A lot happens to water between the municipal reservoir and the glass. Aging pipes under the street and in the house can add impurities that make what comes out of the tap taste or look funny. Which is why many people turn to water-filtering systems that connect right to the faucet.

Need to know: Filtration systems come in various sizes and shapes, with different kinds of filters that should be replaced regularly, according to the manufacturer's recommendations. Some filters fit directly onto the end of the faucet. Others hook into the cold-water line under the sink. Still others sit on the counter near the sink and are connected to the faucet with a hose. (A diverter allows you to bypass the filter when you want to use regular tap water to wash dishes.)

Operating manual: For removing minor impurities, many filters use granulated active charcoal. The carbon in the charcoal absorbs the contaminants that can cause foul odors and bad taste.

What's your problem? All filters are not the same, however. Some remove rust and sediments, some reduce the level and odor of the chlorine municipalities use to treat water, and some tackle bacteria that can cause illness. If you don't need a filter that multitasks every which way - these tend to be on the high end of expensive - shop for a filter designed for your specific water issue.

Supply demands: "Clear water" iron (not yet rusted or oxidized, that is) can't be removed by a filter; a water-softening system is needed. To remove lead caused by water passing through older pipes, an under-the-sink or whole-house filter at the water-supply source is recommended instead of a faucet filter.

Be sure to ask: Can my current faucet accommodate a filter, or do I need a new one? If a new faucet is in order, ask about models with built-in filters.

Installation extras: Find out how complicated it will be to install the filtration system you choose. Something simple will mean you won't have the expense of hiring a plumber, which is likely why most consumers go for the screw-on variety of faucet filters. Under-the-sink filters involve only minor plumbing - unless you have old pipes and not much room to work with - but they also can be used to supply filtered water to the icemaker in your refrigerator.

An ounce of prevention: Some faucet filters reduce the clearance you have to fill pots. Calculate how much clearance you need before you shop for a filtration system.

What will it cost? Faucet filtration systems are priced from $30 to $300, depending on what the filter is being asked to do. But you should also factor in how often the filter itself will need to be replaced and the cost of replacement cartridges. Costly cartridges may make a reasonably priced filtration system more expensive in the long run than a system with a higher price tag.

But replace you must: A clogged filter will just start dumping contaminants back into the water. It's time to change the filter when what comes out of the tap starts to taste or look funny again. Whether you have a water filter or not, it's recommended that you always let water run for several seconds before you use it.

Want Alan J. Heavens' advice on a home-improvement project or purchase? E-mail him at aheavens@phillynews.com or write to him at The Inquirer, Box 8263, Philadelphia 19101.