Described are peritoneal dialysis systems and methods that involve the use of first and second stage filtration of a used dialysate withdrawn from the peritoneal space of a patient. The first filtration stage forms a first retentate containing an osmotic agent and a first permeate containing water and nitrogen-containing waste products of the patient. The second filtration stage acts on the first permeate to form a second retentate containing nitrogen-containing waste products of the patient and a second permeate containing water. At least some of the water from the second permeate is combined with the first retentate to form a regenerated peritoneal dialysis medium containing an amount of the osmotic agent. The regenerated peritoneal dialysis medium can be returned to the peritoneal space of the patient.
