What It Is
Reverse osmosis (RO) is a water filtration process that forces water through a semi-permeable membrane under pressure, removing dissolved minerals, contaminants, and impurities that other filters cannot capture. An RO system typically includes pre-filters to remove sediment and chlorine, the RO membrane itself, and a storage tank. RO is used primarily for drinking and cooking water at a single point of use, rather than whole-house treatment - it produces water slowly and in limited volumes.
What It Does
Standard filters remove particles - sediment, chlorine taste, some chemicals. The RO membrane goes further, blocking dissolved ions and molecules based on size. Properly functioning RO systems remove a broad range of dissolved contaminants: heavy metals like lead and arsenic, nitrates, fluoride, dissolved salts, and many others. The tradeoff is flow rate and water efficiency - RO systems are slow, and for every gallon of purified water produced, they send some water down the drain as concentrate.
RO is often paired with a water softener or pre-filter in homes with hard water. Hard water clogs and shortens membrane life quickly; softening or sediment-filtering the water first protects the RO membrane and extends its service interval.
Where It Is Located
Most residential RO systems are installed under a kitchen sink at the point of use, with a dedicated small faucet at the sink for filtered water. The system includes: a pre-filter housing (sediment), a second pre-filter (carbon for chlorine), the RO membrane housing, a storage tank (typically 2-4 gallons), and a post-filter polishing the water before it reaches the faucet. A drain line connects to the sink drain to remove the concentrate.
How It Works
Normal osmosis moves water from a less concentrated solution through a membrane to a more concentrated one. Reverse osmosis applies pressure to push water the other direction - from the concentrated side (tap water with dissolved contaminants) through the membrane to the pure side. The membrane has pores small enough to block most dissolved ions and molecules but allow water molecules through. Contaminants that cannot pass accumulate in the concentrate stream and are flushed to the drain. The purified water that passes through the membrane collects in the storage tank.
Membranes have a finite lifespan, typically 2 to 5 years depending on water quality and usage. Pre-filters upstream of the membrane protect it from sediment and chlorine damage; replacing pre-filters on schedule (every 6 to 12 months) is the most important maintenance task for extending membrane life.
Common Problems or Signs of Failure
- Slow or no water from the RO faucet - storage tank may be empty, membrane may be clogged, or tank pressure is low
- Water tastes different than normal - membrane may be failing and allowing more contaminants through, or the post-carbon filter is exhausted
- Constant draining at the drain line - check valve failure causing the membrane to drain continuously instead of stopping when the tank is full
- Pre-filters overdue for replacement - reduces membrane protection and water quality output
Can a Homeowner Check It?
- Track filter replacement schedules - pre-filters typically every 6 to 12 months, membrane every 2 to 5 years depending on usage and source water quality
- Check the storage tank pressure - an under-pressurized tank (standard pre-charge is around 7-8 PSI) produces low flow; a bicycle pump can add air if needed
- Test the output water with a TDS (total dissolved solids) meter - if TDS rises significantly, the membrane may need replacement
- Listen for constant drain line flow when no water is being used - this indicates a faulty check valve
When to Call a Plumber
Initial RO installation involves connecting to the cold supply line, adding a drain saddle, and mounting the dedicated faucet through the sink - tasks well within a plumber's scope. If the system develops leaks, a membrane or housing needs replacement, or the system needs to be moved or upgraded, a plumber can handle the work quickly. Filter replacement itself is a homeowner task once the system is installed.
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