The wastewater from a petrochemical complex is characterized by a diversity of pollutants including hydrocarbons, in free and emulsified form, phenols including cresols and xylenols, mercaptans, sulfides, ammonia and cyanides. The wastewater treatment of the reported petrochemical complex is based on a multiple stage treatment approach, consisting of physico - chemical, and biological processes. The biological treatment process could be efficiently protected by preliminary wastewater flow regulation, controlling hydraulic and pollutants loads. Additional protection and improvement of the biological treatment was achieved by preliminary removal of more than 90 percent of the oil found in free and in emulsified form in the raw wastewater. Significant reliability and efficiency could be achieved by a combined biological treatment consisting of aerated ponds, lime clarification and a water cooling industrial system, recycling treated effluent as make-up.