A portable hemodialysis system is provided including a disposable cartridge and a reused dialysis machine. The disposable cartridge includes a dialysate flow path and a blood flow path which flow in opposite directions through a dialyzer. The disposable cartridge includes a filter for removing waste products from the dialysate. The reused dialysis machine possesses a reservoir for dialysate, various sensors including a reservoir level sensor assembly, and a processor connected to the various sensors for controlling and monitoring hemodialysis treatment. The reservoir level sensor assembly includes a vertically extending electrode array for transmitting electrical signals to the processor detecting whether the reservoir possesses sufficient dialysate for therapy, and for detecting whether there is a fault condition during therapy such as a loss of dialysate in the reservoir or an increase in fluid in the reservoir which may indicate a failure in the dialysis membrane.
