A pump cassette comprising: a primary reciprocating pressure displacement membrane pump (820, 828) within a housing of the pump cassette (800, 1100, 1200, 1300, 1400) having a pumping chamber with an inlet (850, 852) 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 (838, 840, 842, 844, 846, 848) of the reciprocating pressure displacement membrane pump, and an outlet fluidly connected to an outlet of the cassette; a first metering pump having an inlet (832, 834) 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 (854, 858) fluidly connected to a third inlet of the cassette, and an outlet (856, 860) fluidly connected to the inlet of the reciprocating membrane pump, wherein 1) the pump cassette (800, 1100, 1200, 1300, 1400) is arranged to receive a first fluid into the pumping chamber of the primary reciprocating pressure displacement 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 (800, 1100, 1200, 1300, 1400) 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 pressure displacement 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. a primary reciprocating pressure displacement membrane pump (820, 828) within a housing of the pump cassette (800, 1100, 1200, 1300, 1400) having a pumping chamber with an inlet (850, 852) 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 (838, 840, 842, 844, 846, 848) of the reciprocating pressure displacement membrane pump, and an outlet fluidly connected to an outlet of the cassette; a first metering pump having an inlet (832, 834) 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 (854, 858) fluidly connected to a third inlet of the cassette, and an outlet (856, 860) fluidly connected to the inlet of the reciprocating membrane pump, wherein 1) the pump cassette (800, 1100, 1200, 1300, 1400) is arranged to receive a first fluid into the pumping chamber of the primary reciprocating pressure displacement 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 (800, 1100, 1200, 1300, 1400) 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 pressure displacement 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. 1) the pump cassette (800, 1100, 1200, 1300, 1400) is arranged to receive a first fluid into the pumping chamber of the primary reciprocating pressure displacement 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 (800, 1100, 1200, 1300, 1400) 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 pressure displacement 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.