Page 368 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 353

The choice of chemicals used to clean a membrane is dependant on the
chemical nature of the foulant. Dissolved organic materials are the
primary foulants in many water-based membrane applications. Even
when mass concentrations of particle exceed those of organic matter,
organic matter may dominate fouling. Organic matter may form a gel
layer on the surface of the membrane or adsorb to the surface or within
the membrane matrix. In gas separations, gas-phase impurities may foul
membranes by adsorption. The possibility of precipitative fouling is a
serious concern in the operation of membranes designed specifically to
remove scale-forming species (e.g., RO, membrane distillation, electro-
dialysis). Common foulants of concern for precipitation include calcium,
magnesium, and iron (primarily ferrous), any of which might precipi-
tate as a hydroxide, carbonate, or sulfate solid. Colloids of all kinds
may accumulate on or near membranes, forming a cake. Biofouling of
membranes is a key concern in applications in which membranes are in
contact with an aqueous environment. Bacteria may colonize mem-
branes forming biofilms that consist of the bacteria and compounds
secreted by the bacteria.
In pressure-driven membranes, these potential causes of fouling
can be represented mathematically by modifying Eq. 29 to include
resistance terms for each of the causes of fouling, including those due
to the changes in membrane permeability over time, R (t), and the for-
m
mation of a cake or biofilm, R (t). One approach to describing the
c
resistance of the cake, R , is to assume that the cake has a uniform
c
ˆ
structure with a specific resistance, R c , and thickness, (that may
c
change over time). An expression for hydraulic permeability, such as
the Kozeny equation, can then be used to predict the specific resist-
ance, assuming that the cake is incompressible and is composed of
uniform particles:
ˆ
R c 5 180s1 2 e c d 2 (33)
2 3
d p e c
where is the porosity of the cake, and d is the diameter of the parti-
p
c
cles that it comprises. This expression predicts that resistance to
permeation by a deposited cake should increase as one over the square
of the diameter of particles comprising the cake. Thus, in comparison
with larger colloidal species, an accumulation of nanoparticles on the
membrane should form a cake of relatively high specific resistance.
The effect of concentration polarization on permeate flux is primarily
due to the accumulation of solutes near the membrane and the resultant
osmotic pressure effect. It will typically be negligible for MF and UF that
reject only particles or macromolecules. The concentration polarization