For most home RO systems, the answer is four. However, if you consider overall taste a factor, then we’re talking five stages.
By their nature, these systems tend to use a number of pre-filters before the incoming feed water from a building’s mains/well supply undergoes reverse osmosis to be purified for the consumer. Many products on the market mention having five, six, or seven stages, but it’s good to understand the difference between the required pre-filter stages and the optional post-filter stages, as well as the main point, the single critical stage at which reverse osmosis occurs. It’s also important to note that, to be considered an actual RO system, a model must have NSF 58 certification.
After a system’s installation and prepping, the crude tap water first encounters a sediment filter (stage 1). A home-system pre-filter tends to be 5-10 microns in gauge. It removes the larger particles of contaminants such as sand, rust and dust. If you live in an area that has a lot of sediment in the supply, it will become clogged faster. This is your treated water’s first line of defense; if it is compromised, there will be problems further down the line, so change it regularly (6-12 months, depending on use and supply conditions).
This first pre-filtration process effectively protects the second and third stages—carbon block filters—so that each can concentrate on removing finer pollutants, including tastes and odors. In most systems, there tend to be two of these secondary pre-filters. Both of these filters’ principal role is to remove organic contaminants, including chlorine and its byproducts, so that these can’t harm the true star of the show, the RO membrane. While the carbon filter in stage 2 catches most, the one in stage 3 helps to mop up any lingering traces of pollutants (such as fluoride) that were missed. Together, they protect the next stage, which is the definitive one in RO.
Thus, we can say that true Reverse Osmosis kicks in at stage 4. Having gone through these first three lines of defense, what gets through to the RO membrane still contains contaminants (such as sodium traces). At this stage, this higher concentrate squeezes through the very fine membrane to come through as weaker concentrate. The membrane will usually be made of thin film composite (TFC), which can handle feed water of up to 11 pH. However, TFC is vulnerable to chlorine attack, hence the reason for having carbon pre-filters to remove it beforehand. Also, as the pores of the membrane are so fine (measuring tiny fractions of a micron), it’s important to prevent the larger contaminants from clogging or damaging it, as water surges through the membrane under considerable force.
Although uncommon, a few systems use a membrane made of CTA (cellulose tri-acetate membrane). These have no carbon pre-filters, and, therefore, the required RO process is the second stage. Using carbon filters with a CTA membrane would cause it to fail; also, CTA membranes need chlorine in the supply to prevent them from fouling with biological contaminants. This material is more durable in handling chlorine; nevertheless, it cannot survive in water with a greater pH of 8.5 (more alkaline than neutral).
The key to the enterprise is the membrane (stage 4 or, in the case of CTA, stage 2), especially how well the membrane surface holds up against the incoming water—if a hole develops, the membrane is finished. While no membrane can last forever, careful maintenance of the less-expensive pre-filters can keep it from damage.
Although these stages are required for RO, a fifth stage polishes the water using a carbon cartridge, making it taste better while keeping it clean as it sits in the tank. Some consider this luxury an essential. Stages 6 and 7 are for alkalinizing the water (RO water tends to be slightly acidic) and anti-bacterial UV ray treatment, respectively.
Also, some more sophisticated models can remove the good pollutants, such as magnesium, and reintroduce them after RO occurs. Note, too, that while the filters catch sediment and debris, the membrane filters water in a different way, meaning that a lot of it gets rejected. A few sophisticated models are able to channel this waste water back around and re-treat it, thereby economizing on water use.
In all, remember that the more stages your system has, the more maintenance and cost you’ll be facing.