Using compression/expansion (pressure differential).

or, similar to the hot side of an air conditioning unit.

No fire needed, just the ability to compress the liquid.

Seems I was inspired by an old junkyard wars tv episode.

Just something to consider.

yeah, it's the "pressure-temperature" curve. One of the reasons people use Pressure Cookers: boil at lower temps.

How this will help in real life, I have no idea.

Could one put water in a pressure cooker, set that on the fire for a while, and have "safe" water???

I think the pertinent fact is the water would still need to boil. The only thing increasing pressure does is increase the temperature at which water boils. NOTE: I probably mixed this up in my original post. This means that the pressure cooker would raise the boiling point (thus allowing cooking at a higher pressure WITHOUT boiling water off).

"Maximum vaporization occurs at the boiling point, 100°C (212 °F) at 1 atmosphere. Conversely, the vapor pressure of water, that is, the pressure of the vapor in equilibrium with the liquid, is 760 mm Hg at 100 °C. If the atmospheric pressure is reduced to say, 525.8 mm Hg, as would be the case if the same kettle is boiled on the top of a mountain, the boiling point will be reduced by several degrees to 90 °C (194 °F). The reason: at this temperature the vapor pressure of water is 525.8 mm Hg."

http://www.unesco.org/webworld/ramp/html/r8707e/r8707e06.htm

Sorry about the earlier typo: I wasn't reading as thoroughly as I should have (studying at the time). Notice also how little the boiling point changes, even with rather significant changes in pressure.

Sure, if you reduce the pressure enough, water will boil at room temperature. But if you are talking about cooking or purifying water, it won't help. It is the temperature that matters for cooking or purifying (killing bacteria). The actual boiling or vaporization point of water has nothing to do with it. On the opposite end of the scale, raising pressure raises the temperature at which water boils, which is what a pressure cooker does. At sea level pressure, you can't get water hotter than 212 degrees farenheit, but by increasing the pressure, you can get water much hotter which is why a pressure cooker can cook foods faster, or break down foods that need a higher temperature than the normal boiling point of water.

No, not lowering the pressure, but raising it, dramatically. If you raise the pressure on a container 2/3 full of water, it should eventually reach temps above 100 C (212 F). Think like with a fire piston.

Water is incompressible so you can't raise its temperature by compressing it. You could raise the temperature of air and transfer the heat to the water, but...

To raise the temperature of 1 gallon of water from room temp to boiling takes over 1000 BTU. That's 778,000 ft-lbs. So if you weigh 200 lbs, you can jump off a ~4000 ft cliff and land on the piston - boiling water!

(My apologies for the non-SI units)

Actually, it does NOT need to boil. It needs to reach around 200 degrees, which kills everything. If it were in a strong enough pressure vessel, it might not boil until 220 or so.

Boiling is just any easy way to see if it the water is pasteurized yet.

200F kills close to everything, but not quite. There are spore-forming bacteria (Bacillus and Clostridium species) that are heat resistant and can withstand 200F and still be viable. Our steam autoclaves operate at 250F under 15 psi, at this temperature and pressure, sterilization is usually a 15 minute process, although depending upon the type of load and how it is packed into the autoclave, sterilization times may be extended to as much as 1 hour.

Pete

I think they were more referring to the 212 degree heat thing as being destructive to bacteria/viruses that are most often considered harmful to humans and take residence in water.

The ones you listed are mostly related to food poisoning (of all things) and are more of a reason to cook your food thoroughly than in relation of trying to purify water. Here's a list (per wikipedia)

Bacillus anthracis is a Gram-positive, facultatively anaerobic, rod-shaped bacterium of the genus Bacillus. An endospore forming bacterium, B. anthracis is a natural soil-dwelling organism, as well as the causative agent of anthrax.

Bacillus cereus is an endemic, soil-dwelling, Gram-positive, rod shaped, beta hemolytic bacteria that causes foodborne illness.

Bacillus coagulans is a species of lactic acid forming Bacillus bacteria, which can contaminate canned food and gives it a flat sour taste.

Clostridium includes common free-living bacteria as well as important pathogens.[2] There are four main species responsible for disease in humans:

C. botulinum, an organism producing a toxin in food that causes botulism.[3]
C. difficile, which can overgrow other bacteria in the gut during antibiotic therapy, can cause pseudomembranous colitis.[4]
C. perfringens, causes a wide range of symptoms, from food poisoning to gas gangrene. Also responsible for enterotoxemia (also known as "overeating disease" or "pulpy kidney disease") in sheep and goats.[5]
C. tetani, the causative organism of tetanus (lockjaw).[6]

Just trying to make sure this doesn't cause confusion for some.
Good info nonetheless