Under-sink reverse osmosis

Reverse Osmosis Systems

Q: Is reverse osmosis water safe to drink?

Yes. In our RO video Aidan calls it "very clean, very safe water." It also takes out most of the minerals; see the minerals question below.

Q: Reverse osmosis vs a water filter: which do I actually need?

Match the technology to the problem. Carbon takes out chlorine taste and smell and costs less: if chlorine taste is your only complaint, carbon is the answer. Carbon does not lower dissolved solids such as sodium, nitrate or TDS; an RO does, and its carbon stages cover taste at the same tap. Carbon filter vs reverse osmosis goes through both.

Q: Do I need the 50 GPD or the 75 GPD system?

Either serves a kitchen tap in most homes, because the tank does the serving and refills in the background. Aidan usually recommends the Pure-75: "That gives the fastest and the highest capacity." The MAW-50 costs less up front and to keep up.

By Aidan · 32-year water expert, Mid Atlantic Water · Updated October 2026

If a water test found lead, nitrate, PFAS or a high TDS (dissolved solids) number in your drinking water, an under-sink reverse osmosis (RO) system treats it at the kitchen tap. A membrane filters out most of what is dissolved in the water, and the clean water is served from its own faucet. It works on city water and on a well.

We sell two. The Pure-75 ($595) is the one Aidan usually recommends: four stages, a 75 gallon a day membrane, and cartridges that change with a quarter turn without shutting off the water. The MAW-50 ($275 with 4 stages, $325 with 5) costs less and uses standard filter housings. Both come with the tank and a lead-free faucet and need no electricity.

Quarter-turn filter changes (Pure-75)

Storage tank + faucet included

No electricity: runs on water pressure

Free shipping to the lower 48 states

RO treats one tap. Hard water, iron, a rotten-egg smell or chlorine taste in every faucet and shower is a whole-house job, and on a well that treatment goes in first, because hardness and iron ruin a membrane. In Aidan's words: "The iron and the hardness will kill the reverse osmosis." Send him your water test and he will tell you whether you need an RO at all. Call 800-460-5810 or text Aidan at 443-277-2204.

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Aidan Water Expert · Mid Atlantic Water Text me your water test, I’ll size your system for you. Tap to text 443-277-2204

Under-Sink Reverse Osmosis Systems

Two under-sink systems. Each comes with a 3.2 gallon storage tank, a lead-free faucet, the tubing and the drain saddle, and runs on your water pressure with nothing to plug in. The Pure-75 ($595) is the one Aidan usually recommends: quarter-turn cartridges, no water shut-off to change them. The MAW-50 ($275 or $325) has standard housings and the lowest filter cost.

Replacement Filters & Membranes

Filters every 6 to 12 months (every 6 on the Pure-75). The membrane every 2 to 3 years on the MAW-50; the Pure-75's maker rates its membrane at 24 months. The Pure-75 kit ($195) is the full set with the membrane. MAW-50 filter packs are $49 (4-stage) or $59 (5-stage), and its TFC-50 membrane is $135. Not sure which you have? Text Aidan a photo.

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Our two systems

Pure-75 or MAW-50: side by side

Both come from our supplier, Nelsen, and work the same way: a sediment filter, a carbon filter that takes out the chlorine, a thin-film membrane, a carbon filter after it, a 3.2 gallon tank and their own faucet. The differences are how the filters change, what upkeep costs and the water each one can take. The prices and upkeep are ours; the ratings and limits are from the maker's sheets.

	Pure-75	MAW-50

Price	$595	$275 (4-stage), $325 (5-stage)
Stages	4: sediment, carbon block, membrane, carbon block	4 or 5: sediment, carbon block (a second carbon filter on the 5-stage), membrane, in-line carbon
Filter changes	Quarter turn, with the water left on	Standard housings and the included wrench; water shut off first
Upkeep	Cartridges every 6 months; membrane rated 24 months; the $195 kit is the full set	Filter pack every 6 to 12 months ($49 or $59); TFC-50 membrane every 2 to 3 years ($135)
Membrane	75 gallons a day	50 gallons a day
Well water it can handle (maker)	Hardness under 17.5 grains, iron under 1 ppm	Hardness under 15 grains, iron under 0.3 ppm, manganese under 0.05 ppm, no hydrogen sulfide (rotten-egg smell)
Electricity	None: runs on 30 to 100 psi water pressure	None: runs on 30 to 100 psi water pressure

Aidan usually recommends the Pure-75: "That gives the fastest and the highest capacity." The MAW-50 costs less to buy and to keep up. Neither is certified as a system to NSF/ANSI 58.

Nelsen rates the membranes at 91 to 98 percent of dissolved solids under its own test conditions. Cold well water and low pressure make less water and take out a little less.

Do you need one?

When an under-sink RO is the right tool

Lead, nitrate and PFAS have no taste, smell or color at the levels found in homes, so the lab report is the only signal. Here is when an under-sink RO is the right tool, and when it is not.

A person reading a water test report at a kitchen table

Your water test found lead, nitrate, PFAS or high TDS

An under-sink RO lowers each of these at the tap you drink from. It does not take everything to zero (Aidan tells nitrate callers to expect about half, up to 70 percent), so test the RO water after install.

YES RO treats the tap you drink from

A hand filling a drinking glass at a kitchen faucet

Tap water tastes salty or mineral

That usually means a high TDS: sodium from a softener, or minerals in the source water. Aidan: "Only reverse osmosis is gonna change the TDS. Nothing else will." Chlorine taste alone is a carbon filter job.

YES RO lowers dissolved solids

Hard water, iron stains or a sulfur smell at every tap

Those are whole-house problems, and on a well they ruin an RO membrane. Treat the whole house first (a softener, an iron filter); add an RO at the kitchen tap only if your test still shows something it should take out.

NO Whole-house treatment first

The two sample bottles and the paperwork from the well water test kit, on a table

Test before you treat

A lab test tells you whether you need an RO at all. The Well Water Test ($199) reads 53 contaminants and values, including lead, nitrate, arsenic, sodium, hardness, iron and TDS; the City Water Test ($199) reads 47. Neither includes PFAS, which is its own $349 test. Send us your result and we size your system free.

TEST IT Free sizing help

Step 2: Match Your System

Match your problem to the right system

Find the line that matches you to see where to start. If the problem is in every faucet and shower, see the note below the rows.

A lab found lead, nitrate or PFAS At the kitchen tap An under-sink RO lowers each of these at the tap you drink from

AIDAN'S USUAL PICK
Drinking water treatment, 75 GPD Pure-75 Quick-Change Twist-Loc From $595
Want the lowest cost to own $275 50 gallon a day membrane, 4 or 5 stages, tank and faucet included; $49 filter packs

VALUE PICK
Value Pick, 50 GPD MAW-50 Reverse Osmosis, 4 or 5 Stage From $275
Haven't tested yet No lab number Dissolved contaminants are invisible. The test decides whether you need an RO

START HERE
Test First Well Water Test (53 contaminants) From $199

Problem at every tap? Hard water, iron, a sulfur smell or chlorine taste in every faucet is a whole-house job, and on a well it goes in before any RO. Chlorine taste only? A carbon filter is the cheaper fix. Still not sure? Text Aidan your water test at 443-277-2204.

Not sure? Call Aidan at 800-460-5810 →

Step 3: Pick a size

What size reverse osmosis system do I need?

Either system serves a kitchen tap in most homes. The tank holds the clean water; once you empty it, Aidan says it takes from 30 minutes to about 3 hours to refill, depending on your water pressure and temperature. The refill time is what limits you, not the daily rating. RO treats one tap: problems in every faucet take whole-house equipment.

	Pure-75 (4 stage)	MAW-50 (4 or 5 stage)

Tank size	Quick-change unit plus a 3.2 gallon tank	Filter unit plus a 3.2 gallon tank
Household	Kitchen tap, most homes	Kitchen tap, most homes
Filter changes and cost	Cartridges every 6 months; membrane rated 24 months ($195 kit)	Pack every 6 to 12 months ($49 or $59); membrane every 2 to 3 years ($135)
Membrane rating	75 gallons a day	50 gallons a day
Price	$595	$275 or $325
	Shop now	Shop now

Gallons per day (GPD)

The membrane's rating in the maker's test (the MAW-50's 50 gallons is at 60 psi and 77 F). Cold well water and low pressure make less. What you notice day to day is the tank: about a gallon at a time, in Aidan's words, then a refill.

Water sent to the drain

An RO rinses what it removes down the drain while the tank fills. Aidan's rule of thumb is about two gallons to the drain for each gallon you drink, and it depends on your water pressure and temperature.

What the stages do

The sediment filter stops grit. The carbon filter takes out the chlorine that would ruin the membrane. The membrane removes the dissolved solids (Nelsen rates ours at 91 to 98 percent). The last carbon filter polishes the taste on the way to the faucet. The MAW-50's fifth stage is a second carbon filter, for heavily chlorinated city water.

When to replace the membrane

Read the tap and the RO faucet with a TDS meter and divide. Under 10 percent of the tap reading is fine, 10 to 25 percent means the membrane is fading, and over 25 percent means replace it. Checking your RO with a TDS meter.

Pure-75 Reverse Osmosis System - Quarter-Turn Quick Change, 75 GPD

Pure-75 (4 stage)

$595

Household

Kitchen tap, most homes

Filter changes and cost

Cartridges every 6 months; membrane rated 24 months ($195 kit)

Tank size

Quick-change unit plus a 3.2 gallon tank

Membrane rating

75 gallons a day

Shop Pure-75 (4 stage)

MAW-50 Reverse Osmosis System - 4 or 5 Stage, 50 GPD

MAW-50 (4 or 5 stage)

$275 or $325

Household

Kitchen tap, most homes

Filter changes and cost

Pack every 6 to 12 months ($49 or $59); membrane every 2 to 3 years ($135)

Tank size

Filter unit plus a 3.2 gallon tank

Membrane rating

50 gallons a day

Shop MAW-50 (4 or 5 stage)

Installation

Installed under one sink by your plumber

We recommend a licensed plumber; Aidan supports your plumber by phone at 800-460-5810. Usually everything goes under one sink: a tap on the cold line, the faucet, a drain saddle on the sink drain, the filter unit and the tank. What ships: the filter unit with its cartridges and membrane, the 3.2 gallon tank, the faucet, tubing, the angle stop adapter, the drain saddle and the tank valve. If your sink has no spare hole, the plumber drills one for the faucet.

30 to 45 min

Plumber time. Aidan's estimate for installing the RO itself, all under one sink.

0 watts

No electricity. Both systems run on your water pressure (30 to 100 psi on the maker's sheets). Nothing to plug in.

Free

Phone support. Aidan supports your plumber by phone at 800-460-5810.

What to have ready

Cold water connection under the sinkThe included angle stop adapter taps the cold supply line. Both systems use push-connect fittings and the included tubing.
A hole for the dedicated faucetEvery system ships with its own lead-free faucet. A spare sprayer or soap dispenser hole works; otherwise your plumber drills one in the sink or counter.
Drain connectionThe membrane rinses what it removes to the drain through the included saddle fitting on the sink drain pipe, above the trap.
Room in the cabinetThe filter unit hangs on the cabinet wall next to the 3.2 gallon tank.
Test your water firstThe maker's hardness, iron and pressure limits are in the comparison table above. Aidan treats about 10 grains of hardness or any iron as the point to treat the house first.

What your plumber does

Your plumber shuts off the cold water under the sink and fits the angle stop adapter on the cold line.
Mounts the faucet in the sink or counter hole and connects its tubing.
Fits the drain saddle on the sink drain pipe, above the trap, and connects the drain tubing.
Hangs the filter unit on the cabinet wall and connects the tubing: cold water to the unit, unit to the tank, tank to the faucet, and the drain.
Opens the tank valve, then the cold water feed, slowly. A soft hiss at the drain is normal while the tank fills. A drip means a fitting needs reseating: push-connect tubing must seat all the way in.
Lets the tank fill, then drains the first tankful or two through the RO faucet before anyone drinks it. Check every fitting for drips again the next day.

Want a second set of eyes before you order? Text Aidan a photo of the space under your sink and your water test. He'll confirm the fit and flag anything your install needs.

Open the system builder

FAQ

Frequently asked questions

Does reverse osmosis remove PFAS?

RO is one of the treatments EPA names for PFAS: research shows RO membranes are typically more than 90 percent effective across a wide range of PFAS. Our two systems are not certified for PFAS reduction, and EPA suggests looking for NSF/ANSI 53 or 58 certification for a PFAS claim. If a lab found PFAS in your water, test the RO water after install. PFAS in drinking water covers the limits and the options.

How much water does reverse osmosis waste?

Aidan's rule of thumb is about two gallons down the drain for each gallon you drink. The drain only runs while the tank refills, and the amount depends on your water pressure and temperature.

Is reverse osmosis water safe to drink?

Yes. In our RO video Aidan calls it "very clean, very safe water." It also takes out most of the minerals; see the minerals question below.

How often do you change reverse osmosis filters?

Filters every 6 to 12 months, the membrane every 2 to 3 years. On the Pure-75 the cartridges change every 6 months and the maker rates the membrane at 24 months; the $195 kit is the full set with the membrane. On the MAW-50, the filter pack ($49 for 4-stage, $59 for 5-stage) every 6 to 12 months and the $135 TFC-50 membrane every 2 to 3 years, sooner on hard or high-TDS water. A TDS meter at the RO faucet tells you when a membrane is fading. More in reverse osmosis maintenance.

Does reverse osmosis remove healthy minerals?

Yes. The membrane does not sort good dissolved minerals from bad ones, so calcium and magnesium come out with the rest. That is also why RO water can taste flat to some people.

Reverse osmosis vs a water filter: which do I actually need?

Match the technology to the problem. Carbon takes out chlorine taste and smell and costs less: if chlorine taste is your only complaint, carbon is the answer. Carbon does not lower dissolved solids such as sodium, nitrate or TDS; an RO does, and its carbon stages cover taste at the same tap. Carbon filter vs reverse osmosis goes through both.

Can I use reverse osmosis on well water?

Yes, and on a well the kitchen tap is often where it matters most, but the water reaching the membrane must be within the maker's limits (hardness under 15 grains and iron under 0.3 ppm for the MAW-50). The order on a well: a sediment filter, then iron or pH treatment if needed, then the softener, then the RO at the kitchen tap, and UV if the well has bacteria. Reverse osmosis for well water explains when RO is the wrong first move.

Tank vs tankless reverse osmosis: which is better?

Tankless units make water on demand with an electric pump, so they need an outlet under the sink. Ours are tank systems that run on your water pressure, with nothing to plug in. The trade-off is the tank: about a gallon at a time, then a refill of 30 minutes to about 3 hours.

Do I need the 50 GPD or the 75 GPD system?

Either serves a kitchen tap in most homes, because the tank does the serving and refills in the background. Aidan usually recommends the Pure-75: "That gives the fastest and the highest capacity." The MAW-50 costs less up front and to keep up.

What exactly does reverse osmosis remove?

Dissolved solids: the salts, minerals and metals in the water. Nelsen rates our membranes at 91 to 98 percent of dissolved solids, and the carbon stages take out chlorine taste and smell. It does not take everything to zero: on calls Aidan tells nitrate callers to expect about half, up to 70 percent, depending on water temperature and pressure. RO is not a bacteria treatment; on a well with bacteria, UV is. The full list is in what reverse osmosis removes.

Why the separate faucet, and can RO feed my fridge or ice maker?

The RO makes clean water slowly and keeps about a gallon ready, so it gets its own faucet for drinking and cooking. A line can also run to a refrigerator's water dispenser or ice maker; your plumber sets it up, and Aidan can tell you what the connection needs.

Learn more

Reverse Osmosis System guides & deep-dives

TDS Meter for Water: What It Tells You (And What It Doesn't)

What the number means, what it cannot tell you, and checking your RO with a TDS meter.

Read the guide →

Best Under-Sink Reverse Osmosis System (2026 Buyer's Guide)

A practical comparison of the best reverse osmosis systems for 2026.

Read the guide →

Carbon Filter vs Reverse Osmosis: Which Do You Actually Need?

They solve different problems: a whole house carbon filter for every tap, an under-sink RO for drinking water.

Read the guide →

Nitrates in Well Water: Health Risks, Safe Levels & How to Remove Them

You cannot see, smell or taste it; the EPA limit is 10 mg/L measured as nitrogen.

Read the guide →

PFAS in Drinking Water: What You Need to Know

On city water your utility's report may already show your number; on a well, only a lab test will tell you.

Read the guide →

Arsenic in Well Water: Health Risks, Testing, and How to Remove It

No smell, no taste, no color: the number on a lab report is the only way to know.

Read the guide →

Personalized recommendation

Want Aidan to size it for you?

Paste your water test below. Aidan replies with the right system (Pure-75 or MAW-50), whether your well needs treatment before the membrane, and what your plumber needs to know.