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At the moment, there are five main types of water��treatment devices, all of which we recently tested at MSR’s water lab in the company’s��headquarters in Seattle, Washington. This week,��we break down��the��technologies used in each. Next, we’ll follow up with reviews.

Also, before we move on, a��quick note about protozoa, bacteria, and viruses—the bugs these devices are designed to catch. Protozoa are the largest and easiest to filter out, but they’re tough to kill with chemicals. Bacteria are smaller than protozoa, but most of the devices on this list can still handle them. Viruses, like norovirus, are the smallest of the bunch and can sneak through most of the filters on this list. But fear not:��Viruses are mostly a concern if you’re traveling internationally. (If you are leaving the country, check out the MSR Guardian below, under Ultrapurifier, or the chemical treatments.)

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Examples: , ,

How It��Works: Chemicals kill bacteria, viruses, and sometimes protozoa. (Note:��Chemical treatments, including the ones above, are tested and controlled by the EPA. The other devices we tested��are not.)

Strengths: Ease of use and portability

Weaknesses:��

• There’s a finite amount of bug-killing agent. If you don’t use enough, there might be leftover bugs. And because the��chemicals attack everything in their path, including bits of leaf debris or pine needles, they get used up quickly in dirty water.��
• They’ll make you sick if you use too much, and it’s not safe to consume large amounts for an extended period of time.��
• Protozoa—particularly cryptosporidium,��which is often present in U.S.��waterways—are highly resistant to chemical treatments. Some chemical treatments, like Potable Aqua Chlorine Dioxide, can kill cryptosporidium��but only after a four-hour wait. For comparison, it takes just 30 minutes for the pills to kill viruses and bacteria.

Bottom Line: Travel with chemical pills as a backup,��or use them in conjunction with other types of water��treatment devices. They’re also great when you’re traveling somewhere with virus-infested water.

Ultraviolet

Examples: , SteriPen ��or

How It��Works: These devices use ultraviolet rays to kill bugs.��

Strengths: UV light is powerful��and will kill bacteria, protozoa, and even viruses if used correctly.

Weaknesses:��

• To be effective, the UV rays need to hit the bacteria, virus, or protozoa head-on. This means UV filters don’t work well in silty water.��
• There’s potential for user error. You need to agitate the water you’re purifying so the rays hit the entire sample. This can be hard to do with some devices.

Bottom Line: Make sure you prefilter or use clear water with this treatment. Follow the directions closely.

Adsorptive

Examples: , ,

How It��Works: An adsorptive is like a tiny magnet that attracts and captures objects. As you push it through water, it grabs bugs, particulates, and chemicals,��leaving clean water behind.

Strengths: When used correctly, an adsorptive filter removes even the tiniest impurities because the magnetic sites attract bugs of all sizes. Adsorptive filters not only remove bacteria, protozoa, and viruses, but also clean out chemicals—think pesticides and fertilizers—which are tiny and difficult to catch with other filter systems.

Weaknesses:��

• There are a limited number of attractive sites in an adsorptive device, and they stop working once they’re saturated. There’s also no way to know when the magnet’s full.
• Silty��water is a problem because particulates fill the spots instead of bugs.��
• If you move water through an adsorptive filter too quickly, the magnet��won’t have enough time to grab the bugs.

Bottom Line: Because it removes chemicals, an adsorptive filter gets rid of bad tastes and odors associated with��pollutants. It will clear out all the bugs as well, but since you can’t tell when it’s full and stops working, this type of water��treatment device is probably best used in conjunction with another type of treatment device.

Microfilter

Examples: , , ,

How It��Works: A microfilter works a bit like a colander, but instead of catching pasta, it filters out protozoa and bacteria. ��

Strengths: The holes are small enough to catch bacteria and protozoa—but��not viruses. The filter will clog, but you can tell when that happens so you know to throw it away and replace it with a new one.��

Weaknesses:

• The holes are not small enough to get rid of viruses,��just bacteria and protozoa. If you’re in a place that has a lot of viruses,��you’ll want to back up your filtration with a chemical tablet or go for an ultrafilter (listed below).��

Bottom Line: Reliable and a great go-to for backpacking in the United��States.

Ultrapurifier

Example: MSR Guardian (available January��1, 2016)��

How It��Works: It uses the��same general principle as a microfilter, but the pores are roughly ten times smaller, so they also filter out viruses.

Strengths: It gets rid of protozoa, bacteria, and viruses, which makes the purifier��ideal for international travel. It also alerts you when the filter��needs to be replaced.��

Weaknesses:��

• It won’t catch really��small particles like chemicals or fertilizers.��
• It’s expensive at $350.

Bottom Line: If you have $350��and do a lot of international travel, this is what you want. Yes, it’s up to��three times more expensive than a normal filter (and��30 times more expensive than chemical tablets),��but it’s the safest, most foolproof��water purification��technology on the market. Anyone who’s had norovirus or giardia��will tell you that $350 is well worth that peace of mind.