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UV Disinfection

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UV Disinfection
Sunlight-based treatment at scale
Sub-Topics
🍶
SODIS Batch System
Organized bottle-based treatment
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Solar Continuous Flow
Gravity-fed UV exposure systems

Sunlight disinfects clear water through its ultraviolet-A rays: filled PET bottles laid in full sun for 6 hours, or for 2 overcast days, are safe to drink, but a rack of 50-100 bottles treats only 100-200 liters a day and the water has no protective residual afterward. It suits the drinking water of a small group when no chlorine is available, and it fails in cloudy water, in heavy rain and for Cryptosporidium.

Part of Water Treatment at Scale

UV disinfection at community scale uses the ultraviolet part of sunlight to inactivate pathogens in water, either in organised batches of bottles or in shallow flowing trough systems.

Sunlight Works Where Chemicals Run Out, Within Strict Limits

The individual method is described in SODIS Method and the science in Solar Disinfection. UV-A rays in the 320-400 nm range damage the DNA of bacteria, viruses and protozoa, and sun warms the water, which helps. Those pages put the required exposure at 6 hours of strong sun for clear skies or partly cloudy days under half cover, and 2 full days when it is overcast. Heavy rain or continuous dense cloud means do not use SODIS.

At community scale, the carrier describes two systems: a batch of bottles (SODIS Batch System) and a continuous shallow trough (Solar Continuous Flow). The batch system is the sound one. The trough needs care, for reasons given on its page.

What UV cannot do matters as much:

  • It needs clear water. Turbid water blocks the light and shields organisms. Hold a filled bottle over printed text: if you cannot read the words, filter or settle the water first.
  • It leaves no residual. Chlorine keeps protecting water in the pipe; sunlight-treated water can be recontaminated by the next dirty hand or cup. Drink from the bottle or pour into a clean cup.
  • It is weaker against Cryptosporidium, which is the most UV-resistant common pathogen; the SODIS pages advise filtration or boiling where it is a risk.
  • It is slow and weather-dependent. No sun, no treatment.

What You Need

  • Clear PET bottles (recycling symbol 1) up to 2 liters; coloured, glass or PVC bottles are not suitable per the SODIS pages
  • Clear water that has been settled or filtered
  • A sunny surface, ideally reflective, such as corrugated metal
  • A routine and a person to run it

Method: Fill, Expose, Collect, Store

Step 1: Clear the water first. Use the settling and sand filter steps if the water is cloudy.

Step 2: Fill the bottles, shake, and lay them flat in the sun, on a reflective surface if you have one.

Step 3: Expose for 6 hours in good sun, or two days if overcast; no use in heavy cloud or rain.

Step 4: Collect in the evening, label with the date, and use in order.

Step 5: Keep the treated water capped until it is drunk.

Worked Example: How Many People Can One Rack Serve?

Inputs: 50-100 bottles of 2 liters; one batch a day.

  • Output = 50 × 2 = 100 L/day up to 100 × 2 = 200 L/day, as the carrier states.
  • At the minimum of 20 L per person per day for drinking, cooking and basic hygiene, 100 ÷ 20 = 5 up to 200 ÷ 20 = 10 people.
  • If the sunlight water is used only for drinking, at an assumed 5 L per person per day, 100 ÷ 5 = 20 up to 200 ÷ 5 = 40 people.
  • Against the carrier’s 2,000 L/day for a village of 20-30, the rack provides 100 ÷ 2,000 = 5% to 200 ÷ 2,000 = 10% of the flow.

The rack is a source of drinking water and a backup, not the village supply.

Common Mistakes

  • Using cloudy water. The light does not reach the germs.
  • Using the wrong bottles. Scratched, coloured, glass or PVC bottles pass too little UV-A.
  • Leaving bottles in the shade for part of the day.
  • Treating the water as protected afterward. There is no residual.
  • Trusting SODIS for Cryptosporidium. Filter or boil instead.

Safety

Sunlight treatment gives no margin for error

Only drink from bottles that have had the full exposure; when in doubt, treat again or boil. Hot bottles can burn skin and should not be handled with bare hands in strong sun. Do not store chemicals, fuel or detergent in bottles that will later be used for drinking water.

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