The invention concerns a bag comprising a first conduit (13C) which extends longitudinally between a flow pump connector (11C) emerging on a first side (68) and a tangential filter connector (11M) emerging on a second side (69); a second conduit (13B) which extends longitudinally from a first side of said conduit (13C) between a supply container connector (11B) emerging on said first side (68), and another tangential flow connector (11N) emerging on said second side (69); a third conduit (13H) which extends from a second side of said conduit (13C), starting at a collecting container connector (11J), until it enters said first conduit (13C); and a fourth conduit (13A) which extends from the first side of said conduit (13C), starting at a transfer pump connector (11A), until it enters said second conduit (13B).