A reversible liquid filtration system for cell culture perfusion, comprising: a bioreactor vessel, for storing the cell culture; a perfusion pump, comprising a reciprocable element which is movable in opposing first and second pumping directions (dF, dR); a filter, comprising a membrane which includes a retentate side and an opposing, permeate side; and first and second bi-directional valves (BV1, BV2), each selectively controllable between open and closed positions, the perfusion pump, the filter, and the first and second bidirectional valves (BV1, BV2), together comprising a fluidic circuit in communication with the bioreactor vessel, wherein, in dependence on the selected positions of the first and second bi-directional valves (BV1, BV2) and the pumping direction (dF, dR), the system is selectively operable between: a reversible circuit flow mode, wherein the cell culture will flow, around the fluidic circuit, in a first circuit flow direction or in a second circuit flow direction which is opposite to the first circuit flow direction; and an alternating flow mode, wherein the cell culture will alternately flow, around only a portion of the fluidic circuit, between a first alternating flow direction and in a second alternating flow direction which is opposite to the first alternating flow direction, and wherein, in each of the circuit flow mode and the alternating flow mode, the perfusion pump is operable to displace the cell culture along the retentate side of the filter to cause a driving fluid pressure (pD) on the retentate side of the filter membrane such that permeate material may pass through the membrane from the retentate side to the permeate side. a bioreactor vessel, for storing the cell culture; a perfusion pump, comprising a reciprocable element which is movable in opposing first and second pumping directions (dF, dR); a filter, comprising a membrane which includes a retentate side and an opposing, permeate side; and first and second bi-directional valves (BV1, BV2), each selectively controllable between open and closed positions, the perfusion pump, the filter, and the first and second bidirectional valves (BV1, BV2), together comprising a fluidic circuit in communication with the bioreactor vessel, wherein, in dependence on the selected positions of the first and second bi-directional valves (BV1, BV2) and the pumping direction (dF, dR), the system is selectively operable between: a reversible circuit flow mode, wherein the cell culture will flow, around the fluidic circuit, in a first circuit flow direction or in a second circuit flow direction which is opposite to the first circuit flow direction; and an alternating flow mode, wherein the cell culture will alternately flow, around only a portion of the fluidic circuit, between a first alternating flow direction and in a second alternating flow direction which is opposite to the first alternating flow direction, a reversible circuit flow mode, wherein the cell culture will flow, around the fluidic circuit, in a first circuit flow direction or in a second circuit flow direction which is opposite to the first circuit flow direction; and an alternating flow mode, wherein the cell culture will alternately flow, around only a portion of the fluidic circuit, between a first alternating flow direction and in a second alternating flow direction which is opposite to the first alternating flow direction, and wherein, in each of the circuit flow mode and the alternating flow mode, the perfusion pump is operable to displace the cell culture along the retentate side of the filter to cause a driving fluid pressure (pD) on the retentate side of the filter membrane such that permeate material may pass through the membrane from the retentate side to the permeate side.