How a Reverse Osmosis Water Purifier Works
Reverse osmosis pushes water through a very fine membrane to leave dissolved salts behind. Here are the filter stages, the waste stream and what it can and cannot remove.
In this article
A reverse osmosis (RO) purifier forces tap water through a membrane so fine that most dissolved salts stay behind. The cleaned water, called permeate, goes to your tap, while the rest, carrying the removed material, is flushed to the drain. Filters before and after the membrane protect it and polish the taste. This article explains each part and the limits.
Osmosis, then the reverse
In ordinary osmosis, water moves through a semipermeable membrane toward the side with more dissolved material. Wikipedia explains that reverse osmosis applies pressure higher than the solution's osmotic pressure, which pushes water the other way: out of the salty side and through the membrane. The pressure needed depends on the water. Wikipedia gives about 2 to 17 bar for fresh and brackish water and 40 to 82 bar for seawater, which has a natural osmotic pressure of roughly 27 bar.
Two streams, not one
A membrane in an RO unit does not simply trap contaminants like a sieve that fills up. Water flows along the membrane surface, and part of it passes through as permeate while the rest leaves as concentrate, also called reject, carrying the salts. The share of the feed water that becomes permeate is called the recovery ratio.
Wikipedia says household units have low back pressure and waste a lot of water, typically producing about 1 litre of usable water for every 3 to 25 litres sent to the drain. Large municipal and industrial plants recover far more, often 75 to 90 percent. A home unit's actual ratio depends on its design and the incoming water pressure, so manufacturers' figures are the better guide for a particular model.
The stages in a typical home unit
- Sediment pre-filter: a cartridge that catches sand, rust and silt. Wikipedia notes that cartridge filtration in RO pre-treatment is commonly rated at 1 to 5 micrometres.
- Carbon pre-filter: activated carbon that takes out chlorine, which can damage some membranes, and improves taste and smell. The CDC says carbon filters mainly improve taste and smell and generally do not remove germs.
- RO membrane: the main separation step.
- Post-filter: usually a carbon cartridge that polishes the water before it reaches the tap.
Wikipedia explains why pre-treatment matters: the membrane cannot be backflushed, so it is prone to fouling, and in large plants pre-treatment also includes chemicals to prevent scale and neutralise chlorine. Home units mainly rely on the cartridges.
What it removes
The US Centers for Disease Control and Prevention (CDC) says RO filters remove germs, meaning parasites, bacteria and viruses, and some chemicals including lead, copper, chromium, chloride and sodium. It says they may also reduce arsenic, fluoride and others, and that removal varies by product, so the label should be checked for the specific chemicals a unit handles.
The CDC points readers to NSF certification marks. It lists NSF Standard 58 as the one that covers reverse osmosis, alongside Standard 42 for taste and odour, 53 for cyst reduction and 62 for distillation. Indian buyers may also see other marks; check what a certificate actually covers before relying on it.
What to know about minerals
Because RO takes out dissolved salts, it also removes minerals such as calcium and magnesium. Wikipedia notes that some studies link long-term low intake of these minerals with health effects, but describes the evidence as low quality. Large plants often add minerals back; some home units include a mineral cartridge for the same purpose. Whether that matters is a question for a doctor, and this article does not give health advice.
Keeping it working
The CDC advises changing filter cartridges regularly, following the manufacturer's recommendations, to keep them working and prevent germ growth. It also says people with weakened immune systems should avoid changing filters themselves, since trapped germs or chemicals could be released. CDC-hosted research on arsenic found that household RO units reduced arsenic well but did not always bring it under the US limit, and that in one Maine study up to a third of such units failed within months to a year of installation. That shows why testing the water and servicing the unit matter.
This is general information; check the manufacturer's specifications and local water-quality guidance for a particular home.
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