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Reverse Osmosis: How Many Stages?

I often get asked by friends and customers alike, “how many stages do I need in my Reverse Osmosis (RO) system?”

It’s a great question that often gets talked about and as there are more and more systems available with 5+ stages it’s important to understand what the different stages do and whether or not they are a good investment.

In order to best understand this it is a good idea to quickly understand what a basic RO system is and how it works.

The most basic Reverse Osmosis only require 2 stages. The first stage, a carbon block does two things. It filters down to a certain micron rating (a micron is a measurement of size, if someone is talking about filtering down to a certain micron level, they are referring to how small a particle would have to be to get through the filter) and it also removes chemicals through chemical reaction (chemicals in the water bond onto the carbon block and get stuck there). In an RO system this would be referred to as ‘pre-filtration’ as it occurs before the RO membrane.

The second stage, the RO membrane itself is a thin film of composite material that filters at a molecular level rejecting the majority of contaminants in the water by size or electrical charge. The holes or ‘pores’ in the film are so fine that water will only flow through under pressure, the more pressure the faster the water can flow through. Since tap water pressure is not that high, the flow rate of the pure water through an RO membrane is quite slow and would take a couple of minutes to fill one glass. To combat this, reverse osmosis systems (as in the picture above) will usually come with a tank. This tank is pressurised and fills up over time from the RO. Then when drinking water is required, the small tap is pressed and the tank empties at a very usable flow rate (5-8 seconds to fill a glass).

This is where the additional stages come in. Because the filtered water is stored in the tank that has an internal rubber bladder, the water can develop a taste, particularly if it is left in the tank for a long time. Enter the 3rd stage – “Post treatment”. A second Carbon filter is placed between the tap and the tank to remove any final taste from water stored in the tank.

So that makes three stages: Pre-filtration, Reverse Osmosis Membrane & Post-filtration. Anything more than this and you’re starting to buy into ‘bells & whistles’ that will cost you a lot of money each time you have to replace them.

4 Stage: People will often put a sediment filter before the initial carbon filter. This sediment filter is like finely wound cotton wool. The benefit of doing this is that the sediment filter will remove particles from the water before they reach the first carbon filter. Carbon filters are more expensive to replace that sediment and are also more prone to blocking up. The Sediment filter is usually a higher micron rating than the carbon and protects the carbon from blocking up to quickly, just as the carbon protects the RO membrane from in turn blocking up too quickly.

5 Stage: Some people count the tank as a stage, others will start to add a never growing list of add-on filters that do an ever increasing list of ‘amazing things’ to the water including re-introducing many of the minerals and such that your filter has just taken out.

More still will have things like anti-scalant pre-filters to stop the RO membrane from scaling. Anti-scalant is a very prevalent technology in large industrial reverse osmosis applications, we know because our parent company have been building containerised reverse osmosis plant for large industry for years. The thing is it doesn’t make commercial sense to buy into this in small residential systems. The cost of using the anti-scalant cartridge outweighs the savings provided from replacing the RO membranes. Those cartridges also work by leeching additional chemicals into the water prior to the Reverse Osmosis membrane, all of which end up being washed into the drain and back into the environment. So they’re a bad idea on that front as well.

6,7 & 8 stage: Marketing, arms race, etc etc, this line of thinking that obviously a 6 stage will be better than a 5 stage and so on is really just a way to get consumers to buy more than they need. You can guarantee that the water quality provided from these additional stages will be no better, as it is the RO membrane itself that does all the hard work and all the systems have one.

The other thing that can’t be stressed enough is that when it comes time to service your 8 stage Reverse Osmosis system, you’ll be paying for 8 different replacement cartridges which is going to take a modest investment into an unnecessary expense.

Summary: Carbon filter + Reverse Osmosis membrane will provide the best water you can get, everything else will end up costing you a lot more for little/no benefit. If you plan on using your system in the home for drinking purposes a tank is highly advised as the flow rate from RO is not practical on tap water pressure.

Having been in this business since 1994 we’ve come up with a select range of quality systems that represent best value for money. Check out our recommended 4 Stage Reverse Osmosis system, it’s what I use myself.

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