A method for processing an initial liquid medium including cells in an initial concentration to obtain a product liquid medium including cells in a product concentration includes supplying the initial medium to a separator via a first supply system, separating the initial medium into various constituents including the product medium, extracting the product medium from the separator via a first outlet system, measuring a physical parameter related to the concentration of the cells in the initial medium using a sensor, and controlling at least one process parameter as a function of the physical parameter. During the step of separating the initial medium, a solution is supplied to the separator at a flow rate via a second supply system. The flow rate of the solution is determined based on the at least one process parameter, and is reduced when the concentration of the cells measured in the initial medium increases.