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Water Pasteurization (SODIS & WAPI)

5 min read

Sunlight makes water safer in two different ways: SODIS leaves clear PET bottles in strong sun for 6 hours (2 days if cloudy) so UV and heat together kill pathogens, while heat pasteurization holds water at 65°C for 30 minutes in any solar collector or cooker. A wax indicator that melts when the water has passed pasteurizing temperature confirms the heat method, so neither method is a guess; boil the water instead whenever you cannot confirm it.

Part of Solar Cooking & Pasteurization

Solar pasteurization uses sunlight, by UV in bottles or by heat in a cooker or collector, to make drinking water microbiologically safer without fuel.

Two Methods, Two Mechanisms

SODIS (solar disinfection). Clear PET bottles up to 2 L, filled with clear water, laid in direct sun for 6 hours, or 2 days if it is cloudy (parent page). UV-A and heat together do the work; the method is endorsed by WHO and detailed on Solar Disinfection. It needs clear water (it fails in turbid water) and it is weaker against Cryptosporidium, which is more resistant, as that page notes.

Heat pasteurization. Heat the water to 65°C and hold it for 30 minutes, in any solar collector or cooker; the parent page says this kills all common waterborne pathogens. Heat does not rely on UV, so a cooker, a collector or a batch heater will do (Box Cooker). Both methods depend on strong sun: under cloud neither reliably gets the water to its dose or its temperature. Published time-and-temperature tables show that the time required falls steeply as temperature rises; the 65°C and 30 minutes of the parent page is a working rule with some margin, and more margin is better than less.

The Indicator: WAPI

A water pasteurization indicator (WAPI) is a small tube with a plug of wax that melts at a set temperature and slides to the bottom, so the user can see that the water went above it. The parent page gives 65°C as the melting point and a glass tube with soy wax as a build. The commercial indicator (recalled from Solar Cookers International’s descriptions, not from this wiki) uses a wax that melts at about 69-70°C, a few degrees above the 65°C pasteurization threshold, so that melting means the threshold was passed. A wax that melts at exactly 65°C leaves no margin.

Homemade soy wax varies widely in melting point, so an uncalibrated tube proves nothing. Calibrate it: lower the tube into a bath and heat slowly beside a reliable thermometer, and note the temperature at which the wax slumps. Use only wax that melts a few degrees above 65°C. Hang the tube in the coolest part of the water, because the coolest spot decides whether the whole batch is safe.

Worked Example: How Long Does A Batch Take?

Inputs: water from 20°C to 65°C is a rise of 45 K; 4.18 kJ/kg·K; 1 kWh = 3,600 kJ.

  1. Heat for 2 L = 2 × 4.18 × 45 = 376 kJ = 0.105 kWh.
  2. Heat for 10 L = 10 × 4.18 × 45 = 1,881 kJ = 0.52 kWh.
  3. A box cooker delivers about 60 W net (Box Cooker). 2 L takes 376,000 ÷ 60 = 6,270 s = 1.7 hours; 10 L takes 31,350 s = 8.7 hours.

So in a box cooker, small batches work and large batches do not. A flat plate collector or batch heater with a larger area is the tool for bigger volumes, and 65°C is easy there; the challenge is the 30-minute hold at the end of the afternoon.

What You Need

  • A thermometer to calibrate and check, if any can be salvaged
  • Clear PET bottles (up to 2 L, no tint) for SODIS
  • A cooker or collector and a lidded vessel for heat pasteurization
  • A WAPI or a thermometer in the water
  • A fall-back: a pot and fuel to boil

Method: Pasteurize By Heat

Step 1: Filter cloudy water first (Water Purification); pasteurization works better on clear water.

Step 2: Fill the vessel, set the indicator, close the lid.

Step 3: Heat in full sun until the indicator trips, then keep the water above that temperature for 30 minutes by closing the cooker and keeping it aimed at the sun.

Step 4: Let the water cool in its closed vessel, and pour it into a clean container.

Common Mistakes

  • Trusting an uncalibrated indicator.
  • Stopping at the moment the wax melts. The 30-minute hold still counts.
  • Treating pasteurized water as free of chemicals. Heat does not remove poisons.

Safety

Scald risk and false confidence

Water at 65°C is hot enough to scald within seconds, so let it cool before pouring it for anyone, especially children and the elderly. Do not drink water you cannot prove reached the temperature; boil it instead. Neither method removes chemical contamination.

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