Sorbent cartridges having a polystyrene sulfonate resin saturated with calcium ions for the performance of kidney replacement therapy are disclosed. Systems and methods having or using a sorbent cartridge, a dialyzer, control components, a cartridge having a polystyrene sulfonate resin, and fluid reservoirs configured to be of a weight and size suitable to be worn or carried by an individual requiring treatment are disclosed. A system for performing kidney replacement therapy has a controlled compliance dialysis circuit, where a control pump controls the bi-directional movement of fluid across a dialysis membrane. The system provides for the monitoring of an inlet and outlet conductivity of the sorbent cartridge to quantify or monitor the removal of urea by the sorbent cartridge.