SGUK Outdoor Skills Challenge 53: Learn How to Solar Treat Water

Clean drinking water is one of the most important things to plan for when spending time outdoors. If your normal water supply is unavailable, understanding different treatment methods can help you make better decisions. One method worth learning about is solar water disinfection, often called SODIS.

This solar water treatment technique uses sunlight to help inactivate certain disease-causing microorganisms in clear water held in suitable transparent bottles. It has important limitations, though: it does not remove chemical contamination, and it is not a reliable solution for every water source or weather condition.

For this challenge, you will learn how the method works, practise preparing a suitable bottle and identify when solar treatment should not be relied upon.

Your Challenge

Your challenge is to explain how solar water disinfection works and demonstrate how you would prepare water for the process.

You will learn about bottle selection, water clarity, sunlight exposure and the method’s limitations. You will also consider why collecting water from a natural source does not automatically make it safe to drink.

You can complete this challenge using clean tap water for your practical demonstration. You do not need to collect or drink untreated water from a stream, pond or other outdoor source.

What You’ll Need

• A clean, clear, transparent PET plastic bottle, preferably no larger than 2 litres.
• Clean tap water for your demonstration.
• A sunny outdoor location.
• A light-coloured or reflective surface on which to place the bottle.
• A notebook or phone for recording your observations.
• A camera or phone to photograph your evidence.

Do not use a coloured, cloudy, scratched or heavily worn bottle for the demonstration. Avoid glass bottles for this method, as they may not provide the required ultraviolet transmission.

Take the Challenge

1. Understand how solar water disinfection works

Solar water disinfection uses sunlight, particularly its ultraviolet radiation, together with the warming effect of the sun, to help inactivate certain microorganisms that can cause illness.

The method is commonly known as SODIS. It has been used in some settings where access to conventional drinking-water treatment is limited, but it is not a universal treatment for every type of contamination.

Solar disinfection does not filter out dirt, remove dissolved chemicals or desalinate seawater. It should not be confused with leaving any container of water in the sun and assuming it will become safe.

Your first task is to explain what sunlight does during solar water disinfection and name one type of contamination that the method cannot remove.

2. Choose a suitable bottle

The traditional SODIS method uses clear PET plastic bottles that allow the relevant sunlight to pass through. Smaller bottles, commonly up to 2 litres, are easier to expose consistently than large containers.

Choose a clean, transparent bottle with no significant cloudiness or damage. Remove any label that blocks sunlight and check that the bottle and cap are clean.

For this demonstration, fill the bottle with tap water and close the cap securely. You are practising the preparation method rather than treating water for drinking.

Take a photograph of your chosen bottle and explain why clarity, cleanliness and size matter.

3. Check the water clarity

Sunlight cannot penetrate cloudy water as effectively as clear water. Suspended particles can shield microorganisms from the radiation, making the process less reliable.

If water is visibly cloudy, the traditional SODIS method recommends clarifying it before treatment, for example by allowing particles to settle and carefully decanting the clearer water, or by using an appropriate filtration step. This does not remove all hazards, and clear-looking water is not necessarily safe to drink.

For your practical demonstration, use clean tap water. Observe how easily you can see through it and explain why you would not simply place muddy water into a bottle and leave it in the sun.

4. Understand the sunlight and exposure requirements

For the traditional SODIS method, clear bottles are laid on their side in full sunlight. A bright, reflective surface beneath them can help increase exposure.

Under suitable sunny conditions, the standard guidance is at least six hours of sunlight. When the sky is heavily overcast, the process may require two consecutive days of suitable exposure. Continuous rain or prolonged poor sunlight makes the method unsuitable to rely on.

These timings are not a guarantee that every water source will become safe. Water quality, bottle condition, cloud cover and other factors affect the result.

For this challenge, place your demonstration bottle in a sunny spot and photograph its position. You do not need to leave the bottle out for the full treatment period because you are using clean tap water and are not preparing drinking water from an untreated source.

5. Know when not to rely on solar treatment

Solar disinfection has significant limitations. It is intended to reduce certain biological hazards in suitable water, not to remove every possible contaminant.

Do not rely on it for water that may contain fuel, pesticides, industrial chemicals, heavy metals or salt. It is also unsuitable when the water remains too cloudy or when the required sunlight exposure cannot be achieved.

If you need drinking water outdoors, use a reliable source wherever possible. Where treatment is necessary, choose a method appropriate to the likely hazards and follow its instructions. Boiling, suitable water filters and approved disinfectants have different capabilities and limitations; a filter, for example, may not remove viruses unless it is specifically designed to do so.

Never drink water from your demonstration bottle on the assumption that sunlight has made it safe. Use clean tap water throughout this challenge.

6. Explain how you would use the method responsibly

Imagine that you are planning an outdoor trip where you may need to treat water. Before relying on SODIS, you would need to consider whether the method is appropriate for the source, whether the water can be made sufficiently clear, whether suitable bottles are available and whether the weather will provide enough sunlight.

You would also need a backup plan. Weather can change, and solar treatment takes time. It should not be your only water-safety strategy when conditions are uncertain.

Write down three limitations of the method and explain what you would do if the water were cloudy, potentially chemically contaminated or exposed to insufficient sunlight.

Check Your Results

Before completing the challenge, make sure you can answer these questions:

• What does solar water disinfection do?
• Why does the traditional method use clear PET bottles?
• Why is cloudy water a problem?
• How long does the method require in suitable sunny conditions, and what changes in heavily overcast weather?
• Why can solar treatment not remove chemical contamination?
• What would you do if the weather did not provide enough sunlight?

If you are unsure, revisit the relevant sections and check that you understand the difference between reducing biological hazards and removing other contaminants.

Complete the Challenge

To complete Challenge 53, demonstrate that you understand the basic principles and limitations of solar water disinfection.

Your evidence should include:

• A photograph of a suitable clear PET bottle prepared for the demonstration.
• An explanation of how sunlight helps inactivate certain microorganisms.
• A description of the usual sunlight exposure requirements.
• Three limitations of the method.
• An explanation of why you would never assume that clear-looking water or water left in sunlight is automatically safe to drink.

Use clean tap water for your demonstration. Do not drink untreated water or deliberately expose yourself to a waterborne illness risk to prove that you understand the method.

How to Submit Your Challenge

Submit your evidence

Once you have completed the challenge, share your evidence on your SGUK member profile so it can be checked.

1. Go to your SGUK member profile.
2. Open Social Activity.
3. Upload a photograph of your evidence to the activity wall.
4. Add the hashtag #SGUKChallenge53.
5. Share it for verification.

Include your written explanation alongside the photograph so it is clear that you understand both the method and its limitations.

Challenge 53 Checklist

Want to Push Yourself?

Compare solar water disinfection with two other water-treatment methods, such as boiling and a purpose-made outdoor water filter. For each one, consider what it can treat, what it cannot reliably remove, what equipment it needs and how weather or water quality affects its use.

Create a simple water-treatment decision guide for an outdoor trip. Include a reliable water source, an appropriate treatment method and a backup plan if your preferred method is unavailable.

The aim is not to find one method that works in every situation. It is to understand the options well enough to choose a suitable approach without taking unnecessary risks.

What’s Next?

If you have not completed the previous challenge, try SGUK Outdoor Skills Challenge 52: Learn How to Avoid Condensation in a Bivvy. Discover how ventilation, careful pitching and moisture management can make overnight trips more comfortable.

You can complete the SGUK Challenges in any order, so choose the skills that interest you and continue developing your practical outdoor knowledge.

Once you’ve shared your evidence and it has been verified, the SGUK Outdoor Skills Challenge 53 is complete and the completed challenges figure will be updated on your profile.

Learn. Practise. Master.

That’s what the SGUK Outdoor Skills Challenge is all about.