WaterSolutions How to Design an RO / NF System

Step 9 - Balance the Permeate Flow Rate

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

Balance the Permeate Flow Rate

The permeate flow rate of the tail elements of a system (the elements located at the concentrate end) is normally lower than the flow rate of the lead elements. This is a result of the pressure drop in the feed / brine channel and the increase of the osmotic pressure from the feed to the concentrate. The ratio of the permeate flow rate of the lead element and the tail element can become very high under certain conditions:

• High system recovery
• High feed salinity
• Low pressure membranes
• High water temperature
• New membranes

The goal of a good design is to balance the flow rate of elements in the different positions. This can be achieved by the following means:

• Boosting the feed pressure between stages: preferred for efficient energy use
• Apply a permeate backpressure only to the first stage of a two-stage system: low system cost alternative
• Hybrid system: use membranes with lower water permeability in the first positions and membranes with higher water permeabilities in the last positions: e.g. high rejection seawater membranes in the first and high productivity seawater membranes in the second stage of a seawater RO system

The need for flow balancing and the method used to balance the flow can be determined after the system has been analyzed with ROSA (Reverse Osmosis System Analysis).

Comparing Actual Performance of DOW FILMTEC™ Elements to ROSA Projection (25KB PDF)

Flow Factor for RO System Design (159KB PDF)

System Operating Characteristics (89KB PDF)