Abstract: A raw fluid containing foreign materials is led to the inlet of a membrane filter equipment (10) which provides on the one hand a concentrate containing more foreign materials than that of the raw fluid, on the other hand a permeate containing less foreign materials than that of the raw fluid. The concentrate leaving the membrane filter equipment (10) is fed back entirely to the inlet of the membrane filter equipment (10) and the fluid is circulated under high pressure until a predetermined concentration value is reached, then the concentrated fluid is discharged and steps are repeated with a further amount of raw fluid. A continuous operation of the membrane filter equipment (10) and a pump (13) ensuring a high pressure in it can be provided by two vessels (34, 35) which are arranged in the fluid circuit and adapted to be switched over.