WaterSolutions How to Design an RO / NF System

Step 10 - Example

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

Example

Given Conditions:

• Feed source: brackish surface supply water, SDI <5
• Required permeate flow = 132 gpm (720 m³/d)
• 6-element pressure vessels to be used

Steps:

1. Brackish surface supply water with SDI <5; total permeate flow = 132 gpm (720 m³/d)
2. Select plug flow
3. FILMTEC™ BW30-365 element (BW element with active membrane area of 365 ft² (33.9 m²))
4. Recommended average flux for surface supply water feed with SDI <5 = 15.0 gfd (25 L/m/h).
5. Total number of elements =
   (132 gpm)(1440 gpd/gpm) / (15 gfd)(365 ft²) = 35
   OR
   (720 m³/d)(41.67 L/h)/(m³/d) / (33.9 m²)(25 L/m²/h) = 35
6. Total number of pressure vessels = 35/6 = 5.83 = 6
7. Number of stages for 6-element vessels and 75% recovery = 2
8. Staging ratio selected: 2:1. Appropriate stage ratio = 4:2
9. The chosen system must then be analyzed using the Reverse Osmosis System Analysis (ROSA) computer program. This program calculates the feed pressure and permeate quality of the system as well as the operting data of all individual elements. It is then easy to optimize the system design by changing the number and type of elements and their arrangement.
   

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