A filtration module comprising: a plurality of stages , each stage having a plurality of channels (841, 841', 841") providing at least one serial flow path, each stage being in fluid communication with each adjacent stage preceding it and being in fluid communication with each adjacent stage that follows it; each of the plurality of channels (841, 841', 841") comprising a filtration membrane and having a length, a membrane area, a void volume, a specific membrane area expressed as a ratio of the membrane area to the void volume, and a dimensionless length expressed as a product of the channel length and the specific membrane area; wherein a dimensionless length of a stage is the sum of the dimensionless lengths of each channel in the longest serial flow path in the stage and the dimensionless length of the module is the sum of the dimensionless lengths of the plurality of stages ; and said filtration module being characterized in that said filtration module is set up as a single-pass tangential flow module without a recirculation loop and comprising means to provide a feed stream to the feed of the filtration module and means to adjust the specific feed flow rate to less than 200 liters per hour per m 2 (lmh); in that said filtration membrane is in the form of a flat-sheet membrane; in that the specific membrane area of at least one channel is greater than 40 cm -1 , the dimensionless length of the module is greater than 2,000 and the dimensionless length of at least one of the plurality of stages is less than 6,000; and in that - the module is internally staged and further comprises a housing, wherein the plurality of stages is disposed within the housing, - or in that the module is externally staged, wherein the stages are coupled together with discrete external connections. a plurality of stages , each stage having a plurality of channels (841, 841', 841") providing at least one serial flow path, each stage being in fluid communication with each adjacent stage preceding it and being in fluid communication with each adjacent stage that follows it; each of the plurality of channels (841, 841', 841") comprising a filtration membrane and having a length, a membrane area, a void volume, a specific membrane area expressed as a ratio of the membrane area to the void volume, and a dimensionless length expressed as a product of the channel length and the specific membrane area; wherein a dimensionless length of a stage is the sum of the dimensionless lengths of each channel in the longest serial flow path in the stage and the dimensionless length of the module is the sum of the dimensionless lengths of the plurality of stages ; and said filtration module being characterized in that said filtration module is set up as a single-pass tangential flow module without a recirculation loop and comprising means to provide a feed stream to the feed of the filtration module and means to adjust the specific feed flow rate to less than 200 liters per hour per m 2 (lmh); in that said filtration membrane is in the form of a flat-sheet membrane; in that the specific membrane area of at least one channel is greater than 40 cm -1 , the dimensionless length of the module is greater than 2,000 and the dimensionless length of at least one of the plurality of stages is less than 6,000; and in that - the module is internally staged and further comprises a housing, wherein the plurality of stages is disposed within the housing, - or in that the module is externally staged, wherein the stages are coupled together with discrete external connections. - the module is internally staged and further comprises a housing, wherein the plurality of stages is disposed within the housing, - or in that the module is externally staged, wherein the stages are coupled together with discrete external connections. <EOS>