A hemodialysis system comprising: a cassette system, comprising a directing fluid circuit, a mixing fluid circuit and a balancing fluid circuit, wherein the directing fluid circuit is configured to direct water from a water supply to the mixing fluid circuit and direct dialysate from the mixing fluid circuit to the balancing fluid circuit, the mixing fluid circuit is configured to mix water from the directing fluid circuit with one or more ingredients to produce dialysate, and the balancing fluid circuit is configured to control the amount of dialysate passing through a dialyzer; wherein the mixing fluid circuit comprises a mixing cassette for mixing water with bicarbonate solution to form water and bicarbonate mixture, and for mixing the water and bicarbonate mixture with acid solution to form dialysate, the mixing cassette configured to: send bicarbonate and water mixture to an external sensor cassette, and receive the bicarbonate and water mixture from the sensor cassette; mix the bicarbonate and water mixture with acid solution to form dialysate if the bicarbonate and water mixture meets pre-established thresholds in the sensor cassette; send the dialysate to the sensor cassette, and receive the dialysate from the sensor cassette; and send the dialysate to drain if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is not within set parameters/thresholds; or send the dialysate to an external dialysate reservoir if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is within the set parameters/thresholds. a cassette system, comprising a directing fluid circuit, a mixing fluid circuit and a balancing fluid circuit, wherein the directing fluid circuit is configured to direct water from a water supply to the mixing fluid circuit and direct dialysate from the mixing fluid circuit to the balancing fluid circuit, the mixing fluid circuit is configured to mix water from the directing fluid circuit with one or more ingredients to produce dialysate, and the balancing fluid circuit is configured to control the amount of dialysate passing through a dialyzer; wherein the mixing fluid circuit comprises a mixing cassette for mixing water with bicarbonate solution to form water and bicarbonate mixture, and for mixing the water and bicarbonate mixture with acid solution to form dialysate, the mixing cassette configured to: send bicarbonate and water mixture to an external sensor cassette, and receive the bicarbonate and water mixture from the sensor cassette; mix the bicarbonate and water mixture with acid solution to form dialysate if the bicarbonate and water mixture meets pre-established thresholds in the sensor cassette; send the dialysate to the sensor cassette, and receive the dialysate from the sensor cassette; and send the dialysate to drain if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is not within set parameters/thresholds; or send the dialysate to an external dialysate reservoir if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is within the set parameters/thresholds. send bicarbonate and water mixture to an external sensor cassette, and receive the bicarbonate and water mixture from the sensor cassette; mix the bicarbonate and water mixture with acid solution to form dialysate if the bicarbonate and water mixture meets pre-established thresholds in the sensor cassette; send the dialysate to the sensor cassette, and receive the dialysate from the sensor cassette; and send the dialysate to drain if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is not within set parameters/thresholds; or send the dialysate to an external dialysate reservoir if it is determined, after the dialysate flows to the sensor cassette, that the dialysate is within the set parameters/thresholds.