| The Membrane Selection Dilemma
Membrane surface smoothness is one of the main factors in
preventing membrane fouling. When organic material comes
into contact with a membrane surface, it inherently finds
places to adhere, thus initiating membrane fouling. If the
foulant has a biological origin, a potential site for a
colony to grow is created wherever a spore attaches to the
membrane.
Traditionally, cellulose acetate (CA) membranes with
relatively smooth surfaces have been used for surface water
applications. While these membranes are typically less
susceptible to fouling from organics, CA membranes have a
shorter life span and need to operate at almost double the
pressure to approach the same salt rejection properties of
polyamide (PA) membranes. These drawbacks led GE Osmonics to
develop an even smoother membrane solution.
An Extra Membrane Layer Creates a Smoother Membrane
Standard polyamide reverse osmosis (RO) and
nanofiltration (NF) membranes consist of a polyester backing
material followed by a polysulfone ultrafiltration support
on which a thin film of polyamide is deposited. This design
has an inherent roughness that can be seen on a microscopic
level. The peaks and valleys observed on the membrane
surface catch colloidal particles and/or contaminants from
the feed water, causing fouling.
The innovative addition of a proprietary intermediary
layer, as well as membrane chemistry adjustment, has
resulted in a membrane with a very smooth and fouling
resistant surface. With this 3-layer membrane design
incorporated in Duraslick elements, GE Osmonics has
succeeded in developing a PA thin film membrane element that
decreases the rate of microbial and particulate attachment,
has superior membrane stability and chemical compatibility,
reduces flux loss, and extends the time between required
cleanings. |
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