Described are peritoneal dialysis systems and methods that involve the use of crossflow filtration of a used dialysate withdrawn from the peritoneal space of a patient. The filtration forms a retentate containing amounts of an osmotic agent and a permeate containing amounts of water and nitrogen-containing waste products of the patient. The retentate, or a fraction thereof, can be returned to the peritoneal space of the patient to return osmotic agent to the patient. The permeate, or a fraction thereof, can be discarded to discard nitrogen-containing waste products of the patient.
