Reverse osmosis is one of the most common desalination methods on the planet, in addition to also being used as a method of general purification of contaminated freshwater sources or tap water. However, Reverse Osmosis does not simply function as a single in-line filter straight from the source, there are pre and post-treatment stages that both optimize the process and improve the water quality & taste. The exact details of these pre and post-treatments vary greatly depending on the water source and target use, I wrote this article to briefly explain the differences between basic and more comprehensive systems as well as the reasoning behind each.
There are between 3 and 7 stages of filtration in reverse osmosis, 3-step systems are aimed at improving clean sources like rainwater and 7 for the purification of highly contaminated sources. There are distinct reasons for each additional step from 3 to 7 as RO systems increase in complexity.
3 Stage: Base Level System
Three-stage reverse osmosis systems are both the least complicated and most common. The three-stage system is the base on which more complex systems are built, it consists of an initial sediment removal filtration, followed by an activated carbon filtration stage and then the water finally passes through the reverse osmosis membrane. These systems are targeted at clean water sources like tap, or rainwater where there is a low sediment load and minimal contaminants.
Sediment filtration: This stage removes insoluble particles that could potentially damage the delicate reverse osmosis membrane like dirt, metal fragments and organic matter. Often in the form of a cotton cartridge filter cartridge (PPF), this stage is necessary to both prolong the life of the reverse osmosis membrane and also increase the efficiency of the whole system, if dirt starts to block up the membrane, less water can pass through!
Carbon filtration: The next step after the sediment removal, this step aims to remove cloudiness, colouration, chlorine and Chloramine as well as tastes, odours and other organic compounds. The activated carbon removes the majority of these from the water however it cannot remove smaller particles like salts, nor is it effective at removing heavy metals, both of which RO is effective at doing. Although RO could remove all of the particles filtered out in this step, it is simply more efficient to get a carbon filter to do the work and it prolongs the life of the RO membrane.
Carbon filtration is actually the most common filtration method used in household water filtration, If you are wondering how it works, I wrote a simple explanation of it here.
Reverse Osmosis: The final step of the three-stage process and the most important. The pre-filtered water is forced through the RO membrane, in the process removing ~99% of the remaining impurities in the water like heavy metals and salts. RO membranes have extremely fine pores that have the ability to block out the vast majority of impurities, however, if they were to be damaged by particles filtered out in previous steps of the process, the membrane of course cannot function properly and contaminants will get through.
4 Stage: Adding Minerals/Taste After RO filtration
Four-stage RO systems are often considered the most complete systems for households. They are a three-step system with a post-reverse-osmosis filtration or remineralization step. Often in the form of a coconut shell filter to remove the inert flavour often associated with RO water as well as giving it a slight sweetness. The other option for a fourth stage is a remineralization filter that is specifically there to add some salts and minerals to the water that were previously removed in the RO stage. Both the coconut shell carbon filter and the remineralization filter are added in due to RO removing everything from the water in a similar manner to distillation, including beneficial minerals like magnesium and calcium that also give taste to the water. The six-stage RO system generally adds in whichever of these two options is not added in at the fourth stage.
5 Stage: Additional Pre-Filtration Between Carbon & RO
Five-stage RO systems represent a four-stage system with an additional pre-filtration stage between the activated carbon stage and reverse osmosis. The filter used here is often, but not always a polypropylene water filter designed to pick up anything one micron or above. The idea of this step is to pick up smaller insoluble particles that can get through the initial sediment filtration, and then pass through the carbon filtration stage. Like the other two stages of pre-filtration, this stage just further improves the efficiency of the whole system by reducing the workload and the functional life of the reverse osmosis membrane. The five-stage systems are simply an improvement on the four-stage in terms of efficiency, flow rate and lifespan so can be applied to high-use cases.
6 Stage: Additional post-RO Mineral/Taste
Much like the fourth stage, the sixth stage adds either a coconut shell carbon filter or a remineralization filter after reverse osmosis, hypothetically speaking, whichever was not added included in four stage system. The reason for this step is to further improve the taste and general hydration properties of the water produced.
7 Stage: Additional Bacteria Removal
Seven-stage reverse osmosis systems add an additional stage after reverse osmosis to kill any residual bacteria. The additional antibacterial capacity involves the inclusion of an ultraviolet treatment stage immediately after reverse osmosis. The reasoning for this step is that, although reverse osmosis will remove the majority of bacteria and pathogens from water, it will not remove all of them, whereas ultraviolet’s strength as a treatment process is its ability to eliminate nearly all microorganisms from water. This can also be substituted for the coarse filtration added in step eight, depending on your individual priorities.
If you are interested in understanding how ultraviolet water treatment works, I suggest you have a look at my quick guide on the topic here.
8 Stage: Coarse Sediment Removal
The eighth stage involves additional sediment removal, it is usually in the form of a sand filter. The reason for this stage is when the source water is muddy or has a high sediment load like a lake, river or even some underground water sources. This high concentration of course particles will quickly block up the standard cotton filter cartridge and render the whole system useless. Of course, this step is not necessary if the source water is relatively transparent like rainwater, tap water or even a slightly murky well or lake water, it can also be substituted for the UV treatment added in the seventh stage depending on priorities.
Sand filters may sound strange however they are extremely common and widely used around the world, If you are curious, I explained their various applications here in a short article.
