How to Design an RO / NF System

Step 2 - Select the Flow Configuration and Number of Passes

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 the Flow Configuration and Number of Passes

The standard flow configuration for a membrane system is plug flow, where the feed volume is passed once through the system. Concentrate recirculation is common to smaller systems used in commercial applications, as well as in larger systems when the number of elements is too small to achieve a sufficiently high system recovery with plug flow. Concentrate recirculation systems can also be found in special applications like process liquids and wastewaters.

An RO / NF system is usually designed for continuous operation and the operating conditions of every membrane element in the plant are constant with time. In certain applications, however, a batch operation mode is used (e.g. in treating wastewater or industrial process solutions) when relatively small volumes (batches) of feed water are discharged non-continuously. The feed water is collected in a tank and then periodically treated. A modification of the batch mode is the semi-batch mode, where the feed tank is refilled with feed water during operation.

A permeate staged (double pass) system is the combination of two conventional RO systems where permeate of the first system (first pass) becomes the feed for the second system (second pass). Both RO / NF systems may be of the single-stage or multi-stage type, either with plug flow or with concentrate recirculation. The production of water for pharmaceutical and medical use are typical applications of permeate staged systems. As an alternative to a second pass, ion exchange may also be considered.

Batch vs. Continuous (53KB PDF)
Single Module System (57KB PDF)
Plug Flow (171KB PDF)
Permeate (double pass) Staged System (58KB PDF)
Special Design Possibilities (35KB PDF)

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