UV water treatment: what a portable UV device can and cannot do

Portable UV devices can kill parasites, bacteria and viruses in small amounts of clear water, according to CDC. They are a disinfection step, not a filter, and they work only where the light reaches the water. Cloudy water blocks that light, so a UV unit needs clear water, a charged battery and the device's own instructions. UV does not remove chemicals.

What CDC says a UV unit kills

CDC's travel page covers UV in its own section, separate from the chemical disinfectants. It says portable units that deliver ultraviolet light can kill parasites, bacteria and viruses in small amounts of clear water. That claim has two limits built into it. The first is volume: the wording is small amounts. The second is clarity: the wording is clear water.

CDC does not give a dose, a treatment time or a volume for any device. Those come from the device's own instructions, which this page does not replace.

Clear water first

CDC says to filter water before using UV light to disinfect it, and that UV light does not work well in cloudy water because small particles can block germs from the light. The emergency page adds that, if the water is cloudy, you should first filter it through a clean cloth, paper towel or coffee filter, or let it settle and then draw off the clear water. The cloudy water guide covers that sequence in full.

Clear water is the condition the device needs, not proof that germs are absent. CDC says water containing germs sometimes looks clean, so the clear-water step and the device's own instructions both matter. EPA's advice is to consider how the water looks, and to filter it if needed, before treatment. The water decisions tool shows the clarity step before the UV step when cloudiness is set to yes.

Dose, volume and the device's instructions

A UV device is made for a stated volume of water and a stated treatment time, and the instructions set both. CDC says to follow the instructions on any product used to treat water, so the same rule applies here. Do not treat more water, or treat it in less time, than the instructions allow.

Store treated water in clean containers with covers. EPA gives that advice for boiled and disinfected water.

What to check in a device's instructions

The instructions set the limits that CDC leaves to each product. Check these before the trip, not at the water's edge.

• The volume the device is rated to treat in one cycle, and whether that rating assumes clear water.
• The treatment time for that volume, and any stated minimum before the cycle counts as complete.
• Whether the device expects a pre-treatment step, such as filtering or settling, before the UV step.
• How the device is powered, how much charge one cycle uses, and any stated limits in cold conditions.
• How the device shows that a cycle finished or failed, and whether the water must be stirred or the bottle moved during the cycle.
• The operating temperature range, which matters on cold mornings when both the water and the battery are near freezing.

Power, batteries and cold

A UV unit is only as available as its battery charge. The power budget tool converts a battery's capacity into usable energy, using the efficiency it assumes by default. That conversion is for the battery, not the device. The device's own power draw and its treatment time decide how much water one charge covers, and the device's instructions give those figures. Litres per charge cannot be worked out without the device's rated draw and cycle time, so check those figures before relying on one battery.

The worked example below uses a 2,600 mAh battery at 3.7 V, which is the tool's default cell voltage. The efficiency is the tool's assumed 80% end-to-end figure, not a measured one for any product.

usable Wh = mAh ÷ 1,000 × V × efficiency
2,600 mAh:  2.6 × 3.7 × 0.80 = 7.70 Wh

Cold can reduce what a battery delivers, so a figure taken in a warm room may overstate what is available on a cold morning. The cold-weather batteries guide covers the cold side of that limit, and the phone charges guide is the related reading for phone-sized loads.

Recontamination and a backup method

Treated water can be contaminated again by dirty containers or hands. CDC's travel page describes no protection that lasts after UV treatment. Keep the treated water in clean containers with covers, as EPA advises, and avoid pouring it back into a vessel that has held untreated water.

A UV unit depends on a battery and on clear water, so carry a backup method. CDC lists boiling first among the options, and filtering followed by disinfection second. A stove and pot are one backup, and the boil guide sets out the timing at each elevation.

What UV leaves in the water

UV is a disinfection step. CDC says boiling or disinfecting does not make water containing harmful chemicals, toxins or radioactive material usable, and EPA says disinfection does not destroy heavy metals, salts or most other chemicals. A UV device does not remove these either, so a source with a chemical concern needs a different source. The water decisions tool returns "choose a different water source" when a chemical hazard is unknown or suspected.

For the other portable options, the filter guide sets out what a filter removes, and the chemical guide covers the disinfectants that work on the same germs.