Purification of Water: Which Method To Use?

Clean, drinkable, temperature controlled water at the turn of a tap gets my vote for the greatest invention of human civilization, and the biggest driver of improvement in human health. If its delivery fails though, its critical that preppers have a good water purification method on hand. But there are so many choices! Which to choose?

No method of purification is perfect

First, don’t expect perfection. There’s a wide variety of ways water can be ‘bad’, and no one method reliably removes all of them. When we need ultrapure water (say, for laboratory work), we have to apply two or three purification methods to the same batch of water to get reliable results. One key then is to know which methods are good against which threats. What threats?

Microbes are the most notorious water contaminant, with good reason. Water that appears perfectly pristine, such as a burbling mountain stream, can and often does harbor microorganisms that cause disease. These include (in order of biggest to smallest particle size) protists, bacteria, viruses, and potentially even prions. Prions are the most recently discovered infective particles, and we’re not even sure they will transmit through water (they’re usually transmitted by being eaten or licked up).

Toxic molecules

Toxic molecules, usually organic molecules, are often man-made. Residues of gasoline, oils, lubricants, pesticides, herbicides, industrial chemicals, and who knows what else escape into the environment and get washed into the water supply. Many of them taste nasty, but some would be undetectable by taste.

Heavy metals and other ions are another concern. They can be residues from burned fuel, or industrial waste (mine tailings are notorious), or naturally occurring in some soils. Lead, mercury, and arsenic are real problems. There have been whole civilizations that were killed off by heavy metals just by moving onto the wrong patch of ground and drinking its water.

Big particles like silt are the most obvious but least troublesome of water contaminants.  I drank water somewhat gritty with rust at times as a kid, and it didn’t kill me. More importantly, these particles are easy to see, easy to remove (hankercheifs often work), and you can see when the job is accomplished. It’s not a bad idea to run any water that looks like it’s got particles through a cloth prefilter before getting serious about purification.

Podcast? You betcha, here it is:

Reverse Osmosis:  Good … for now

If you’re looking for purification of your home water supply in case your supplier isn’t providing good water, reverse osmosis might be a good choice.  The theory here is that pressurized water is forced through a membrane that has pores just big enough for the water.  Almost nothing else can flow through.  The only real risks are from certain small molecular toxins and failures due to flawed membranes.

From a prepping perspective, though, I’m not impressed. These systems need a lot of pressure to move the water; the membranes have a very limited life and require multiple prefiltration steps, and it’s very hard to tell if the membrane has failed. They also only yield about 10-15% of pure water, with 85% of what you started with leaving as waste — and potentially overloading a septic or sewer system.

For those of you on the coast without much fresh water at hand, reverse osmosis is a nice choice for desalination.  With the ocean right there, the low efficiency and water disposal would not be problems after all.

Distillation does a good job, but isn’t very mobile

I’ve done another post on distillation, since it does such a good job and can be a good prepper option, at least for a place you mean to stay put for a while.  You can find that post here.

Distillation turns the water into vapor (by boiling it), because only a few types of molecules can move into the vapor form with it in any quantity.  Then you cool the vapor and collect the purified water.  It takes a lot of energy to boil the water, but it’s pretty darned reliable and takes only low-tech gear; stuff one can often scrounge. Basically all microbes (and prions, which are hard to remove) and most of the heavy metals are gone.  Most molecular toxins are well removed; a small number are not.

Size-based Filters are easy to use but vary in ability

These filters run water through pores of various sizes. None of these are very effective against organic chemicals or heavy metals.  How good they are against microbes depends on their pore size. (1)

T-shirts and the like: remove particulates only.  The water will look better but won’t be much safer. Cloth is still useful as a prefilter, as the particulates quickly clog the better filters if not removed first.

Microfilters have a pore size of averaging 0.1 micron. Many light, portable filters are in this category (including Lifestraws and the portable Katadyn models). They’re good against cysts, protists and bacteria, but not viruses. Cholera? You’re protected. Rotavirus or hepatitis? No help.

Ultrafilters have pore sizes around 0.01 micron. They’re good against everything a microfilter is good against, plus do some good against viruses. Some portable filters fall in this category.

Nanofilters have teensy pores; averaging 0.001 micron. They’re effective against all microbes and can even remove some chemicals; but they’re not reliable against the latter. While Berkey didn’t want to be specific about pore size, it was specific about removing well over 99% of viruses, so it seems to fall in this category.

(By the way: I’m mentioning filters we like well enough to own. We have no financial relationship with any of these companies.)

So why not just go with the nano, to be sure? Finer filters clog easier, tend to flow slower, and are often easier to damage; so going finer than needed for conditions is a bad trade.

Carbon filters mostly absorb chemicals

Carbon filters do work as rough size filters (usually 0.5 microns or more). Their real strength is in absorbing many chemical compounds. They’re used as prefilters for reverse osmosis because chlorine ruins reverse osmosis membranes and carbon absorbs chlorine. Organic chemicals tend to be removed well by carbon. They don’t remove small microbes or heavy metals.

I consider carbon filters a useful prefilter for some uses, but I wouldn’t want to drink random water filtered only through carbon.

Chemical treatments and UV kill microbes

Bleach and water purification tablets fall in this category, as do the cute little UV-light emitters hikers can get to stir their water bottles. They kill the microbes nicely if used according to directions, but you’ll get no help with chemicals or metals. In fact, the chemicals add chemical taste. Some contain iodine, so are not suitable for all people.  Bleach is dead cheap by not very stable (more on that here). With UV sources, there’s always the question of power supply.

So what to pick?

First question: What are the hazards?  In most wilderness areas, microbes are the main risk.  I just use a virus-capable filter when I hike, with a tiny bottle of purification tablets in case that filter gets damaged. If I was bugging in in a city and having to draw run-off, I’d want to distill it because of the organic molecules and heavy metals.

Next question: What scenario are you prepping for? Filters, chemicals, and UV are most portable. Of those, UV is likely to have the shortest lifespan…except in winter.  When filters get wet then freeze, Bad Things tend to happen.

Salty and I keep a variety of options to suit different needs.

1) A Guide to Drinking Water Technologies for Home Use. 2014. Centers for Disease Control and Prevention. https://www.cdc.gov/healthywater/drinking/home-water-treatment/household_water_treatment.html

*By Øyvind Holmstad [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

**By Colby Fisher [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons