Reverse Osmosis: Does it Remove Calcium from Drinking Water?

Reverse Osmosis: Does it Remove Calcium from Drinking Water?

Reverse osmosis is a water treatment process that is often used in residential settings. One of the most common questions people have about reverse osmosis is whether or not it removes calcium from drinking water.

Reverse osmosis is very effective at removing calcium from drinking water. Several studies and decades of experience from thousands of homeowners demonstrates that RO can remove 99+% of calcium. One significant issue with reverse osmosis is that the calcium often fouls the membrane and forms scale on the particle filter – this degrades the filter’s performance and increases the maintenance costs significantly. Depending on the amount of calcium and hardness you have, a water softener may be a better option for treating your water.

In this blog post, we will explore the question about how well reverse osmosis treats calcium. We’ll also provide you with information about how reverse osmosis works, challenges with hard water, and whether RO is right for treating your drinking water.

About Calcium

Calcium is the most abundant mineral in our bodies. It is essential for building and maintaining strong bones and for carrying out many important functions. It is also found in the majority of tap water and well water used for drinking.

What is calcium

Calcium is a mineral that is found in many foods and in most drinking water. It is necessary for a healthy body. Calcium helps with muscle movement, nerve function and blood vessel contraction all throughout your entire system.

In water, calcium is present as an ion. It is also part of total dissolved solids (TDS) which is the total amount of minerals in water. The calcium ion is usually Ca+ and the concentration is generally given as mg/L (milligrams per liter) or ppm (parts per million).

Fouling and scaling caused by calcium

Calcium and magnesium are the two dissolved solids that are known as hardness. Hardness is a measure of these two minerals and is associated with scaling and fouling of pipes.

Hard water is not harmful to your health, but it can cause plumbing problems. The calcium and magnesium in hard water can form a scale on the inside of pipes. This scale can build up over time, damaging water appliances and reducing the flow of water through your pipes.

Health concerns with calcium

Calcium does not pose any health concerns when it is present at typical levels in drinking water. In fact, calcium is necessary for good health. However, too much calcium can cause problems.

Excess calcium can bind with other minerals in the body and form deposits. These deposits can build up in the kidneys and cause kidney stones. Too much calcium can also increase the risk of heart disease. It is important to note that many health experts discount these concerns, so you should consult your doctor with questions.

In general, the level of calcium in drinking water is not a health concern. However, if you have hard water, it can cause plumbing problems.

Drinking water standard for calcium

Across the United States and Canada, the calcium concentration of water varies from 1 to 135 ppm.

There is no drinking water standard for calcium because it does not cause disease or illness in concentrations typically found in drinking water. The EPA has not established a primary MCL or secondary MCL for calcium.

Reverse Osmosis

Reverse osmosis is a widely used treatment method that reliably removes a wide range of contaminants from drinking water. It is effective at reducing the concentration of many impurities such as lead, PFAS, and heavy metals.

How RO worksHow Reverse Osmosis Works

Reverse osmosis systems use a semipermeable membrane to filter impurities from water. The membrane allows water to flow through while trapping dissolved solids and other contaminants. The system applies pressure to the water, forcing it through the membrane and removing any impurities.

The membrane is constructed of a semi-permeable polymer that is spirally wrapped around a central tube. The water is forced through the membrane, and the impurities are filtered out. The clean water is then collected on the other side of the membrane, while the contaminants are retained in a concentrated brine. This brine waste is then flushed down the drain.

Reverse osmosis systems can be used to treat both municipal water supplies and private wells. This treatment technology is effective at removing a wide range of contaminants, including salt, bacteria, viruses, pesticides, and more.

RO system componentsCalcium Does RO Remove it Spiral Wrapped Membrane

Reverse osmosis systems typically consist of three modules:

  1. pretreatment
  2. reverse osmosis membrane
  3. post-filter

Pretreatment protects the membrane from fouling and damage caused by chlorine. It usually consists of a sediment filter and an activated carbon filter. The sediment filter removes particles such as dirt and rust, while the activated carbon filter removes chlorine and other chemicals.

The reverse osmosis membrane is the heart of the system. It’s a thin film composite (TFC) that remove contaminants from water. The TFC is made of several layers of material, including a semi-permeable membrane.

The post-filter is the final stage of filtration that removes any remaining impurities from the water. This is typically a second carbon filter that ensures the water is safe to drink and tastes great. It also includes a particle filter to remove any remaining turbidity.

Some RO systems include a water storage tank, which stores the clean water until you’re ready to use it. This is desirable because the flow rate through the filter is slow. Having a storage tank ensures you can fill a glass or a cooking pot quickly with clean water without having to wait.

How Well Does Reverse Osmosis Treat Calcium

Reverse osmosis removes calcium from water by a process called ion exchange. This is where the calcium ions are attracted to the negatively charged membrane. The calcium ions then pass through the pores in the membrane while the rest of the water molecules are blocked.

Reverse osmosis membranes have tiny pores with a diameter of around 0.0001 microns. Many minerals, including calcium, are smaller than 1 micron in size. This makes them too small to see. However, they are larger than the membrane pore size, so they are readily removed as the water is filtered through the RO system.

How much calcium can reverse osmosis remove

Reverse osmosis filters remove 95 to 99.9% of calcium from drinking water. A small, under-sink RO filter can remove 95% or more of the calcium in the water. A large, whole-house RO filter can remove 97% to as much as 99% of calcium from water.

Membrane fouling caused by calciumRO Sediment Pre-Filter

Calcium and magnesium are the two minerals that make up hardness. Hard water is known to foul reverse osmosis filters. It forms scale on the pre-filters and also within the pores of the RO membrane. These deposits reduce the flow rate through the filter and decrease the treatment efficiency of the RO system.

Many people with hard water install a water softener upstream of the unit. The softener removes the hardness and prevents the fouling issues caused by hard water.

Poor taste

Calcium is a mineral that makes water taste good. A lot of springs have high mineral content, which is why people say the water tastes great. However, when calcium is removed from water, it can taste flat, bitter, or metallic.

Reverse osmosis systems that remove a high percentage of dissolved minerals can leave water with a flat taste or even no flavor at all. This change in flavor is especially noticeable if you’re used to how your water tastes and then add a reverse osmosis system.

Many people add the minerals back to their RO treated water to improve its taste. This can be done by using a mineral cartridge or by adding drops of mineral water to the RO water.

Does Reverse Osmosis Remove the Good Minerals from My Water

Reverse osmosis removes virtually all of the minerals in water. Should we be removing beneficial minerals like calcium from our drinking water? There are many conflicting opinions on this topic, and it can be very confusing when you’re trying to decide what’s right for you and your family.

Is reverse osmosis water bad for you?

Many people are concerned about drinking RO treated water because our bodies need minerals like calcium for good health. They ask – should I use a treatment method that removes beneficial minerals like calcium from my water?

Fortunately, we get most of these minerals from our food, and not from our drinking water. Nutritionists point out that the minerals in food, especially plant-based foods, are more readily absorbed by our bodies than minerals in drinking water.

The WHO published a study that concluded that people who drink reverse osmosis treated water do not suffer from mineral deficiencies.

Where can I get the calcium I need?

Calcium is found in many foods. You can get recommended amounts of calcium by eating a variety of foods, including the following:

  • Dairy products such as milk, yogurt, and cheese
  • Fish with bones such as canned sardines and canned salmon
  • Vegetables such as Bok choi, broccoli, and kale
  • Supplemented beverages like fruit juices, soy milk and almond beverages
  • Breakfast cereals that add calcium

If you are concerned about the health effects of drinking reverse osmosis treated water, consult your doctor. This article is merely providing information, but your health is important, and your physician can give you the best advice.

Factors to Consider with Reverse Osmosis

If you’re concerned about the quality of your drinking water, a reverse osmosis filter may be a good option for you. RO filters are able to remove a variety of contaminants from your water, including lead, mercury, and chlorine. However, there are a few factors you should consider.

Installation cost of reverse osmosis

Depending on your home’s plumbing, installing an RO system can be relatively simple or quite complex. If your home has a standard under-sink configuration, you can likely install the filter yourself. However, if your home has multiple faucets or you need to drill new holes for the filter, it’s best to hire a professional.

A whole-house RO system can be expensive to install, especially if you have to modify your home’s plumbing. An under-sink unit, on the other hand, can often be installed by a good do-it-yourselfer for a reasonable cost. Each home is unique, so you’ll have to determine which system is right for you.

Maintenance cost

RO filters have sediment filters and activated carbon filters that have to be replaced. A sediment filter will usually last for about six months before it needs to be replaced. Carbon filters have to be replaced every 3 to 6 months, depending on how much water you use and the amount of chlorine in your water.

The cost of replacement filters can vary, but you can expect to spend around $60 to as much as $200 per year on them. Whole house filters are 3 to 4 times as much as under-sink filters.

While this may seem like a lot, it’s important to remember that RO filters can remove a variety of contaminants from your water, making it safer and healthier to drink.

Whole-house versus under-sink unitsWhole House RO System Diagram

Another thing to consider is how much water you’ll need to filter. RO filters are typically sized according to the number of gallons they can process per day. If you have a large family or use a lot of water for cooking and bathing, you may need a larger filter.

Reverse osmosis systems are available in a wide range of sizes, from small countertop units to large industrial systems. They can be used for point-of-use (POU) applications, such as filtering water for a kitchen sink, or point-of-entry (POE) applications, such as filtering all the water coming into your home.

Deciding which type of RO system is right for you depends on a number of factors, including your budget, the size of your home, and your water quality. Read my article on under-sink RO systems and whole-house RO systems for a detailed explanation.

NSF certificationNSF certification for reverse osmosis filters

Reverse osmosis filter manufacturers often make claims about how well their units remove contaminants. The best way to be certain of their claims is to look for units that have been certified by NSF International.

NSF is an independent organization that tests and certifies products to ensure they meet strict standards for quality and safety. If a product has been certified by NSF, you can be confident it will perform as advertised.

The applicable standard for point-of-use reverse osmosis systems is NSF/ANSI 58. It covers the following parameters:

  • Materials of construction and leaching of harmful substances
  • Mechanical integrity
  • Removal of total dissolved solids (TDS)
  • Efficiency rating
  • Recovery rating
  • Contaminant concentration reduction


Does reverse osmosis remove calcium carbonate?

Reverse osmosis removes virtually all calcium carbonate in water. This mineral is effectively trapped by the RO membrane and flushed down the drain.

What is the best way to remove calcium from water?

The most effective way to remove calcium from water is by using a water softener. Water softeners can remove up to 99% of calcium (and hardness) in water, making it ideal for those who want to protect their pipes and plumbing from hard water.

Does reverse osmosis remove hardness?

Reverse osmosis removes both calcium and magnesium – the two minerals that comprise hardness. However, over a short period of time, the RO filter becomes fouled, and its performance drops off. You may need a water softener if you have hard water.


Reverse osmosis can remove virtually all of the calcium in your drinking water. However, hard water can foul the RO filter, and you may need a water softener to remove the hardness.

Removing calcium from your water can affect its taste, and you may want to add the minerals back to your water to improve its taste. You can do this by using a mineral cartridge or by adding drops of mineral water to your RO water.

Boch Richard

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

Recent Posts