Membrane and Desalination Technologies - TCE Moodle Website

580 J.P. Chen et al.
l up to 40% humic substances,
l low inorganic content, and
l high microbiological counts including bacteria and fungi.
Biofouling is perhaps the most difficult fouling threat to overcome. No system operator can
afford to ignore the potential for biofouling. Biological organisms include bacteria, algae, and
fungi. Of these, bacteria cause the majority of problems in membrane treatment systems for a
variety of reasons. Many bacteria can easily adapt to the environment inside the membrane
treatment system. With the organic compounds concentrated on the membrane surface, the
bacteria multiply rapidly and are rejected by the membrane. These bacteria have a number of
defense mechanisms. For example, some have fimbriae, which stick out from all sides of the
cell. These allow the bacteria to attach themselves, and remain attached, to the surface of the
membrane or feed space. In addition, bacteria may secrete a mucous capsule, or slime, which
coats the cell and protects them from any harsh elements entering their environment.
The susceptibility of membranes to biological fouling is significantly dependent on the
membrane composition. CA membranes are easily attacked by bacteria. Disinfection of feed
water is required for such membranes. PA membranes are also susceptible to bacterial attack,
but TFC membranes are generally quite resistant. Chlorination can be used for CA membranes,
but must be followed by dechlorination for PA and other membranes, usually by the
addition of sodium metabisulfate. In ultrapure water production, sterility is often maintained
by ultraviolet (UV) sterilizers (41).
A recent study has shown that some RO membranes are more prone to biofouling than
others. The study involved a bioadhesion assay which utilizes a model bacterium, SW 8,
known to adhere to membranes. Examination of the bacteria adhered to the membranes using
optical microscopy revealed that membranes that are less susceptible to bioadhesion are
hydrophilic in nature (39). Further investigation carried out on RO simulators that consisted
of flat sheet membranes to simulate spiral-wound module revealed biofilm characteristics
under field emission scanning electron microscope. Microorganisms covered the surface of
all types of RO membranes used in the experiment to a density of about 2.25 10 8 cells/cm 2
(39). Bacteria of different shapes (mostly rod shaped) with dimensions between 1 and 3 mm
were observed. The organisms appeared to excrete extracellular polymeric substances (EPS).
Understanding membrane fouling creates the foundation for researchers to devise an
approach to counter or minimize fouling in order to maintain high RO performance. One
strategy to alleviate membrane fouling is feed pretreatment to reduce or remove fouling
constituents in the feed. A proper membrane cleaning and regeneration protocol to remove
foulants periodically from the membrane surface is also essential in maximizing RO
efficiency.
4.2. Feed Pretreatment
The success of RO process is highly dependent on appropriate feed pretreatment. Pretreatment
must be effective in reducing RO fouling potential in a reliable and consistent manner.
Feed pretreatment continues to be extensively studied. In the development of a pretreatment
program, the focus is on removing as many fouling constituents in the feed water as possible