What is a Cellulose Acetate membrane – CA membrane definition

/ / Technology, Water Treatment
cellulose acetate membrane pores reverse osmosis desalination

What is a Cellulose Acetate Membrane

The thin semi-permeable film of the first Reverse Osmosis membranes was developed in the late 1950s at UCLA (University of California Los Angeles). It was made of cellulose acetate (CA) polymer. A Cellulose Acetate membrane have a three-layer structure similar to that of a Polyamide Thin Film Composite TFC membrane. The main structural difference is that the top two layers (the ultrathin film and the microporous polymeric support) are made of different forms of the same Cellulose Actetate polymer.

Cellulose is a polymer that is made up of repeating units (monomers) of C6H10O5. (Note: A monomer is a molecule which comes together with other identical monomers to form a chain of monomers, called a polymer). The number of acetates on the cellulose molecules affects the semi-permeability and other characteristics of the membrane. In general, the following are some of the most important differences between diacetate and triacetate membranes.

cellulose polymer cellulose acetate membrane

cellulose polymer cellulose acetate membrane

In a TFC membrane these two layers are made of completely different polymers. The thin semi-permeable film is polyamide, while the microporous support is polysulfone. Similar to a TFC membrane, a Cellulose Acetate membrane have a film layer that is typically about 0.2 um thick. But the thickness of the entire membrane (about 100 um) is less than that of a TFC membrane (about 160 um). One important benefit of a Cellulose Acetate membrane is that the surface has very little charge and is practically uncharged. As compared to a TFC membrane, which have a negative charge and can be more easily fouled with cationic polymers. If such polymers are used for source water pre-treatment.

cellulose acetate membrane pores reverse osmosis desalination

cellulose acetate membrane pores under microscope

In addition, a Cellulose Acetate membrane have a smoother surface than a TFC membrane. Which also renders them less susceptible to fouling. Cellulose Acetate membrane have a number of limitations. Including the ability to perform only within a narrow pH range of 4 to 6 and at temperatures below 35 C (95 F). Operation outside of this pH range results in accelerated membrane hydrolysis, while exposure to temperatures above 40 C (104 F) causes membrane compaction and failure. In order to maintain the Reverse Osmosis concentrate pH below 6, the pH of the feed water to the cellulose acetate membrane are reduced to between 5 and 5.5. This results in significant use of acid for normal plant operation and requires Reverse Osmosis permeate adjustment by addition of a base (typically sodium hydroxide) to achieve adequate boron rejection.

Cellulose Acetate membrane experience accelerated deterioration in the presence of microorganisms. Since they’re capable of producing cellulose enzymes and bioassimilating the membrane material. However, they can tolerate exposure to free chlorine concentration of up to 1.0 mg/L. Which helps to decrease the rate of membrane integrity loss due to destruction by microbial activity.

Since Cellulose Acetate membrane have a higher density than a Polyamide TFC membrane. They create a higher headloss when the water flows through the membranes. Therefore a cellulose acetate membrane operates at higher feed pressures, which results in elevated energy expenditures. Despite their disadvantages, cellulose acetate membrane have high tolerance to oxidants (chlorine, peroxide, etc.). As compared to a PA TFC membrane. Cellulose Acetate membrane are used in municipal applications for saline waters with very high fouling potential. Mainly used in the Middle East and Japan in seawater reverse osmosis plants, and for ultrapure water production in pharmaceutical and semiconductor industries.