A pumping cassette comprising: a housing comprising at least a first fluid inlet, where a first fluid enters the cassette, and a first fluid outlet, where the first fluid exits the cassette; a second fluid inlet, where a second fluid enters the cassette, and a second fluid outlet, where the second fluid exits the cassette; at least one balancing pod within said housing, wherein said at least one balancing pod includes a balancing pod A and a balancing pod B, each balancing pod comprising a membrane forming two balancing chambers; and at least one reciprocating positive-displacement membrane pump within said housing, wherein said at least one positive-displacement membrane pump includes a pod pump A and a pod pump B and each said positive-displacement pump is arranged to pump a volume of said second fluid from the positive-displacement pump (4828, 4820) to the balancing pods (4812, 4822), wherein said pumping cassette is arranged such that: when the first fluid flows into the balancing pod A, the membrane thereof forces a volume of second fluid contained therein out of the balancing pod A and the second fluid flows out of the second fluid outlet; at the same time, pod pump B includes a volume of second fluid and this is pumped to balancing pod B, whereby a volume of first fluid contained in balancing pod B is displaced by the volume of second fluid and flows to the first fluid outlet and exits the cassette; a volume of the second fluid is pumped from pod pump A to balancing pod A and displaces first fluid in the balancing pod A, and the first fluid displaced from the balancing pod A flows to the first fluid outlet; and when the first fluid flows to balancing pod B it displaces second fluid in the balancing pod B, forcing the second fluid to flow out of the cassette through the second fluid outlet, whereby said balancing pod (4812, 4822) balances the flow of the first fluid and the flow of the second fluid such that the volume of said first fluid equals the volume of said second fluid. a housing comprising at least a first fluid inlet, where a first fluid enters the cassette, and a first fluid outlet, where the first fluid exits the cassette; a second fluid inlet, where a second fluid enters the cassette, and a second fluid outlet, where the second fluid exits the cassette; at least one balancing pod within said housing, wherein said at least one balancing pod includes a balancing pod A and a balancing pod B, each balancing pod comprising a membrane forming two balancing chambers; and at least one reciprocating positive-displacement membrane pump within said housing, wherein said at least one positive-displacement membrane pump includes a pod pump A and a pod pump B and each said positive-displacement pump is arranged to pump a volume of said second fluid from the positive-displacement pump (4828, 4820) to the balancing pods (4812, 4822), wherein said pumping cassette is arranged such that: when the first fluid flows into the balancing pod A, the membrane thereof forces a volume of second fluid contained therein out of the balancing pod A and the second fluid flows out of the second fluid outlet; at the same time, pod pump B includes a volume of second fluid and this is pumped to balancing pod B, whereby a volume of first fluid contained in balancing pod B is displaced by the volume of second fluid and flows to the first fluid outlet and exits the cassette; a volume of the second fluid is pumped from pod pump A to balancing pod A and displaces first fluid in the balancing pod A, and the first fluid displaced from the balancing pod A flows to the first fluid outlet; and when the first fluid flows to balancing pod B it displaces second fluid in the balancing pod B, forcing the second fluid to flow out of the cassette through the second fluid outlet, whereby said balancing pod (4812, 4822) balances the flow of the first fluid and the flow of the second fluid such that the volume of said first fluid equals the volume of said second fluid. when the first fluid flows into the balancing pod A, the membrane thereof forces a volume of second fluid contained therein out of the balancing pod A and the second fluid flows out of the second fluid outlet; at the same time, pod pump B includes a volume of second fluid and this is pumped to balancing pod B, whereby a volume of first fluid contained in balancing pod B is displaced by the volume of second fluid and flows to the first fluid outlet and exits the cassette; a volume of the second fluid is pumped from pod pump A to balancing pod A and displaces first fluid in the balancing pod A, and the first fluid displaced from the balancing pod A flows to the first fluid outlet; and when the first fluid flows to balancing pod B it displaces second fluid in the balancing pod B, forcing the second fluid to flow out of the cassette through the second fluid outlet, whereby said balancing pod (4812, 4822) balances the flow of the first fluid and the flow of the second fluid such that the volume of said first fluid equals the volume of said second fluid.