By Rehan Iqbal
Next to chlorination and fluoride, no form of water treatment gets more attention than reverse osmosis. It’s listed as the primary form of treatment used for many brands of bottled water. It may finally make desalinating ocean water affordable, giving coastal areas the drinking water they need without draining in-land aquifers. It’s also promoted by manufacturers as the best way to get clean, great tasting drinking water.
Once you start looking into these filters, their features can get confusing. What exactly does reverse osmosis remove? Is it the best choice for removing contaminates, or are you better off using alternatives? Can it cause more issues than it solves, or make your water taste bad? Does it remove minerals your body needs? If reverse osmosis is so good at filtering water, why does it come with other filters? Why do these systems use their own water tap, instead of treating all of your home water? We’re here to answer these questions and help you figure out how you can use reverse osmosis correctly to improve your municipal or well-sourced tap water.
What Water Quality Problems Can a RO System Treat?
Here’s a list of the most common contaminates in water, the effectiveness of RO filters on them, and alternative treatment systems.
Reverse osmosis is the best method for removing lead. The activated carbon filter in most systems can remove about 95% of lead in water. Lead molecules are larger than water molecules, so they can’t pass through the RO membrane. This filtration isn’t perfect, but you end up with just over 99% of the lead removed by the time the water leaves the filter. NSF includes lead testing under standard 58.
However, lead can be a problem for whole house reverse osmosis systems. Water with low levels of total dissolved solids (TDS) leach materials from pipes as they pass through them. This includes the lead solder used to join metal plumbing pipes. Manufacturers recommend replacing these pipes with polyethylene tubing before installing a point of entry RO system.
Arsenic occurs naturally in rocks. Arizona, Maine, and New Hampshire have bedrock with high levels of arsenic, but this chemical can be found almost anywhere. There are two types of arsenic: arsenic 3 and arsenic 5. Arsenic 3 cannot be removed from water. However, oxidizing arsenic 3 turns it into arsenic 5. This type of arsenic can be removed by water treatment systems. Oxidizing filters are also used to remove metals and kill pathogens.
Arsenic 5 doesn’t make water unsafe for cleaning and washing, so point-of-use reverse osmosis is one of the most cost effective ways to remove this contaminate. An RO filter removes between 80 and 99% of arsenic from water. If you need a highly effective filter, look for one that is NSF 58 certified for this mineral. Ion exchange systems can remove some arsenic, but these systems have stronger attraction to other metal ions, making them less reliable than an RO system.
Sodium chloride is common table salt, and it’s the same salt found in sea water. Reverse osmosis water filtration was invented to remove this salt, letting submarines generate their own fresh water supply. The pores in the membrane are small enough to trap salt ions in the waste water.
An RO filter is about 99% effective at removing salt from water. There isn’t a good alternative to an RO system for removing salt from water, aside from distillation. This is rarely used for home water treatment, since the energy required to boil water makes this process expensive.
Copper contamination comes almost entirely from plumbing systems. If your water is acidic, has low levels of total dissolved solids (TDS) or high levels of sulfides, it can corrode copper pipes in your home. Copper gives water an unpleasant metallic taste. High levels of copper can cause intestinal distress, but this is rare.
Reverse osmosis is effective at removing copper from water. This contaminate is covered under the NSF 48 standard. Alternatively, copper can be removed with ion exchange systems, like salt-based water softeners. You can also stop contamination by raising water pH with calcite at the point of entry, or by replacing copper pipes with non-reactive polyethylene.
Whole house reverse osmosis systems can increase copper contamination at the tap, since it removes dissolved solids. This makes it easier for acidic water to pick up copper from pipes as it passes through the plumbing system. Over time, this leaching can lead to pitting and eventually pinhole leaks in copper pipes.
More than likely, you’ve heard about this contaminate from the movie “Erin Brockovich.” It’s based on a real world case of water contamination from a natural gas pumping station.
Chromium exists in two forms: trivalent chromium (chromium-3) and hexavalent chromium (chromium-6.) While chromium-3 is an essential nutrient, chromium-6 is highly carcinogenic. This metal can switch between both forms, depending on a number of environmental factors. Testing and treatment covers both types of chromium.
While ion exchange systems are used for large scale filtration, it requires specialized media that you won’t find in home water softeners or nitrate treatment systems. The only practical way to remove this contaminate in your home’s water is with a reverse osmosis filter certified for chromium treatment.
Pesticides and Herbicides
Pesticides and herbicides include a long list of chemicals with an equally wide range of health effects. Even if the levels of these chemicals aren’t high enough to cause health concerns, they can be a major water quality issue. Even small amounts will make your water taste terrible.
These chemicals are small enough that they have no problem passing through the pores of a reverse osmosis filter. However, they can be filtered out by activated carbon. Since RO systems include activated carbon filters, they’re usually effective at removing these contaminates. Alternatively, you can add a whole house or under-sink activated carbon filter. These filters are also used the remove chlorine, so they’re included in chlorine-based oxidation treatment systems.
Reverse osmosis doesn’t remove dissolved gases. However, KDF and catalytic carbon can remove small amounts of hydrogen sulfide. This gas makes water smell like rotten eggs. Of course, if you have hydrogen sulfide in your water, you don’t want to smell it anywhere. The most effective treatment for this gas is oxidation. By spraying water through air or mixing it with hydrogen peroxide or chlorine, this gas breaks down, forming a solid that is filtered out of the water.
Reverse osmosis also isn’t effective on methane. This gas isn’t poisonous, but it is flammable. To prevent methane buildup in well water, the well needs a casing above ground with a vent. This lets the methane escape before it collects in the well or enters your plumbing system.
Although great for dental health in small amounts, high levels of fluoride can cause bone issues. Reverse osmosis is the most effective way to remove fluoride from water. It filters out up to 99% of this mineral, and works on all water. There is an NSF/ANSI 48 certification test for this contaminate.
Activated alumina absorbs fluoride, but it only works with water that has a pH under 6.5. Bone charcoal removes about 90% of fluoride in water, while other carbon filters, including those included with RO systems, do not.
Radioactive elements like radium release energy that breaks down DNA. Over time, this can cause cancer. Radium is common in rocks, and leaches into low pH water. It also comes from mining runoff. While most radium is excreted by your body, some of it can attach to bones. Long term exposure leads to bone cancer, although it’s hard to estimate the exact amount of radium where this becomes an issue.
Instead of measuring radium directly, water tests measure the amount of alpha particles given off by the water in picocuries. If your tap water has 5 pCi/L, over one year, you’re exposed to as much radiation drinking this water as you are from a chest X-ray.
Ion exchange water softeners can remove this radioactive element, as can softening water with lime (calcium hydroxide.) In both cases, the radium switches places with other ions. Granular activated carbon removes up to 70% of radium. However, the most effective treatment method is reverse osmosis, which filters out almost all radium. There is an NSF 48 test for effectiveness of this filtration in RO systems.
Sulfates make water taste bitter, while hydrogen sulfide makes water smell like rotten eggs. High levels of sulfates can have a laxative effect. This normally isn’t a problem for adults, but it can cause diarrhea in babies.
If your water has high levels of sulfates, the only solution is oxidation. This can be a system that adds chlorine or hydrogen peroxide to the water, or sprays water through oxygen-rich air. Reverse osmosis filters cannot remove sulfates or hydrogen sulfide. However, activated carbon works well for water that contains 1 mg/l or less sulfur. The filter, which is included with most RO systems, may be enough to remove this contaminate from your water.
Calcium carbonate is one of the minerals that increases water hardness. It leaves mineral deposits on pipes, plumbing fixtures, appliances and on any surface near faucets.
Reverse osmosis removes calcium. However, under sink systems are not an effective way to treat this mineral. If you want to prevent clogged pipes and limescale, you need a whole house solution, like a water softener or water conditioner.
Calcium is also a nutrient our bodies need, and it improves the taste of water. Some RO systems include a remineralization filter that adds some calcium back to the water before it exits the tap.
Magnesium is found in granite, sandstone, limestone and dolomite. Water that runs through these rocks picks up this mineral. Magnesium in your water is both good and bad. It’s a necessary mineral for nutrition, and it improves the taste of water. It also makes the water hard. When magnesium passes through your plumbing system, it leaves deposits on pipes, inside appliances and on and around faucets.
Reverse osmosis removes magnesium, but that won’t stop hard water problems throughout your home. It’s better to deal with hard water by adding a whole house water softener or conditioner. Remineralization filters add back magnesium, improving the flavor and nutrition of water.
Potassium permanganate is used to oxidize hydrogen sulfide, iron and manganese, so they fall out of suspension. Water treatment systems that use this chemical pair it with manganese-coated aluminum silica or anthracite together with manganese-treated green sand. If your treatment system is working correctly, there should be no potassium permanganate left in the water when it reaches your taps.
If it does make it out of the treatment system, an RO filter will remove this chemical. Potassium hydroxide, a form of potassium found naturally in water, usually isn’t a health issue, except in extremely high amounts. RO membranes remove 95% or more of this contaminate.
Long term exposure to nitrates can cause cardiovascular issues, but they’re mostly a threat to babies. Unlike adults, babies don’t have a digestive tract that can break down nitrates into other chemicals. Once they enter the blood stream, nitrates keep red blood cells from working. This leads to methemoglobinemia, AKA “blue baby syndrome.” This is why you should always test well water for nitrate levels before bringing a baby into your home.
Reverse osmosis isn’t just effective at removing nitrates. It’s also often the cheapest option for removing these chemicals from tap water. Nitrates are only a hazard if they’re consumed, so there isn’t any extra benefit to treating all the water in your home. Look for a filter that meets NSF standard 58 for nitrate removal.
Phosphorous is found naturally in rocks, but most problems with phosphorous in water come from fertilizer contamination. Soil has a limited capacity for storing this mineral. Once that’s exceeded, it enters the water table.
Although inorganic phosphorous from fertilizer runoff is generally considered safe, it affects the flavor of water. High levels of phosphorous makes water taste sweet, bitter or metallic. Both reverse osmosis filters and activated carbon filters are effective at removing phosphorous. Since RO systems have both types of filters, they have no problem dealing with this contaminate.
What is Reverse Osmosis, and How is it Used to Treat Water?
If you have water separated by a permeable barrier, dissolved solids will pass through this barrier until the water on both sides has the same mineral content. For example, if one side has clean water and the other side has salt water, the salt will pass through the barrier until the water on both sides has the same salinity. This process is called “osmosis.” Reverse osmosis uses pressure to push dissolved solids through the barrier, concentrating them on one side. In an RO filtration system, the water with high levels of dissolved solids goes down the drain, while the clean water goes to the tap.
A reverse osmosis filter is made from several layers of cellulose acetate, polysulfonate and polyamide, These layers have different densities, creating a skin with a support structure. The pores in this filter can be as small as 0.0001 microns. This is large enough for water molecules to pass through, while leaving most contaminates behind. However, this filtration layer isn’t perfect. Over time, holes form, letting some contaminates pass through the barrier.
Reverse osmosis treatment systems are more than a single RO filter. Pre-filtering removes iron and chlorine, which are harmful to RO filter media. RO filters are expensive, so most systems have additional particulate, activated carbon and catalytic carbon filters. These filter out the majority of contaminates, so the RO membrane has less to remove.
Since reverse osmosis is mostly used to improve drinking and cooking water quality, some systems have a remineralization filter. This adds back calcium and magnesium, improving the water’s flavor.
Under-sink RO systems use water pressure to push water through the filter media. The high resistance reduces flow to a trickle. To compensate for this, most systems come with a storage tank. This gives the filter system time to generate clean water before it’s needed.
Whole house reverse osmosis filters are available, but they are prohibitively expensive. These require additional pumps to push water through the filter, along with large pre-filter systems. It’s more cost effective to use alternative water treatment systems for home use, saving reverse osmosis for drinking water.
Even if you can afford a whole house system, reverse osmosis can cause plumbing issues. Since filtered water doesn’t have dissolved solids, it’s ready to pick up new solids from anything it comes in contact with. This includes copper pipes and lead solder used in plumbing systems. Not only does this contaminate the water, it also dissolves plumbing connections, eventually causing leaks.
What Does it Mean When a Filter is Certified by NSF/ANSI Standard 58?
NSF and ANSI are two standards bodies that work with industry groups to address safety and quality issues. NSF/ANSI standard 58 tests the effectiveness of reverse osmosis drinking water systems. To meet this standard, the filter has to meet performance metrics for safety, structural integrity, efficiency and total dissolved solids (TDS) removal. There are additional tests under this standard for specific contaminates, but this testing is not required for certification.
Instead, if a manufacturer claims their RO system removes a specific contaminate, it must meet the testing requirements for that contaminate under NSF/ANSI 58. The contaminates covered under this standard don’t include everything an RO system removes. For example, these systems are effective at removing chlorine, since it must be removed by the carbon filters to protect the reverse osmosis filter.
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