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What is reverse osmosis (RO) and how does a water filter work?

Reverse Osmosis (RO) is one of the most effective technologies for deep water purification. Its main component is a special semipermeable membrane that reduces not only particles visible to the naked eye from the water, but also a large portion of substances dissolved in the water.

This is precisely what distinguishes reverse osmosis from ordinary mechanical, carbon, or ultrafiltration systems.

RO systems are particularly often used for filtering drinking water and, as a rule, are installed under the kitchen sink.

What does reverse osmosis mean?

In order to understand how reverse osmosis works, we must first examine the principle of ordinary osmosis.

During osmosis, water moves naturally through a semipermeable membrane from a less concentrated environment toward an environment with a higher concentration.

In the case of reverse osmosis, this process proceeds in the opposite direction.

Through the influence of water pressure, water passes through a special RO membrane. The membrane retains a large portion of the substances dissolved in the water, while the filtered water moves to the subsequent stages of the system.

As a result, the system produces two streams:

  • filtered water, which is delivered to the consumer;
  • concentrate, into which the substances retained by the membrane enter and which flows into the drain.

How finely does an RO membrane filter?

The filtration level of a household reverse osmosis membrane is approximately 0.0001 microns (μm).

This is a significantly smaller scale than in the case of ordinary mechanical or ultrafiltration systems.

For comparison:

  • Mechanical filters mainly retain relatively large particles;
  • An ultrafiltration membrane can work at approximately the 0.01 μm level;
  • The effective filtration scale of an RO membrane is approximately 0.0001 μm.

However, it is important to understand that reverse osmosis is not just a very fine "sieve." The efficiency of separating substances is influenced by membrane properties, water pressure, temperature, the molecular and ionic characteristics of the substances, and other factors.

What does a reverse osmosis system reduce in water?

A correctly selected RO system can significantly reduce many types of dissolved substances and contaminants present in water.

Including:

  • dissolved salts;
  • calcium and magnesium;
  • total dissolved solids (TDS) of water;
  • nitrates and nitrites;
  • certain types of heavy metals;
  • lead;
  • chromium;
  • copper;
  • sulfates;
  • chlorides;
  • sodium;
  • fluorides;
  • certain forms of arsenic;
  • other inorganic impurities.

An RO membrane also creates an effective barrier for many microorganisms. If the water source carries microbiological risk, the filtration scheme should be selected based on water analysis and specific conditions. Additional information can be found in the article "Bacteria and Viruses in Water - Microbiological Contamination and Ways of Protection".

The efficiency of reducing a specific substance depends on the membrane used, the design of the system, the quality of the incoming water, and operating conditions. Therefore, in the presence of a specific contaminant, it is always important to check the technical data of the appropriate system.

How does a multi-stage RO filter work?

A household reverse osmosis system does not consist of just one membrane. A high-quality RO system, as a rule, uses several consecutive stages.

1. Mechanical pre-filtration

At the first stage, relatively large mechanical particles are removed from the water:

  • sand;
  • rust;
  • sediment;
  • solid particles that entered from the pipes.

This stage also protects the subsequent filters and the RO membrane from excessive load.

2. Activated carbon filtration

Then the water passes through an activated carbon filter.

The main purpose of carbon filtration is to reduce substances in the water such as chlorine and various organic compounds. It also helps to improve unpleasant tastes and odors in the water.

This stage is also important for protecting the RO membrane, because some types of membranes are sensitive to free chlorine.

3. Reverse osmosis membrane

This is the main stage of the system.

Under the influence of pressure, water passes through a semipermeable RO membrane, where water molecules are separated from dissolved salts and many other substances.

The filtered water moves to subsequent stages, while the concentrated waste water flows into the drain.

4. Final carbon filter

After the RO membrane, many systems use a so-called Post Carbon Filter.

Its purpose is the final correction of the water's taste and odor immediately before delivery to the tap.

5. Mineralization

Not all RO systems have a mineralizer.

The membrane also reduces a large portion of minerals from the water, so through an additional mineralization stage, the water is re-enriched with certain minerals.

Such a system may contain, for example, a mineralization cartridge containing calcium and magnesium.

Depending on the system configuration, additional filtration stages may also exist.

Why does reverse osmosis have a storage tank?

A classic household RO membrane processes water at a certain speed and, when the tap is opened, cannot always provide the amount of water the consumer needs at a specific moment.

Therefore, many RO systems use a special pressurized storage tank.

Filtered water is collected in the tank gradually, and when the tap is opened, the already prepared water is delivered to the consumer.

There are also tankless, direct flow RO systems, which use higher-capacity membranes and, usually, a pump.

Why does reverse osmosis need a drain?

An RO membrane does not constantly accumulate the substances separated from the water in the cartridge.

A portion of the substances retained by the membrane is expelled from the system along with a separate stream of water and enters the drain.

This is a natural and necessary part of how reverse osmosis works.

Therefore, an RO system has:

  • an incoming water line;
  • a filtered water line;
  • a drain line.

The amount of water used by reverse osmosis is not the same in all systems. Efficiency is influenced by the membrane, system design, water pressure, temperature, and other operating conditions.

Does RO reduce water hardness and limescale?

Yes.

Water hardness is mainly caused by calcium and magnesium ions dissolved in the water.

The RO membrane reduces a large portion of them. Consequently, the probability of limescale formation in water filtered by reverse osmosis is also significantly reduced.

Because of this, when using RO water, the amount of scale in a kettle or coffee machine can be drastically reduced.

However, we must consider an important distinction here.

A household RO system installed under the sink treats only a specific point of drinking water.

If hard water causes problems throughout the whole house, for example:

  • in the boiler;
  • in the shower;
  • on faucets;
  • in the washing machine;
  • in the heating system;

In this case, a water softening system installed on the central line is a more appropriate solution.

RO and ultrafiltration - what is the difference?

These two technologies are often confused with each other.

Ultrafiltration - UF

Ultrafiltration effectively reduces:

  • mechanical particles;
  • turbidity;
  • bacteria;
  • colloidal particles.

At the same time, a large portion of minerals and salts dissolved in the water remains.

Therefore, UF is a good choice when we want to preserve the natural mineral composition of the water and the water does not suffer from high TDS or hardness.

Reverse Osmosis - RO

RO is a much deeper membrane processing technology and also reduces a large portion of salts and minerals dissolved in the water.

Therefore, reverse osmosis is more appropriate when there is a need for:

  • TDS reduction;
  • reduction of water mineralization;
  • nitrate reduction;
  • heavy metal reduction;
  • deeper processing of drinking water.

Is mineralization of RO water necessary?

A reverse osmosis membrane does not distinguish between "beneficial" and "non-beneficial" minerals. Its purpose is to reduce a significant portion of substances dissolved in the water.

Therefore, the amount of calcium and magnesium in water after RO is also significantly reduced.

If the consumer wants more mineralized water or prefers its taste, a mineralizer can be added.

For example, some multi-stage RO systems use a mineralization cartridge after the membrane, which re-enriches the water with certain minerals.

Mineralization is not necessary for the RO system to work - it is an additional stage.

What does TDS mean and why does it decrease after RO?

TDS - Total Dissolved Solids refers to the total amount of substances dissolved in water.

It may include various dissolved ions and salts, including:

  • calcium;
  • magnesium;
  • sodium;
  • chlorides;
  • sulfates;
  • other mineral substances.

Since the RO membrane reduces a large portion of these dissolved substances, the TDS indicator after reverse osmosis is, as a rule, significantly lower.

A TDS meter is not a complete test of water safety.

It shows the total level of substances dissolved in the water based on electrical conductivity, but cannot determine specifically which substances are in the water.

Does an RO system need water pressure?

Yes.

The reverse osmosis process depends on water pressure.

If the incoming water pressure is insufficient, the performance of the membrane and the efficiency of the system may decrease.

In such a case, an RO system equipped with a pump is used, which creates the necessary operating pressure for the membrane.

This is precisely why considering water pressure is important when selecting a system.

How often are RO filters changed?

Not all elements in an RO system have the same resource.

Pre-mechanical and carbon cartridges are, as a rule, changed more often than the RO membrane.

The duration of the membrane's operation depends on:

  • the quality of the incoming water;
  • water consumption;
  • water hardness and TDS;
  • the condition of pre-filtration;
  • water pressure;
  • requirements of the specific membrane manufacturer.

Therefore, when changing cartridges and membranes, it is important to follow the service schedule defined for the specific system. Additionally, you can read "When should you change a water filter cartridge?".

In what case is it worth choosing reverse osmosis?

An RO system is an especially good choice if:

  • you want high-quality drinking water at home;
  • the water has high TDS;
  • the water is distinguished by high mineralization;
  • you are troubled by scale formed in a kettle;
  • reduction of salts dissolved in the water is needed;
  • nitrates or other specific chemical impurities have been detected in the water;
  • you want deeper processing of drinking water than ordinary carbon or UF filtration can provide;
  • you want to reduce the need to constantly purchase bottled water.

If the specific water problem is unknown, evaluating water quality before selecting a system is the best decision.

How to select an RO system?

When choosing a reverse osmosis system, one should not pay attention only to the number of filtration stages.

One must consider:

  • initial water quality;
  • TDS;
  • water hardness;
  • water pressure;
  • the family's water consumption;
  • membrane performance;
  • whether the system has a pump;
  • whether mineralization is needed;
  • storage tank volume;
  • availability of cartridges and membranes;
  • cost of subsequent maintenance.

A 7-stage system does not automatically mean that it is better than a 5-stage system in all cases.

The most important thing is the purpose of each stage and how well the specific system corresponds to water quality and consumer needs.

If you are still in the selection stage, you can review our guide: "Water Filter - How to select a drinking water filter?".

AQUAREA's Reverse Osmosis Systems

AQUAREA offers household RO systems of various configurations, including:

  • standard reverse osmosis systems;
  • systems equipped with mineralization;
  • models with additional filtration stages;
  • systems equipped with a pump;
  • models with membranes of various performance capabilities.

When choosing a system, it is important to consider not only the price or number of stages, but also water quality, water pressure, and the consumer's specific requirements.

View Reverse Osmosis (RO) systems

Frequently Asked Questions - FAQ

Can one drink water filtered by reverse osmosis?

Yes. A correctly selected and maintained household RO system is intended for processing drinking water. If desired, an additional stage of mineralization can also be used.

Does reverse osmosis remove all minerals from the water?

An RO membrane reduces a large portion of minerals dissolved in the water, but the final result depends on the membrane and operating conditions. In systems with a mineralizer, the filtered water then additionally passes through a mineralization stage.

Does reverse osmosis reduce limescale?

Yes. RO reduces the amount of calcium and magnesium that cause water hardness, so the formation of limescale in filtered water is significantly reduced.

Does an RO filter need electricity?

Not all models. In the case of sufficient water pressure, many classic RO systems work without electricity. Models equipped with a pump, however, do need electricity.

Why does an RO filter have a drain pipe?

Substances retained by the membrane must be expelled from the system along with a concentrated stream of water. This water goes into the drain and is a natural part of the reverse osmosis process.

Is RO better than ultrafiltration?

This depends on water quality and the consumer's goal. UF retains a large portion of minerals dissolved in the water, while RO significantly reduces them and the TDS. In the case of high mineralization, hardness, or certain dissolved chemical impurities, RO is a more appropriate technology.

Is a water analysis necessary before installing an RO system?

In the case of municipal water, a full laboratory analysis is not always necessary, though knowing the water source, TDS, hardness, and pressure simplifies selecting the system correctly.

In the case of a well or an unknown source, water analysis is especially important.

Conclusion

Reverse osmosis is a water treatment technology that is particularly effective when simply improving the taste and smell of water is not enough and there is a need for significant reduction of substances dissolved in the water and total mineralization.

A correctly selected RO system can become a stable source of daily drinking water and significantly reduce dependence on bottled water.

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