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Advanced Membrane Fouling Characterization 105
resistance than that made up of same colloidal particles only. The organic matters can fill up
the voids between the colloidal particles, narrowing and reducing the pathways the feed water
can take to reach the membrane surface. Ionic strength and pH of feed water affect the
membrane–foulant and foulant–foulant interactions by altering the surface properties of
membrane and foulants. Generally, a high ionic strength and low pH enhance the accumulation
of foulants on membrane surface.
2.2. Types of Fouling in RO Processes
Membrane fouling is a complicated process and fouling behavior varies according to the
types of feed water and operating conditions. According to the nature of foulants, membrane
fouling in RO processes can be commonly classified as colloidal fouling, organic fouling,
biological fouling, and scaling.
Colloidal particles are major foulants in all kinds of membrane processes. The size of
colloidal particles ranges from a few nanometers to a few micrometers. They are ubiquitous
in natural waters and examples of inorganic colloids include metal oxides, clay minerals,
colloidal silica, and silicon. There are also plenty of colloidal particles of organic and
biological origins. Most colloids carry negative surface charge in pH range of natural waters.
Under the drag force of permeate flux, these colloidal particles will accumulate on the RO
membrane surface to form a cake layer. The formation of this cake layer of the deposited
colloidal particles adds on an additional resistance to the membrane resistance. This type of
fouling is known as colloidal fouling.
Organic fouling is another prevalent problem in water treatment and reclamation plants. A
common organic foulant is the natural organic matter (NOM), which is a complex heterogeneous
mixture of different organic macromolecules from degradation and decomposition of
living organisms. Another organic foulant is the residual organic matter in the effluent of
wastewater treatment plants. The ability of organic foulants to foul membranes includes their
affinity for RO membrane, molecular weight, and functionality. Negative functional groups on
organic polyelectrolytes may be repulsed by the negatively charged membrane surfaces of RO
membranes. Greater charge density on the membrane surface is often associated with higher
hydrophilicity. Because most RO membranes are made of hydrophobic polymers, organic
matters in the feed water usually tend to be preferentially adsorbed onto the membrane
surfaces.
Biological fouling or biofouling occurs when living microorganisms in the feed water are
transported onto the membrane surface where they absorb or adhere, forming a thin fouling
layer. Once attached, microorganisms may grow and multiply at the expense of nutrients in
the feed, forming a biological film or biofilm on membrane surface. Biofilm can be detected
directly with microscopic inspection of fouled membrane surfaces, or indirectly with measurements
of consequences of biofilm on membrane properties and system performance, such
as solute rejection and permeate flux. Biofouling not only increases the membrane resistance,
but also biodegrades the membranes through hydrolysis of membrane polymers.
Fouling by precipitation of the sparingly insoluble minerals to form an impermeable layer
on the membrane surface is commonly known as scaling. As the salt concentration in the feed