Osmosis Pressure Literature Review 2022

LITERATURE REVIEW
Osmotic Pressure
Osmosis is the movement of water/Solvent from a region of low
concentration (hypotonic) to a more concentrated solution (hypertonic)
through a semipermeable membrane. Solvent/water continues to move
across the semipermeable membrane as an isotonic (equal concentration on
both ends) is achieved. At the same time the process of osmosis can be
reversed through additional pressure in the regions of high concentration
with respect to that in low concentration region. The pressure which is
needed to avert the passage of the water/solvent through the
semipermeable membrane and unto the highly concentrated solution is
equal to the osmotic pressure. Presently, the process of osmosis finds its
applications finds it’s applications in food processing, power generation,
water treatment and the novel techniques for the regulated drug release
(Cath T.et al.,2006).
There are three categories of osmotic occurrences which takes place during
the process of osmosis and they include;
(FO) Forward Osmosis
(RO) Reverse Osmosis
(PRO) Pressure Osmosis

Forward osmosis refers to the process which makes application of the
osmotic pressure gradient between the concentrated draw solution and the
dilute feed solution to drive water/solvent across a semipermeable
membrane and hence creation of greater pressure at the concentrated draw
solution and driving the solvent/water from the feed solution via the
membrane while rejecting the solutes, hence separating the water/solvent
from feed solution via the membrane while rejecting solutes,hence
separating water/solvent from diluted draw solution.
(Cath T.et al.,2006); Mecha A.,2018; Zhang S.et al.,2016). For the forward
Osmosis, osmotic pressure gradient (∆π) > 0, where hydrostatic pressure
(∆P) = 0 (Cath T. et al., 2006; Zhang S. et al., 2016). Reverse Osmosis is the
most commonly used osmosis process, whereby the hydrostatic pressure
exerted to the concentrated solution is normally higher compared to the
osmotic pressure gradient between the two regions which pushes the solvent
from the hypertonic solution to the hypotonic solution via the semipermeable
membrane. For the Reverse Osmosis, the hydrostatic pressure For the
Reverse Osmosis, the hydrostatic pressure (∆P) > (∆π) osmotic pressure
gradient (Cath T. et al., 2006; Zhang S. et al., 2016). During incidences when
hydrostatic pressure is too high, there is normally a risk of the solid particles
blocking the membrane and hence affecting its functionality in a manner
which can never be reversed. In addition to that, the hydrostatic pressure
will be limited to a given n threshold because of the energy pumping needs.
Reverse Osmosis technology is currently being used in majority of the

modern desalination plants which are constructed all over the globe.
(Zhang S. et al., 2016). Pressure Retarded Osmosis is seen as the
intermediary step allying the Forward Osmosis and the Reverse Osmosis
whereby the solvent/water permeates from the dilute feed solution into the
partially pressurised concentration draw solution through the semipermeable
membrane where hydrostatic pressure (∆P) < osmotic pressure (∆π). At this
moment, the solvent/water flows at higher volumes at the pressurised draw
solution region, which is then used in generating power (Achilli A. and
Hickenbottom K., 2016; Cath T. et al., 2006; Zhang S. et al., 2016). The
illustration 1 below shows the three categories of osmotic phenomena which
takes place during the osmotic process.
Fig. 1 Schematic illustration of (FO) Forward Osmosis, (PRO) Pressure
Retarded Osmosis and (RO) Reverse Osmosis processes (Zhang S. et al.,
2016)

In the early 1980’s Lee et al. characterised the (3) three osmotic process as
RO when ∆P > ∆π, PRO when ∆π > ∆P and FO when ∆P = 0./ Figure 2 below
shows the relationship between pressure gradient, hydrostatic pressure
and water flux difference in RO, FO and PRO PROCESS. The Forward
Osmosis occurs the moment at which the hydrostatic pressure difference is
nil. Reversed Osmosis occurs when the applied pressure difference is
normally larger as compared to the osmotic pressure gradient. The PRO
process takes place where the difference in the pressure applied is usually
between zero and the flux reversal point.
Fig. 2 Flux directions and driving forces for FO, PRO, and RO (Lee et al.,
1981)