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 further includes a filter for removing waste products from the dialysate as well as one or more flow sensors for measuring the flow of dialysate in the dialysate flow path. The preferred flow sensor includes a rotatable spoked wheel in the dialysate flow path which is rotated by the flow of dialysate. The spoked wheel includes one or more magnets which are rotated with the rotation of the spoked wheel. The flow sensor further includes a magnetic field sensor in the reused dialysis machine which is connected to a processor for monitoring the rotation of the spoked wheel to determine flow of fluid in the dialysate flow path.