A method for cleaning membranes and an inlet side of a membrane filtration module of an apparatus includes filling a basin with a treatment fluid; feeding an incoming fluid to the treatment fluid present in the basin and mixing and treating it with the latter fluid so as to obtain a fluid mixture; feeding the fluid mixture to the membrane filtration module; discharging permeate filtered by and retentate retained by the membranes; and periodically closing the fluid mixture feed-through line and opening the flushing discharge line, so that at least the inlet side of the membranes and the connection chamber of the membrane filtration module disposed underneath it are flushed.