A system for cleaning feed water of variable quality, the system comprising an inlet for selectively delivering feed water (FW) to one or other of at least two feed chambers (2,4), each feed chamber having a delivery pipe (2i, 4i) for delivering feed water to a reverse osmosis or nanofiltration (8); a pump (6) to deliver the feed water from one of the chambers (2, 4) through its associated delivery pipe (2i, 4i) to the reverse osmosis or nanofiltration (8) to create a concentrated feed stream and a product water stream (PW); return pipes (2R, 4R) for selectively returning the concentrated feed stream to one or other of the at least two feed chambers (2, 4); a product water outlet for removal of the product water (PW); and means for switching the delivery of the concentrated feed stream between the selectable return pipes (2R, 4R) upon detection of a predetermined reduction in the efficiency of the RO or NF process within one or other of the feed chambers (2, 4). The pressure of the concentrated feed stream is reduced to atmospheric pressure prior to its delivery back to the chamber and the feed stream passes through a desaturation unit (20).