A dialysis system comprising: a module that is configured to be connected to a dialysis machine having a dialyzer configured to remove one or more substances from a dialysis solution as the dialysis solution passes through the dialyzer, the module comprising: a fluid line (311, 312, 313, 314, 315, 323, 336, 326) configured to be, when in use, in fluid communication with the dialyzer; a sorbent cartridge in fluid communication with the fluid line; and a sodium control system in fluid communication with the fluid line, the sodium control system being adapted to actively alter a sodium concentration of dialysis solution passing through the fluid line as the dialysis solution exits the sorbent cartridge; the sodium control system comprising a conductivity sensor that is adapted to send a conductivity signal indicating the conductivity of the dialysis solution as the dialysis solution exits the sorbent cartridge, the conductivity sensor being in communication with the sodium control system, and a processor configured to receive the conductivity signal from the conductivity sensor, wherein the processor is configured to compare the conductivity signal to a threshold value lower than a prescription value and cause the sodium control system to stop actively altering the sodium concentration if the conductivity signal is greater than the threshold value but below the prescription value, and add dilution water to the solution exiting the sorbent cartridge if the conductivity signal is greater than the prescription value. a fluid line (311, 312, 313, 314, 315, 323, 336, 326) configured to be, when in use, in fluid communication with the dialyzer; a sorbent cartridge in fluid communication with the fluid line; and a sodium control system in fluid communication with the fluid line, the sodium control system being adapted to actively alter a sodium concentration of dialysis solution passing through the fluid line as the dialysis solution exits the sorbent cartridge; the sodium control system comprising a conductivity sensor that is adapted to send a conductivity signal indicating the conductivity of the dialysis solution as the dialysis solution exits the sorbent cartridge, the conductivity sensor being in communication with the sodium control system, and a processor configured to receive the conductivity signal from the conductivity sensor, wherein the processor is configured to compare the conductivity signal to a threshold value lower than a prescription value and cause the sodium control system to stop actively altering the sodium concentration if the conductivity signal is greater than the threshold value but below the prescription value, and add dilution water to the solution exiting the sorbent cartridge if the conductivity signal is greater than the prescription value. a conductivity sensor that is adapted to send a conductivity signal indicating the conductivity of the dialysis solution as the dialysis solution exits the sorbent cartridge, the conductivity sensor being in communication with the sodium control system, and a processor configured to receive the conductivity signal from the conductivity sensor, wherein the processor is configured to compare the conductivity signal to a threshold value lower than a prescription value and cause the sodium control system to stop actively altering the sodium concentration if the conductivity signal is greater than the threshold value but below the prescription value, and add dilution water to the solution exiting the sorbent cartridge if the conductivity signal is greater than the prescription value. compare the conductivity signal to a threshold value lower than a prescription value and cause the sodium control system to stop actively altering the sodium concentration if the conductivity signal is greater than the threshold value but below the prescription value, and add dilution water to the solution exiting the sorbent cartridge if the conductivity signal is greater than the prescription value.