Reverse osmosis is one of the most effective water purification technologies available for home use — but the name doesn’t make it obvious what’s actually happening. “Reverse osmosis” sounds technical, and explanations often make it more complicated than it needs to be. Here’s a plain-English explanation of how a residential RO system works, what each stage does, and what you actually get out of it.

The Basic Principle: Pressure and a Very Fine Membrane

Osmosis is a natural process where water moves through a semi-permeable membrane from a region of low solute concentration to high solute concentration — essentially, water “wants” to dilute the saltier side. Reverse osmosis uses applied pressure to do the opposite: push water from the more concentrated (contaminated) side through the membrane, leaving the contaminants behind.

The RO membrane has pores at 0.0001 microns — small enough that only water molecules pass through. Everything dissolved in the water — lead, arsenic, nitrates, fluoride, PFAS, dissolved salts, bacteria, viruses — is rejected and flushed away in a separate drain stream called the concentrate (or reject water).

This physical separation is what makes RO different from carbon filters, which adsorb contaminants onto filter media but can’t remove dissolved inorganic compounds. RO doesn’t adsorb — it physically excludes.

The Stages of a Residential RO System

Stage 1: Sediment Pre-Filter (5 micron)

Before water reaches the delicate RO membrane, it passes through a sediment filter that catches particles — sand, silt, rust flakes, and debris. This protects the membrane from physical damage and clogging. Sediment filters typically need replacement every 6–12 months, depending on incoming water turbidity.

Stage 2: Carbon Pre-Filter

An activated carbon filter removes chlorine and chloramines from the water before it reaches the membrane. This is essential because polyamide thin-film composite (TFC) membranes — the type in all modern residential RO systems — are rapidly damaged by free chlorine. One or two carbon pre-filter stages is standard. This stage also reduces VOCs, taste, and odor compounds.

Stage 3: The RO Membrane

The core of the system. A spiral-wound polyamide TFC membrane, typically 50–100 GPD rated capacity for residential use, achieves 97–99.5% rejection of dissolved solids. Feed water is split into permeate (clean water) and concentrate (reject water).

The concentrate is continuously flushed down the drain. Standard residential membranes produce 3–4 gallons of reject water per gallon of permeate — this is the trade-off inherent in RO’s high removal efficiency. Modern high-efficiency membranes improve this ratio to 1:1 or 2:1.

Stage 4: Storage Tank

RO membranes produce water slowly — 50–100 gallons per day for most residential units. A pressurized storage tank (typically 2–4 gallons) stores treated water so it’s available on demand. Water production continues until the tank is full, then pauses. When you run the RO faucet, water flows from the tank.

Stage 5: Post-Carbon Polish Filter

A final activated carbon filter polishes taste and removes any residual taste or odor from the tank before the water reaches the faucet. This stage also provides a final protection barrier if there’s any microbial growth in the storage tank during extended disuse.

Optional Stage 6: Remineralization

RO removes calcium and magnesium along with contaminants. Many systems add a remineralization stage — typically calcite or dolomite media — that restores these minerals at controlled levels. This raises pH from the ~6.5 of pure RO permeate to ~7.5–8.0 and improves taste. Optional in residential systems but commonly included in quality 6- or 7-stage units.

What Residential RO Removes (Approximate Rates)

• Lead: 99%+
• PFAS (PFOA/PFOS): 95–99%
• Arsenic: ~95%
• Nitrates: 83–92%
• Fluoride: 85–95%
• Chromium: 95%+
• TDS/dissolved salts: 97–99.5%
• Bacteria and viruses: >99% (physical exclusion by pore size)

Maintenance: What You Actually Need to Do

A residential RO system requires very little active maintenance:

• Pre-filters (stages 1 and 2): Replace every 6–12 months
• Post-carbon filter: Replace every 6–12 months
• RO membrane: Replace every 2–5 years (longer with clean feed water)
• Storage tank check: Ensure pressure is at 5–8 PSI when empty (use a tire gauge on the Schrader valve)

Most people replace filters once or twice a year on a simple schedule — the most involved part is about 10 minutes under the sink with no plumbing knowledge required.

AMPAC USA’s under-sink RO systems deliver 5–7 stages of filtration in a compact under-counter unit — easy to install, easy to maintain, and certified to remove the contaminants that matter most to your household.