Boiling vs. UV Disinfection

March 24, 2020

Maintaining a rolling boil for one minute is recommended for disinfecting water. The energy required for disinfection by boiling is approximated here as the energy required to raise the temperature of one gallon of water from room temperature to the boiling point.

The specific heat of water, the quantity of heat necessary to increase the temperature of water by one degree Celsius, varies from 4.18 kilojoules per kilogram at 20°C (68°F room temperature) to 4.22 kJ/kg at 100°C, the boiling point. A value of 4.2 kJ/kg is used here for the specific heat of water.

A gallon of water weighs 8.3 pounds at room temperature and sea level. There are 2.2 pounds in a kilogram. To raise one gallon of water from room temperature to boiling requires
8.3 lbs / (2.2 lbs/kg) x 4.2 kJ/kg/°C x (100°C – 20°C) = 1268 kJ

Independent lab testing of the UV-H2O-Box confirmed that it meets the requirements of the NSF International Protocol P231 for Microbiological Water Purifiers. The directions for use of the water box call for exposing each batch of water to the UV light for one minute. The energy required for UV disinfection of one gallon of water is approximated as the energy required to light the germicidal bulb in the UV-H2O-Box for one minute.

The G14T5L4 germicidal UV lamp used in the UV-H2O-Box draws 14 watts of power. There are 3.6 kilojoules in a watt-hour. Each batch of water contains 1 gallon. The energy required for a one-minute exposure is
14W x 1 minute x (1 hour/60 minutes) * 3.6 kJ/Wh = 0.84 kJ

UV is more than 1500 times more energy efficient than boiling
Our figure for the energy required to boil a gallon of water, 1268 kilojoules, is a lower bound, since it doesn’t account for the inefficiency of transferring heat from the fire to the water itself, nor does it account for the energy required to keep the water at a boil for the recommended time. Even so, 1268 kJ is 1500 times larger than the 0.84 kJ that it takes to operate the germicidal fluorescent tube in the UV-H2O-Box for one minute.