How Water Filters Work

By: Amy Hunter
Sure -- splash water on your face. But don't drink it before you filter it.
George Doyle/Getty Images

When you're wading in a cool mountain stream, and you can see your to­es in the sand, you assume the water's safe to drink. But even water that looks perfectly clear may harbor a variety of bacteria, parasites and protozoa that can make you very ill. You can't tell from smelling or taking a small taste whether water is safe to drink. Even water from spigots in campgrounds may not be. When you're camping, unless there's a sign that specifically states that water is safe to drink, you should take precaution and treat it.

Portable water filters are probably the easiest and safest way to treat water for drinking. There are many choices in water filters. The least expensive may cost less than $20, while more expensive filters can cost several hundred. For the average user, it's possible to find a perfectly functional water filter for less than $100.


You should expect water from different areas to have its own unique flavor, but the process of filtering the water doesn't make it taste fun­ny. Water filters work by capturing the microscopic life that lives in abundance in freshwater.

The consequences of drinking untreated water can be severe. Water that tastes fine can contain bacteria, parasites and protozoa that cause nausea, diarrhea, loss of appetite, fatigue and vomiting. Some of the diseases spread through a contaminated water supply are botulism, cholera and dysentery. Parasitic infestations from contaminated water can lead to the development of rashes, muscle aches, fever, chills, coughs, neurological symptoms, jaundice and malnutrition. While most illnesses from drinking contaminated water cause discomfort for a few days or weeks, others can be deadly.


Water Filter Basics

The communities of bugs that live in fresh water supplies include giardiasis and cryptos­poridium. The purpose of the water filter is not to kill the creatures, but to capture them inside the filter and prevent you from ingesting them. The effectiveness of the water filter is determined by what is known as the pore-size efficiency. This is the measurement of the size of the openings in the water filter. These measurements are microscopic.

The measurement used to describe the size of the filter's openings is called a micron. One micron is 1/1,000 of a millimeter. Any water filter with a micron size of one or less will remove parasitic eggs and larvae from the water as well as protozoa. To remove bacteria, the micron size must be less than 0.4 microns.


Regardless of the type of water filter, they all work in the same basic way. An intake hose is used to draw water into the filter. If you have a filter inside your water bottle, you'll fill the water bottle up first and place the filter inside. If you have a standalone filter, you can scoop water into a pail or put the intake hose directly into the water source.

Once inside the intake hose, the water is pressed through the filter, either manually, for a freestanding filter, or through suction, with a water bottle filter. The filter traps any microorganisms that may be living in the water -- and, like we said, it doesn't kill them. Once the debris is captured in the filter, clean water passes through and is ready for drinking. The area where the clean water exits the filter is called the filter outlet.


Choosing a Water Filter

water filter
Campers wash dishes in the water. 
David De Lossy/Getty Images

The style of water filter that you choose depends on your personal needs. Many people favor water bottles with the filter built in. Others choose a small hand filter that allows them to purify water as they pull it from the water source and dispense it into separate containers. If you have a water pack system that fits into your backpack, it's possible to purchase a water filter system that coordinates with it.

How you decide what water filtering system will work best for your needs depends on a variety of factors. If you travel with youngsters and plan on filtering their water, you'll probably want a separate hand filter. Many hikers like to add powdered drink mixes to their water. If you're one of these hikers, you probably don't want a water bottle filter. This way you can have separate drinking containers, one with your drink mix and the other with plain water. If you add drink mix to a water bottle filter, you won't be able to drink or filter any water until the bottle is empty.


If you're bacteria, protozoa and parasites should be more than adequate. If you're headed for a Third World country or an area where the water may be exposed to sewage, your filter also needs to kill viruses.

Water filters that are equipped to kill viruses most often contain an iodine filtering system. Like iodine chemical tablets (which we'll learn more about later), this can adversely affect the flavor of your water. If you want the insurance of a filter that kills viruses, take along some ascorbic acid -- even the granulated orange flavored powders available at grocery stores will work. After filtering the water through the iodine system, add some of the powder. Ascorbic acid neutralizes the iodine, improving the taste. It also neutralizes the effectiveness of the iodine, so make sure that you follow the manufacturer's instructions for how long to let the water set in the iodine filter before adding anything else to the water.

Regardless of what type of water filter you choose, you don't want a situation where you are surrounded with water but have nothing to drink. A flow rate of one liter per minute is a good average to shoot for when shopping for a water filter.


Caring for Your Water Filter

The very design of the water filter means that it will eventually clog. Remember, it's not killing those creepy crawlies in the water; it's trapping them in the filter, where they remain. The first sign that your filter may need some maintenance is that it becomes difficult to pump. Don't force the issue. If you try to force the water through the pump, you may wind up with microscopic bugs in your water supply.

To get the longest life out of your water filter, it's important to follow the manufacturer's instructions on care and maintenance. But there are some general tips to keep in mind. You can count on filtering about 100 gallons of water before your water filter needs to be changed. Proper care and a little preventative maintenance will go a long way in reaching this goal.


You can scrub some water filters to clean them. If your filter is one of these, scrub it gently with a toothbrush when it becomes difficult to pump. If your water filter cannot be scrubbed, it may still be possible to clean it. Some water filters can be immersed in clean water and rinsed gently. Of course, this is something that must be done at home with clean tap water.

If your water filter comes with a pre-filter, be sure to use it. A pre-filter captures some of the larger debris before it enters the main filter. If your water filter doesn't have a pre-filter, and the water around you has a lot of sediment, make your own, using a coffee filter or clean shirt.

Some filters can be backwashed. To backwash your filter, remove the intake hose and reattach it to the filter outlet. When you pump water with the hose attached this way, it will send clean water through the filter, loosening debris that may be clogging it up. It's important to sanitize the filter with a solution of one capful of bleach to one quart of water before using it again. Pump the solution through the filter and let it dry thoroughly before storing it away or using it.


Water Filter Troubleshooting

A young man shaves at an outdoor water source in Italy.
Wilfried Krecichwost/Getty Images

One of the best ways to increase the life of your water filter is to start with the cleanest water that you can find. While a babbling brook may seem cleaner than a pool of standing water, this is not necessarily so. Flowing water stirs up dirt and sand on the bottom of the water, making it easy to pull that into the water filter. For the cleanest water, choose a pool of standing water, and don't let the hose touch the ground. If this isn't possible, dip some water into a pan and let it set for at least an hour before running it through your filter. This will give the sediment time to settle to the bottom.

Some of the most common problems with water filters are also easily preventable. It's important to be gentle with the filter. If you drop it, it may appear fine and continue to work, but the inside may develop small cracks. The microorganisms that you're trying to filter out can easily pass through these cracks without you knowing it -- until you get sick.


If you're camping in cold weather, it's important to remember what happens to water when it freezes. If your water filter freezes while it's damp, the water inside can expand, causing cracks and leaks to develop. Tuck the water filter inside your clothes during the day, and sleep with it at the bottom of your sleeping bag at night.

Finally, make it a habit to keep your water filter dry. When you're not using the water filter on your trip, carry it outside your backpack in a mesh bag. When you return from your trip, flush the filter with a weak bleach solution and let it dry thoroughly before packing it away. This will prevent bacteria from growing inside of the water filter.


Other Ways to Purify Water

Water filters aren't the only way to purify your drinking water. Boiling is one traditional method. But while boiling is highly effective, it does have its drawbacks. For one, it's time consuming to purify a significant amount of water this way. And, while warm water may be a treat when you're on a winter hike, it's certainly not when the weather is toasty. Even after allowing the water to cool, you'll be drinking tepid water.

Boiling also has a negative effect of the taste of the water. Some people describe the taste as "flat." You can reduce this somewhat by pouring the boiled water back and forth between two clean containers as it cools. This process aerates the water, improving the taste. Probably the biggest drawback of boiling is that you'll use a lot of fuel to boil enough water to keep you hydrated.


Chemical tablets are another option for purifying water. These tablets typically contain iodine. They do a good job of killing bacteria in the water and making it safe to drink, but they have a negative effect on the water's taste. Water treated with iodine is bitter and has a lingering aftertaste.

Also, while they're inexpensive, once the bottle of chemical tablets is open, it has a limited lifespan. Another drawback of chemical water treatment is that it doesn't always work against some types of protozoa. Finally, if the weather is particularly cold or the water is filled with sediment, the chemical tablets will take longer to work.

The biggest drawback of chemical treatment is the adverse health effects it can have on some people. For people with thyroid disease, immunodeficiency and some other health concerns, chemical tablets containing iodine can be a serious threat. Also, if you're pregnant, it's important to speak with a doctor before using iodine for water purification.


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More Great Links

  • Getchell, Dave. "Water Filter Maintenance." Backpacker. March 1998. (February 3, 2009)
  • Hostetter, Kristin. "Water Filter First-Aid." Backpacker. December 1998 (February 3, 2009)
  • Hodgson, Michael. "Camping for Dummies." 2000.