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One of the surest ways of purifying water is distillation water is boiled and only things that evaporate at or below 100C are captured. This does lead to the potential issue of things like petroleum products with a lower boiling point, getting distilled into your collection chamber.

It would seem that if you pre-boil your water in an open container, any contaminates that boil at temps lower then 100C would get cleared.

When should I boil water before distilling it? How can I know how long it should pre-boil in an open container?

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    I'm voting to close this question as off-topic because it doesn't appear to be applicable to outdoor activities. – Ben Crowell Feb 10 '17 at 18:05
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    @BenCrowell The question is about purifying water. How is that not on topic? There are 45 questions on the tag. – James Jenkins Feb 10 '17 at 18:26
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    VTC as not an activity you should need to perform in the outdoors. The type of contaminates that would cause you to distill are hydrocarbons and heavy metals. Those are nasty. There are plenty of sources for drinking water that don't have those contaminates. They don't occur in a natural outdoor setting. If they do occur in your outdoor setting then go somewhere else. You should never pre-boil hydrocarbon in an open container as that is fire hazard. You should never be camping or hiking where water is contaminated with hydrocarbons let alone consider drinking it. – paparazzo Feb 10 '17 at 21:05
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    @BenCrowell I understand your objections and I sympathize but ultimately I feel like this question is on topic. I think it would be hard to find a situation where this would be required in real life which is why I sympathize. The underlying concern that you could still get contamination in a still has some merit though even if it was something I would never worry about. That is why I think it is on topic. – Erik Feb 11 '17 at 5:49
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    @paparazzi distillation is also used to remove salt so on a hypothetical desert island would be completely on topic. – Chris H Feb 12 '17 at 12:39
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I still think this is off topic but I don't agree with a posted answer.

If you are talking hydrocarbons they are pretty toxic to humans even in small concentrations. A spill minor compared to the size of the ocean will kill marine life.

What you are describing is simple batch distillation and I don't think you could get water contaminated with hydrocarbons safe enough to drink even in a lab with that process.

Heptane is right at the boiling point of water.

I have a degree in chemical engineering and have worked in refineries but I never explicitly worked in waste water. Water contaminated with hydrocarbon went through some pretty extensive processing to be discarded to public water systems. They use filtration, distillation (not simple - column), and even some enzymes. Some times they just apply enough heat and burn it. Some times they can run it through an existing plant and make finished product out of it. Drinking water is never finished product to my knowledge.

Find cleaner water. Water not contaminated with hydrocarbons is readily available. That kind of contamination is just not common in nature.

Heavy metals you could address but again that kind of contamination is rare in nature.

Batch distillation should work for remove salt or any solids but not like you could fabricate in the outdoors. Not sure for urine as there are some liquid contaminates.

Your depiction of pre boil is off. You plain boil off the lighter but you should collect it as it is combustible.

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    I agree with a lot of what you say. Depending on the contaminate you aren't going to have the tools in the woods to make it safe for drinking. – Erik Feb 12 '17 at 3:18
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    Another case in point is that of alcohol. Despite ethanol and water having quite different boiling points you can't get either pure by simple distillation. – Chris H Feb 12 '17 at 14:52
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    @ChrisH Alcohol is getting into another thing. And I would not say quite different. – paparazzo Feb 12 '17 at 15:16
  • I mean it's a well-known example of a hydrocarbon compound that can't be separated from water as easily as boiling points might suggest, which seems to be the point you're making – Chris H Feb 12 '17 at 17:25
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    @ChrisH But read your words - "Despite ethanol and water having quite different boiling points". I consider their boiling points to be pretty close in the overall scheme of things and I would not call alcohol a hydrocarbon. No need to get into azeotrope for this question. – paparazzo Feb 12 '17 at 17:41
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Distillation of water will not remove all hydrocarbons or chromium-6, without very sophisticated equipment. It can remove arsenic, salt, and other such nasty compounds. Source:

5 Things To Know About Chromium-6 In Drinking Water

In practice, large-scale water purification systems expect water to contain bacteria, parasites, viruses, sediments, chemicals, and VOC. In a typical setup, such as NYC, which processes 1.4 billion gallons a day, and which takes appx 8 hours for water to enter into, and exit, the filtration systems, rest assured that distillation is not a part of this process.

You can read about how NYC handles its water, and if you are so inclined, you can arrange for a visit to their facilities, which requires special precautions and arranging, and may not be suitable for everyone. Nevertheless, I worked there for several years and found the treatment processing fascinating. Here is the link:

New York City’s Wastewater Treatment System (1)

and

New York City's Wastewater Treatment System (2)

So you don't need (or want) distillation to remove the worst of the worst. Having said that, you can't just drink VOC-contaminated water, like you could drink disease-infected water, and just deal with the consequences later on. That's because the human body reacts violently to VOC-contaminated water. Here are some studies on the effects of drinking such contaminated water:

The Health Effects of Oil Contamination: A Compilation of Research

Since the bulk of questions here on this forum deal with personal outdoor issues, I'll relegate my answer to that kind of scenario.

There are many products on the market for treating water, and all involve either chemically treating the water, filtering the water, or just boiling the water - and none of them will remove VOC or chemical compounds like chromium and arsenic. Nor will they remove compounds which are solutions - like salt. VOCs are volatile organic compounds, like oil and benzene. If you are in a situation where you have to drink this water or go without, you'll have to decide that on your own, based on information such as nearest other water area, your tolerance, and other things.

But distillation is an impractical form of water purification for most individuals. The heat needed to distill it would kill off most common bacteria, virus, and parasites. What's left are the things which cannot easily be removed by distillation, leaving you with a conundrum best handled by not bothering to distill it at all. Unless you were dealing with salt water - then you have no choice but to distill. Just know that it wouldn't get rid of all VOCs.

I think, then, the best way to treat water without products like tablets and filtration systems, or distilling, is to make use of what is already in nature.

So you ask "when should you boil it before distillation", my answers is "all the time - but skip the distillation step". Since you mentioned a concern for petroleum (VOC) contaminant, you should use the soil - sand, if possible - to filter and absorb such contaminants. In other words, you think about mimicking what NYC does with their water, only on a smaller scale:

  1. Remove large sediments

  2. Let stand (that's what the big circular vats are doing to the water, only you don't have vats)

  3. Siphon off water about 3/4 from the top - that removes water between the floating oils and the sunken solids. Do this without disturbing the water as much as you can.

  4. Next up is to filter the water using sand - lots of it. This will absorb some of the VOCs present, though not all of it.

  5. Last, you can boil water to kill off microorganisms.

NYC and other municipal systems go through steps like aeration, ozonation, and chlorination, and these are impractical methods for most individuals.

If you were truly concerned about removing VOC, then you must use aeration with activated carbon, or use a reverse osmosis filtration method:

VOCs: Volatile Organic Chemicals in Private Drinking Water Wells

Note also there are no purification methods for radioactive contaminants in water, and there are many sites in the US with this problem:

Defenders of the Black Hills - Hesapa O'nakijin

Radioactively Contaminated Sites

By the way, there are some great articles on purification from the US military, although I haven't found much on oil-contamination:

Small Wastewater Treatment Systems Using Soil Purification Method

and also from the CDC (also little on oil contamination):

A Guide to Drinking Water Treatment and Sanitation for Backcountry & Travel Use

Emergency Disinfection of Drinking Water

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First and foremost try to find better water. If your water is so polluted that this is a serious concern then I strongly feel you should find less polluted water to purify. This is true regardless of what method you're using to purify the water. For an extreme example I wouldn't try to collect water to purify from a port-a-potty if there was a river a couple of miles away.

If for some reason the only water you have is extremely polluted then I'd follow these steps.

  1. Collect the water and let it stand for a period of time to allow particulates and oils to separate from the water.
  2. After skimming the oil from the top transfer the water without the particulates into another vessel.
  3. Set up your water vessel and heat source so you can vary the temperature of the water. The goal is to have a system in place so you can hold the water at a relatively constant temperature below boiling for a period of time. One primitive way of doing this would be with a tall tripod over a camp fire. That will allow you to adjust the temperature of the water by adjusting the distance the water vessel is above the flames.
  4. After holding the water vessel at a high temperature that is still below boiling for a long period of time you can be relatively certain that the substances that have a boiling point lower than water will have evaporated.
  5. Setup your water vapor collection system, and bring the water to a boil. Since we have boiled off most of the things with a lower boiling point all we should be collecting are things at or close to water's boiling point.
  • I don't agree you can make water contaminated with hydrocarbon safe for drinking with batch distillation even in a lab. – paparazzo Feb 11 '17 at 18:11
  • @Paparazzi I agree that the closer the contaminant's boiling point is to water the harder this will be. Overall the goal is to reduce the concentration of any contamination which this procedure will accomplish. Depending on the contaminate and your ability to control the temperature you will have varying degrees of success. – Erik Feb 11 '17 at 19:53
  • Reduce concentration is not good enough. The objective is safe drinking water. I will post an answer. – paparazzo Feb 11 '17 at 20:44

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