Short definition

A reverse-osmosis (RO) system is an under-sink drinking-water filter that uses a semi-permeable membrane to remove dissolved solids — including most salts, fluoride, lead, arsenic, and many other contaminants. A standard residential layout runs the cold supply through a sediment filter, a carbon filter, the RO membrane, a small storage tank, a final carbon polish, and out through a dedicated drinking-water faucet at the sink.

What it is

An RO system breaks drinking water into two streams. Permeate is the cleaned water that passes through the membrane and reaches your faucet. Concentrate (also called reject water) is the discharge that carries the rejected dissolved solids to the drain.

The standard residential layout has five stages:

  1. Sediment pre-filter — catches sand, rust, and silt that would clog the membrane.
  2. Carbon pre-filter — removes chlorine and chloramines (which damage RO membranes) plus taste and odor.
  3. RO membrane — the heart of the system. A semi-permeable membrane lets water molecules through and rejects almost everything dissolved in the water.
  4. Storage tank — a 2- to 4-gallon pressurized tank under the cabinet that holds permeate water until you draw from the faucet.
  5. Carbon post-filter — polishes any residual taste or odor from the storage tank before water reaches the faucet.

Discharge from the membrane (concentrate) drains through a drain saddle clamped onto the sink tailpiece, or through an air-gap fitting built into the dedicated faucet. The air gap is the code-required backflow protection — without it, a sewer backup could contaminate the RO output.

A modern tankless RO design produces water on demand without the storage tank, using a higher-pressure pump. Tankless units are smaller, run quieter, and often hit better waste ratios.

Why it matters to a homeowner

Two things drive most homeowner RO decisions in Washington.

Source water. If you’re on Seattle’s Cedar / Tolt water or Tacoma’s Green River supply, the source water is soft, low-TDS, and well-treated. RO is optional — most people who install it want fluoride removal or trace contaminant reduction, both of which are personal-choice decisions rather than necessity.

If you’re on a private well in rural Snohomish, Mason, Jefferson, Kitsap, or the Olympic Peninsula, RO is genuinely useful. Some WA wells exceed EPA standards for arsenic or nitrate; both are removed effectively by reverse osmosis.

Waste water. Older RO systems wasted 4 gallons of concentrate for every 1 gallon of drinking water they produced — a meaningful issue on a metered city supply or a well with limited yield. Modern systems hit 1:1 to 2:1 ratios, and tankless designs reach 1:1 or better. If you’re shopping in 2026, the waste ratio is published on the spec sheet.

The certification to look for is NSF/ANSI 58 for the RO performance, NSF/ANSI 53 for health-effect contaminants, and NSF/ANSI 42 for aesthetic (taste, odor, chlorine) reduction. Premium systems carry all three.

Common failure modes

  • Drain saddle leak at the tailpiece pierce point — common older-design failure. Air-gap faucet drains are now preferred.
  • Air-gap faucet “spitting” — when the drain saddle clogs, reject water backs up into the air-gap faucet and dribbles onto the counter. Symptom of a downstream clog or kinked drain hose.
  • Membrane fouling — high-sediment input water clogs the membrane. Replace.
  • Slow flow at the faucet — storage tank air pre-charge has bled off. Recharge through the Schrader valve to about 7 psi (manufacturer-specific).
  • Mineral scale on the faucet aerator — RO water plus minerals from the faucet itself. Descale.
  • Cracked supply tubing — ¼-inch plastic compression line nicked or kinked. Replace.

Filter and membrane lifetimes

  • Pre-filter and post-filter (carbon, sediment): 6 to 12 months typical replacement.
  • Membrane: 2 to 5 years residential, depending on input water and use volume.
  • Storage tank: indefinite, but the air pre-charge needs occasional service.

Common variants and what an RO system is not

  • Tank-RO vs. tankless RO. Tank-RO holds purified water in a pressurized 2-to-4-gallon tank; tankless produces on demand at higher pressure. Tankless is newer, more space-efficient, and avoids the air-gap drain saddle in many designs.
  • RO with remineralizer. A post-treatment cartridge adds back trace minerals (calcium, magnesium) for taste; popular in 2024–2026 systems.
  • Whole-house RO vs. point-of-use RO. Whole-house RO is rare in residential installs (high cost, bulk water-waste); point-of-use under the kitchen sink is the standard.
  • RO vs. carbon filter. A carbon filter removes chlorine, taste, odor, and some pesticides — but not dissolved solids, fluoride, or arsenic. RO removes all of those. Different systems for different jobs.
  • RO vs. distiller. A distiller boils-and-condenses (electricity-intensive). RO uses pressure across a membrane (no electricity for the RO stage itself).

Washington water-quality context

  • Seattle (Cedar / Tolt) and Tacoma (Green River): soft, low-TDS surface water. RO usually unnecessary for taste; some homeowners install for fluoride or trace-contaminant removal.
  • Spokane: harder, well-influenced supply; RO more common.
  • Private wells (rural counties): RO is a meaningful upgrade for arsenic, nitrate, and hard-water deposits. The Washington Department of Health publishes water-testing guidance for private wells; a baseline test before installing any treatment system is the right starting point.