How To Disinfect Water With Sunlight

Sunlight kills microbes and disinfects clear water in a plastic water bottle after six hours (sunny conditions) or 2 days (cloudy conditions) making the water safe to drink.

Have you ever refilled your plastic water bottle when hiking but you’re not sure if it’s ok to drink? Sunlight may be your only good option to disinfect the water. So, here’s everything you need to know about using sunlight to make water safe to drink.

Disinfecting water with sunlight has been endorsed by the World Health Organization as an “emergency treatment of drinking water.” In this post, we cover everything on solar disinfection, where it’s used, and how you go about doing it yourself.

Sunlight water disinfection

The solar disinfection method was developed in the 80’s in Lebanon as a cost-efficient, easy way of disinfecting water with minimal equipment.

It was first used to disinfect water before being used for drinking. A decade later, the Swiss Federal Institute for Environmental Science and Technology began exploring ways to use solar disinfection (also known as SODIS) as a household water treatment mechanism in developing countries.

How does it work?

The disinfection properties of sunlight are primarily because of the effect of solar UV rays on microbes.

In addition to visible light, solar radiation is comprised of ultraviolet rays with wavelengths ranging between 100 and 400 nanometers (nm).

UV rays can be divided into three wavelength bands:

  • UVA (315-400 nm)
  • UVB (280-315 nm)
  • UVC (100-280 nm).

Of these, UVC has the highest germicidal properties. UVC rays penetrate the genetic material of microbes and break them down, and effectively kill them.

UVC light from the sun effectively kill bacterial species including:

  • pseudomonas,
  • staphylococcus,
  • clostridium,
  • salmonella, and
  • Escherichia.

UVC light from the sun also destroys viral strains of:

  • influenza,
  • rotavirus,
  • hepatitis, and
  • poliovirus

Algae of the Chlorella family are also eliminated by UVC.

Destroying microbes with the combined action of oxygen and light is called photo-oxidative damage. Additionally, common bacterial species can be thermally inactivated at a temperature of 65°C (150°F).

By allowing sunlight to directly shine through water for an extended period of time, you can effectively eliminate microbes and disinfect the water.

Keep in mind that sunlight cannot remove any dissolved minerals, chemicals, or suspended particles (like sand and other impurities) from the water.

This does mean that you are not actually “purifying” the water with sunlight, but you are disinfecting it.

How to disinfect water using sunlight

Using sunlight to effectively eliminate microbes in drinking water is a fairly straightforward process. The Center for Disease Control details the following steps to use sunlight disinfection:

  1. Fill 2/3rds of a transparent PET plastic bottle with clear water. Keep in mind that you need to use clear water otherwise it will not necessarily be completely disinfected. Turbid water will need to be pre-treated before disinfecting (more information on this below)
  2. Shake the water bottle to agitate and oxygenate the water.
  3. Then, completely fill the bottle with water.
  4. Place water bottle horizontally in direct sunlight. This is to maximize the surface area of impact.
  5. You may place the bottle horizontally on the roof, the ground, or any flat surface such as the top of your pack when hiking. Leave it for a minimum of six hours under direct sunlight.
  6. If it’s cloudy, the water needs to be left undisturbed for two days for effective disinfection.
  7. To make the process more efficient, you can paint or put some black tape on the bottom surface of the bottle – black absorbs more heat, and helps disinfect the water more effectively.
  8. You may also place the bottle on metal or even black plastic. Placing the bottle on a reflective surface also maximizes heat and radiation.
  9. After you wait the appropriate 6 hours (for sunny conditions) or 2 days (for cloudy conditions) the water is safe to drink (not considering other pollutants and chemicals).

How effective is sunlight water disinfection?

Sunlight disinfection is a proven way to reduce the number of disease-causing microbes in water, provided the water is exposed to sunlight for the appropriate time.

It is one of the cheapest methods to disinfect water. No additives and chemicals are added, which means that sunlight water disinfection does not alter the taste or smell of the water. There is minimal risk of re-contamination if the water from the bottle is not transferred before use.

A study by the Swiss Federal Laboratories for Materials Testing and Research showed that the concentration level of toxic chemicals in sunlight disinfected water was far below WHO guidelines.

This method is simple to use and requires no electricity. A simple transparent PET plastic water bottle will do. You can use this technique to disinfect water practically anywhere in the world where there is sunlight.

Limitations of sunlight water disinfection

While sunlight is a great way to reduce microbial content in water, it is important to know what it can and cannot kill. As explained above, many varieties of bacteria, viruses, spores, and algae are inactivated by sunlight. But its disinfection action ends there.

Since sunlight disinfection does not use any physical or chemical filtration methods, it cannot remove:

  • Nitrates,
  • Sulfates,
  • Carbonates,
  • Sodium,
  • Potassium,
  • Magnesium,
  • Heavy metals,
  • Effluents, and
  • Other dissolved or suspended particulate matter.
  • The sunlight disinfection method does not alter the Total Dissolved Solids (TDS) level at all.

This means that cloudy, turbid water purified only by sunlight may not be fit for consumption. Such water may need to be pretreated and filtered.

This method also requires a supply of clean, intact PET clear plastic bottles (not colored bottled). Surprisingly, other studies using glass bottles produced inconclusive results.

Which water is best for sunlight disinfection?

If you’re out camping and hiking and come across water from a clear stream or river, you can use sunlight to kill any microbes.

However, if you are unsure of the source of the water, we would not recommend sunlight.

For example, if you think your water may be contaminated by agricultural runoff, sewage, or industrial effluents, sunlight alone will not make the water fit for consumption.

Avoid still or foul-smelling water.

How do I disinfect cloudy water?

For cloudy, turbid water with sediments, you will need to use a basic filter first or allow the sediment to settle and decant off the clear liquid before using sunlight to disinfect the water.

Use a basic filter first

Run the water through a layered cloth filter (or piece of clean clothing) multiple times and catch as much debris as possible. Then, let the remaining water sit still for a while so that the sediments settle at the bottom. Transfer the clear water to another container carefully and then you can go ahead and disinfect it.

Use a flocculant

You can use a flocculant to quickly settle the finer sediments (your silts and clays) and decant the clear liquid off the top. The flocculation process groups non-settling particles together to form larger, heavier sediments (floc) at the bottom of the vessel. Clean water is decanted off from the top and the floc is left behind.

These are good floculants that you can use on cloudy water

  • Table salt,
  • Bentonite clay,
  • Ferric chloride, or
  • Alum.

The Bottom Line

Sunlight disinfection is a great way to harness solar power to disinfect water. However, make sure to check the water is clean and clear of sediment before trying to disinfect it and before drinking.

Russell Singleton

Russell has a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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