What is Reverse Osmosis? Reverse osmosis (RO) is a water‐purification process that forces tap water under pressure through a very fine, semipermeable membrane. This membrane has tiny pores that only water molecules can pass through, so dissolved salts, minerals, metals and other impurities are left behind. In other words, high pressure pushes water out of the “dirty” side toward the “clean” side of the membrane, reversing the natural flow of osmosis. According to one description, “reverse osmosis is a water purification process that removes contaminants … by using pressure to force it through a semipermeable membrane”. In practice, the RO process yields pure water on one side of the membrane and a concentrated waste stream on the other. (As Express Water notes, applying extra pressure “stops or reverses” the normal flow of water so that it flows away from the impurities.) The end result is that nearly all dissolved solids and impurities are blocked by the membrane, producing very clean water.
How Is Reverse Osmosis Water Made? Reverse osmosis drinking water systems typically use multiple filter stages. A common setup has 3–5 stages:
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Stage 1: Sediment prefilter. The incoming tap water first goes through a sediment filter that catches large particles like dirt, rust, sand and silt. This protects the next stages from clogging.
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Stage 2 (and 3): Carbon prefilters. The water then passes through one or two carbon block filters. These remove chlorine, chemical odors, and many organic compounds. Removing chlorine is important because it can damage the RO membrane.
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Stage 3 (or 4): RO membrane. Next, the water is forced under pressure through the core RO membrane. This is a very tight filter (pore size ~0.0001 microns) that blocks virtually all dissolved solids. The membrane removes up to 99.99% of total dissolved solids (TDS) and tough contaminants like salts, lead, arsenic, fluoride, and many others. In this step, pure water permeates through the membrane into a storage tank, while the remaining concentrate (“reject” water) carrying the concentrated impurities is sent to the drain. (Importantly, RO systems split the feed water flow so that only the clean portion passes through. As Express Water explains, one stream is pushed through the membrane, and a second stream carries the rejected salts and contaminants away to waste.)
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Stage 4 (or 5): Polishing filter. Finally, the purified water flows through a final filter (often another carbon filter) that “polishes” the water, removing any lingering tastes or odors before you drink it. After this, the clean water is delivered to your faucet.
In summary, RO water is made by pre-filtering tap water for sediments and chlorine, then pushing it through the RO membrane to strip out dissolved impurities, then using a post-filter to improve taste. The result is exceptionally pure drinking water.
What Contaminants Does Reverse Osmosis Remove? RO is effective against a very broad range of pollutants. In practice, it removes almost all dissolved inorganics and most organic chemicals, plus many microbes. Key examples include:
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Heavy metals and minerals: RO membranes remove virtually all heavy metals and many minerals. For example, Express Water notes that its RO membranes can eliminate “lead, arsenic, chromium, [and] radium” and similar contaminants. In general, reverse osmosis will remove dissolved metals like lead, mercury, copper and more. (By removing these harmful metals, RO water is much safer to drink.)
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Other dissolved chemicals: Most dissolved inorganic chemicals are removed. This includes compounds like nitrates, fluoride, chloride, sulfate, potassium and sodium salts. (Basically, all of the common minerals that contribute to “hardness” and TDS in water are filtered out.) RO also reduces many toxic chemicals such as arsenic and radium. Note that some chemicals, like chlorine, are removed by the carbon prefilters before the RO membrane. After full treatment, the RO water is nearly free of any dissolved solids.
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Microorganisms and particles: The RO membrane’s pores are extremely small (around 0.0001 micron), so bacteria, viruses, protozoan cysts (like Giardia or Cryptosporidium), and other microbes cannot pass through. In fact, certified RO systems are tested to remove parasites and bacteria. As CDC guidance notes, reverse osmosis filters will remove germs (bacteria, parasites, viruses) in addition to chemicals. (Express Water specifically points out that Giardia and Cryptosporidium are much larger than the membrane pores.) The sediment filter stage also traps any large particles or turbidity.
In short, reverse osmosis strips out almost all solids and contaminants except dissolved gases. It removes metals and minerals (so TDS plummets), most inorganic chemicals (nitrates, chlorides, fluoride, etc.), and it blocks bacteria, viruses and cysts due to the tiny membrane pores. This leaves you with water that is effectively purer than ordinary tap or bottled water.
Benefits of Drinking Reverse Osmosis Water: Because RO water is so clean, it offers several health and taste advantages:
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Cleaner, safer water: By removing potentially harmful substances, RO water reduces exposure to contaminants that can affect health. For example, eliminating lead, arsenic or nitrates from drinking water means your body isn’t ingesting those toxins. As one industry source notes, RO systems remove impurities like heavy metals, chemicals and microorganisms, making them a top choice for health-conscious consumers. In practice, people who switch to RO often feel reassured that their water is nearly free of pollutants.
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Better taste and smell: Without chlorine, sulfates, iron or other unpleasant compounds, RO water often tastes fresher. Many users report a crisp, clean flavor. In fact, reverse osmosis water “tastes better” precisely because it has had chlorine, sulfur or other off‑flavors removed. This means tap water that may have tasted metallic or chlorinated becomes neutral and refreshing. It can also improve the taste of coffee, tea, and cooking (since unwanted minerals or chemicals are gone).
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Environmental and cost benefits: Having pure water on tap means less need for bottled water. Over time an RO system can save money and cut plastic waste. While not strictly a health or taste benefit, it’s another plus – you get high-quality drinking water at home without the hassle and cost of buying jugs.
Overall, RO water gives you consistently clean-tasting water and confidence about water safety. Many people choose RO water because it feels fresher on the palate and may be better for long-term health by keeping contaminants out of their bodies.
What Does pH Mean? The pH scale measures how acidic or basic (alkaline) water is, on a scale of 0 to 14. Pure water has pH 7.0, which is considered neutral. Values below 7 are acidic, and above 7 are basic. In practical terms, typical tap water is usually around neutral pH (roughly 6.5–7.5). A water with pH between 6 and 7 is essentially neutral (only slightly on the acidic side of 7). Express Water reports that its RO-filtered water comes out in the pH 6–7 range, meaning it is very close to neutral. This is expected, since removing minerals often brings water closer to pH 7.0. In summary, pH is just a measure of acidity/alkalinity, and Express Water’s RO water having pH ~6–7 means the water is neither strongly acidic nor basic (i.e. it’s neutral and safe).
What Does TDS Mean? TDS stands for Total Dissolved Solids, which is the total amount of dissolved minerals, salts and inorganic substances in water. It is usually measured in milligrams per liter (mg/L) or parts per million (ppm). Common TDS components are calcium, magnesium, sodium, chloride, etc.. A higher TDS means more dissolved substances. For drinking water, a lower TDS generally indicates purer water. (For context, the U.S. EPA considers up to 500 mg/L (ppm) acceptable for taste, and water under ~300 mg/L is often rated “excellent”.) Reverse osmosis water typically has extremely low TDS, because the membrane strips out nearly all minerals and salts. Express Water’s RO systems produce water with TDS around 2 ppm, essentially removing 99%+ of dissolved solids. In practice, that means there are virtually no impurities dissolved in the water. A TDS of 2 ppm is far below the EPA’s taste guideline (500 ppm), so it indicates very pure water.
In short, pH tells you if water is acidic or basic (RO water is usually near neutral pH 6–7) and TDS tells you how many total minerals are dissolved in it (RO water has extremely low TDS). Express Water’s RO filtration produces nearly neutral (pH ~6–7) water with only ~2 ppm TDS, meaning the water is almost completely free of dissolved substances.
Sources: Authoritative water science sources and Express Water’s documentation were used. For example, EPA and Culligan describe pH and TDS measures, and industry guides explain how RO membranes remove contaminants. (Where Express Water’s specific product values are given, they reflect the company’s published data and typical RO performance.)