What’s the Difference Between RO Stages and Passes?

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Reverse osmosis lingo feels like a language unto itself sometimes, and it is easy to get a little confused.

One of the common mix-ups is between RO stages and passes. A lot of people believe they are the same thing.

In reality, they are pretty different. Let’s clarify!

Key Takeaways

  • A 2-stage RO system is one where the wastewater is collected after one round of RO filtration, and directed to a second round of filtration, extracting even more clean RO water and reducing wastewater.
  • A reverse osmosis pass refers to how many separate RO membranes the filtered water moves through. When the water moves through multiple passes, each pass filters the permeate to make it as clean as possible.

What’s the Difference Between RO Stages and Passes?

So, what’s the difference between RO stages and passes?

In an RO system, the key distinction between a stage and a pass lies in their treatment of water. During a stage, the water is divided into permeate (purified water) and concentrate (wastewater). Permeate is collected for use, while concentrate is directed to a second RO membrane. Then, the second RO membrane purifies the wastewater from the first membrane, reducing overall water wastage.

2 Stage RO System Diagram

A 2-pass RO system functions slightly differently. The first RO membrane separates the water as usual, but the purified water or permeate undergoes a second round of treatment by the second membrane. So the already purified water undergoes an additional level of refinement, creating ultra-pure water.

Reverse Osmosis Stage

Again, the feed water enters the RO membrane as a single stream and comes out as two types of water: concentrate (dirty) and permeate (clean). The concentrate from the first stage is then used as the feed water for the second stage in a two-stage RO system. The clean water collected in the first step is mixed with the clean water obtained in the second stage.

Reverse Osmosis Pass

Think of a pass as a standalone RO system. In a double-pass RO system, the clean water from the first pass becomes the feed water for the second pass. This means the water goes through two RO systems, resulting in even higher-quality clean water.

2 Pass RO System Diagram

Pros and Cons of Single Pass vs Double Pass Reverse Osmosis Systems

There are several pros to having a single/double pass RO system, such as:

  • Double-pass systems produce higher-quality permeate water and allow for the removal of carbon dioxide gas.
  • By introducing caustic between the first and second pass, the pH of the permeate water can be raised, converting CO2 into bicarbonate and carbonate for increased rejection in the second pass.
  • Single-pass RO systems cannot perform this process due to the risk of scaling the membranes when cations like calcium are present.

Cons of a double pass vs a single pass:

  • Additional energy costs from the extra pass.
  • Higher investment and maintenance costs due to the need for more membranes.
  • Maintenance of double-pass RO systems is slightly more involved as two sets of membranes must be cleaned or replaced.
  • The amount of wastewater produced with double-pass RO can be up to multiple times higher than that of a single pass RO system.

If you have any questions about RO stages and passes 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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