A system for cleaning feed water of variable quality, the system comprising: (a) an inlet for selectively delivering feed water 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; (b) a pump to deliver the feed water from one of the chambers (2, 4) through its associated delivery pipe (2i, 4i) to a delivery line connected to the reverse osmosis or nanofiltration membrane to create a concentrated feed stream and a product water stream; (c) a return line connected to return pipes (2R, 4R) for selectively returning the concentrated feed stream to one or other of the at least two feed chambers (2, 4); (d) a product water outlet for removal of the product water; (e) 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); wherein the predetermined reduction in the efficiency of the RO or NF process is detected by a predetermined maximum salt concentration corresponding to the maximum osmotic pressure at which the reverse osmosis or nanofiltration membrane can operate; (f) a desaturation unit provided in the return line between the reverse osmosis or nanofiltration brine and the feed chamber, the desaturation unit being arranged to remove contaminants selected from a group consisting of salts, dissolved salts, sparingly soluble salts or any combination thereof, from the feed stream after its passage through the reverse osmosis or nanofiltration membrane, wherein the desaturation unit is a fluidised bed reactor, and a pressure exchanger is provided that is configured to reduce the pressure of the concentrated feed stream in the return line to substantially atmospheric pressure, wherein the pressure exchanger is downstream of the RO or NF and upstream of the desaturation unit; and wherein the switching means is adapted to enable the delivery of feed water from the first chamber through a first delivery pipe to the reverse osmosis or nanofiltration to be recycled through its return pipe to the first chamber until the predetermined reduction in efficiency of the RO or NF process is detected in that chamber whereupon the switching means enables feed water to be delivered from the second chamber though a second delivery pipe to the reverse osmosis or nanofiltration to be recycled through its return pipe to the second chamber until the predetermined reduction in efficiency is detected in the second chamber. (a) an inlet for selectively delivering feed water 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; (b) a pump to deliver the feed water from one of the chambers (2, 4) through its associated delivery pipe (2i, 4i) to a delivery line connected to the reverse osmosis or nanofiltration membrane to create a concentrated feed stream and a product water stream; (c) a return line connected to return pipes (2R, 4R) for selectively returning the concentrated feed stream to one or other of the at least two feed chambers (2, 4); (d) a product water outlet for removal of the product water; (e) 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); wherein the predetermined reduction in the efficiency of the RO or NF process is detected by a predetermined maximum salt concentration corresponding to the maximum osmotic pressure at which the reverse osmosis or nanofiltration membrane can operate; (f) a desaturation unit provided in the return line between the reverse osmosis or nanofiltration brine and the feed chamber, the desaturation unit being arranged to remove contaminants selected from a group consisting of salts, dissolved salts, sparingly soluble salts or any combination thereof, from the feed stream after its passage through the reverse osmosis or nanofiltration membrane, wherein the desaturation unit is a fluidised bed reactor, and a pressure exchanger is provided that is configured to reduce the pressure of the concentrated feed stream in the return line to substantially atmospheric pressure, wherein the pressure exchanger is downstream of the RO or NF and upstream of the desaturation unit; and