How to Design an RO / NF System

Step 7 - Select the Number of Stages

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

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Select the Number of Stages

The number of stages defines how many pressure vessels in series the feed will pass through until it exits the system and is discharged as concentrate. Every stage consists of a certain number of pressure vessels in parallel. The number of stages is a function of the planned system recovery, the number of elements per vessel, and the feed water quality. The higher the system recovery and the lower the feed water quality, the longer the system will be, with more elements in series. For example, a system with four 6-element vessels in the first and two 6-element vessels in the second stage has 12 elements in series. A system with three stages and 4-element vessels, in a 4:3:2 arrangement has also 12 elements in series. Typically, the number of serial element positions is linked with the system recovery and the number of stages as illustrated in Table 1 for brackish water systems and Table 2 for seawater systems.

Table 1 Number of Stages of a Brackish Water System

System Recovery (%)	Number of Serial Element Positions	Number of Stages (6-element vessels)
40 - 60	6	1
70 - 80	12	2
85 - 90	18	3

One-stage systems can also be designed for high recoveries if concentrate recycling is used.

In seawater systems the recoveries are lower than in brackish water systems. The number of stages depends on recovery as shown in Table 2.

Table 2 Number of Stages of a Seawater System

System Recovery (%)	Number of Serial Element Positions	Number of Stages (6-element vessels)	Number of Stages (7-element vessels)	Number of Stages (8-element vessels)
35 - 40	6	1	1	----
45	7 - 12	2	1	1
50	8 - 12	2	2	1
55 - 60	12 - 14	2	2	----

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