This paper describes waste water treatment technology in production processes at Rijeka Oil Refinery. A very important part of the manufacturing process also makes care of the wastewater of the Rijeka Oil Refinery, which are divided depending on the type of production and consequently, it purifies. All wastewater generated in the Rijeka Oil Refinery is processed on the respective purification systems and are discharged in special places in accordance with the legal provisions. The introductory part of the paper deals with the general refinery systems: when petroleum refineries appear, their division, working patterns, the problems they face in their production processes, environmental protection, a brief overview of foreign refineries and the differences between the refinery systems in our area. The types of wastewater in the refinery systems, their classification and more detailed characteristics of industrial waste water are also mentioned. Following the general remarks on wastewater, waste pipes in the area of the Rijeka Oil Refinery have been developed in more detail, their distribution, description and disposal. Legislation is an inevitable concept, and are summarized in the law under which the Rijeka Oil Refinery manages and supplies waste water from different plants and production processes. In the final part of the paper, accent is placed on the most common waste water treatment processes at the Rijeka Oil Refinery. In the mentioned refinery wastewater treatment technology on wastewater treatment plants, it is based on preprocessing, primary and secondary treatment, all dependent on the prior use of water and their level of pollution. From the work it can be concluded that the discharge of untreated wastewater into natural receivers, watercourses and the sea could cause a number of adverse environmental impacts, including potentially harmful effects on human health, as well as the overall treatment of process wastewater treatment and the implementation of a comprehensive quality assurance The product produces the protection of the recipient and preserves the natural biological balance of the marine ecosystem.