Boiling

Boiling water for purification.
Photo: Kopernick

Boiling is considered the world’s oldest, most common, and one of the most effective methods for treating water. If done properly, boiling kills or deactivates all bacteria, viruses, protozoa (including cysts) and helminths that cause diarrheal disease. Pathogens are killed when the temperature reaches 100 degrees Celsius. In high-lying areas, where the boiling temperature of water can be as low as 80 degrees Celcius, a longer boiling time is necessary to kill all pathogens.

Organizations recommend boiling both for water treatment in developing countries and to provide safe drinking water in emergency situations throughout the world. Although boiling time recommendations vary significantly, from 0-20 minutes, to make water safe for consumption the water simply must reach the boiling point of 100°C. However, studies in developing countries have documented incomplete inactivation of bacteria in boiled water. This disparity between the laboratory and field results is attributed to users not heating the water to the boiling point and/or recontamination of boiled water in storage. To date, there have been no peer reviewed studies assessing the health impact associated with boiling water, although some case-control studies in cholera outbreaks have noted boiling as being protective against cholera.

Contents

Suitable conditions

Boiling is applicable in area with a good fuel supply, a cultural tradition of boiling, and where water is stored safely after boiling.

Advantages

Disadvantages

- Simple - Uses common knowledge - Gernerally uses locally available materials - Proven inactivation of all bacteria, viruses and protozoa, even in turbid or contaminated water - Socio-cultural acceptance of boiling for water treatment, particularly in tea-consuming cultures - Easy and very effective

- Inefficient - Requires a considerable amount of energy (wood, charcoal, fuel) - Time consuming - Does not remove suspended or dissolved compounds - Lack of residual protection against contamination - Potential for burn injuries - Potentially high cost of carbon-based fuel source (with concurrent deforestation risk) and the opportunity cost of collecting fuel - Potential user taste objections - Potential for incomplete water treatment if users do not bring water to full boiling temperature - Indoor pollution: increased risk of respiratory infections from indoor stoves or fires - After boiling, water needs cooling down (cannot be consumed immediately) - Deforestation (approx. 1 kg wood/1 litre water) - CO2 emission

Highly effective for: Somewhat effective for: Not effective for: - Bacteria - Viruses - Protozoa - Helminths - Ca, - Mg - TSS - TDS

Treatment process: Thermal disinfection Inlet water criteria: Any water can be boiled

Construction, operations and maintenance

Several solar cookers at work boiling water to make it safe to drink. Photo: Nasa.gov

Operation

Water is heated over a fire or stove until it boils. Different fuel sources can be used depending on local availability and cost (e.g. wood, charcoal, biomass, biogas, kerosene, propane, solar panels, electricity). Water bubbling as it boils provides an easy to recognize visual indicator and does not need a temperature gauge.

Recommended boiling times vary between organizations. The World Health Organization recommends that water be heated until it reaches the boiling point (WHO). If the boiling point is reached, boiling is effective at inactivating all the bacteria, viruses, and protozoa that cause diarrheal disease. The Centers for Disease Control and Prevention, recommends a rolling boil of 1 minute, to ensure that users do not stop heating the water before the true boiling point is reached (CDC, 2009).

CAWST recommends boiling water for 1 minute and adding 1 minute per 1000 metres of elevation.

Recontamination of boiled water is a major problem. Water is often transferred from the pot into dirty storage containers which then make it unsafe to drink. Water should be stored in the same container in which it was boiled, handled carefully, and consumed within 24 hours to prevent recontamination.

Some people believe that boiled water tastes flat. This is caused by dissolved oxygen escaping from the water as it boils. This can be reduced by vigorously stirring or shaking cooled water to increase its dissolved oxygen content.

Almost all households have the equipment required to boil water.

Manufacturing

Needed materials are fuel (e.g. wood, charcoal, biomass, biogas, kerosene, propane, solar panels, electricity, a pot and a lid. Labour is needed for the regular collection of some fuels (e.g. wood, charcoal, biomass).

There is potential for burn injuries; caution should be maintained around stoves and fires and when handling hot water. Cause of respiratory infections associated with poor indoor air quality; improved stoves can be used to reduce illness and death from indoor fires and stoves.

Treatment Efficiency

	Bacteria	Viruses	Protozoa	Helminths	Turbidity	Iron	{{{extra_Field}}}
Laboratory	Up to 100%	Up to 100%	Up to 100%	Up to 100%	0	0	{{{lab:extra}}}
Field	97-99%	not available	not available	not available	not available	0	{{{field:extra}}}

Maintenance

Pots used for boiling may need to be replaced.

Costs

Captial Cost	Operation Cost	Replacement Cost	Estimated 5 years Cost	Cost/liter treated
US$ 0*	US$ 0 - 2/month and up	US$ 0	US$ 0 - 120	US$ ~0.01

(*) Households are assumed to already have a pot and fire/stove for cooking Note: Program, transportation and education costs are not included.

Manuals, videos, and links

• Household Water Treatment and Safe Storage, Fact Sheet 4.1: BOILING. Provided by: CAWST (Centre for Affordable Water and Sanitation Technology)

Acknowledgements

This article is based on a factsheet from Centre for Affordable Water and Sanitation Technology (CAWST), which is gratefully acknowledged.

• Household Water Treatment Options in Developing Countries: Boiling. Centers for Disease Control and Prevention (2009). Atlanta, USA.

• Clasen, T. Microbiological Effectiveness and Cost of Disinfecting Water by Boiling in Semi-urban India. London School of Hygiene and Tropical Medicine, 2008.

• Clasen, T., Thao, D., Boisson, S., et al. Microbiological effectiveness and cost of boiling to disinfect drinking water in rural Vietnam. Environmental Science and Technology; 42(12): 42:55. 2008.

• Household Water Treatment and Safe Storage Following Emergencies and Disasters: South Asia Earthquake and Tsunami. World Health Organization.