When a product is sold as "5 stage," it usually means a sequence of five filters in a line, most often under a kitchen sink, ending in a dedicated faucet. Most of these systems include reverse osmosis, which is a very fine membrane that pushes water through while holding back most dissolved substances.
This is a drinking-water system, not a whole-house one. It treats a small trickle at one faucet, and the stages are arranged so each one protects the next. Knowing that order helps you tell a good system from a well-priced box of extras.
The five stages, in order
Stage one: sediment
A fine sediment cartridge catches grit and scale. Its main job is protecting the stages after it, especially the membrane.
Stage two: carbon block
A carbon cartridge takes out chlorine-type tastes and odors. It also guards the membrane, which does not tolerate disinfectant well.
Stage three: second carbon or fine sediment
Some systems repeat carbon or use a finer particle filter here. This stage is a place where manufacturers differ, so ask what yours does.
Stage four: the RO membrane
The reverse osmosis membrane is the heart of the system. It separates clean water, which collects in a small tank, from a stream of reject water that goes to the drain.
Stage five: polishing carbon
A short inline carbon cartridge sits after the tank and cleans up any taste the tank or tubing picked up before water reaches the tap.
Some five stage systems swap one of those stages for a remineralizing cartridge, which adds a little mineral back so the water does not taste flat. Others add a UV stage. The count on the box tells you very little until you know which stages are actually installed.
Where Millbrae water changes the picture
Mild hardness, chloramine and what that means for a membrane
The 2025 Millbrae water quality report lists hardness between 8.1 and 112 parts per million, with an average of 62. That is on the soft side of typical tap water, with the top of the range reaching moderately hard, so scale buildup on an RO membrane is less of a threat here than in hard-water areas. That means you can focus on protecting the membrane from disinfectant instead of dealing with heavy scale.
Millbrae's supply is 100 percent SFPUC water, disinfected with chloramine. A membrane is sensitive to disinfectant, so the carbon stages ahead of it do real work, and a worn carbon cartridge is the most common way a membrane is shortened in life. Replace carbon on schedule, even when the water tastes fine.
A note on honesty: reverse osmosis is more than most people need for Millbrae tap water. It makes sense when you want the cleanest-tasting drinking and cooking water, when someone in the household is cautious about specific contaminants or when you want to remove the taste of chloramine without a special cartridge. If you are happy with the tap, a simple carbon filter at the sink may be enough.
The part nobody mentions: the drain connection
Reverse osmosis wastes water. For every bit of clean water that goes to the tank, a portion goes down the drain as reject. How much depends on membrane, pressure and tank state. Your household water meter reads the total, so it is worth knowing before you add one.
The reject line must connect to the sink drain correctly, with an air gap, usually through the faucet body, so dirty drain water cannot be drawn back into the clean line. A saddle clamp on the drain pipe is common, but it must be placed above the trap and tight, or it leaks under the sink.
| Part | What it does | Common install trouble |
|---|---|---|
| Feed valve | Takes water from the cold supply | Wrong supply line, or hot line tapped |
| Tubing runs | Carries water between stages | Kinks, loose fittings, tight bends |
| Storage tank | Holds treated water for the faucet | Air charge low, weak flow at faucet |
| Air gap faucet | Dispenses water and protects the drain | Needs a hole in the sink or counter |
| Drain saddle | Sends reject water away | Leaks, slow drain, noise |
Faucet hole, space and the rest of the cabinet
The dedicated faucet needs a hole in the countertop or sink deck. Many sinks already have a spare hole, but a granite or quartz top may need a specialist to drill. Check before you commit, because the faucet is the part you see every day.
Under the sink, the cartridges, the membrane housing and the tank take surprising room. A garbage disposal, a water heater tank on the same wall or deep storage already under the sink can leave no space. Some owners relocate the tank to an adjacent cabinet or garage with a longer tubing run.
Maintenance, in practical terms
Sediment and carbon cartridges are the frequent changes. The membrane lasts longer, but only if the first stages are kept fresh. The tank can lose its air charge over time, which shows up as a faucet that trickles or a tank that never feels full.
- Sediment and carbon cartridges: change on the schedule printed for your system, sooner if flow drops
- Membrane: replace when the water tastes off or the output slows even with fresh cartridges
- Tank air charge: check if the faucet weakens while the cartridges are new
- Fittings and tubing: look for drips whenever you open the cabinet
DIY or what people try
- Buying a complete kit and drilling the counter without checking for pipes
- Tapping the hot supply by mistake
- Skipping the air gap and sending reject water straight into the drain pipe
- Changing cartridges in the wrong order
Professional approach
- Confirming the supply line and drain layout before cutting
- Setting the faucet and air gap correctly in the sink or countertop
- Pressure testing and flushing the system before use
- Labeling cartridge dates so changes are simple
Do you need a whole-house system, too?
A five stage system treats the drinking water only. If the concern is sediment, shower water or protecting appliances, a water filtration and softener setup at the main is a different tool. The two can work together, with a whole-house stage first and a five stage unit at the kitchen sink.
If you are adding the system during a remodel, bring it up with the plumber before the cabinets go in. Plumbing installation rough-ins for the supply, the drain and a faucet hole are easy while walls are open and awkward afterward.
We cover Millbrae and nearby towns, including the one our Burlingame plumber page describes. The full list of towns is on the service area page, and the 24/7 line is the best way to start.
Choosing between RO and a simple sink filter?
Call the 24/7 line. We will ask about your sink, your cabinet and what you want from the water, and give a clear price before work starts.
Not automatically. The number counts cartridges, not results. A well-built three stage unit with a good membrane can beat a five stage with filler stages. Ask what each stage does and how often it is changed.
The carbon stages ahead of the membrane do most of that work, and the membrane handles part of the remainder. Because chloramine is harder on carbon than free chlorine, ask about cartridge type and plan on regular replacement.
Some people notice it. The membrane removes dissolved minerals, and a remineralizing stage can bring a little back. Taste is personal, so try the water before you decide on an add-on.
It sends a share of the feed water to the drain as reject. The proportion varies with pressure and the membrane, so ask the installer for the expected range. A shutoff valve at the tank helps limit it.
Often yes, with longer tubing, but runs should stay protected, short and level enough for good flow. Long runs or cold garages can create problems, so ask before you pick a location.
No. Hardness is a smaller issue for a membrane in Millbrae. The chloramine disinfectant is, so the carbon stages stay important even when the water is mild.
Sources and standards: The City of Millbrae 2025 Water Quality Report. The California Plumbing Code (based on the Uniform Plumbing Code).