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How many stages do I need my water filter to have, what is the difference between a three stage reverse osmosis system and a four, five, six or even seven stage reverse osmosis system?

This is a common question that we hear regularly there are many systems available, many of which unfortunately are no more than marketing hype designed to make the system seem more beneficial……….THREE stage water filter, SEVEN stage Reverse Osmosis system certainly sounds better than single stage water filter or three stage reverse osmosis system, in reality however it is more to do with the cartridge that you choose to use in your system rather than how many stages the system has. Here is our simple guide in how to work out what is best for your requirements.


Where to Start?

What do you want to remove from the water? E.g. you may only want the water to taste better which would mean using a 1 micron chlorine taste and odour reduction cartridge, on the other hand you may want to remove specific contaminants such as lead or fluoride from the water which would require the use of a specialised carbon cartridge.

Once you have an idea regarding the contaminants that you wish to remove from the water you can start to get an idea as to how many stages that your water purification unit requires.

We believe that the less stages you have to use to achieve the water quality that you desire the better, less cost up front and less cost in replacement cartridges, why use a seven stage system when you can achieve the same results with a two stage.

There are two main options when it comes to water filtration systems, Reverse Osmosis or Carbon Filtration here is our take on both:


Single Stage or Two Stage?

In the majority of cases a two stage filtration system is the best choice, the first stage is an inexpensive sediment filter which is used only to prevent the more expensive carbon filter from being blocked by silt or sand. If your water quality is good you may not need to use a sediment filter.

Carbon Filters are not all created equal, we have good quality cartridges which are great for removing chlorine, taste and odours from the water, if you want a cartridge to remove any other contaminants you should opt for a cartridge that has been certified by a body such as the NSF, it is all too easy to state that a carbon cartridge will “remove” lead or cysts the real question is what percentage and for how long? The table below outlines the cartridges available and the contaminants that they are CERTIFIED to remove.

As you can see from the table above if the correct combination of cartridges are chosen, the tap water coming to your home can be treated to high standards by using either a one stage or two stage filtration system, there really is no need to go to the expense of three or four stage water filtration.

What About Reverse Osmosis?

There are so many Reverse Osmosis Water Filters on the market, it has become very confusing for the end user to decipher which one is best for their requirements. Here is a basic 101 on how Reverse Osmosis Systems work and what everyone should look out for when it comes to stages and how many are required.

The heart of every reverse osmosis system is the membrane, in fact if the system does not have an NSF 58 certification it is not a reverse osmosis system.

The reverse osmosis membrane is generally TFC which is very susceptible to chlorine attack, therefore this chlorine must be removed from the water using a carbon filter and as we know it is good practice to use a more inexpensive sediment filter to protect the carbon filter. This leads to three stages being required for most reverse osmosis systems.

STAGE 1 Sediment Filter to Remove Silt and Sand (protects stage 2 carbon filter)

STAGE 2 Carbon Filter to remove chlorine and/or chloramines (protects reverse osmosis membrane)

STAGE 3 Reverse Osmosis Membrane removes organic and inorganic compounds such as Fluoride

So why do suppliers offer 4 Stage Reverse Osmosis Systems and even 7 Stage Reverse Osmosis Systems?

An extra stage after the reverse osmosis membrane is easily explained, the water produced by the system is not very palatable and requires a carbon cartridge to “polish” the water, also reverse osmosis water generally is held in a holding tank where it can pick up odours or even non harmful bacteria again a fourth stage carbon filter will ensure that the water reaches the tap in pristine condition.

After the fourth stage it becomes a little more difficult to make an argument that more stages are required, it is a fact that reverse osmosis water is slightly acidic therefore the use of an alkalising carbon filter could be justified however there are now carbon filters available which will alkalise the water as well as performing the job of the fourth stage “polishing” filter

The only other possible option after all this would be to fit a de ioniser, we do not recommend this for drinking water as if it is not maintained it could result in traces of the de ionising resins being present. 

In Summary using any more stages than is required for your water filtration system only results in more maintenance and unnecessary cost.