have been employed for the evaluation of control limits regarding turbidity in particular with a view to checking, analyzing and finally improving the quality of the process. Furthermore, a lot of knowledge about these quality characteristics is revealed.