Both reverse osmosis (RO) and nanofiltration (NF) membranes have been increasingly used for water purification and desalination. However

the salt rejection of NF membranes is quite different from that of RO membranes

which makes a significant distinction in their process designs. This work started from the performance investigation of a single NF membrane element and then focused on the process design of the NF system for surface water treatment. In experimental tests

it was found that the observed rejection of the NF element becomes nearly constant when the concentrate flow is large enough

while the membrane flux of the NF element is quite stable regardless of the water flow across the membrane surface. These findings can be used to instruct the process design of the NF system for surface water treatment. In process design

a two-stage arrangement is sufficient for the NF system to reach the highest water recovery

while the RO system requires a three-stage arrangement.