I wanted to create a quick, rough guide to help people when deciding what water purification method they would need (or combination thereof) to ensure safe drinking water.

I came up with a quick chart to show the relative strengths and weakness of various methods. A couple of things to note:

This is a quantized analog to digital graph -- in other words, while in reality it's not this clear cut and there are almost always exceptions, it serves as a general "rule of thumb" guide.





The threat types generally increase in size from left to right, however they are not to scale and in reality these threats often overlap with regards to size. For instance, there are some bacteria smaller than the largest viruses, and there are some protozoa larger than crypto. Indeed, the largest bacteria are visible to the naked eye, but these are extreme cases.

Additionally, when I selected what method covers what threat, I realize that some filters may be able to filter out some of the larger viruses, so technically the coverage would dip a little bit into the virus category. Further, some filters add chemical means as a stage of filtration to kill viruses in which case technically it would fully cover the virus category -- but these are exceptions rather than the rule, so to err on the side of safety we assume filters do not cover viruses.


My intention is not to spread disinformation, so I present this here for review to see if you agree that it is generally correct.

Is there anything I need to change? Should I add any treatment methods such as UV treatment and distillation, or threat types such as heavy metals? One of the problems with adding things like UV filtration is that it is less effective the more turbid the water which this graph doesn't go into.

Regards

Your graph is generally correct, but the trick is going from the graph to a specific unit or procedure that will achieve safe drinking water. You probably should include UV filtration, which is becoming popular with wilderness users. Heavy metals could be a consideration in some urban situations, but are not much of a hazard in wilderness.

Selecting a treatment method is actually a fairly complex process, and many factors should be considered in making a decision. Your graph is a good beginning, but only a beginning.

Nice job. How about including Alum (aluminum sulfate) treatment as well.

Does it make sense to add a column for chemical contamination?

There's also reverse-osmosis, which AFAIK filters all of those.

aluminum sulfate is a flocculant IIRC, I know it would help with some of the smaller particulate matter but will it kill any nasties?


Thinking of adding the following

Methods:
-Distillation
-Reverse Osmosis
-UV (Incl SODIS)
-Chemical Treatment (Bleach)


Threats:
-Seawater
-Chemically Contaminated Water


So now we're getting into territory I know little about, so it's good to have all of your inputs. What will cover what?

UV for instance -- I thought it handled viruses and bacteria no problem but had trouble penetrating the oocysts. Turns out, it seems to be the opposite: bacteria and oocysts like crypto it does well, but viruses take a much higher dose.

Also chemical contamination can include such a wide range of things, is this chart too general to include it?

Will things like distillation generally work sufficiently for almost all chemical contaminators that it's a good idea to include, or are there enough exceptions that it would be risky? Should we have several types of chemical contaminators on the list (ie, contaminators with a boiling point below 212 and contaminators with a boiling point above 212, or oil based vs. non oil based, etc)

By the way, if anyone can think of a better way to organize the information I'm all for it, I just wanted to make it rule-of-thumb useful for your average new person to see a chart and think to themselves "Oh, I thought all I needed was this filter but it turns out I need to either chemically treat or boil my filtered water as well..."

Very useful for my work, please do add UV.

Here in Appalachia, one has to be cautious of their water sources. Coal mining, before it was well regulated, released several toxins and heavy metals from within the earth, out into the water ways. Most streams and rivers in my area contain certain amounts of lead, mercury, arsenic, cadmium, selenium, and hexavalent chromium, even 40 years later.

Both natural springs, and rain are plentiful here in KY, so finding treatable water isn't a problem. One just has be be aware of their water source.

In response to your question: It is your project so do what you think is best. I do believe an addition or two could benefit others, if you are up to it, though. Chemical contamination is a widespread phenomenon in North America. Your prerogative, Bro.

Chemical contamination is a complex issue.

RO will filter some chemicals, such as salts, but not others such as alcohols. Further, some chemical contaminates can degrade the RO membrane.

Distillation is better way to remove chemicals, however, any chemicals with a lower boiling point than water will boil off first and thus must be discarded before collecting the water. Without test equipment it is difficult to know when the contaminates have stopped boiling off.

Conversely, after the water has boiled off, then you may start getting other chemical contaminates boiling off. Again it is difficult to know when to stop collecting the water.

You could just discard the first and last ten percent of the starting quantity of contaminated water. This would eliminate most of the chemical contaminates. However, if you have very little water to begin with, then loosing 20 percent via this method may not be the best solution.

In a survival situation water is the number one necessity, so one may have to take the risk of unknown chemical contamination rather than die of dehydration.

Your nose can help in determining chemical contamination. If it smells like chemicals then you should avoid it. However, if it doesn't smell like chemicals, it doesn't mean that it isn't contaminated.

Your sense of taste can also help in determining some chemicals. Of course, you run the risk of getting either chemical posioning and/or biological infection just by tasting untreated water (even if you spit it out). You could treat for biological contamination first, then taste it.

As, I said, chemical contamination is a complex issue.

Chemical contaminates are going to be a concern in any populated area.

If available, rainwater is the best source of water--provided you collect it directly.

In high mountain streams, you mostly have to just deal with biological contaminates. This is why backpacking purification systems don't generally treat water for chemical contaminates. It is also why they will probably not be sufficient in an urban situation.

I agree with what others have said about UV, it should definitely be included; you may also want to include different levels of filtration.

It would likely also be handy to define the type of situation the chart is intended to be used for (e.g., larger scale urban disaster, backcountry use, etc) ... this might also help determine if you should include things like chemical contamination in your chart.

In the past, I've found the following chart helpful:



Source: MEC