Water Purification

Four kinds of germs

Clear, cold water from a northern lake or a spring can still make you sick. The CDC sorts the risks into four groups ¹:

• Bacteria: E. coli, Salmonella, Shigella, Campylobacter.
• Viruses: norovirus, hepatitis A, enteroviruses, rotavirus. The smallest.
• Giardia: a protozoan parasite with a tough cyst stage.
• Cryptosporidium: a protozoan with an even tougher oocyst shell that shrugs off chlorine.

No single method except boiling handles all four in every case. Most treatment choices come down to knowing which method misses which group ¹.

Boil

Boiling is the most reliable way to kill germs: bring water to a rolling boil for 1 minute, or 3 minutes above 6,500 feet ¹. Nowhere in Wisconsin is close to that elevation, so 1 minute is enough. Older guidance, including the Army survival manual, adds a minute per 300 meters of elevation or says to boil 10 minutes anywhere ²; the CDC’s shorter times are current.

Boiling costs fuel and time and leaves the water hot, but it works on cloudy water too.

Filters and purifiers

Filters strain germs out ¹:

• Pores of 0.3 micron or smaller remove bacteria.
• A filter certified for “cyst” and “oocyst” reduction removes Giardia and Cryptosporidium.
• Filters don’t remove viruses.

Purifiers are devices rated to remove or kill viruses too, and handle all four groups ¹.

Cloudy water: filter it first. If you can’t boil, the CDC’s next-best option is a filter, especially for cloudy water, followed by another treatment ¹. Even a bandanna or coffee filter as a pre-filter keeps silt from clogging the real filter and from shielding germs from chemicals and UV.

Chemicals and UV

• Chlorine and iodine kill bacteria and viruses, are only somewhat effective against Giardia, and don’t kill Cryptosporidium ¹.
• Chlorine dioxide kills bacteria, viruses, and Giardia and is somewhat effective against Cryptosporidium ¹.
• UV light works on all four, but only in clear water ¹.

Follow the product label, including the contact time ¹. Chemicals need time to work; drinking right after dropping in a tablet defeats the point.

Pairing methods

Since each method has a gap, combine two that cover each other ¹:

• Certified filter + chlorine dioxide: the filter catches the parasites, the chlorine dioxide handles viruses. Effective on all four ¹.
• Pre-filter + boil, or pre-filter + UV for clear results.

Rainwater collected in clean containers is usually safe, but water from lakes, ponds, springs, and streams should be treated, especially near human settlements ².

Try it

Take your filter on a day paddle and treat lake water three ways: a filter alone, a filter plus chlorine dioxide tablets (read the contact time on the package and set a timer), and a pot boiled for one minute on a stove. Note how long each takes, how much fuel the boil uses, and how a cloth pre-filter changes the filter’s flow on murky water.

What to take from this

Treat lake, river, and spring water. Boiling is most reliable: 1 minute, or 3 above 6,500 feet. Filters (0.3 micron or smaller, cyst-certified) remove bacteria and parasites but not viruses; purifiers handle viruses too. Chlorine and iodine miss Cryptosporidium; chlorine dioxide is better; UV needs clear water. Filter cloudy water first, follow contact times, and pair a filter with a disinfectant to cover all four groups.