An apparatus for ultrafiltration of a patient being overhydrated due to congestive heart failure, comprising a tube set including a connector for connection to a patient line for access to the peritoneal cavity of the patient. A flow pump is arranged for addition and removal outflow and inflow (recirculation) of fluid from/to the peritoneal cavity. An osmotic agent peristaltic pump is arranged for replenishment of glucose solution to the fluid added to the peritoneal cavity for promoting ultrafiltration. The glucose is replenished intermittently for keeping a concentration of glucose substantially constant in the peritoneal cavity. The flow pump comprises a pressure chamber with rigid walls and a flexible pump bag arranged therein. An air pump pressurizes the chamber for outflow of fluid from the peritoneal cavity by a sub pressure and inflow of fluid to the peritoneal cavity by an overpressure, which pressures are maintained within safe limits.