Why I Use a 5 Stage Reverse Osmosis System
By Matthew Barta, Water Quality Association - Master Water Specialist
After using a five stage RO system for over 25 years I won’t change. Here’s why.
I grew up on a dairy farm in rural Minnesota, drinking healthy and great-tasting water from our farm’s well. I remember very clearly, as a kid, every time our parents took us to visit relatives or friends in the “Cities,” we would always make sure to drink lots of water before we left home because we could not stand to drink the “city water.” At a young age, I learned that not all water is created equal.
Fast forward to the post-college years. Volunteering in Asia, we faced the same problem of trying to find good drinking water. Fortunately, while in Taiwan, my wife met Morgan Yang, the owner of Puricom, an international manufacturer of reverse osmosis water filters, and he agreed to donate RO filter systems to the three schools we were helping.
This was my first introduction to reverse osmosis, but having a background in biology, I understood the concept of this new technology right away. Little did I realize at that time that this friendship with Morgan, combined with my biology major background, would lead to a much deeper understanding of water filtration and representing Puricom at water industry trade shows around the world.
But I'm getting ahead of myself. So, why have I continued to use a five-stage RO system for over 25 years without changing? The answer lies in two simple reasons: First, the water tastes awesome. Second, and probably even more important, I know that the water is free of any contaminants and, therefore, is as healthy as you can get.
What is Reverse Osmosis?
The core of reverse osmosis system is the very fine, semi-permeable RO membrane. Water is forced through this membrane under pressure, leaving larger molecules, dissolved solids (like salts), and contaminants behind. Only water molecules are small enough to pass through, resulting in very pure water.
You can find more detailed information about RO membrane technology on the web if you want a deeper understanding, or you can refer to About Formal Water Purification Knowledge, but let’s get to the main point of this article.
Why Five Stages? What Does That Mean?
Although the RO membrane is the heart of the system for removing contaminants, the membrane itself is very delicate and sensitive to physical and chemical damage. While the first three stages of the system filter out much of the contaminants, their main purpose is to remove anything from the water that could damage the membrane. Here is why there are three stages before the membrane.
Stage 1: Polypropylene (PP) Pre-filter: Sediment Removal
This filter is made of a very fine mesh material that physically strains any sand, dirt, or other particles that could damage the membrane by scratching or plugging it. As the front line of the filtering process, this filter is the first to get plugged up and therefore needs to be replaced most often. By filtering out dirt and sediment, it also protects the second stage—the carbon filter—from getting clogged with sediment, reducing the need for frequent replacements.
Stage 2: Activated Carbon Filter: Chlorine and Chemical Removal
The RO membrane is very sensitive to chemical deterioration from chlorine. If exposed to chlorine, the chlorine will very quickly deteriorate the membrane’s polyamide surface. Because chlorine is added to nearly all municipal water, it is essential that it be removed. If not removed, the chlorine could deteriorate the RO membrane in as little as six months or less. The activated carbon filter removes this chlorine.
Activated carbon is a very fine porous material that acts like a micro-sponge. Water can flow through this sponge-like structure, and chlorine and other organic molecules will be adsorbed onto the surfaces of the carbon. Eventually, the surfaces of the carbon will be completely covered and no longer able to absorb chlorine. Therefore, it is extremely important to change the carbon filter regularly at the recommended time.
There are two common types of carbon filters: Granular Activated Carbon (GAC), which consists of loose granules through which the water flows, and Carbon Block, which is made of finer carbon particles mixed with an adhesive to form a porous 'block' through which the water can flow.
Stage 3: Fine Polypropylene (PP) Filter: Fine Sediment Removal
The activated carbon in the second stage may have fine particles that flake off, which could scratch the membrane. The third-stage PP filter, finer than the first stage (usually 1 micron), will remove any fine particles of carbon, as well as any other particles that may have passed through the first stage.
If the first three stages are properly maintained and changed on time, the RO membrane will usually last for 3 years or more. However, proper and timely filter changes are important.
Stage 4: RO Membrane
This is the stage that removes the bulk of chemicals and other contaminants from the water, down to a molecular level. It works through a combination of both physical separation and an electrostatic barrier on the surface of the membrane. This barrier is actually only a very thin coating of material on the surface of a multi-layered filter sheet. As water flows over the surface of the membrane, only the smaller water molecules can pass through the membrane. The polar electrostatic charge that pushes ions and polar molecules away from the surface of the membrane also helps to prevent the membrane from clogging.
Stage 5: Post Filter
Because the RO filtering process is fairly slow, traditional RO systems collect the pure water in a storage tank. This tank has low pressure (typically 5 to 20 psi), which is low enough to prevent too much back pressure on the membrane, but high enough to provide a good flow to the faucet. Since these tanks usually have a rubber bladder inside, there is a possibility of the water taking on an off taste if stored for an extended period. To ensure that the water going to the faucet is perfectly clean, a post-carbon filter between the tank and the faucet will 'polish' the water to make sure it is perfectly clean and great tasting.
In addition to carbon for polishing the water, the post filter may contain calcium or magnesium granules to reintroduce these healthy minerals back into the water. This will also improve the taste of the water.
More post filter types：https://www.puricom.com/en/product-cate-third/post-filter-cartridges
The Total Picture
Each stage of a 5-stage reverse osmosis (RO) system plays a vital role in ensuring the production of high-quality drinking water. The sequence of stages works together to protect the system's integrity and enhance water purity, making it crucial that no stage be omitted.
While a five-stage system provides comprehensive water treatment, there are RO systems available with six or more stages. These additional stages can offer further enhancements to water quality, such as additional mineralization or advanced contaminant removal, and should be evaluated based on the specific benefits they bring to the overall system.