04 Water Purification: Physical Treatment: Heat Treatment
Description
This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
04 Water Purification: Physical Treatment: Heat Treatment
Boiling is one guaranteed way to purify water of all pathogens. Most
experts feel that if the water reaches a rolling boil it is safe. A
few still hold out for maintaining the boiling for some length of
time, commonly 5 or 10 minutes, plus and extra minute for every 1000
feet of elevation.. If one wishes to do this, a pressure cooker would
allow the water to be kept at boiling with out loosing the heat to
evaporation. One reason for the long period of boiling my be to
inactivate bacterial spores (which can survive boiling), but these
spore are unlikely to be waterborne pathogens. Hepitatis A, a viral
pathogen, may not be inactivated with much of a safety margin unless
water is allowed to boil for 1 minute.
African aid agencies figure it takes 1 kg of wood to boil 1 liter of
water. Hardwoods and efficient stoves would improve on this.
Water can also be treated at below boiling temperatures, if contact
time is increased. A commercial unit has been developed that treats
500 gals of water per day at an estimated cost of $1/1000 gallons for
the energy. The process is similar to milk pasteurization, and holds
the water at 161 deg. F for 15 seconds.. Heat exchangers recover most
of the energy used to warm the water. Solar pasteurizers have also
been built that would heat three gallons of water to 65 deg. C and
hold the temperature for an hour. A higher temperature could be
reached if the device was rotated east to west during the day to
follow the sunlight.
Regardless of the method, heat treatment does not leave any form of
residual to keep the water free of pathogens in storage.
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