Why does a reverse osmosis system produce wastewater?

Some of the water is used to flush away substances that do not pass through the membrane. The volume of wastewater varies depending on system design, pressure, and operating conditions.

What is the lifespan of a reverse osmosis membrane?

Lifespan depends on incoming water quality, volume treated, pressure, pre-filtration quality, and membrane model. The manufacturer's recommendations remain the primary reference.

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What Is Reverse Osmosis?

Q: Does reverse osmosis remove all pollutants?

No. Performance depends on the contaminant, membrane, complete system, and operating conditions. You need to check the documented performance for the substances you want to reduce.

What is reverse osmosis? How it works, benefits, and limitations

Reverse osmosis is a water treatment technology that uses a semipermeable membrane and pressure to reduce a large proportion of dissolved substances and many contaminants present in water. It is used in homes as well as in commercial and industrial applications.

Updated on September 11, 2026, by the Josmose team.

Reverse osmosis at a glance

Reverse osmosis involves passing water under pressure through a very fine membrane. Some of the water passes through this membrane and becomes treated water, while another portion carries away some of the retained substances. This technology can significantly reduce dissolved salts and many contaminants, but it requires regular maintenance and also produces a certain amount of wastewater.

How does reverse osmosis work?

Reverse osmosis works by passing water through a semipermeable membrane. Unlike natural osmosis, the system applies pressure to force some of the water through this membrane.

The membrane mainly allows water to pass through while retaining a large proportion of the dissolved substances present in the incoming water. Actual effectiveness, however, depends on many factors: membrane type, pressure, temperature, water composition, and how well the system is maintained.

The process therefore produces two streams:

treated water, also called permeate or RO water;
wastewater, which carries away some of the retained substances.

The different stages of a reverse osmosis system

A home RO system generally does not rely solely on its membrane. Several stages work together.

1. Pre-filtration

Before reaching the membrane, water generally passes through one or more pre-filters.

These can help trap particles and reduce certain substances that could damage the membrane or impair its performance.

See: filters for RO systems .

2. The reverse osmosis membrane

The membrane is the heart of the system. This is the stage at which a large proportion of dissolved substances are separated from the water produced.

Performance varies depending on the membrane type, its capacity, the available pressure, and the quality of the incoming water.

See: membranes for RO systems .

3. Post-filtration

After the membrane, some systems use an additional stage, such as an activated carbon filter, to improve certain characteristics of the water before dispensing it.

4. Optional remineralization

Reverse osmosis also reduces the concentration of many dissolved minerals. Some systems therefore add a remineralization filter cartridge after the membrane.

This cartridge does not filter the water further: it works after reverse osmosis to add certain minerals, depending on its composition.

Learn more: remineralization solutions .

5. Tank or direct flow

Once treated, water can be stored in a tank or produced directly on demand, depending on the unit's design.

To understand the differences: direct-flow or tank-based RO system?

What can reverse osmosis reduce?

Reverse osmosis is primarily used to reduce the concentration of many substances dissolved in water. Exact performance varies depending on the membrane, operating conditions, and substances involved.

Substance	Potential reduction	Note
Dissolved salts	Yes	This is one of the main uses of reverse osmosis.
Nitrates	Yes	The level of reduction depends on the system and operating conditions.
Certain dissolved metals	Yes	Performance depends in particular on their chemical form.
Certain PFAS	Reduction possible	Performance must be verified using documented data.
Microorganisms	The membrane can act as a barrier	Membrane integrity and system hygiene remain essential.

Performance is not the same for every RO system. For a specific substance, you should consult the technical data or tests for the system in question.

Reverse osmosis and PFAS

PFAS are a family of persistent chemicals that can be present in the environment and in some water sources.

Reverse osmosis is one of the technologies that can be used to reduce the concentration of certain PFAS in water. However, results depend on the membrane, the complete system, and its operating conditions.

To explore this topic further: read our in-depth article on PFAS and their filtration .

What are the benefits of reverse osmosis?

Significant reduction of dissolved substances: reverse osmosis is particularly suitable when your goal is to substantially reduce mineral content and TDS.
Action on many contaminants: depending on the system used, it can help reduce several families of substances present in water.
Versatile technology: it can be used on a residential, commercial, or industrial scale.
Production of water with low mineral content: this can be desirable for certain specific uses.
Can be combined with other treatments: pre-filtration, activated carbon, remineralization, UV, or other stages depending on the installation.

What are the limitations of reverse osmosis?

Reverse osmosis is effective for many uses, but it also has several requirements and drawbacks to consider.

Wastewater: some of the water supplied to the system is used to carry away retained substances.
Mineral reduction: the process also reduces the concentration of many dissolved minerals.
Pressure requirements: some installations need a booster pump when the water supply pressure is insufficient.
Regular maintenance: pre-filters, the membrane, and any post-filters must be replaced according to the manufacturer's recommendations.
Cost of consumables: the unit's purchase price does not represent its total cost over several years.
Space requirements: some systems require space under the sink, especially when a tank is used.

Learn more: RO systems: benefits and drawbacks .

Reverse osmosis, ultrafiltration, or activated carbon: what are the differences?

Not all filtration technologies meet the same needs. Your choice depends on what you want to reduce in your water.

Criterion	Reverse osmosis	Ultrafiltration	Activated carbon
Reduction of dissolved salts	Significant	Limited	Limited
Wastewater	Yes	Depends on the system	Generally no
Reduction of certain organic substances	Yes, depending on the system	Variable	One of its main uses
Dissolved minerals	Substantially reduced	Mostly retained	Mostly retained

Ultrafiltration is therefore not simply a “more eco-friendly” version of reverse osmosis: it is a different technology that does not treat the same substances in the same way.

Learn more: what is ultrafiltration? .

Which RO system should you choose?

Understanding reverse osmosis is the first step. To choose a unit, you then need to compare the specific features of the different systems.

The main criteria are:

the flow rate you need;
whether or not the system has a tank;
the available pressure;
the amount of wastewater;
space requirements;
optional remineralization;
the cost of filters and the membrane.

To continue your research:

How do you install an RO system?

Many home RO systems are designed so you can install them yourself using a detailed user manual. However, the difficulty depends on the model, your existing plumbing, and the connections required.

An under-sink installation generally requires:

a cold water supply;
a drain connection;
enough space for the unit;
possibly an electrical outlet if the system has a pump;
installation of a dedicated or compatible faucet.

Before any installation, always follow the user manual for the model you are using.

You can also consult: our FAQ and advice for RO systems .

If you have any doubts about your installation, it may be best to hire a professional.

What maintenance should you plan for a reverse osmosis system?

An RO system needs regular maintenance to maintain good performance and protect its membrane.

Item	When should you take action?	What to monitor
Pre-filters	According to the manufacturer's recommendations	Time in use, flow rate, water quality
Membrane	Based on its performance and the manufacturer's recommendations	TDS, flow rate, quality of the water produced
Disinfection	According to the system's recommendations	Maintenance, extended periods of inactivity, component replacement

For more information: how to maintain your RO system .

Ready to choose an RO system?

Compare technologies, flow rates, wastewater, maintenance, and the different models available.

View the guide to choosing your RO system View RO systems

Frequently asked questions about reverse osmosis

What is reverse osmosis?

Reverse osmosis is a water treatment process that uses pressure to push water through a semipermeable membrane. This membrane significantly reduces many dissolved substances.

What is the difference between osmosis and reverse osmosis?

Osmosis is the natural movement of water through a membrane. Reverse osmosis, by contrast, uses external pressure to move water in the desired direction and produce water containing fewer dissolved substances.

Does reverse osmosis remove all pollutants?

No. Performance depends on the contaminant, the membrane, the complete system, and operating conditions. It is therefore best to check documented performance for the substances you want to reduce.

Does reverse osmosis reduce PFAS?

Reverse osmosis can help reduce the concentration of certain PFAS. However, the level of reduction depends on the membrane and the installation used.

Why does an RO system produce wastewater?

Some of the water carries away substances that do not pass through the membrane. Wastewater volume varies considerably depending on the system's design, pressure, and operating conditions.

Does RO water still contain minerals?

Reverse osmosis significantly reduces the concentration of many dissolved minerals. Some systems then add a remineralization filter cartridge to reintroduce some of them.

How long does a reverse osmosis membrane last?

Its lifespan depends on the quality of the incoming water, the volume treated, pressure, pre-filtration quality, and the membrane model. The manufacturer's recommendations remain the main reference.

Reverse osmosis or ultrafiltration: which should you choose?

Reverse osmosis is generally chosen when you want to significantly reduce dissolved salts and mineral content. Ultrafiltration retains more dissolved substances. Your choice therefore depends primarily on the quality of the incoming water and the results you want.

Sources and references

For topics related to water safety, we recommend consulting publications from health and scientific authorities first.

ANSES – Assessment of health risks associated with microorganisms in water intended for human consumption

About this guide

This content is written by the Josmose team based on our experience with water treatment solutions, the equipment's technical specifications, and the sources cited in the article.

Josmose sells RO systems, membranes, and consumables mentioned on or accessible from this page.