The particularly powerful means supplied by commercial microcomputers made it possible to build this two–stage ozonation plant designed to ensure the chemical demand in oxidizing agent ofthe water in a first step and the viricidal action of this oxidizing agent ina second step. This plant was designed with one single ozone production unit and for quite variable flows of water tobe treated. Now, because such means are available, it is impossible to separate the design of acomplex ozonation plant from that of its automatic control system in particular since one cannot be looking for an economical and reliable system without conducting an overall analysis of the problem.