Dow Water & Process Solutions

Steps to Design an RO / NF Membrane System

Accurate system design is critical to getting the most from your water treatment system. Dozens of factors can impact the productivitiy of your system. That is why FilmTec's ROSA (Reverse Osmosis System Analysis) design software program is intended to offer the most precise design recommendations to optimize your system performance.

An entire RO / NF water treatment system consists of a pretreatment section, the membrane element system, and a post-treatment section. The membrane element system includes an inlet for feed water and outlets for permeate and concentrate.

RO / NF system performance is typically characterized by two parameters, permeate (or product) flow and salt rejection. These should always be referenced to a given feed water analysis, feed pressure, temperature, and recovery. The goal of the designer of an RO / NF system for a certain required permeate flow is to minimize feed pressure and membrane costs while optimizing permeate quality and recovery.

The steps to design such a membrane system are described here. Dow also offers the ROSA computer program to carry out designs.

How to Design a RO / NF Membrane System:

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