A dialysis system comprising: a fresh dialysis fluid membrane pump; a spent dialysis fluid membrane pump; a first balance chamber having a fresh dialysis fluid compartment connected fluidly to the fresh dialysis fluid membrane pump and a spent dialysis fluid compartment connected fluidly to the spent dialysis fluid membrane pump; and a second balance chamber having a fresh dialysis fluid compartment connected fluidly to the fresh dialysis fluid membrane pump and a spent dialysis fluid compartment connected fluidly to the spent dialysis fluid membrane pump, wherein: (i) the first and second balance chambers (340a, 340b) are provided in a cassette, the cassette having first and second portions (96, 98) and a flexible membrane that define the fresh and spent dialysis fluid compartments (344a, 344b, 342a, 342b) for each balance chamber, (ii) the fresh dialysis fluid compartment of the first balance chamber and the fresh dialysis fluid compartment of the.second balance chamber each have a single port for (a) receiving fresh dialysis fluid from the fresh dialysis fluid membrane pump, and (b) delivering fresh dialysis fluid from the respective fresh dialysis fluid compartment (344a, 344b), and (iii) the spent dialysis fluid compartment of the first balance chamber and the spent dialysis fluid compartment of the second balance chamber each have a single port for (a) receiving spent dialysis fluid from the spent dialysis fluid membrane pump, and (b) delivering spent dialysis fluid from the respective spent dialysis fluid compartment (342a, 342b). a fresh dialysis fluid membrane pump; a spent dialysis fluid membrane pump; a first balance chamber having a fresh dialysis fluid compartment connected fluidly to the fresh dialysis fluid membrane pump and a spent dialysis fluid compartment connected fluidly to the spent dialysis fluid membrane pump; and a second balance chamber having a fresh dialysis fluid compartment connected fluidly to the fresh dialysis fluid membrane pump and a spent dialysis fluid compartment connected fluidly to the spent dialysis fluid membrane pump, wherein: (i) the first and second balance chambers (340a, 340b) are provided in a cassette, the cassette having first and second portions (96, 98) and a flexible membrane that define the fresh and spent dialysis fluid compartments (344a, 344b, 342a, 342b) for each balance chamber, (ii) the fresh dialysis fluid compartment of the first balance chamber and the fresh dialysis fluid compartment of the.second balance chamber each have a single port for (a) receiving fresh dialysis fluid from the fresh dialysis fluid membrane pump, and (b) delivering fresh dialysis fluid from the respective fresh dialysis fluid compartment (344a, 344b), and (iii) the spent dialysis fluid compartment of the first balance chamber and the spent dialysis fluid compartment of the second balance chamber each have a single port for (a) receiving spent dialysis fluid from the spent dialysis fluid membrane pump, and (b) delivering spent dialysis fluid from the respective spent dialysis fluid compartment (342a, 342b). (i) the first and second balance chambers (340a, 340b) are provided in a cassette, the cassette having first and second portions (96, 98) and a flexible membrane that define the fresh and spent dialysis fluid compartments (344a, 344b, 342a, 342b) for each balance chamber, (ii) the fresh dialysis fluid compartment of the first balance chamber and the fresh dialysis fluid compartment of the.second balance chamber each have a single port for (a) receiving fresh dialysis fluid from the fresh dialysis fluid membrane pump, and (b) delivering fresh dialysis fluid from the respective fresh dialysis fluid compartment (344a, 344b), and (iii) the spent dialysis fluid compartment of the first balance chamber and the spent dialysis fluid compartment of the second balance chamber each have a single port for (a) receiving spent dialysis fluid from the spent dialysis fluid membrane pump, and (b) delivering spent dialysis fluid from the respective spent dialysis fluid compartment (342a, 342b).