Reverse Osmosis Maintenance: The Real Filter Schedule and Yearly Cost

Reverse osmosis system maintenance is the part nobody quotes you on. A salesman quotes you $450 for the unit under the sink. The filters that keep it running never come up. Eighteen months later the water tastes flat, the tank fills at half speed, and you learn that a membrane runs $50 to $120 and the sediment cartridge should have been changed three times by now.

The filter schedule that actually applies

Every manufacturer prints intervals in months. Months are a proxy for gallons, and gallons are a proxy for how dirty your feed water is. A couple on soft city water and a family of six on a sediment-heavy well will finish the same cartridge months apart. Treat the table as a starting point.

The ranges overlap and the gallons column is wide. That is honest, not vague. A 5 micron cartridge loaded with fine silt from a shallow well can be spent in four months. The same cartridge on chlorinated city water, behind a whole house sediment filter, coasts past a year.

The one interval I would not stretch is the carbon pre-filter. It is not there for taste. It strips chlorine before the water reaches a thin-film composite membrane, and chlorine damage to TFC film is cumulative and invisible. Run a dead carbon block six extra months on chlorinated water and a five-year membrane becomes a two-year one. A $15 cartridge guarding a $90 part is the best trade in the system.

Remineralization is the opposite case. Replace the cartridge if your unit came with one, but most homes do not need to add one. It puts back a little calcium and magnesium for taste, and drinking water is not where a US diet gets its minerals.

Why TDS creep beats the calendar

Buy a handheld TDS meter. They cost $12 to $25, and the cheap ones differ from the good ones mainly in how long they hold calibration. What matters is not the reading at your RO tap on its own. It is the ratio between feed water and product water.

feed (cold tap)  = 320 ppm
RO tap, new      =  12 ppm
rejection = 1-(12/320) = 96.3%

same feed, 30 months later
RO tap           =  60 ppm
rejection = 1-(60/320) = 81.3%
drop of 15 points, membrane is done

A healthy residential membrane rejects 90 to 98 percent of dissolved solids. When rejection falls roughly 15 percentage points below what your own system did when new, replace it. Not before. On soft water behind a carbon pre-filter changed on time, a membrane can still hold 94 percent in year four. On a hard well with no softener, eighteen months is a plausible life.

Two cautions on the meter. Take the feed reading from the cold line that feeds the unit, not the bathroom sink. Take the RO reading after thirty seconds of running, so you are not measuring water that sat in the tank overnight. Stagnant tank water always reads higher.

Sources disagree here. Manufacturers publish membrane life in years because a schedule has to fit on a label and cover every customer. Technicians talk in rejection percentages because that is what they measure at the faucet. Neither is lying. The calendar figure is a conservative average across all feed waters, so on good water it makes you replace too early, and on bad water it lets a spent membrane run for months. One caveat: some warranties are written around the printed schedule.

Checking tank pressure with a tire gauge

Slow flow at the faucet, a tank that feels heavy but delivers half a glass, a system that runs and runs. Usually that is not the membrane. It is the bladder tank losing its air charge.

• Close the tank ball valve, open the RO faucet and drain it completely. The last of it comes out as a dribble.
• Find the Schrader valve under the cap. Same valve as a car tire.
• Check with a low-range tire gauge. Empty, you want 7 to 8 psi on most residential tanks. Some specify 5 to 7, so read your label.
• Add air with a bicycle pump in short bursts. Not a compressor, unless you enjoy rupturing bladders.
• If water sprays out of the valve, the bladder has failed. A replacement tank runs $40 to $90.

Do this once a year, at the same time as the pre-filter change. Ten minutes. It is the highest-value thing most owners never do. Low feed pressure at the house produces the same symptom, so if the tank checks out and flow is still poor, look upstream, which is where the walkthrough of how reverse osmosis works and what pressure does to the waste ratio picks up.

Sanitizing the system once a year

An RO system is a dark, wet, room-temperature plastic environment. Biofilm grows in those. On city water it is rarely a health problem, but it produces a musty taste and colonizes the carbon post-filter, which is why some systems taste swampy in year three on fresh cartridges.

The routine happens during a filter change, cartridges out and the membrane pulled and set aside dry. Shut the feed, drain the tank, put roughly two tablespoons of unscented household bleach into the first empty housing, reassemble everything except the membrane, let the system fill, then wait thirty minutes. Flush the tank twice, completely, before the membrane goes back. Bleach destroys TFC membranes, so it never sees the sanitizer.

How often to do it is genuinely contested. Manuals say every filter change, because they write one instruction for every customer including the ones on well water. Plenty of technicians say that on chlorinated city water, with pre-filters changed on time, it is mostly theater, since the feed arrives carrying a disinfectant residual. Well water, a unit that sat through a long vacation, or a tank you have never drained is where the bleach earns its thirty minutes.

On a private well, sanitizing the unit is downstream housekeeping and does nothing about the source. A musty taste there deserves a test, not a bleach flush. Total coliform is the standard screening indicator, and although private wells sit outside the EPA total coliform rule that covers public systems, it is the same test a certified lab runs. Your county health department can name the certified labs in your state. A positive result puts the fix at the well, and the choice between shock chlorination and a UV disinfection system is a separate decision. I am not a licensed plumber.

What a year of ownership actually costs

Here is the arithmetic most product pages leave out, for a four-stage under-sink unit on city water, with the membrane spread across its life instead of dumped into one year.

The low column and the high column are not the same household, so almost nobody pays $42 and almost nobody pays $213. Call it $70 to $180 a year. The low end is generic 10-inch cartridges bought in multipacks. The high end is well owners, who burn through sediment cartridges two or three times as fast, and anyone locked into proprietary sealed filters.

That last group deserves a warning. Proprietary quick-change cartridges tie you to one supplier at roughly two to three times the price of standard 10-inch cartridges, and the annual bill can land near $200 on a unit that cost less to buy. That belongs in the purchase decision, not in the discovery eighteen months later, which is why it shows up in the comparison of under-sink RO systems and what actually fits in the cabinet.

Water and sewer for the reject stream is real and small at point-of-use volumes. A household drinking and cooking with 15 gallons a week from a 3:1 system sends about 2,300 gallons a year to the drain. Combined water and sewer is billed per thousand gallons and the rate swings hard between districts, so call it a few dollars a year in a cheap city and closer to $40 in an expensive one. Whole house RO is a different arithmetic and usually a mistake.

Doing it yourself or paying someone

Service plans run $120 to $300 a year and include cartridges plus a visit. Against $42 to $213 in parts, you are paying $50 to $200 for the labor and, more honestly, for the reminder. Some people need the reminder and there is nothing wrong with buying it. The real exception is a rental or lease-to-own unit, where the cartridges are sealed and opening anything voids the contract. Own the hardware and the whole tool kit is a tire gauge, a TDS meter, and the plastic wrench that shipped in the box.

The failure worth respecting is the tubing. Push-fit fittings reseat easily and they also reseat wrong. After any work, leave the cabinet door open and set a paper towel under every joint overnight.

Mistakes that cost people real money

• Replacing the membrane while leaving a dead carbon pre-filter in place. The new membrane starts dying on day one.
• Never flushing a new membrane. Most need 10 to 20 minutes of run-to-drain to clear the storage preservative, and the first full tank goes down the sink.
• Feeding an RO system hard water with no softener ahead of it. Scale shortens membrane life noticeably above roughly 10 grains per gallon, so sizing a softener by grain capacity protects the RO as a side effect.
• Over-tightening housings. The o-ring seals, not the torque. Cracked sumps leak slowly and ruin a cabinet floor.
• Buying cartridges with no certification. Look for NSF/ANSI 42 for taste and chlorine, 53 for health contaminants such as lead, 58 for the RO system as a whole, and 401 for emerging contaminants. A cartridge with no listed standard is a plastic tube with something inside it.

A calendar you can actually keep

Pick one date and keep it. Mine is the first weekend of a chosen month. Sediment and carbon pre-filters out. Post-carbon out. TDS reading taken and written on masking tape stuck to the tank. Tank pressure checked. Sanitizing done while the housings are open. Ninety minutes a year, plus a mid-year pre-filter swap if your water warrants it.

Write the numbers down. Three years of TDS readings on a strip of tape tells you more about your membrane than any manufacturer chart, because it is your water, your pressure, your usage. It is also what tells you whether the taste change you noticed last month is the membrane, the post-filter, or the utility doing something seasonal.