Because no filter media can efficiently tackle all types of contaminants on their own, most water filter systems are constructed in a way to combine several stages of filtration to achieve the desired filtration efficiency and water purity. In some cases, having a certain number of filtration stages is compulsory (e.g. UV water filters need a pre-filter), in other cases, some stages are optional (e.g. remineraliser filters in RO water filters).

4 stage reverse osmosis system 

Reverse osmosis water filters are a typical example of a multi-stage water filter. These systems can have up to six or even seven stages of filtration, but are all these stages compulsory? How many filtration stages should an RO system have?

Most water filtration experts agree that 4 filtration stages are more or less required for reverse osmosis filters. These four stages include the sediment pre-filter, a carbon block pre-filter, a reverse osmosis membrane, and a carbon-block post filter.

Pre-filters have the role of removing larger contaminants, silt, rust, sand, chlorine and other organic chemicals, chlorine by-products, finer pollutants and other contaminants that may damage or clog the RO membrane. These pre-filters usually have a micron rating between 5 and 10, and have to be changed at more frequent intervals (every 6-12 months). It helps to think of these pre-filters as guardians of your RO membrane, which is actually the heart of an RO system, responsible for carrying out the reverse osmosis filtration. Because the RO membrane is vulnerable to the damaging effects of chlorine, a carbon block pre-filter is absolutely necessary to maintain the integrity of the membrane, especially if it’s made of thin film composite (TFC). Cellulose tri-acetate membrane (CTA) is less common, and while these do have a good chlorine resistance, they can’t handle water with a pH above 8.5. The TFC on the other hand, can handle feed water with a pH of up to 11. RO membranes usually have a service life of up to 2 years. The last compulsory stage is the post-carbon block filter, which acts as a final polish for water removing any pollutants missed by previous stages. These last filters have to be changed less frequently, usually every 12 months.

Optional stages in RO systems are remineraliser filters or alkaline filters, which add healthy minerals to water that may have been lost because of the RO filtration process. Alkaline filters also reduce the acidity of water. While these filters are not compulsory, some manufacturers do recommend them. Some 4-stage RO systems like the Watts 4 Stage Reverse Osmosis (RO) Water Filter System are expandable, meaning that they allow you to add one of these optional filters.

Now that you know which stages are compulsory and which are optional in an RO system, check out our 4-stage and 6-stage reverse osmosis filters and see which one will deliver better-tasting, contaminant-free filtered water: