Cloudy water: let it settle or filter it before any treatment step

Let cloudy water settle, or strain it through a clean cloth, paper towel or coffee filter, and draw off the clear water before you boil or disinfect it. CDC and EPA both give those two clearing options. Cloudiness matters because it changes how well the next treatment step works, and clear-looking water can still carry germs.

Why cloudiness changes the next step

CDC says UV light does not work well in cloudy water, because small particles can block germs from the light. EPA says disinfection does not work as well when water is cloudy or coloured. Those are the two sources' own points, and they run in the same direction: the clearing step comes before any treatment that depends on light or on a chemical reaching the germs. CDC's emergency page makes the same point about solar disinfection, which also does not work well in cloudy water.

The table sets out each treatment against cloudiness, using the wording of CDC and EPA guidance.

A prefilter is one way to protect the main filter from clogging. The manufacturer's guidance decides how a prefilter is fitted and cleaned, so follow that rather than improvising one. EPA advises considering how the water looks, and how to filter it if needed, before treatment.

Settling and straining, step by step

CDC's emergency guidance gives the sequence for cloudy water, and the steps below follow it. EPA's options for the same step are the same two, cloth or settling.

1. Pick the clearest water you can reach, from the clearest part of the source. Where you have a choice, EPA says flowing water is generally better than still, stagnant water.
2. Either strain the water through a clean cloth, paper towel or coffee filter, or let it settle until the particles drop out. CDC and EPA give no settling time.
3. Draw off the clear water from the top, and leave the sediment behind.
4. Treat the clear water by boiling it, by disinfecting it, or by filtering it and then disinfecting the filtered water. A UV device is used on clear water only.
5. Store the treated water in clean containers with covers, as EPA advises for settled and filtered water.

1  Settle the water, or strain it through clean cloth, paper towel or coffee filter
2  Draw off the clear water from the top, leaving the sediment
3  If it still looks cloudy, clear it again before the UV pass
4  Treat the clear water with the UV device, following its instructions

Boiling cloudy water

Once the water is clear, the boil follows the same rule as any other water. CDC says to bring clear water to a rolling boil for 1 minute, or for 3 minutes above 6,500 ft (1,981 m). Clearing the water does not change that time, and the elevation decides which figure applies. The boil guide sets out the elevation table and the EPA threshold, which is lower.

Choosing where to collect water

Collect from the clearest part of the source you can reach. EPA says to avoid water with floating material in it, or with a dark colour or a questionable odour. EPA advises against using water with those signs, so they are a reason to look for another source before treatment is considered.

Clarity at the collection point does not change the source question. CDC says that drinking straight from a clear mountain stream is generally not safe, and it notes that water with germs sometimes looks clean. A stream that looks clean may still need the full treatment sequence.

Clarity is not a germ test

CDC says water with germs that can make you sick, including bacteria, viruses and parasites, sometimes looks clean. A clear result after settling is therefore a step in the process, not evidence that the water needs no treatment. The guidance deals with germs through the treatment itself: a rolling boil for the time the altitude requires, a disinfectant used as its label says, or a UV pass on clear water.

EPA's point about cloudiness is about the treatment, not a test: disinfection works less well when the water is cloudy. Clearing the water is therefore a step before treatment, not a result that replaces it. EPA's doubling rule for bleach covers cloudy, coloured or very cold water, and the chemical guide sets out the figures.

What clearing leaves in the water

Settling and straining change how the water looks. They do not show whether germs are present, and they do not remove dissolved chemicals, salts or fuel. EPA says boiling and disinfection do not destroy heavy metals, salts or most other chemicals, and CDC says water containing harmful chemicals, toxins or radioactive material cannot be made usable by boiling or disinfecting. A clear sample from a source with a chemical concern still needs a different source.

For the treatment step after clearing, the boil guide sets out the timing, the filter guide covers what a filter removes, the chemical guide covers bleach and iodine, and the UV guide covers the device limits.