Water Purifier Buying Guide: RO, UF and Filters Compared

A water purifier buying guide starts with one question: what is wrong with your water? Pick the unit that fixes that problem, then check its upkeep and cost. A filter that does not match your problem is wasted money.

Start with your water, not the box

Learn what is in your tap water first. Ask your water supplier for its yearly water quality report. If you use a private well, test it at a certified laboratory. Test results tell you what to remove.

People buy a unit for three common reasons. Some want better taste. Some want to remove a named contaminant, such as lead. Some live with a person whose immune system is weak and want extra care. Each reason points to a different kind of unit.

This guide is general information, not medical advice. If someone in your home has a health condition, ask a doctor how to treat their drinking water.

Source: U.S. EPA, Water Health Series: Filtration Facts (2005).

Carbon filters: simple and cheap

A carbon filter traps contaminants on the surface of carbon. Pitchers, tap filters and under-sink filters often use it. It is good at improving taste, and some models also reduce lead.

It has limits. A carbon filter by itself is not made to remove every germ. Each filter also holds only so much water. After that, it clogs and stops working well. Check how often the cartridge must be changed and what a new one costs before you buy.

A simple pitcher can cost under 20 US dollars. That makes carbon the lowest cost way to start.

Source: U.S. EPA, Filtration Facts (2005).

UF and fine filters: look at the hole size

Every filter is a screen with tiny holes. The smaller the holes, the more it removes. Hole size is measured in microns. Bacteria such as E. coli are about 0.2 to 4 microns wide. Viruses are far smaller, from 0.004 to 0.1 microns.

Ultrafilters, known as UF, have very small holes. The EPA says only a few filters, such as ultrafilters and reverse osmosis units, have holes small enough to block viruses. Always read the label. Look for an absolute rating, which is the largest hole. A nominal rating is only the average hole, so it tells you less.

Like any filter, a UF unit must be changed or cleaned on the maker's schedule. Ask the seller how much water it wastes and what its parts cost.

Source: U.S. EPA, Filtration Facts (2005).

Reverse osmosis: strong, but it wastes water

A reverse osmosis unit, or RO, forces water through a thin membrane under pressure. The membrane leaves contaminants behind. RO removes most chemical contaminants, and the EPA lists it as effective against germs, lead and nitrates.

The trade-off is waste water. The EPA says RO units use about three times as much water as they treat. For every glass you drink, two more go down the drain. Think about this if water is scarce or your bill is high.

RO also costs more. Prices range from a few hundred dollars up. Plan for membrane and filter changes too.

Source: U.S. EPA, Filtration Facts (2005).

Compare upkeep, waste and cost side by side

Use this short list when you compare models.

• Carbon: lowest price, best for taste, filter changes often, no waste water.
• UF: very small holes, blocks viruses on some models, filter care needed, ask the seller about waste water.
• RO: highest price, widest removal, about three times more water used than treated.

Ask the seller for the price of every replacement part for two years. A cheap unit with costly cartridges can end up more expensive than an RO.

Source: U.S. EPA, Filtration Facts (2005).

Check for independent proof before you pay

Look for a mark from an independent group that tests to ANSI and NSF standards. The EPA names NSF International, Underwriters Laboratories and the Water Quality Association. Their tests check claims about contaminant removal, the materials and the build quality. NSF also runs surprise audits of factories.

For germs called cysts, such as Cryptosporidium, look for a unit certified to NSF Standard 53 for cyst removal. If a unit has no mark, ask the maker for proof of its claims in writing.

Last, follow the maker's schedule for filter changes. An old filter does not work as promised. Write the change date on the unit when you install it.

A purifier is not needed for every home. If your tap water already meets the rules where you live and you only dislike the taste, a carbon filter is often enough.

Source: U.S. EPA, Filtration Facts (2005).

Sources

• U.S. Environmental Protection Agency, Water Health Series: Filtration Facts, 2005.