Reverse Osmosis (RO) is a membrane-based demineralization
technique used to separate dissolved solids, such as ions, from
solution (most applications involve water-based solutions, which is
the focus of this work). Membranes in general act as perm-selective
barriers, barriers that allow some species (such as water) to selectively
permeate through them while selectively retaining other dissolved
species (such as ions). Figure 1.1 shows how RO perm-selectivity
compares to many other membrane-based and conventional filtration
techniques. As shown in the figure, RO offers the finest filtration currently
available, rejecting most dissolved solids as well as suspended
solids. (Note that although RO membranes will remove suspended
solids, these solids, if present in RO feed water, will collect on the
membrane surface and foul the membrane. See Chapters 3.7 and 7 for
more discussion on membrane fouling).