A method of performing a peritoneal dialysis treatment includes connecting a disposable unit to a source of water. The disposable unit includes at least a first container holding a sterile concentrate containing an osmotic agent, a second container holding a sterile concentrate containing electrolytes, an empty sterile mixing container, and a tubing set with a pre-attached peritoneal fill/drain line. The method also includes receiving a prescription command by a controller, indicating at least the fill volume and desired final concentration of the osmotic agent to be used for a current fill cycle under said treatment, and pumping a quantity of the concentrated osmotic agent that is at least sufficient to achieve the desired final concentration into the mixing container. The contest of the mixing container are mixed, further diluted or concentrated, and then flowed to a patient.