Assembly for withdrawing and filtering partial volumes of process fluid

A method for withdrawing and filtering a partial volume of a particle-containing process fluid enclosed in a container, the method comprising the following steps:

• providing an aperture in a wall of the container,
• providing a filtering device having a filtering screen having a filter surface and adapted for rotation about an axis of rotation, wherein said surface is rotated around said axis at a speed sufficient to cause at least a substantial area of said surface to move at a tangential speed of at least approx. 0.3 meters/second, withdrawing means for withdrawing filtered process fluid and rotating means for rotating said surface around said axis,
• fixing said filtering device in said aperture such that the filtering screen is immersed in said process fluid,
• establishing a pressure differential through said filtering screen for causing said process fluid to flow therethrough to said withdrawing means, said flow of the process fluid having a linear velocity, and
• rotating said surface around said axis, while ensuring free access of said process fluid to at least a portion of said surface
• and wherein the method does not include a step of relying on a narrow gap between a stationary body and said rotating filter surface
• and wherein the linear velocity of said process fluid through the filtering screen is at least approx. 0.2 meters/second less than said tangential speed
• and wherein the method is used for essentially continuous, on-line filtration of biological process fluids.