Reverse Osmosis Filter Systems

A plain guide to how reverse osmosis works, what it removes, and what to check before choosing a system for your home or business.

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How a reverse osmosis system works

A reverse osmosis (RO) system pushes water under pressure through a semi-permeable membrane. The membrane has pores so small that most dissolved salts, minerals and many other contaminants cannot pass through. The treated water, called permeate, is collected in a storage tank for use, while the rejected material is flushed to drain.

The main stages

  1. Sediment pre-filter.

    Catches sand, rust and other particles that could damage the membrane.

  2. Carbon pre-filter.

    Reduces chlorine and some organic compounds that can shorten the membrane's life.

  3. RO membrane.

    Separates dissolved salts, minerals and many contaminants from the water.

  4. Storage tank and post-filter.

    Holds the treated water and improves its taste before it reaches the tap.

What reverse osmosis can remove

RO is effective against many dissolved substances, including salts, nitrates and heavy metals such as lead. When the system is properly installed and maintained, it can also reduce many bacteria and viruses. Performance depends on the membrane, the pre-treatment and the condition of your water, so a water test is the best way to confirm what a system will do for your supply.

What to check before installing

  • Water pressure: most systems need a minimum inlet pressure, and a booster pump may be needed if yours is low.
  • Waste water: part of the incoming water goes to drain, and the ratio varies by system.
  • Mineral content: RO removes most minerals, so some households add remineralisation.
  • Maintenance: pre-filters and the membrane need scheduled replacement.
  • Space and drainage: the unit needs room near the point of use and a drain connection.

Reverse osmosis questions and answers

What does a reverse osmosis filter system remove?

A reverse osmosis (RO) system removes most dissolved salts and minerals, and reduces many other contaminants such as nitrates, heavy metals and some microorganisms. How well it performs depends on the system and on your water supply, so a water test is the best way to confirm what a system will remove in your home or business.

How does reverse osmosis work?

Water is pushed under pressure through a semi-permeable membrane with pores so small that most dissolved substances cannot pass through. The treated water, called permeate, is collected in a storage tank. The rejected water carries the removed material to drain.

Does a reverse osmosis system waste water?

Yes. Part of the incoming water is sent to drain with the rejected contaminants. The proportion varies with the system design, the water pressure and the water quality. Ask for the expected ratio before you buy.

Does reverse osmosis remove minerals from drinking water?

Yes. Most dissolved minerals, including calcium and magnesium, are removed along with other dissolved substances. Some households add a remineralisation stage to put some minerals back. Your installer can advise on whether this suits your supply.

How often should the filters in an RO system be changed?

Sediment and carbon pre-filters and the membrane are replaced on a schedule set by the manufacturer and adjusted for your water quality and usage. Regular pre-filter changes help protect the membrane. Agree a maintenance schedule when the system is installed.

Does a reverse osmosis system work with low water pressure?

Most RO systems need a minimum inlet water pressure to operate properly. Check the manufacturer's stated minimum for your model. If your pressure is below it, a booster pump can be fitted to support the system.

Do I need a water test before choosing an RO system?

A water test shows which contaminants are present, which helps determine the right pre-treatment stages and membrane for your supply. WaterSource Ireland offers a free water analysis, which you can book from the contact section of this site.

Not sure which system you need?

Book a free water analysis and we will recommend a system for your water. If you use a private well, see our well water treatment page, or read the frequently asked questions about system components and operation.

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