An apparatus for ultrafiltration of a patient being overhydrated, comprising a patient tube (22) for access to a peritoneal cavity of the patient； a flow pump (41, 43) for removal of a fluid via said patient tube from the peritoneal cavity and to a pump bag (41) and for filling of the fluid from the pump bag (41) to the peritoneal cavity； a glucose bag (26), comprising concentrated glucose solution； a glucose pump (16) for addition of concentrated glucose to said fluid； whereby the glucose is diluted in said fluid and replenished intermittently for keeping a concentration of the osmotic agent substantially constant in the peritoneal cavity. The flow pump comprises a pump chamber having rigid walls and a flexible pump bag being inserted in said pump chamber, whereby the exterior of the flexible pump bag is exposed to a positive or negative pressure inside said pump chamber. The pump chamber is elongated and is provided with a lid (62) at one end and a cap (61) at the other end, which both are removable for cleaning the interior of the pump chamber. The pump chamber comprises an overpressure when the lid is going to be removed after a treatment and/or a sub pressure when the lid is going to be inserted before a treatment.
