A method of filtering a liquid feed, comprising: a) passing a liquid feed in a single pass mode through a single pass tangential flow filtration (SPTFF) system that comprises: a plurality of filtration modules that are stacked to form levels, wherein each level comprises a single filtration module, and the plurality of filtration modules are fluidly connected, each filtration module comprising: a manifold segment comprising a first manifold for receiving and carrying the feed into the filtration module, a second manifold for receiving and carrying retentate out of the filtration module, and a third manifold for receiving and carrying permeate through the filtration module; a plurality of TFF cassettes that are stacked on one or both faces of, and are fluidly connected to, the manifold segment; a feed inlet on the first filtration module in the system; a retentate outlet on the last filtration module in the system; and valves positioned in the first and second manifolds, the valves permitting parallel flow between manifold segments when the valves are open, and serial flow between manifold segments when the valves are closed; the SPTFF system having a liquid flow path that is parallel through the cassettes in each filtration module and serial through the manifold segments in adjacent filtration modules by coupling of the first manifold in each manifold segment to the second manifold of an adjacent manifold segment, such that the retentate of one module serves as the feed for the next module, except that the first manifold in the first filtration module in the system is connected to the feed inlet and the second manifold in the last filtration module in the system is connected to the retentate outlet; and b) recovering the retentate and permeate from the system in separate containers (14, 15) without recirculation through the SPTFF system, thereby filtering the liquid feed. a) passing a liquid feed in a single pass mode through a single pass tangential flow filtration (SPTFF) system that comprises: a plurality of filtration modules that are stacked to form levels, wherein each level comprises a single filtration module, and the plurality of filtration modules are fluidly connected, each filtration module comprising: a manifold segment comprising a first manifold for receiving and carrying the feed into the filtration module, a second manifold for receiving and carrying retentate out of the filtration module, and a third manifold for receiving and carrying permeate through the filtration module; a plurality of TFF cassettes that are stacked on one or both faces of, and are fluidly connected to, the manifold segment; a feed inlet on the first filtration module in the system; a retentate outlet on the last filtration module in the system; and valves positioned in the first and second manifolds, the valves permitting parallel flow between manifold segments when the valves are open, and serial flow between manifold segments when the valves are closed; the SPTFF system having a liquid flow path that is parallel through the cassettes in each filtration module and serial through the manifold segments in adjacent filtration modules by coupling of the first manifold in each manifold segment to the second manifold of an adjacent manifold segment, such that the retentate of one module serves as the feed for the next module, except that the first manifold in the first filtration module in the system is connected to the feed inlet and the second manifold in the last filtration module in the system is connected to the retentate outlet; and a plurality of filtration modules that are stacked to form levels, wherein each level comprises a single filtration module, and the plurality of filtration modules are fluidly connected, each filtration module comprising: a manifold segment comprising a first manifold for receiving and carrying the feed into the filtration module, a second manifold for receiving and carrying retentate out of the filtration module, and a third manifold for receiving and carrying permeate through the filtration module; a plurality of TFF cassettes that are stacked on one or both faces of, and are fluidly connected to, the manifold segment; a manifold segment comprising a first manifold for receiving and carrying the feed into the filtration module, a second manifold for receiving and carrying retentate out of the filtration module, and a third manifold for receiving and carrying permeate through the filtration module; a plurality of TFF cassettes that are stacked on one or both faces of, and are fluidly connected to, the manifold segment; a feed inlet on the first filtration module in the system; a retentate outlet on the last filtration module in the system; and valves positioned in the first and second manifolds, the valves permitting parallel flow between manifold segments when the valves are open, and serial flow between manifold segments when the valves are closed; b) recovering the retentate and permeate from the system in separate containers (14, 15) without recirculation through the SPTFF system, thereby filtering the liquid feed.