kota's “memex”

pasteurization

A full boil (for any time) is orders of magnitude more reduction than you need and far more purity than the food you'll be eating anyway. In regular field use you can use a WAPI, a dead simple 4-10g device that uses oil in a tube to detect pasteurization. They've been around for decades and work perfectly without electricity, at any elevation, and in any conditions.

Alternatively, you can follow the boil 3 add 1 method. You can boil 3/4th of your water and then add in the final 4th which will still be hot enough to pasteurize, but will save you fuel and time. This ratio is chosen for worst case scenario (at freezing, 4000m). At sea level you can boil 5 add 2.

chemical

There are three main categories of chemical water treatment.

iodine

35 mins foregoing cryptosporidium

This is the classic "old school" chemical compound. It tastes pretty bad, fails to kill cryptosporidium, and can be harmful when used over long periods of time (or short periods of time if you have any sort of hypo-thyroid issues).

elemental chlorine (free available chlorine, bleach, etc)

40 mins foregoing cryptosporidium

This is probably what your local city water treatment uses. It works extremely well and extremely light. However, when used on water with bio matter (dirt, shit, etc) instead of pre-filtered water like your local water treatment plant is doing the effectiveness can be much lower and there can be harmful byproducts created such as trihalomethanes and haloacetic acids.

It also similarly is not completely effective at treating cryptosporidium especially if you get the dosage wrong which, in the field, you will get it wrong. Specifically, the natural variations in Ph and water temperature could call for 1-6x the normal dosage, so you would need thermometers, ph strips, and charts.

chlorine dioxide

4 hours
30-40 mins foregoing cryptosporidium
15 mins forgoing viruses like microfilters

Although is it similar in name this is not just "regular" free chlorine. Unlike regular chlorine it IS capable of treating cryptosporidiums at practical dosages and times. It's also up to 5 times faster at treating giardia AND unlike chlorine, which adds a nasty taste, chlorine dioxide will be more neutral and can even kill the taste of common algaes and odor producing phenolic compounds such as tannins (leaf taste).

It seems to be relatively safe, especially when compared to the other two chemical treatment methods. There can be some irritation in the throat and nose if consuming large quantities, but there doesn't seem to be any more serious side-effects. It's also not "so new that there's no data" many commercial water treatment plants use it instead of free chlorine or in addition to including Niagara falls which has been using it since 1944.

brands for backpacking

filtration

microfilters (0.1 microns)

A water filter is only rated to handle bacteria and cysts, but will not generally work against viruses.

The standard is generally 6 log for bacteria and 3 log for cysts.

life straw

The thing your family will give you if you're into hiking.

bacteria: 99.999999
cysts:    99.999

katadyn befree

Similar to the life straw based on some sleuthing, but notably they don't actually advertise having 5 log reduction of cysts and instead just say they "meet or exceed EPA standards" which is only 3 log. Log 5 is probably good enough for drinking out of the vast majority of backcountry streams and lakes, which will have probably less than 65 cysts per liter. However, drinking from an actual cattle pond can be MUCH worse at 14000 cysts per liter and you'll definitely want something stronger (7 log or better) (or to pasteurize) in such cases.

bacteria: 99.999999
cysts:    99.999

sawyer mini

They website only claims 6 log cysts removal, which is still better than the others, but they do have a lab report showing 7 log results. This is probably good enough to drink from shit filled cattle ponds and still have your chances of getting giardia be less than 1/1000.

bacteria: 99.999999
cysts:    99.99999

ultrafilters (0.01 microns)

In general the next class up of filters is "ultrafilters" which have 10 times denser filters and are able to tackle viruses.

These are generally heavier and bulkier than microfilters. Think of the old-school pump style filters. They're nice for car camping, but difficult to use for backpacking.

activated carbon

This is the method used in brita filters and many other common home water filters. Unlike microfilters and ultrafilters the method isn't just smaller holes, but rather it's using a material similar to charcoal which attracts particles in the water to a surface. Generally speaking the carbon itself, without added materials or filters, is able to capture very tiny particles and metals (including chlorine and iodine) which affect the taste of the water, but the much larger viruses, bacteria, and cysts will float on by.

improvised crushed charcoal and a cloth

In 2019 the Wilderness Medical Society published a paper saying that typically with improvised crushed charcoal and cloth based filters you could reduce bacteria and viruses by 50-85% and larger cysts by 99% which is actually quite a bit better than nothing.

The reason charcoal works so well is that it's rather wrinkly and has a shockingly large surface area of 2 - 5m²/g which is the same as a 6x9ft tarp.

activated charcoal / carbon

A much improved version can be produced by baking charcoal in a kiln at a high heat while blowing it out with either steam of chemicals. This helps remove impurities, but also greatly increases the surface area by making it even more porous: 3000m²/g.

It's quite good at removing the taste of chemical disinfectants such as iodine and chlorine. However, although it will often "capture" bacteria, viruses, and cysts, it will not "kill" them without additional materials or filtration.

This quirk is actually used in combination with more traditional microfilters to create an equivalent to ultrafilters which can remove all 3 microbes effectively.

sawyer s3 select water purifier

It's literally a Sawyer Micro Squeeze on top of a bottle which contains a foam + activated carbon material in it. You're meant to add dirty water to the bottle and filter it through the micro squeeze. I imagine it works well, but I don't really like the idea of having to keep this bottle around and the whole thing is very expensive.

aquamira frontier max

A modular and versatile inline filter which internally combines microfiltering with activated carbon to protect from all 3 classes of microbes as well as meeting EPA 42 which indicates that it removes chlorine and zinc taste from the water.