The 5 Stages of a Home RO System
The 5 What of the What?
You'll often hear Reverse Osmosis Systems (RO Systems) referred to as 5 stage, 4 stage, 6 stage, 7 stage or more. But what does that mean? Basically, a "stage" is talking about a filtration step. So if a system uses 5 steps of filtration, it would be a 5 stage system.
Great - but why so many? Each of the stages (or steps) of filtration has a specific purpose in the system. It's not a one-filter-fits-all type of situation - some remove chemicals, some remove dirt, and so on. But we'll dive more into those specifics in a moment. First, we should answer a question you've probably got in the back of your mind, because that will help understand the different stages.
What is a pre-filter? And why is that different from a post-filter?
Remember that when we are talking about an RO, it's all about the RO membrane. The RO membrane is the rock star of the system and all the other filters (or stages) are just supporting band members. So if a filter comes before the membrane (meaning that the water passes through it first), it's called a pre-filter. Usually pre-filters are in place to knock out the easy-to- filter particles and chemicals which would damage an RO membrane if it was exposed.
Oppositely, any filter that comes after the membrane (meaning the water passes through it after it passes through the membrane), it is referred to as a post-filter.
So, now that' we've got that cleared up, let's dive into each of the typical 5 stages and what each one does.
Stage 1: Sediment Filter Cartridge (Pre-Filter)
The first filter the water passes through is a five micron sediment filter cartridge. This is made of tightly wound fibers that quite simply trap the dirt inside. A quality filter (like the ones we use) will have what's referred to as a "graded pore structure" - which means that the pores along the outside of the filter are larger in size to trap the bigger particles, and they gradually become smaller as they reach the center. This means the entire depth of the filter is used rather than all of the dirt caking up around the surface - so you can go longer between change-outs.
A 5 micron sediment filter cartridge removes dirt, sand, rust, grit, and other suspended matter from water. This protects the rest of the filtration stages and equipment from damage and clogging due to buildup of sediment.
Typically a sediment filter will need to be replaced every 3-6 months, depending on the incoming water quality. Dirtier water will clog the filter faster, and vice versa. You may notice that the tank is taking longer to fill if this filter is clogged.
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What is a Micron? 
Stage 2: Block Carbon Cartridge (Pre-Filter)
Next, the water passes through a carbon filter which absorbs chemical contaminants from the water including:
- Chlorine
- Chlorine By-Products - Trihalomethanes (THMs): A Group of organic chemicals suspected of being carcinogenic.
- Bad Tastes & Odors
- Turbidity (Cloudy Water)
- Herbicides, Pesticides, & Insecticides
- Volatile Organic Chemicals (VOCs): Organic Chemicals that turn into vapor.
The type of carbon filter used may vary from manufacturer to manufacturer. Some prefer to use granular activated carbon (GAC) which is loose carbon media enclosed in a plastic casing, but we prefer carbon block filters since they don't release any carbon fines. This is the same carbon material compressed into a solid block, and wrapped in a mesh netting.
Look for systems using carbon filters that are made from coconut shell carbon, as these are sustainable and cleaner compared to the traditional coal-based carbon.
Typically a carbon filter will need to be replaced every 3-6 months, depending on the incoming water quality. We recommend replacing this filter at the same frequency as you replace your sediment filter, with no more than 6 months between change-outs.
Stage 3: Block Carbon Cartridge (Pre-Filter)
No, you're not seeing double. This is the same filter used in stage 2!
Next, the water passes through another carbon filter which absorbs any remaining chemical contaminants from the water. This filter is in fact identical to stage 2, which may seem redundant, but is necessary to ensure complete chemical removal from the water. This is especially important for the USA since United States has a higher chlorine concentration than some other countries. That's not necessarily a bad thing, since USA municipal water is generally considered microbiologically safe to drink (thank goodness!), and keeping water clean as it travels through miles of pipeline to your home is no small feat. Needing a second carbon filter for complete chlorine removal is small price to pay for disease-free drinking water. But now that the water has safely reached your sink, the chlorine has fulfilled its destiny and can safely be removed.
This filter also protects the membrane from exposure to any residual chlorine that makes it through the first filter, which would quickly damage the membrane.
Like stage 2, this carbon filter will need to be replaced every 3-6 months, depending on the incoming water quality. We recommend replacing this filter at the same frequency as you replace your other pre-filters, with no more than 6 months between change-outs.
Stage 4: The Reverse Osmosis Membrane
Now that the water has traveled through the sediment and carbon filters to remove particles and chemicals, it's finally ready to meet the star of the show: The RO Membrane.
The RO membrane is the heart of the system. It uses reverse osmosis technology to separate dissolved solids from the fresh water at a molecular level. The pure water passes through the membrane into the product water stream which is delivered to the storage tank, while the contaminants stay on the outside of the membrane to be flushed down the drain. (For more information, check our A-Z Guide to Reverse Osmosis.)
The RO membranes used in our Home RO systems (M-T1812A50 & M-T1812A100) are certified to NSF/ANSI 58 for the reduction of Arsenic, Barium, Cadmium, Chromium (Hexavalent), Chromium (Trivalent), Copper, Turbidity, Fluoride, Lead, Radium 226/228, Selenium, and TDS.
The RO membrane is recommended to be replaced every 1-2 years, depending on your usage, water quality, and how well you've kept up with you pre-filter maintenance. You can typically tell when it's time to change a membrane by an increase in product water TDS..
Stage 5: Carbon Polishing Filter (Post-Filter)
The last stage of filtration occurs as the water flows from the storage tank directly before being dispensed from the faucet. In traditional RO systems, the good water from the membrane fills up a storage tank to wait until it is needed. This ensures that when you turn on the faucet, you've got a nice robust stream of water that is faster than it is actually produced by the membrane element. (The system will automatically refill the tank when water is taken, so that it is ready for the next use.)
The final carbon polishing filter is there to "polish" the flavor and refresh it, in case it has picked up any flavors while sitting in the tank.
This is usually an in-line filter, meaning the carbon is encapsulated inside of the hardshell filter body and the water lines connect directly to the ends (rather than being installed into a housing). The filter contains the same coconut shell carbon as the stages 2 & 3, but in a loose media form (also known as Granular Activated Carbon).
You may notice that our Tankless RO Systems do not have this final step, and the reason for that is because those systems have high flow membranes that produce the water on demand, rather than filling up a tank.
It is recommended to change this filter every 6 months.
Stage 6 and Beyond...
Now that we've discussed the 5 main stages of a standard home reverse osmosis system, and you've got a pretty solid understanding of what each stage does, you may be wondering why you've seen systems advertised with 6, 7, or even 8,9,10 stages. The answer is simple: These systems are being offered with additional filtration features added.
One example is our Alkaline Water Reverse Osmosis System, which has an Alkaline mineral cartridge installed directly after the post filter, which infuses the water with alkaline minerals. So although the rest of the filters are all unchanged, the addition of the alkaline cartridge makes it a 6-stage system. The same is true when other features are added. These can include deionization (DI) filters, ultraviolet (UV) sterilization, and more!
I've even seen sellers counting a single filter as multiple stages because it has multiple types of minerals inside - so take the number of stages advertised with a grain of salt. Now that you're in-the-know, just remember your basic 5 stages and look at any add-on features for their benefits without worrying too much about the number of stages beyond 5.
Conclusion: Every Filter has a Part to Play
In a nutshell, an RO system is designed to be an efficient, reliable, and practical solution to ensure you have high-quality water at your fingertips, and each stage has its own role. The pre-filters are in place to clean up any sediment and chemicals before the water reaches the membrane. The RO membrane does the heavy lifting and rejects up to 99% of total dissolved solids. Post filters (or polishing filters) are there to refresh and enhance the water as it is dispensed. It's that simple.
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