What Is a Single-Pass Reverse Osmosis System?
Written by: Gene Fitzgerald // Last Updated: Sep 13, 2023
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Two common configurations for reverse osmosis systems are single pass and double pass.
A critical difference between the two lies in the treatment process.
Let’s look into them a little more!
Contents
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Key Takeaways
- A single-pass reverse osmosis system is one in which water passes through a single RO membrane once.
What Is a Single-Pass Reverse Osmosis System?
So, what is a single-pass reverse osmosis system?
A single-pass reverse osmosis system can be understood as one independent RO system with a single RO membrane. In a single-pass system, the water passes over the RO membrane once and gets divided into the permeate (purified water) and the reject (wastewater) stream.
Single-Pass vs Double-Pass RO Systems
A double-pass RO system works slightly differently than a single-pass system. In a double-pass setup, the first RO membrane separates the water as usual – into permeate and reject.
The difference is then the purified water goes through a second round of processing by a second RO membrane. This means that the already purified water undergoes an extra filtration step.
While this process yields exceptionally pure water, the amount of purified water obtained is relatively low compared to the initial input. Therefore, this method is chosen when the goal is to achieve extremely high water purity without much concern for the amount of wastewater produced.
Time for some math:
Let’s say our 2 RO membranes have a recovery rate of 33% each. Then, out of every 9 gallons of input water, we get a single gallon of highly purified water (9 gallons * 33% *33% = 1 gallon).
Further Reading:
What Is a Single/Double-Stage Reverse Osmosis System?
What is a single or double-stage reverse osmosis system and how does it differ to single/double-pass RO?
A single-stage RO system is essentially the same as a single-pass RO system. There is a single reverse osmosis membrane being used. With a double-stage reverse osmosis system, however, the reject water – not the permeate water – undergoes a second filtration step in order to lower the overall amount of wastewater produced.
In other words, the wastewater produced by the first RO membrane is being recycled and fed to the second RO membrane. Both permeate water streams are then combined into one.
If we do the same math as before: We get 3 gallons of purified water from the first RO membrane (9 gallons * 33% = 3 gallons), and another 2 gallons of purified water from the second RO membrane (6 gallons * 33% = 2 gallons). This makes a total of 5 gallons RO water.
If you have any questions about single-pass RO systems please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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