Learn the order of steps for making water safe when the taps fail: clearing, filtering, boiling and chemical treatment, and what each one removes.
- Use stored or bottled water first, then clear, filter and disinfect untreated water.
- Boiling and bleach kill germs but do not remove chemicals.
- Always follow local advisories and the product label.
When a storm or outage knocks out your water supply, or a boil-water advisory is issued, you may have to make questionable water safe yourself. The good news is that the methods are simple. The tricky part is knowing which method deals with which problem, and in what order to use them.
This guide explains the main options, what each one does and does not remove, and a sensible order of steps. It follows the general approach of Ready.gov and the US EPA, and it is not a substitute for any notice from your local health authority.
Start with the safest water you already have
Before you treat anything, use water that is already safe. That means your stored drinking water, sealed commercial bottled water, and water from a tap that your local authority says is fine. If you have not stored any yet, see how much water to store and plan for about 4 litres (1 gallon) per person per day for drinking and basic hygiene.
Only treat untreated sources, such as a lake, river, rain barrel or pond, when you have no better option. Water from these sources can carry germs and chemical contamination, and treatment does not fix every problem.
Step 1: Clear cloudy water first
Cloudy water makes every later step less effective, especially chemical treatment. Let the water sit until the particles settle, then pour the clearer water off the top through a clean cloth, paper towel or coffee filter into a clean container. This does not make the water safe to drink. It only prepares it for the steps below.
Step 2: Choose a disinfection method
You generally have three routes, and you can combine them.
- Boiling. Bring the water to a rolling boil for at least 1 minute. Above about 2,000 metres (6,500 feet) elevation, official guidance is to boil for 3 minutes. Let it cool on its own and store it in a clean, covered container. Boiling kills germs such as bacteria, viruses and parasites.
- Chemical treatment. Plain unscented household bleach or purification tablets can disinfect clear water. Details are in our guide on treating water with bleach or tablets.
- Filtering. A portable filter can remove many germs and particles, depending on its design. Check the product label for exactly what it is rated to remove. Our emergency water filter guide explains how to read those ratings.
What each method removes
No single method handles everything, which is why the label and the source both matter.
- Boiling deals with living germs. It does not remove chemicals, fuel, heavy metals or salt. Boiling water that has chemical contamination can concentrate some of it.
- Bleach and tablets deal with many germs when used correctly on clear water. Some hardy parasites may survive standard doses, which is one reason filtering first is often suggested. They do not remove chemicals.
- Filters vary a great deal. Some remove bacteria and parasites but not viruses. Some also reduce certain chemicals through carbon or other media. Only the manufacturer's tested claims tell you what a given filter does.
The EPA suggests that if you are filtering, a filter with an absolute pore size of 1 micron or smaller is a reasonable starting point for removing some parasites, and that you may still need to disinfect afterwards.
A sensible order of steps
- Use stored or commercial bottled water first if you have it.
- If you must use an untreated source, let it settle and strain out the visible particles.
- If you have a filter, filter the water according to its instructions.
- Disinfect by boiling, or by chemical treatment if you cannot boil.
- Store treated water in clean, covered containers and label it with the date.
Keep treated water safe
Use clean containers with tight lids, and keep your hands and utensils clean when you pour. Treated water that sits in a dirty bucket becomes unsafe again. Also use treated or bottled water for brushing teeth, washing dishes, making ice and preparing food, not only for drinking.
If you do not have a way to heat water during an outage, read how to cook without power for safe options. Build your supplies ahead of time using our water hub, and add water to your 72-hour checklist.
Quick answers
How long should I boil water?
Bring it to a rolling boil for at least 1 minute, or 3 minutes at elevations above about 2,000 metres (6,500 feet). Let it cool before use.
Is a filter enough on its own?
It depends on the filter. Many do not remove viruses, so check the label and consider disinfecting afterwards.
Can I treat water that smells of fuel or chemicals?
No. Home treatment does not remove those contaminants. Use bottled water until officials say the supply is safe.