Reverse osmosis drinking water, and what it actually changes

What reverse osmosis removes

RO works by pushing water under household pressure through a semi-permeable membrane whose pores are small enough to block dissolved substances, not just particles. Water passes; most of what is dissolved in it does not. That makes it categorically different from a filter, which strains or adsorbs.

A typical under-sink system is a sequence rather than a single device. A sediment stage catches grit that would otherwise damage the membrane. A carbon stage removes chlorine, which is important because chlorine degrades the membrane. The membrane does the actual work. Then a post-carbon polish improves taste on the way to the tap, and treated water is held in a small storage tank because the membrane produces slowly.

In practice, RO substantially reduces total dissolved solids, the aggregate measure of everything dissolved in the water, and that is what people notice as a change in taste. It also addresses a wide range of dissolved contaminants, which is why it is the usual recommendation when someone has a specific concern about their drinking water rather than a general preference.

The relevant Arizona point: water here is mineral-heavy, and high dissolved-solid content is exactly what RO is good at. That is why it is a common choice in this state for drinking and cooking water even in households perfectly happy with their supply for everything else.

What RO does not do

Four things worth stating plainly, because they are the source of most confusion.

It is not a water softener. An under-sink RO unit treats one tap. It does nothing for your water heater, your shower glass, your laundry or your valve cartridges, because none of those are downstream of it. If scale is your problem, you want softening, and plenty of households have both, for different reasons.

It does not produce water quickly. The membrane works slowly, which is why there is a storage tank. Draw the tank down and you wait for it to refill. This is normal, not a fault, and it is why RO suits drinking and cooking rather than filling a bathtub.

It does not run without maintenance. Stages have service intervals, and a neglected system performs worse than no system while still costing you counter space.

It does not fix pressure or flow in the house. Different problem entirely, see water pressure regulator.

Under-sink or whole-house

Almost always under-sink, and it is worth understanding why rather than assuming it is a compromise.

Under-sink treats the water you drink and cook with, which is a tiny fraction of household consumption. The equipment is small, the membrane is inexpensive relative to whole-house scale, and the waste water involved is modest because the volume produced is modest. One dedicated tap, sometimes a second outlet to the refrigerator.

Whole-house RO exists but is a substantially different proposition: much larger membranes, a large storage arrangement, repressurisation to serve the house, and considerably more waste water because every gallon used for showering, laundry and irrigation is being produced through a membrane. It is occasionally justified, a specific supply problem affecting all use, sometimes on a difficult well, but for the common goal of better drinking water it is a large amount of equipment for a small amount of benefit.

Our honest recommendation for most Arizona households: RO at the kitchen tap for drinking and cooking, softening or filtration for the rest of the house if the water warrants it. Two tools, two jobs.

The waste water question, answered straight

This comes up constantly and deserves a real answer rather than either defensiveness or alarm.

RO separates water into treated product and a concentrate stream carrying the rejected dissolved solids, and that concentrate goes to drain. So yes, an RO system sends more water down the drain than it delivers to the tap. Older or poorly configured systems can be notably wasteful; better modern systems, and correct installation with adequate incoming pressure, improve the ratio considerably. Incoming pressure matters here, a system running on low pressure wastes more and produces less.

Two things put the figure in perspective without dismissing it. The volume in question is drinking and cooking water, which is a very small share of what a household uses, the concentrate from an under-sink system is a fraction of what a single shower or a load of laundry consumes. And a permeate-pump or high-efficiency configuration reduces the ratio meaningfully, which is worth specifying in a state where efficiency is a live concern.

What we would not do is tell you it is nothing. Ask what ratio the system you are being offered achieves, and whether a permeate pump is included. That is a fair question and a good installer will have the answer.

Tank pressure: the reason most RO systems “stop working”

If you take one practical thing from this page, take this.

The storage tank has an air bladder that pushes treated water out to the tap. That bladder is pressurized, and over years it loses pressure. When it does, the symptom is a slow trickle at the RO tap even though the system is producing normally, and it is very frequently misread as a failed membrane or exhausted filters.

Checking it is straightforward: with the tank empty of water, the air pressure at the tank’s valve should sit within the manufacturer’s stated range, and it can usually be topped up with a bicycle pump. A bladder that will not hold pressure needs a new tank, which is an inexpensive part.

Before replacing a membrane because flow has gone slow, check the tank. It is the cheaper answer and it is more often the right one.

Remineralisation, and whether you need it

Because RO removes most dissolved content, it removes beneficial minerals along with everything else. That produces two effects people notice: a flat or slightly blank taste, and water that is mildly aggressive in the chemical sense.

A remineralisation stage adds a small controlled amount of mineral content back after the membrane. Most people who try it find the taste noticeably rounder, and it moves the water back toward neutral. It is an inexpensive addition and it is the single most common reason someone dislikes their new RO water and then likes it.

Is it necessary? Not for safety at a drinking tap. Nutritionally, the mineral contribution of drinking water is small relative to diet. Where it earns its place is taste, and for the small number of households plumbing RO into an ice maker or a coffee setup where flat water is noticeable.

Our position: try it. If the taste of RO water is the reason someone abandons the system, this is the fix, and it is cheap enough to include from the start.

Filters, membranes and realistic intervals

An RO system is only as good as its last service, and the stages have genuinely different lifespans.

Sediment and carbon pre-filters are the frequent items, typically changed on a schedule of months rather than years. The carbon stage is doing protective duty as well as taste duty, because chlorine reaching the membrane shortens its life. Neglecting pre-filters is the usual reason a membrane fails early.

The membrane lasts years when the pre-filters are maintained. Replaced too often it is money wasted; run on exhausted pre-filters it fails well before its time.

The post-carbon polish is a taste stage, changed on its own schedule.

The tank is not a filter but needs its pressure checked, as above.

Two Arizona-specific notes. Sediment loading varies with your supply, and on a private well it can be considerably higher, pre-filters then need changing more often, and a dedicated sediment stage upstream can protect the whole arrangement. And if the house sits empty for months, water standing in the system is worth flushing thoroughly on return rather than drinking straight away.

Fridge lines, pot fillers and second outlets

Running RO water to a second point is common and worth planning at installation rather than adding later.

The refrigerator is the usual second outlet, for drinking water and ice. It is a straightforward run if the fridge is reasonably close to the sink. Two caveats: the tank is now serving two draw points, so a small tank empties faster and a larger one may be worth specifying; and a long run to a distant fridge reduces flow, so an ice maker may fill slowly.

A dedicated faucet at the sink is standard, and it needs its own hole in the sink or countertop. On a stone counter that means drilling, which is a specialist step worth confirming is included.

A pot filler or second sink is possible but pushes the system harder. Discuss it before choosing a unit, because it changes the sizing.

RO alongside a softener

These work well together and the ordering matters.

Where a house has a softener, the RO system normally sits downstream of it, on softened water. That is generally beneficial: softened feed water reduces scaling on the membrane and extends its life. RO then removes the sodium that ion exchange introduced, along with everything else, so the drinking tap does not carry it.

This is the arrangement most Arizona households with both end up in, and it is a genuinely sensible combination: the softener protects the plumbing and appliances, the RO handles what you drink, and each covers the other’s blind spot. If you are installing both, doing them together saves a visit and lets the pipework be arranged once.

If your concern is chlorine taste or sediment throughout the house rather than dissolved minerals at one tap, look at whole-house filtration instead, it is a different answer to a different question.