In this work the structure and the installation of a flexible and low cost SCADA system is presented. An ordinary PC with the appropriate interface and software operates the system. The system was installed to an old desalination plant in parallel with the existing old type conventional automation system, which is using relays, timers, etc. The automation system allows remote control and supervision of the plant at reasonable low cost. On the other hand, the system can be used for additional safety measures for the RO plant. The analysis of the data which are collected could be used for the prediction of possible breakdowns of the RO plant. The design and installation of the automation system, which includes the software and hardware, is simple and easily accessible. The system's architecture and logic are presented in this paper in detail. The hardware which has been used is the Adam 4000 modules and the software is the VisiDaq 3.1. The operation of the system is simple and user-friendly and can be altered with very little knowledge in programming. The installation of the system has been proved quite useful in preventive maintenance, taking into account the peculiarity of this particular reverse osmosis (RO) plant. The system has reduced the labor cost, and has increased the labor productivity of the operation due to the remote supervision of the process.