A filtration device that includes a cylindrical body and a filtration part. The cylindrical body has a first open end and a second closed end with an end wall. The filtration part is on a circumferential portion of the cylindrical body and has through-holes.

[Object] To provide a method for isolating and purifying a minute useful substance. [Solution] Disclosed is a method for isolating and purifying a minute useful substance. The method includes filtering a liquid containing a minute useful substance through a hollow fiber membrane. The hollow fiber membrane has an inner diameter of 0.2 mm to 1.4 mm and a molecular weight cut-off of 100000 to 1000000. The filtering includes a first filtration process of press-fitting the liquid containing the minute useful substance from a first opening on one end side of the hollow fiber membrane and filtering the liquid to separate the liquid into a permeate and a first concentrate, and a second filtration process of press-fitting the first concentrate from a second opening on the other end side of the hollow fiber membrane and filtering the first concentrate to separate the first concentrate into a permeate and a second concentrate. A concentrate is produced in which a concentration of the minute useful substance is increased by filtration in which the first filtration process and the second filtration process are alternately performed a plurality of times at a membrane surface velocity of 0.3 m/sec to 2 m/sec.

A filtration system, having a filtration unit comprising a housing with a sample inlet, a sample outlet, one or more valves, a first actuator, and a second actuator and a third actuator; and a cassette adapted to fit the filtration unit, the cassette having a filter; a first reservoir in fluid communication with the filter, the first reservoir being arranged to couple to the first actuator for imparting flow to a cell-containing solution in a first direction through the filter; a second reservoir in fluid communication with the filter, the second reservoir being arranged to couple to the second actuator for imparting flow to a cell-containing solution in a second direction through said filter, wherein the second direction opposes the first direction; and a third reservoir.

A cassette comprising: a first reservoir arranged to couple to a first actuator for imparting flow to a cell-containing solution in a first direction through a filter; a second reservoir arranged to couple to a second actuator for imparting flow to a cell-containing solution in a second direction through the filter; the filter being in fluid communication with the first and second reservoirs, and when connected to a filtration unit is in further fluid communication to a sample inlet, and a sample outlet; wherein, the cassette is disposable and housed by a connector being removeably adjoinable to the filtration unit, the filtration unit comprising the first and second actuators and one or more valves.

A flow-through apparatus is provided. The flow-through apparatus includes a housing having a fixed volume and having an inlet, an outlet, and a cavity. The flow-through apparatus also includes a first filter in fluid communication with the inlet and disposed within the cavity and a seal attaching the first filter to the housing. The first filter has a mesh size of about 100 microns or less and a first internal volume less than the fixed volume of the housing.