What Do Reverse Osmosis Systems Do? | Water Purification Explained

If you’ve ever wondered what reverse osmosis systems do, the short answer is that they produce cleaner drinking water by physically filtering out substances dissolved in your tap water. The CDC confirms that RO systems remove some chemicals including lead, copper, chromium, chloride, and sodium, and may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorous. That’s why they’re a popular choice for homeowners who want better-tasting water without relying on single-use plastic bottles.

Understanding how the technology works, what it removes, what it leaves behind, and where it falls short will help you decide if an under-sink RO unit belongs in your kitchen.

How Reverse Osmosis Works

RO reverses natural osmosis. Instead of water moving toward a concentrated solution, pressure pushes feed water against a semi-permeable membrane, forcing water molecules through while most dissolved contaminants stay behind. The water that passes through is called product water or permeate; the concentrated waste stream is called reject, brine, or concentrate.

The membrane itself is remarkably fine. That’s how it catches things you can’t see or taste.

Most residential systems don’t rely on the membrane alone. A typical under-sink setup includes:

• A sediment prefilter to catch dirt and rust that could clog the membrane.
• A carbon prefilter to remove chlorine and chloramine, which can damage the membrane.
• The RO membrane itself, where the main separation happens.
• A storage tank, since RO production is slower than normal faucet flow.
• A post-carbon filter to polish taste and odor before the water reaches your glass.

Some systems add remineralization cartridges, specialty filters, or UV light depending on your water’s specific problems and your goals.

What RO Systems Remove — And What They Don’t

That’s why they’re effective against heavy metals, nitrates, and many other dissolved substances that carbon filters alone can’t touch.

RO is not universal removal. It produces a reject stream, so it doesn’t convert all feed water into product water — some goes down the drain as concentrate. And because it removes calcium and magnesium along with the bad stuff, some homeowners add a remineralization cartridge to put beneficial minerals back in.

What About the Waste Water?

Yes, RO systems do waste water. The reject stream carries the concentrated contaminants to the drain, which means a portion of your input water never reaches your glass. Efficiency varies by system and water pressure; modern units with permeate pumps are noticeably more efficient than older designs.

If you’re on a septic system or pay for metered water, that’s a real consideration. It’s also one of the main reasons some homeowners weigh RO against simpler carbon-only filtration, which wastes nothing but removes far less.

Is an RO System Right for Your Home?

RO systems fed by municipal or well water both work, and the CDC lists RO as a legitimate home water treatment option. They’re especially valuable if your water tests high for lead, nitrates, or total dissolved solids. They’re less necessary if your water is already clean and you just want better taste — a carbon filter may do the job at a fraction of the cost.

Maintenance matters. Sediment and carbon prefilters need replacing on schedule, and the membrane itself degrades over time. A system with clogged prefilters loses efficiency and can even damage the membrane, so skipping maintenance costs you more in the long run.

If you’re considering a larger setup for a workshop, farm, or commercial building, our tested roundup of the best commercial reverse osmosis systems compares the top units by output, maintenance needs, and real-world performance.