There is disclosed herein a pump cassette comprising: a primary reciprocating membrane pump having a pumping chamber with an inlet of the reciprocating membrane pump fluidly connected to a first inlet of the cassette； a mixing chamber having an inlet fluidly connected to an outlet of the reciprocating membrane pump, and an outlet fluidly connected to an outlet of the cassette； a first metering pump having an inlet fluidly connected to a second inlet of the cassette, and an outlet fluidly connected to the inlet of the mixing chamber； and a second metering pump having an inlet fluidly connected to a third inlet of the cassette, and an outlet fluidly connected to the inlet of the reciprocating membrane pump, wherein 1) the pump cassette is arranged to receive a first fluid into the pumping chamber of the primary reciprocating membrane pump via the first inlet of the cassette, and to receive a second fluid via the third inlet of the cassette, the second fluid being pumped by the second metering pump into the pumping chamber of the primary reciprocating membrane pump, 2) the pump cassette is arranged to receive a third fluid via the second inlet of the cassette, the third fluid being pumped by the first metering pump into the mixing chamber, and 3) a pre-determined volume of first fluid and second fluid are mixed in the pumping chamber of the primary reciprocating membrane pump, and then pumped into the mixing chamber to be mixed with a pre-determined volume of third fluid pumped by the first metering pump.
