Do I need a reverse osmosis system under the kitchen sink?

You already filter the water somewhere. Maybe it is a pitcher in the refrigerator, maybe the refrigerator’s own cartridge, maybe a whole-home system in the garage. And still the question comes up: should there be a reverse osmosis unit under the kitchen sink? The honest answer depends on what is in your water and what you want from the one tap you drink from.

What RO removes that other filters do not

A carbon filter, whether in a pitcher or a whole-home tank, removes chlorine, chloramine, taste, and odor, and traps some organic contaminants. It does not remove anything dissolved as a mineral or a salt. Reverse osmosis pushes water through a membrane fine enough to reject most dissolved solids: hardness minerals, sodium, nitrates, fluoride, arsenic, many metals, and much of what a lab report lists under total dissolved solids. The result is water with very little in it, which is why it tastes different: clean and slightly flat, like good bottled water. Coffee, tea, and ice made from it taste noticeably cleaner, and the kettle stops scaling.

A whole-home system treats every gallon in the house for the things that matter to the house: chlorine and chloramine for skin, hair, and seals, and hardness for the water heater and appliances. RO treats a few gallons a day for the things that matter to the glass. They are different jobs.

Remineralization and taste

Because RO takes the minerals out, some people find the water flat. Most systems accept a remineralization cartridge as a final stage, which adds a small amount of calcium and magnesium back and raises the pH slightly. It is a taste preference, not a health requirement. Try the water plain first; many households never add the stage.

The drain connection and wastewater

This is the part that surprises people. An RO membrane works by rejecting what it does not let through, and the rejected water has to go somewhere. Every unit has a drain line that ties into the sink drain with a saddle fitting or an air gap faucet, and while the tank fills, a share of the water runs to the drain. Older units were wasteful about it; newer designs and permeate pumps are far more efficient, but the drain connection is not optional. It also means RO belongs at a sink, not in a closet, and that the drain has to be in reach.

What goes under the sink

Tank versus tankless RO

• Tank systems are the traditional design: a set of prefilters, the membrane, a small pressurized storage tank, and a final carbon polish before the faucet. They are simple to maintain, and they deliver a good flow because the tank has already done the work. The tank takes up space under the sink and needs its air charge checked every few years.
• Tankless systems use a larger membrane and a pump to make water on demand. They free up cabinet space, waste less, and never serve stale water from a tank. They need a power outlet under the sink, and the replacement parts are specific to the unit.

Either can be installed well. The cabinet, the outlet, and how much water you draw at once decide which fits.

The separate faucet and the ice maker line

RO water comes out of its own small faucet on the sink deck or countertop, not the main faucet, because the flow is low and the water is precious. Most sinks have a knockout or a sprayer hole that takes it; if not, drilling a hole in a granite or quartz counter is a job for the right bit and a steady hand. The refrigerator’s ice maker and dispenser can be fed from the RO line with a tee and a length of tubing, which is where the clear ice and clean water are most noticed. The run has to be sized so the refrigerator does not starve the faucet, and a long run behind cabinets is the kind of thing that leaks quietly for a year.

Filter change schedule

An RO unit is only as good as its cartridges. The sediment and carbon prefilters protect the membrane from grit and chlorine, and chloramine in particular shortens membrane life if the prefilter is spent. Plan on changing prefilters and the final carbon stage every six to twelve months and the membrane every two to three years, sooner on a well with high solids. If the flow slows to a trickle or the water starts tasting like tap water again, the schedule slipped. A tank that has gone flat, a drain that runs constantly, or a drip under the cabinet are the common failures, and each is a small fix if it is caught early.

A complement, not a replacement

If you are on a whole-home carbon and conditioning system, RO adds the last step for the glass: dissolved solids, sodium, and the flat clean taste. If you are on a well with nitrates or high TDS, RO at the kitchen tap is the practical way to get safe drinking water without treating every gallon in the house. What RO cannot do is protect the water heater, the dishwasher, or your skin, and a unit sized for one faucet cannot supply a shower. Treat the house with whole-home equipment and treat the glass with RO.

Condos, space, and why installation is plumbing work

In a condo, the under-sink cabinet is often small and the drain is shared with a neighbor’s stack, and the association may have rules about what can be tied into it. A tankless unit or a compact tank system usually fits, and the association’s approval is worth a question before buying. Changing cartridges is a homeowner job, and so is checking the tank charge. The installation itself is plumbing: a tee on the cold supply, a saddle or air gap on the drain, a hole in the sink deck, and a line to the refrigerator. A drain saddle installed on the wrong side of the trap or a supply tee that was overtightened onto an old angle stop is a leak inside a cabinet, and a licensed plumber is the right person to do it.

If you are wondering whether RO adds anything for your water, tell us what you have now, whether you are on city water or a well, and what the glass tastes like. We will tell you honestly whether it is worth adding.