Reverse Osmosis: What It Does, What It Costs, and When You Need It

The Short Answer

Reverse osmosis forces water through a semi-permeable membrane under pressure, leaving dissolved contaminants behind. It is the most thorough dissolved-contaminant removal method available for residential use — more comprehensive than carbon, more practical than distillation for a point-of-use drinking water system, and effective against a longer list of contaminants than any other single mechanism.

It is also slow, wastes water, removes beneficial minerals along with harmful ones, requires multiple prefiltration stages to protect the membrane, and is almost universally installed by people who do not understand what it is actually doing or whether they need it. The membrane does not care whether the contaminant is harmful or beneficial — it removes both with equal indifference.

RO is a polishing step. It runs last in a treatment train, on water that has already been cleaned up by the stages before it. It is not a whole-house solution for most people — it is a point-of-use system for drinking and cooking water, producing a relatively small volume of treated water slowly. Understanding those constraints is what separates a good RO installation from an expensive one solving a problem you do not have.

How It Actually Works

Osmosis is what happens naturally when water moves through a semi-permeable membrane from a less concentrated solution to a more concentrated one — the water moves toward the higher concentration to equalize the two sides. Reverse osmosis applies pressure to force water to move in the opposite direction: from the more concentrated side (your tap water, with its dissolved contaminants) through the membrane to the less concentrated side (the permeate, or product water), leaving the dissolved contaminants behind in the concentrate that goes to drain.

The membrane is the working element: a thin-film composite polymer with pores so small that dissolved ions, molecules, and most compounds cannot pass through. The size exclusion is real but is not the whole story — RO rejection is also driven by charge repulsion (the membrane carries a slight negative charge that repels anions) and diffusion dynamics. This is why RO is not simply “a very tight mechanical stage” — the mechanism includes both size exclusion and chemistry, which is why it removes dissolved ionic compounds that no mechanical stage can touch.

What emerges from the membrane side is the permeate: water that has been stripped of most of what was dissolved in it. What does not pass through the membrane is the concentrate, also called reject water or brine, which carries the accumulated rejected contaminants and is flushed to drain. The ratio of permeate to concentrate — the recovery rate — is the source of RO’s most significant criticism: a conventional residential RO system sends roughly two to four gallons to drain for every gallon of product water it produces. Newer high-efficiency RO systems improve this significantly, but some waste is inherent to the mechanism.

RO production is slow. A typical under-sink unit produces 50 to 100 gallons per day — not per hour. To make treated water available on demand, a storage tank (typically two to four gallons, pressurized) sits between the membrane and the tap, accumulating product water between uses. The membrane runs continuously to refill the tank; the tap delivers from the tank. The tank is where most RO maintenance problems begin and where most RO owners stop paying attention.

Whole-house RO systems exist and are used in situations where the water quality problem is severe enough to warrant treating every drop in the house — extremely high TDS, severe arsenic or nitrate contamination, or source water that is genuinely compromised across the board. They are a different animal entirely: professional-specification equipment, large holding tanks measured in hundreds of gallons (because the membrane produces water far more slowly than whole-house demand requires, and the tank bridges that gap), substantial installation cost, and ongoing membrane replacement at a scale that dwarfs the under-sink version. Anyone heading in that direction is working with a water treatment professional and a detailed water analysis — it is not a consumer product. For the purposes of this page, RO means point-of-use under-sink treatment, which is what the vast majority of residential installations are.

What It Removes — and What It Does Not

RO removes an unusually broad list of dissolved contaminants: lead, arsenic, nitrate, fluoride, chromium, barium, radium, perchlorate, PFAS, most dissolved salts, hardness, and a wide range of other inorganic compounds. It also removes most bacteria and viruses — not through disinfection but through size exclusion — though RO is not certified or positioned as a disinfection technology and should not be relied upon as the sole microbiological barrier on a contaminated source. It removes pharmaceuticals and many organic compounds to varying degrees depending on molecular size and charge.

What RO does not reliably remove: certain volatile organic compounds (VOCs), chlorine and chloramines (which actually damage the membrane — they must be removed upstream by a carbon stage), dissolved gases including hydrogen sulfide and radon, and some pesticides with small molecular size. It also does not remove what is not there — a common oversight when RO is installed on city water where the only real concern is taste.

The minerals question for RO is the same as for distillation: calcium, magnesium, potassium, and sodium are removed along with everything harmful. The water coming out of an RO system is soft, low in TDS, and tastes flat to many people. Remineralization cartridges downstream of the membrane are available and increasingly common. For people on a sodium-restricted diet who have a water softener upstream of the RO, removing the added sodium is one of the genuine use cases for RO that is not about contamination at all.

The Prefiltration Dependencies

RO membranes are expensive and fragile. They foul, they degrade, and they fail — and the things that damage them are common in tap water. This is why RO always runs last in the treatment train and why the stages before it are not optional add-ons but functional requirements.

Sediment fouls the membrane physically, blocking pores and reducing flow. A sediment stage comes first.

Chlorine and chloramines oxidize and destroy thin-film composite membranes — the type used in virtually all residential RO systems. A carbon stage must come before the membrane on any chlorinated municipal source. Without it the membrane’s useful life is measured in months, not years.

Hardness causes scaling — calcium and magnesium precipitate on the membrane surface as the concentrate becomes increasingly concentrated. On very hard water, a softener before the RO protects the membrane and improves recovery rate. On moderately hard water, some RO systems include an antiscalant cartridge instead.

Iron and manganese foul membranes rapidly and are very difficult to remove once deposited. On well water with iron or manganese, oxidation and mechanical removal come before everything else in the train.

The treatment train for a complete RO system on a typical municipal source is: sediment → carbon → RO membrane → carbon post-filter → tap. On a well with hard water and iron it is: oxidation/mechanical removal → sediment → carbon → softener → RO membrane → carbon post-filter → tap. Each stage is protecting the next. Removing any stage to save money tends to cost more in membrane replacement.

Do You Need One?

RO makes genuine sense when: your water contains lead, arsenic, nitrate, fluoride, PFAS, or other dissolved inorganic contaminants that carbon and ion exchange either cannot handle or are the wrong tool for; when you want the most comprehensive point-of-use drinking water treatment available from a single system; when you are on a softener and want to remove the added sodium from drinking water; or when your water has multiple dissolved concerns and you want one system that handles most of them.

You may not need RO if: your only water concern is taste and chlorine — a carbon stage does that for a fraction of the cost, wastes no water, and requires less maintenance; if your water tests clean for all health contaminants — RO on safe water is removing beneficial minerals and wasting water to solve a problem that does not exist; or if your concern is microbiological rather than chemical — UV is the right tool for that, not RO.

The question before buying an RO system is: what is actually in my water, and does RO remove it better than a simpler and cheaper alternative? For many people on municipal water with no documented contaminant concerns, a carbon stage and a water test would serve them better than an RO system. For someone with arsenic in their well water, RO is one of the most reliable treatments available.

This is the profile where the “test first” principle matters most. RO is the most expensive residential treatment option, produces the most water waste, requires the most maintenance, and has the most complex installation. Buying one without knowing what is in your water is the most common and most expensive mistake in residential water treatment.

The Tradeoffs

Water waste is the most significant criticism of RO and the one least mentioned by sellers. A conventional residential system wastes two to four gallons for every gallon produced. Over a year that is tens of thousands of gallons of water sent to drain. High-efficiency permeate pumps and zero-waste RO systems have improved this significantly — some systems now approach 1:1 recovery — but conventional systems remain wasteful, and the water waste is ongoing for the life of the system.

Mineral removal is the tradeoff most people notice in the glass. RO water tastes different — flatter, sometimes described as empty. Whether this matters depends on the person. Remineralization addresses it and is worth doing if mineral taste is important to you.

Maintenance is more involved than most sellers communicate. The sediment and carbon pre-filter cartridges need replacement every six to twelve months. The membrane needs replacement every two to five years depending on water quality and usage. The post-filter carbon needs replacement annually. The storage tank needs periodic sanitization — a neglected tank is a bacterial growth site. None of this is technically difficult, but it requires a schedule and the discipline to follow it.

Production rate and tank size create a practical constraint. The storage tank on most under-sink systems holds two to four gallons. A family that uses treated water for drinking, cooking, and pets can deplete the tank between refill cycles. If the tap runs dry mid-meal, the tank is undersized for the usage pattern.

Can You DIY This?

Yes, and under-sink RO installation is within reach for anyone comfortable with basic under-sink plumbing. The system connects to the cold water supply line under the sink, mounts the membrane housing and filter canisters in the cabinet, and runs a dedicated line to a separate small tap on the sink deck. Most systems come with push-connect fittings and detailed instructions. The installation is more involved than a single filter housing but less involved than a water softener.

The part that trips up DIY installations most often is the drain connection — the concentrate line connects to the drain, usually via a saddle clamp on the drain tailpiece. Done correctly it drains by gravity during production; done incorrectly it can siphon or fail to drain, causing the system to malfunction.

Membrane replacement is the ongoing DIY maintenance task. Turn off the feed water valve, relieve pressure from the tank, remove the membrane housing, slide out the old membrane, slide in the new one, reassemble. The first batch of water after a membrane change should be discarded. Most people who struggle with maintenance skip the tank sanitization — find the manufacturer’s procedure for your specific system and do it at each membrane change.

Sizing an RO system for a well requires knowing the feed water pressure (RO needs at least 40 psi, ideally 60 psi or above to perform well), the TDS and specific contaminants in the water (membrane selection and pre-treatment needs depend on these), and your daily volume requirement.

How and When to Buy It

NSF/ANSI 58 is the standard for RO systems. It covers contaminant reduction claims, materials safety, and structural integrity. An RO system certified to NSF 58 for a specific contaminant — lead, arsenic, nitrate, PFAS — has been independently tested to reduce that contaminant. The certification listing shows which contaminants were tested and at what reduction percentage. An RO system without NSF 58 certification, or one certified for a contaminant not relevant to your water, is not solving your problem.

Rejection rate is the performance spec that matters: what percentage of a given contaminant the membrane rejects under standardized test conditions. A 97% rejection rate for nitrate means 3% passes through — on 50 ppm nitrate feed water, product water would contain roughly 1.5 ppm. Whether that is adequate depends on your starting concentration and your target. The rejection rate is on the NSF listing; the marketing does not always lead with it.

Recovery rate and water waste are the specs most sellers do not volunteer. Ask: at my feed water pressure and TDS, what is the ratio of product water to waste water? A system with a permeate pump or a zero-waste design is more efficient but costs more. For most households the efficiency gain is worth the cost, particularly in areas with water scarcity or high water rates.

Tank-less RO systems — which produce water on demand without a storage tank — exist and eliminate the tank maintenance and sanitization concern. They require higher feed water pressure to produce water fast enough for on-demand delivery. If your feed pressure is adequate, tank-less is worth considering for the reduced maintenance burden alone.

Membrane type matters. Thin-film composite (TFC) membranes are the standard for chlorinated municipal water after a carbon pre-filter. Cellulose triacetate (CTA) membranes are chlorine-tolerant but lower performing and less common. For well water, membrane selection should be informed by the specific water chemistry — a membrane specified for low-TDS municipal water may foul quickly on high-TDS well water.

Buy it when your water test says you need it, not before. RO is the right answer for a specific set of dissolved contaminant problems. It is an expensive, high-maintenance, water-wasting solution to problems that cheaper mechanisms cannot solve. When those problems are present in your water, it earns every penny. When they are not, it is solving nothing.

The Deep End

The thermodynamics of reverse osmosis explain both its energy cost and its water waste. Natural osmosis requires no energy — water moves spontaneously from low to high concentration across a semi-permeable membrane, driven by the free energy difference between the two sides. Reversing that process requires work: the applied pressure must exceed the osmotic pressure of the feed water, which is proportional to the dissolved solids concentration. Typical municipal water has an osmotic pressure of 5 to 15 psi; seawater is 350 to 400 psi. A residential RO system operating at 60 to 80 psi is working comfortably above the osmotic pressure of tap water — which is why pressure is the limiting factor for performance at low feed pressures and why pressure boosters improve output.

As water passes through the membrane and is removed as permeate, the remaining feed water becomes increasingly concentrated in rejected contaminants. This concentration polarization effect means the contaminant concentration at the membrane surface is higher than in the bulk feed water, which reduces rejection efficiency and accelerates scaling. The concentrate flush — the reject stream that carries the concentrated contaminants to drain — serves to manage this effect by continuously removing the concentrated layer. This is why some waste is inherent and why zero-waste systems that fully recycle the concentrate require additional engineering to manage concentration polarization.

Membrane fouling is the practical lifetime constraint on any RO system. Fouling takes three forms: particulate fouling (sediment and suspended solids blocking pores — addressed by upstream sediment removal), scaling (mineral precipitation on the membrane surface — addressed by softening or antiscalants), and biofouling (bacterial growth on the membrane — addressed by maintaining adequate upstream disinfection and periodic sanitization). A membrane that is well-protected upstream and properly maintained can last five years or more; one that is not can fail in months.

The RO membrane’s rejection mechanism is not pure size exclusion, which is what distinguishes it from ultrafiltration and nanofiltration membranes. Rejection is governed by solution-diffusion: contaminants dissolve into the membrane material on the high-pressure side and diffuse through it, driven by the concentration gradient across the membrane. Water diffuses through much faster than dissolved salts because of its higher affinity for the membrane material and its smaller effective size. Compounds that are highly soluble in the membrane material — certain organic solvents — can diffuse through more readily than their size would suggest, which explains why some VOCs are not well-rejected by RO membranes despite being larger molecules than the ions that are rejected at 97% or above.

New to this? See how RO stacks up against the 5 most common types of water filtration.

None of this picks your system for you — your water does. Test it first, then choose the mechanism that matches what is actually in it.