The two types of semipermeable membranes that are used most often for demineralization are cellulose acetate and thin film composites. Cellulose acetate (CA), the first commercially available membrane. The cellulose acetate membrane is asymmetric meaning that one side is different from the other side. The total cellulose acetate layer is 50 to 100 microns thick; however, a thin dense layer approximately 0.2 microns thick exists at the surface. This thin, dense layer serves as the rejecting barrier of the membrane.

Researchers realized the need for a membrane with better flux and rejection characteristics than those properties of cellulose acetate. The approach to developing a better membrane was to improve the efficiencies of the thin rejecting layer and the porous substrate so the thin composite membrane was developed.

In the production of the thin composite membrane, the semipermeable membrane is separate from the support layers, and this construction enables membrane manufacturers to select polymers that will produce membranes with optimum dissolved solids rejection and water flux rates.

The rate of flow through the membrane is dependent on the applied pressure, while the mineral flux is not dependent on pressure. As the pressure of the feed water is increased, the flow of water through the membrane increases while the flow of mineral remains constant. Therefore, the quantity and the quality of the purified product (permeate) should increase with increased pressure. This result occurs because more water is present to dilute the same amount of mineral.

Definition of Flux

The term flux is used to describe the rate of water flow through a semipermeable membrane. Flux is usually expressed in gallons per day per square foot of membrane surface or in grams per second per square centimeter.

The average membrane flux rate of a reverse osmosis system is an important operating guideline. In practice, most reverse osmosis systems will require periodic cleaning. It has been demonstrated that the cleaning frequency can be dependent on the average membrane flux rate of the system. Too high a flux rate may result in excessive fouling rates requiring frequent cleaning. Some industry guidelines for acceptable flux rates are:

table 10.1.GIF

Mineral Rejection

The purpose of demineralization is to separate minerals from water: the ability of the membrane to reject minerals is called the mineral rejection. Mineral rejection is defined as:

$LaTeX: Rejection\:\%\:=\frac{\left(1-Production\:Concentration\right)}{Feed\:Water\:Concentration}\times100$

Mineral rejections may be calculated for individual constituents in the solution by using their concentrations.

The basic equations that describe the performance of a reverse osmosis membrane indicate that rejection decreases as the feed water mineral concentration increases, because the higher mineral concentration increases the osmotic pressure. Also, as the feed mineral concentration increase (TDS) rejection decreases at a given feed pressure. Rejection improves as feed pressure increases.

Typical rejection for most commonly encountered dissolved inorganics is usually between 92 and 99-percent. Divalent ions such as calcium and sulfate are better rejected than monovalent ions such as sodium or chloride.

Most demineralization applications require the use of a membrane with high rejection rates (greater than 95-percent). However, some applications can use a membrane with lower rejection rates (80-percent) and lower operating pressures (less than 150 psi). The membranes that fit this classification are commonly referred to as softening or nanofiltration membranes. These membranes produce the same quantity of water as standard RO membranes at lower operating pressures.

Softening or nanofiltration membranes are seeing widespread use for demineralization of municipal water supplies that require high rejection rates for hardness and THM formation potential, and moderate TSD rejection.