9-3. PROPERTIES OF RO AND NF MEMBRANES
9-3-1. Membrane Material
The materials most widely used in RO and NF are cellulosic derivatives and polyamide derivatives.
Cellulosic acetate (CA), the common commercial material, is not tolerant to temperatures above 30°C and tends to hydrolyze when the pH is less than 3 or greater than 8. It is susceptible to biological degradation and degrades with free chlorine concentrations above 1 mg/L. Most membrane manufacturers guarantee integrity of membranes if the chlorine concentration and contact time are within specified limits.
Polyamide (PA) membranes are generally resistant to biological degradation, are stable over a pH range of 3 to 11, and do not hydrolyze in water. Under similar pressure and temperature conditions, PA membranes can produce higher water flux and higher salt rejection than CA membranes. However, PA membranes are more susceptible to fouling and cannot tolerate free chlorine at any concentration (MWH, 2005).
9-3-2. Membrane Configuration
The membrane units are fabricated in either a spiral-wound configuration or a hollow-fiber configuration.
The spiral-wound configuration is shown in Figure 9-4. Two sheets of flat-sheet membrane are joined along three sides with the active membrane layer facing out. A spacer is placed between the membrane sheets to keep them from touching. The open end of the envelope formed by the two sheets is attached to a perforated central tube that collects the permeate. The spiral-wound elements are typically 1 m long and 0.3 m in diameter. The area for a 1 m long element would be about 30 m2. Individual elements have a permeate recovery of 5 to 15 percent. To achieve higher recoveries, elements are placed in series. Typically, four to seven elements are arranged in series in a pressure vessel (MWH, 2005).