A water ioniser is a device that changes the pH value of water and separates it into alkaline and acidic water. It does this by electrolysis: an electric current produces negatively and positively charged ions. The alkaline water is used as drinking water, the acidic water for cleaning. A water ioniser therefore mainly changes the pH value and the taste of the water.

A reverse osmosis system, by contrast, works on the principle of reverse osmosis. It forces the water through a semi-permeable membrane that holds back impurities such as salts, minerals, heavy metals, bacteria and viruses. The filtered water can be used for drinking, cooking or technical applications. A reverse osmosis system aims to remove as many substances from the water as possible.

The main difference is therefore the purpose: a water ioniser changes the pH value, a reverse osmosis system purifies the water thoroughly. Which device suits you depends on what matters more to you.

Osmosis membranes: how do they differ?

The heart of every reverse osmosis system is the membrane. In household systems you will mainly come across two types:

• TFC membrane (thin film composite, made of polyamide): fitted in most household systems today. It holds back dissolved substances very extensively but is sensitive to chlorine. That is why an activated carbon pre-filter sits in front of it.
• CTA membrane (cellulose triacetate): it tolerates chlorine better, but holds back fewer dissolved substances and only works within a narrower pH range. It is rarely found in new household devices.

Beyond that, membranes differ in three respects:

• Output: usually stated in GPD (gallons per day), it tells you how much filtered water is produced in a given time. It says nothing about filter quality.
• Design: replacement membranes only fit the housing and connections of the device they are made for. That is why the AORA GT600, the AORA GT800 and the W23 each have their own replacement membranes. A membrane from another device will usually not fit.
• Replacement interval: it depends on the local water quality and the amount used. On the W19 and the W23, the RO membrane should be replaced after 12 to 24 months. On the AORA GT800, we recommend replacing the membrane after about 18 months and the PPC pre-filter every 6 months.

Can an ioniser and reverse osmosis be combined?

Yes, the two methods are not mutually exclusive. An ioniser can be connected after a reverse osmosis system. There are also reverse osmosis devices with a built-in additional module, for example the W23 with hydrogen generator. If your main aim is to remove substances from tap water, reverse osmosis is what counts. If you care about pH value or taste, the ioniser is what counts.