Reverse Osmosis Systems

How an RO Filtration System Works

An RO filtration system works in stages, with each stage protecting the next. Household water pressure drives water through the sequence, and the contaminants the membrane rejects are flushed to the drain.

The Semipermeable Membrane

The membrane's pores are around 0.0001 micron, small enough to pass water molecules while holding back dissolved solids at a near-ionic level. That's why RO reaches contaminants that carbon alone can't.

Pre-Filters, Membrane and Post-Filters

A sediment pre-filter catches grit, and a carbon pre-filter reduces chlorine that would otherwise damage the membrane. The membrane does the heavy lifting. A post-carbon filter then polishes taste on the way to the faucet. Systems with more stages add UV, pH correction, or alkaline minerals after the membrane.

What Reverse Osmosis Reduces

RO is the broadest single technology for dissolved contaminants. How much a specific system reduces depends on its own testing and certification, and on your feed water.

Start with a water test kit so you know which of these you actually need to address.

Choose an RO System by How You'll Use It

The right reverse osmosis system depends less on stage count and more on where it goes and what the water is for.

Under-Sink Reverse Osmosis Drinking Water Filter Systems

A reverse osmosis drinking water filter system installs under the kitchen sink with its own faucet and storage tank. The Zoi range runs from the 5-stage Zoi Alpha Pure to the 8-Stage UV+pH+Alkaline Zoi Pi Pure. In between are the 6-Stage UV Zoi Gamma, 6-Stage pH Zoi Epsilon, 6-Stage Alkaline Zoi Zeta, 7-Stage pH+Alkaline Zoi Mu, 7-Stage UV+pH Zoi Theta, and 7-Stage UV+Alkaline Zoi Iota, so you add only the stages your water needs. The 4-Stage Twist Loc Nelsen Reverse Osmosis System swaps cartridges with a twist; no housing wrench needed.

Portable RO Systems

The Mikro portable range connects to a faucet with no permanent plumbing, built for renters, RVs, and tight kitchens. It covers the Mikro Alpha 4-Stage, Mikro Gamma 3 Stage with UV sterilization, Mikro Epsilon 3 Stage with re-mineralization, Mikro Delta 4-Stage PLUS DI, and Mikro Omega 3 Stage Aquarium RO/DI, each with a 75 GPD membrane.

Aquarium RO/DI Systems

Reef tanks need water purer than drinking-grade RO. The 4-stage Aquarium RO/DI Psarion Omega Reef System adds a deionization stage, and the 5-Stage Aquarium RO/DI Psarion Psi Reef System runs dual DI for reef chemistry that has to stay stable.

High-Output Tankless RO

The Pentair High Output Tankless RO On Demand Reverse Osmosis System produces water as you draw it, with no storage tank taking up cabinet space. It suits larger households and homes feeding a refrigerator or ice maker.

Certified and Low-Waste Systems

For independently verified performance, the 5-Stage PT-505 Vertex Gold Seal Certified Reverse Osmosis System is made in the USA and carries the WQA Gold Seal. Its booster-pump sibling, built for high-TDS or low-pressure water, adds auto-flush. Browse the full range of Gold and Platinum Seal certified RO systems. To cut drain water, the Vertex Green Machine Supreme pairs a 1:1 Revtex membrane with a permeate pump.

Sizing: GPD, Water Pressure and TDS

GPD (gallons per day) is the rated output, and it should match real demand rather than the biggest number available. A 50 GPD system covers drinking and cooking for a smaller household, while larger families or homes feeding a fridge do better with 75 GPD or a tankless unit. Real output falls with lower water pressure, colder water, and higher incoming TDS. Homes with low pressure or high TDS benefit from a booster pump.

Tank vs Tankless RO

Tank systems store purified water for steady pressure at the faucet and run without electricity, but they take more cabinet space and refill slowly after heavy use. Tankless systems save space and produce water on demand, but they need power. Choose by cabinet space, daily demand, and whether an outlet is convenient under the sink.

Whole Home Reverse Osmosis Water System: When It Makes Sense

A whole home reverse osmosis water system treats every tap in the house. It's uncommon in homes because it processes far more water than you drink, needs large storage and repressurization, and sends a significant volume to drain. Most households get the same drinking-water result from point-of-use RO at the kitchen sink, paired with whole-house filters or a softener for the rest of the home.

RO Filter Replacement: Filters and Membrane

RO filter replacement keeps the system performing as rated. Sediment and carbon pre- and post-filters are typically changed every 6–12 months; hard or high-TDS water shortens that. The membrane lasts longer and is due when the TDS of your purified water starts to climb, which a TDS meter shows at a glance.

AFW groups reverse osmosis water filter replacement filters by system type, so you can buy a full set in one order. Look for drinking RO filter kits, portable RO filter kits, and aquarium RO filter kits.

Reverse Osmosis on Well Water

Iron, hardness, sulfur, and sediment foul or scale an RO membrane far faster than treated city water does, so well water needs pre-treatment first. Install a sediment filter for grit and iron and sulfur filters where iron is present. Add a water softener where hardness runs above a few grains per gallon. Then place RO at the sink as the final polishing step for drinking water.