What Is a Double-Pass Reverse Osmosis System? (+ Diagram)

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A 2-pass or double-pass reverse osmosis system differs from a 2-stage reverse osmosis system, so it is important to know what both are!

The last thing you want is to purchase something that is more complicated than you need.

Today, we will look at the 2-pass RO system in detail and give you a bonus diagram to help clear up any confusion!

Key Takeaways

  • A double-pass reverse osmosis system is a method where water is purified through an RO membrane. Then, that purified water is directed to pass through another RO membrane to be filtered again, ensuring even purer water.

What Is a Double-Pass Reverse Osmosis System and How Does It Work?

So, what is a double-pass reverse osmosis system and how does it work?

A double-pass reverse osmosis system, also known as a DPRO, is a method where the purified water from the first RO membrane is directed to a second RO membrane to produce even purer water.

This system consists of two RO membranes, known as the first pass and second pass. The first pass has a higher capacity for water flow compared to the second pass. A break tank is required between the two systems to store and supply the purified water (permeate) from the first pass to the second pass for further filtration.

While this process produces exceptionally pure water, the quantity of purified water obtained is relatively low compared to the initial amount of input water. Hence, this method is preferred when the objective is to achieve extremely high water purity and the amount of wastewater generated is not a significant concern.

Double-Pass Reverse Osmosis System Diagram

2 Pass RO System Diagram

A ‘pass’ in an RO system refers to a single stage of the filtration process. So, you can think of a ‘pass’ as one full cycle of purification through the RO membrane.

A single-pass RO system means the water goes through the RO process once, while a double-pass RO system means the water goes through the RO process twice, one after the other. The output, or ‘permeate’, from the first pass becomes the input, or ‘feed water’, for the second pass, as indicated on the diagram.

The advantage of a double pass system is that it produces a much higher quality of purified water, or ‘permeate’ because the water has essentially been filtered twice.

Benefits: Why Use an RO System with 2 Passes?

A double pass RO system offers numerous advantages, including:

  • Double-pass systems generate higher-quality permeate water and facilitate the removal of carbon dioxide gas.
  • By introducing caustic between the first and second pass, the pH of the permeate water can be elevated, effectively converting CO2 into bicarbonate and carbonate to enhance rejection during the second pass. Single-pass RO systems cannot perform this process due to the risk of membrane scaling caused by cations like calcium.

Reverse Osmosis Stages vs Passes

A two-stage reverse osmosis system operates differently from a two-pass system. The process begins with the first RO membrane, which separates the water into permeate, which is the purified water, and concentrate, also known as wastewater. The permeate is collected for utilization, while the concentrate is directed to the second RO membrane. Here, the second RO membrane receives the wastewater from the first one for further purification. The aim is to reduce the overall amount of water that is discarded as waste.

To illustrate this, let’s say an RO membrane wastes 50% of the input water. If you start with 4 gallons of input water, the first RO membrane will produce 2 gallons of clean water and 2 gallons of wastewater. These 2 gallons of wastewater will then be directed to a second round of purification, reducing it by another 50%. As such, in the end, only 1 gallon goes to waste instead of 2.

2 Stage RO System Diagram

In a 2-pass system, the first RO membrane separates the water as usual. However, the purified water (instead of the wastewater) is processed a second time by the second RO membrane. This means that the already purified water undergoes an additional round of treatment. This process produces exceptionally pure water, but also a lot of wastewater (3 gallons in our example above).

If you have any questions about double pass reverse osmosis please don’t hesitate to leave a comment below!

About the Author Gene Fitzgerald

Gene Fitzgerald is one of the founders of BOS and currently head of content creation. She has 8+ years of experience as a water treatment specialist under her belt making her our senior scientist. Outside of BOS, Gene loves reading books on philosophy & social issues, making music, and hiking.
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