Gravity filters and reverse osmosis (RO) systems are two of the most popular ways to filter drinking water at home. Both can do an excellent job, but they work in very different ways.
How a gravity filter works
You pour water into an upper chamber, and gravity pulls it slowly through filter elements into a lower chamber. Most quality gravity filters use dense activated carbon. Contaminants are captured on the carbon's enormous internal surface as water moves through. The pace is slow, and that gives the media time to work.
- No installation, plumbing or electricity
- Keeps naturally occurring minerals like calcium and magnesium
- No wastewater: every gallon you pour in comes out to drink
- Portable, and works in a power outage
- Filters in batches, so you refill the top chamber through the day
How reverse osmosis works
RO pushes water under pressure through a semi-permeable membrane with extremely fine pores, usually with carbon pre- and post-filters. It removes a very wide range of dissolved substances, including minerals.
- Very broad removal, including dissolved salts and minerals
- Filtered water on tap at the sink
- Needs under-sink installation and some plumbing
- Sends part of the water to the drain as wastewater
- Removes minerals, so some people add remineralization stages
- Several filters and a membrane to replace on different schedules
Minerals: removed or kept?
This is the biggest practical difference. RO strips out naturally occurring minerals along with contaminants, which some people find makes water taste flat. Carbon gravity filtration leaves them in. Neither choice is right for everyone, but if you like the taste of mineral-rich water, gravity filtration keeps it.
Cost over time
RO systems cost more up front once you include installation, and they need periodic filter and membrane changes. A gravity filter like Aquelia costs nothing to install. Its filters last up to 6,000 gallons, so the running cost works out to under 2¢ a gallon.
Which should you choose?
- Choose RO if your water report shows high dissolved solids, nitrates or other contaminants that call for membrane filtration, and you're happy to install under the sink.
- Choose a gravity filter if you rent or can't modify plumbing, want minerals kept, want no wastewater, or want a system that works without power.
Whatever you choose, start with your local water report so you're filtering for what's actually in your water.
About Aquelia
Aquelia is an all-glass gravity filter with black carbon elements tested by SGS. See the lab results.