T The conventional 3-phase separator installed cannot anticipate the increasein fluid flow in line with the increase in the production rate from the well so thatthe carried-oil content in the wastewater from the separator always exceeds the operating threshold. Optimization of separation is a necessity to be able to reduce the oil-carrying content so as not to burden the wastewater treatment unit. The purpose of this study was to test the application of a hydrocyclonedevice at the inlet of the first stage 3-phase horizontal separator, and obtain a range of production and operating parameters to optimize the separation process in orderto reduce the oil-carrying content in wasted water below the threshold. Hydrocyclone is widely used as an auxiliary device that can optimize oilseparation by minimizing oil-carrying into the waste water stream. During the research, a simulation was conducted to compare the installation of a hydrocycloneat the inlet of a 3-phase horizontal separator that is currently installed and without the installation of a hydrocyclone.The hydrocyclone design was also improved by changing the model basedon flow characteristics with CFD simulation.The correlation of carried over-oil content with water-cut, production flow rate, pressure, and temperature was carried out to obtain optimum operating andproduction parameters from the 3-stage separation with Hysis simulation. Based on the results of the study, it was concluded that improving thehydrocyclone design obtained and fluid flow characteristics with CFD and applying hydrocyclone at the inlet of a 3-phase horizontal separator increased the efficiency of the production fluid separation process so as to reduce the oil-carrying content in the wastewater flow below 20 mg/l.The results of this study are in line and support the de-bottlenecking plan toincrease oil production from 175,000 bopd to a production target 225,000 bopd byOil Trench – 1.