principal driving mechanism for separation, i.e., pressure and electric
potential [1]. Each of the two categories is described in more
detail below along with the pretreatment technologies utilized.
3.1. Pressure driven membranes
In this approach, scaling precursors (primarily calcium, barium
and silica) are typically removed from the first stage RO (primary
RO) concentrate using chemical treatment techniques followed
by a second stage RO system (secondary RO) [31–34]. This type
of treatment results in an overall feed water recovery greater than
95%. ZLD is achievable when the final concentrate from the secondary
RO is further treated with thermal technologies [35]. Several
methods are available for the removal of scaling precursors from
the primary RO concentrate. These methods are described in the
following sub-sections.