a range of about 2 5 feet to 3 0 feet clear space between cassettes can be from zero virtually touching to about two or three inches the recycle wall 28 may also carry some of the fluid services e g air in permeate water out if desired for the ir internal recycle and ras or return flow exiting the mbr the channel 34 can be a long trough that runs down the length of the reactor which could be much longer in length than the example schematically shown this improves flow distribution and can eliminate equipment necessary to conventional plants fig4 is a schematic view in transverse section showing essentially the mbr arrangement of fig3 e and 3f here the recycle wall or compartment 28 is shown between pairs of cassettes 22 arranged longitudinally the cassettes of a pair can be virtually touching or about one to three inches apart in this configuration the actual flat sheet membranes are parallel to the plane of the cassette 39 s side seen in fig4 the liquid surface of the mixed liquor is shown at 33 with the membrane assemblies cassettes or units submerged within the liquid a return zone or overflow area is indicated at 40 as part of the trough structure 34 with an open side 42 facing the mixed liquor in the tank volume 44 at each side of the recycle.