Sunday, April 22, 2012

Water Pasteurization

Humans can't survive very long without drinking water. In some cases, dehydration can kill you in a matter of hours. So the ability to find, collect, or in some way produce water is very high on the survival priority list.

But just having water is not good enough — the water must be sufficiently pure to keep from making you sick. Illness caused by drinking impure water can speed up the dehydration process as a result of vomiting and diarrhea, killing you even faster.

We've talked about water purification before, focusing on the most common traditional methods — boiling, filtering, and chemical treatment. But now we're going to talk about a totally different technique that is faster, easier, cheaper, and more effective than the other methods. The technique is called pasteurization. Yup, the same thing that is done to milk to ensure its safety for consumers.

Pasteurization is a process involving heat. But it's a much lower temperature than boiling. In fact, you can pasteurize drinking water at a temperature less than 150º F.

When it comes to killing water-borne pathogens, pasteurization is relatively swift (6 minutes) and deadly. Worms, protozoa cysts (Giardia, Cryptosporidium, Entamoeba) are killed quickly at 131ºF (55ºC). Bacteria (E.coli, Salmonella, Shigella, Cholera) and Rotavirus are killed at at temperature of 140ºF (60ºC). And Hepatitus A virus is killed at a temperature of 149ºF (65ºC). This is relatively cool water when compared with boiling.

NOTE: Pasteurization of milk requires slightly higher temperature, but the numbers above are sufficient for purifying drinking water.

So why would anyone go to all the trouble to boil drinking water? Good question. There are two answers. First, boiling is an old technique that predates cheap and accurate thermometers. And the second answer is that if you have no means available to verify the temperature of pasteurization you can absolutely know you're going to have safe water if you bring it to an easily verifiably rolling boil. So boiling is still a valid technique for killing all water-borne pathogens.

But if you can in some way verify that the water temperature is at least 149ºF, and you can keep it at that temperature for 6 minutes, you can produce safe drinking water without boiling, filtering, or using chemical treatment. Fortunately, there are easy ways to verify the temperature.

One way is to use an inexpensive cooking thermometer. Stick the probe into the water and watch the dial indicator rise to the desired temperature.

Another way is to use a WAPI (water pasteurization indicator) — a cheap and reusable device made of a small polycarbonate tube in which a solid wax plug is contained. The plug melts at 149 ºF, so with the tube submerged in the warm water, the wax goes suddenly liquid when the temperature has been reached. This lets you know that pasteurization is complete. As the tube cools, the wax solidifies again, allowing repeated use of the WAPI.

The beauty of water pasteurization is that it can be accomplished by using a solar cooker that requires no fuel at all except the sun. We'll talk more about solar cookers in future posts, including plans for making your own.

But for now, add pasteurization to your slate of water purification techniques. It's quick, safe, and requires a lot less fuel than bringing water to a boil.

1 comment:

  1. Yes this is very nice post Thanks for such a nice post!!