How to Design an RO / NF System

Step 3 - Select Membrane and Element Type

Step 1: Consider Feed Source, Feed Quality, Feed / Product Flow, and Required Product Quality
Step 2: Select the Flow Configuration and Number of Passes
Step 3: Select Membrane and Element Type
Step 4: Select Average Membrane Flux (Design Flux)
Step 5: Calculate the Number of Elements Needed
Step 6: Calculate the Number of Pressure Vessels Needed
Step 7: Select the Number of Stages
Step 8: Select the Staging Ratio (Array Ratio)
Step 9: Balance the Permeate Flow Rate
Step 10: Example

Select Membrane and Element Type

Elements are selected according to feed water salinity, feed water fouling tendency, required rejection and energy requirements. The standard element size for systems greater than 10 gpm (2.3 m3/hr) is 8-inch in diameter and 40-inch long. Smaller elements are available for smaller systems.

For high quality water applications where very low product salinity is required, ion exchange resins are frequently used to polish RO permeate.

Membrane Type Feed TDS (ppm) System Permeate Flow (gpm) Permeate Quality (ppm)
TW <5000

4-inch element: max. 25
8-inch element: min. 10

<50
XLE, LE <1000 4-inch element: max. 25
8-inch element: min. 10
<50
BW, FR <5000

4-inch element: max. 25
8-inch element: min. 10

<50
SW 3000 - 15000

4-inch element: max. 25
8-inch element: min. 10

<150
SWHR, SWHR LE 10000 - 50000

4-inch element: max. 25
8-inch element: min. 10

Varies (<500)
NF <1000

4-inch element: max. 25
8-inch element: min. 10

<150

These recommendations are not binding, but suggestions to select the membrane element for a system that has to be designed. The final choice depends also on specific requirements and operating conditions of the system.

Element Characteristics (79KB PDF)

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