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Membrane Processes 341

Transport Principles
for Membrane Processes
The driving force for transport across a membrane may be physical (as
in the case of pressure), chemical, or electrical, acting individually or
in concert. In theory, a gradient across the membrane in any param-
eter that affects the chemical potential of a compound can be used as
a driving force for transport across the membrane. Examples of driv-
ing forces and some corresponding membrane processes are listed in
Table 9.2.
Pressure-driven membrane processes have been developed at large
scale for the treatment of water (both potable and wastewater) and
other fluids. In water filtration, pressure-driven membrane processes
are typically differentiated based on the range of materials rejected.
Microfiltration (MF) and ultrafiltration (UF) are pressure-driven
processes that use porous membranes to separate micron-sized and
nanometer-sized materials, respectively. Nanofiltration (NF) and
reverse osmosis (RO) use dense membranes to separate solutes rang-
ing from macromolecules, nanoparticles, and larger ions in the case of
NF to simple salts and very small molecular weight organic compounds
in the case of RO. Unlike pressure-driven processes in which solvent
passes through the membrane, electrodialysis involves the passage of
the solute through the membrane. In electrodialysis, ions pass through
a semipermeable membrane under the influence of an electrical poten-
tial and leave the water behind, whereas in RO, water passes through
the membrane leaving the ions behind. Polymer electrolyte membranes
used in fuel cells allow for the transport protons under a concentra-
tion gradient, while rejecting the fuel (for example, hydrogen) that is
introduced to a catalyst at the “concentrate” side of the membrane. In
membrane distillation, solvent (e.g., water) evaporates and is trans-
ported across a porous membrane under the driving force of a tem-
perature gradient.
The driving force for transport reflects the differences in the available
energy on the two sides of the membrane. Analogous to water flowing
downhill, material is transported from one side of the membrane to the
other if and only if that transport decreases the total available (or free)
energy of the system. Any spatial gradient in energy, E, can be interpreted

TABLE 9.2 Categorization of Membrane Processes by Driving Force
Driving force Examples of membrane processes
Temperature gradient Membrane distillation
Concentration gradient Dialysis, pervaporation
Pressure gradient Reverse osmosis, ultrafiltration
Electrical potential Electrodialysis, electro-osmosis