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Heat treatment (boiling)

Heat treatment (boiling)

Heating water is the traditional method of water treatment.

It is the only method that is guaranteed to kill all microorganisms. It is important to note that heating water does not remove chemical contaminants or make seawater drinkable.

In the past, authorities recommended boiling water for 3–10 minutes plus one additional minute for every 330 metres (1000 feet) above sea level to account for the decline in boiling temperature at altitude. Recent research indicates that boiling time is irrelevant for all pathogens, except hepatitis A and B, which need at most one minute of boiling time, because by the time water reaches the boiling point of 100ºC at sea level, all harmful microorganisms have been inactivated or destroyed.

The reason for this is that the time required to kill or deactivate microorganisms  decreases exponentially as the temperature increases. Since the time required to boil water adds several minutes to the kill time, even at high altitude where the boiling temperature is lower than 100ºC, most pathogens are already dead by the time water boils.

Just to be sure, water should be brought to a rolling boil for two minutes before being consumed. If it isn’t possible to boil water, heating to above 55ºC for a prolonged period of time (see table below) perhaps by setting a container on a car dashboard or on rocks in hot and sunny conditions can go a long way towards making it safer to drink.

Bacteria

Bacteria are killed easily by heating water. Common pathogenic bacteria such as Salmonella and Campylobacter are destroyed in 30 minutes at 55ºC, less than one minute at 65ºC and instantly at 100ºC.

Viruses

Viruses respond to heat in the same way as bacteria. Inactivation occurs in 20–40 minutes at 60C, less than one minute at 70ºC and almost instantly at 100ºC.

Notable exceptions are hepatitis A and B which are unaffected by temperatures as high as 56ºC and require about one minute at 100ºC to ensure deactivation.

Protozoa

Giardia, Cryptosporidium, E. histolytica and other pathogenic protozoa are killed easily and quickly by heat. Giardia cysts perish within ten minutes at 50ºC, five minutes at 55C and immediately in boiling water. Cryptosporidium and E. histolytica are killed at similar rates.

Parasitic Worm Eggs and Larvae

Worm eggs and larvae are also killed easily by heat. Eggs, which are more resistant than larvae, are killed at temperatures above 55ºC.

Below is a refernece table for heating times and temperature required to make drinking water safe from different pathogens. In all cases a rolling boiler for 2 minutes ensures that water is safe from pathogens.

Water Heating Temperatures and Times for Pathogens
Pathogen 55ºC 75ºC 100ºC (boiling at sea level)
Bacteria 30 minutes <1 minute Instant
Viruses except
Hepatitis A & B
30–50 minutes <1 minute Instant
Hepatitis A & B No affect <10 minutes 1–2 minutes
Protozoa 5 minutes 1–2 minutes Instant
Parasitic worm eggs & larvae 1–2 minutes <1 minute Instant

To Boil or not to Boil?

Ultimately, boiling is a highly effective method of ensuring that water is microbiologically safe to drink because it kills all harmful pathogens.

Boiling is particularly useful in areas that have high concentrations of human and animal waste because it guarantees that even the most dubious water can be made safe for drinking, although it might still have an unappealing taste.

Disadvantages of boiling as a primary method of water treatment include no effect on chemical contaminants and the burden of carrying large amounts of stove fuel or the environmental impact of open fires in some recreation areas. Hot water also has a low level of satisfaction in hot conditions unless you are making soup, tea, coffee or hot chocolate.