Page 364 - Water and wastewater engineering

REVERSE OSMOSIS AND NANOFILTRATION 9-3

Concentrate

Bacteria, MF UF NF RO
protozoa, algae, Concentrate
particies 0.1
m

Viruses and small colloids Concentrate
Dissolved organic matter Concentrate
2
2
and divalent ions (Ca ; Mg )

Monovalent species (Na ; Cl )
Water

Pore 0.1
m 0.01
m 0.001
m Nonporous
sizes
FIGURE 9-2
Common constituents removed by membrane processes.
MF microfiltration;
UF ultrafiltration;
NF nanofiltration;
RO reverse osmosis.

9 -2 THEORY

Osmosis
Osmosis is defined as the spontaneous transport of a solvent (in this case, water) from a dilute solu-
tion to a concentrated solution across an ideal semipermeable membrane that impedes passage of
the solute (ions in solution) but allows the solvent (water) to flow. This is shown schematically in
Figure 9-3 . The system will reach equilibrium when the hydrostatic pressure on the saline water side
balances the force moving the water through the membrane. This is noted as the osmotic pressure
in Figure 9-3 b. If pressure is exerted to overcome the osmotic pressure, the solvent (pure water) will
flow from the saline side to the fresh water side. The semipermeable membrane will not allow the
passage of molecules other than water and gases. This is noted as reverse osmosis in Figure 9-3 c.

Osmotic Pressure
The driving force for diffusion is typically described as a concentration gradient. A more rigorous
explanation is a gradient in Gibbs energy. The general form of the Gibbs function is
∂ G V ∂ P S T ∂∂ u n ii (9-1)

where G Gibbs energy, J
V volume m, 3
P pressure, Pa
S entropy, J/K
T absolute temperature, K

u chemical potential of solute i, J/mole
i
n i amount of solute i in solution, moles