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The ballasted flocculation (BF) process is widely used in the water treatment process to cope with climate change. Microsand and magnetite are commonly used, which act as a seed in ballasted floc. The recovery of the ballast in BF process can affect the operating cost. In this study, we evaluate various sizes of magnetite particle with a specific gravity of above 5 in order to apply the hydrocyclone as a ballast recovery system in the BF process. The conventional purpose of the hydrocyclone is to separate solids and liquids, while the ideal recovery system in BF process should separate the ballast from the sludge and liquid. Computational fluid dynamics modeling was prepared to estimate the optimal conditions for the ballast recovery and the sludge removal ratio. The average ballast recovery ratio shows up to 99.9% varying with the inlet flow velocity. As the inlet flow velocity increases, the average sludge removal ratio decreases until 2 m/s, but it changes to increase after 2 m/s. The highest ratio is approximately 91% at 3.5 m/s of inlet flow velocity. This difference is due to the Reynolds number, which are summarized as the change of the particle size, the inlet flow velocity, and the specific gravity. In addition, the separation efficiency of each particle shows a fish-hook shape related with the Reynolds number. In BF process, the ballast recovery ratio should be maintained high, while the sludge recovery ratio should be maintained low. Criteria for the ballast and sludge recovery can be reached by maintaining the range of particle size, simultaneously satisfying this requirement at a certain inlet flow velocity. In other respects, for optimal ballast recovery and sludge removal, above a certain size of ballast should be used, and the sludge needs to be pulverized to a certain size as the pretreatment of the hydrocyclone.