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 blood leak sensor assembly, and a processor connected to the various sensors for controlling and monitoring hemodialysis treatment. The blood leak sensor assembly includes a light source for emitting light through the dialysate flow path and a light sensor for receiving light having passed through the dialysate flow path. The processor is connected to the light source for determining whether blood has leaked from the blood flow path into the dialysate flow path. Preferably, the light source emits light having two peak wavelengths.