Frequently Asked


  • What's the problem with waterborne diseases?

    Each day, almost 2.000 children under the age of 5 die as a result of drinking contaminated water. Worldwide, there are 660 million people without access to an improved drinking water source. Altogether, 80% of all illnesses and deaths in developing countries derive from insufficient or contaminated drinking water supply.

    Waterborne diseases are mainly caused by the ingestion of water, which is contaminated with different kinds of pathogens. The three most important classes of pathogens are bacteria, viruses and protozoa, all of which lead to diarrheal diseases. Examples are Cholera, Polio or Cryptosporidiosis.

    There are already a number of water disinfection methods for low-income households. Examples are boiling water, chlorine tablets, water filters and solar water disinfection (SODIS). SODIS is a process, in which the UV-radiation of the sun inactivates certain harmful pathogens in the water. WADI™ is a SODIS indicator that was developed to visualize this process and to make it more reliable and practicable for people, who are affected by waterborne diseases.

  • What is solar water disinfection (SODIS)?

    SODIS is a natural process, in which the UV-radiation of the sun inactivates certain harmful pathogens in the water. The process only requires PET-bottles, which are filled with contaminated water and are then exposed to the sun. The duration of this disinfection process varies and is determined by the sun’s UV intensity. Today, SODIS is used by around five million people worldwide. 

    SODIS is endorsed by the World Health Organization (WHO, 2007) as a simple and cheap water disinfection method and is recommended by UNICEF (2009) as a viable method for household water treatment. The SODIS method was researched and developed by the renowned EAWAG research institute in Zurich who today have 20 years of in-depth experience with this water treatment method. Research provides scientific evidence for the functionality and safety of SODIS. Please find relevant studies on

  • Where is SODIS best applied?

    As SODIS is dependent on favorable climate, it is recommended to use the method anywhere between 35° north and 35° south of the equator, where, on average, annual sunshine hours are abundant. This includes most of Latin America, all of Africa and South- and Southeast-Asia, and thereby covers the majority of the countries with people lacking access to improved drinking water sources.

  • What are the advantages and disadvantages of SODIS?

    Research suggests that SODIS is a water treatment method which is highly effective against bacteria, and less effective against viruses and protozoa. SODIS is considered the cheapest method of water treatment available, as only PET-bottles and the sun are required for its application. Furthermore, it is simple to apply and does not change the taste of water. Another advantage of SODIS is that the treated water can be directly consumed from the PET-bottles, which implies zero to very low risk of recontamination of the water.

    As compared to alternative water disinfection methods, disadvantages of SODIS are that it requires relatively low turbidity waters, and that the water used for SODIS must not be contaminated with metals or chemicals. Also, effectivity of SODIS depends on the climate. SODIS is best used in geographical areas that guarantee sufficient sunshine throughout the year. What is more, SODIS is a slow process and people need to wait before the water can be safely consumed.

    The main limitation of SODIS, however, concerns the difficulty to estimate the time needed for the sun to disinfect the contaminated water. The duration of the process depends on the UV-radiation of the sun, which in turn depends on a number of parameters: the prevalent weather conditions (clear, cloudy, rain), the sea level (the higher, the faster), the degree of latitude (the closer to the equator, the faster) and possible air pollution. All affect the process duration, which can range from as little as a couple of hours to two days or even more. WADI™ overcomes this shortcoming by indicating the point in time at which the water is safe* to drink. Usually, and as evidenced by WADI™, the SODIS process takes a few hours only (around three to four) in case conditions are favorable.

    *based on at least a 3log10 (99.9%) reduction of common coliform bacteria in the water.

  • What is WADI?

    WADI™ is an easy-to-use, solar-powered UV-measurement device that visualizes the process of SODIS. A happy smiley face on the WADI™ display indicates the point in time at which the UV-radiation of the sun has made contaminated water in a PET-bottle safe* to drink. 

    WADI™ continuously measures the sun’s radiation, and compares these measurements with scientifically tested data. As soon as the incident UV-dose reaches that of the reference values stored in WADI™, the device indicates its happy face. At this point, at least 99.9% of common coliform bacteria in the water have been inactivated, provided that the water used was not turbid. WADI™ makes SODIS more reliable and practicable for affected people in low-income countries.

    *based on at least a 3log10 (99.9%) reduction of common coliform bacteria in the water.

  • How does WADI indicate the process of SODIS?

    WADI™ visualizes the process of SODIS in five steps. When starting the process, the reset button on top of the WADI™ must be pressed during exposure to sunlight (energy). Note that the button must be pressed hard enough, so that a clear mechanical “click” can be heard from WADI™. A sad face, along with one flashing progress bar will appear. Over time, a second, third and fourth bar will appear along with the sad face. Once WADI™ and the nearby water have received sufficient UV-radiation, the sad face on the WADI™ display will turn into a happy smiley face, along with four constant progress bars, meaning that the water is ready for consumption.

  • What happens over night, when there is no energy supply to power WADI?

    WADI™ is powered by a solar cell, and does not contain any batteries. As a result, continuous light exposure is required to ensure operation of WADI™. It is therefore important to never cover the solar cell during the disinfection process. Upon dark (i.e., sunset), WADI™ stores the last measured UV dose value, and continues measuring upon renewed light exposure (i.e., the next morning). Because of the fact that the pathogens in the water re-grow over night when there is no UV exposure, WADI™ adapts its measurements accordingly and continues them at a lower starting point as compared to the last measurement before dark. 

    In case the process had already finished upon dark (i.e., the happy smiley face had already appeared), the happy smiley face will re-appear upon renewed light exposure. This ensures that people can check whether the water is ready for consumption even when coming home from work at dark, by simply illuminating the solar cell with any type of light available locally.

  • What are the advantages of SODIS in combination with WADI?

    WADI™ was developed especially for the base of the economic pyramid (BoP), and presents one of the first sustainable, practicable and affordable Solution for households in low-income countries. While competitors also try to meet the needs of the BoP market in the water disinfection sector, many of the competitive products available are either too expensive, need defined maintenance, or require electricity for operation. Specifically, the combination of SODIS and WADI™ has four main advantages:

    (i) Sustainability: In comparison to other water disinfection methods, WADI™ requires neither maintenance nor batteries for use, and comes with a product guarantee of two years (a higher usage time is expected).

    (ii) Practicability: Produced according to the International Protection Code 65 (IP65), WADI™ is completely dust- and rain-proof. Also, WADI™ is small, light-weight and is therefore easy to carry.

    (iii) Prevention: WADI™ is an effective prevention tool for times of water crises. It does neither require spare parts nor maintenance, and can be re-used in emergency situations.

    (iv) Environment: WADI™ requires no batteries and chemicals for use, and there are also no CO2 emissions involved in the daily use of WADI™. In comparison, boiling of water, which is a commonly used water treatment method in low-income countries, leads to significant CO2 emissions in the countries affected.

  • Which kinds of bottles can be used for SODIS?

    Only bottles made of PET (polyethylene terephthalate) shall be used for SODIS, as they guarantee a safe and proper use. PET (also: PETE) is commonly used for carbonated beverage, water bottles and many food products. 

    Scientific studies confirm that when SODIS is applied correctly, the use of PET-bottles does not pose any harm to human health. Please see below for citations from publicly available studies.

    “Negative genotoxicity results were obtained for water samples that had been in PET bottles and exposed to normal SODIS conditions (strong natural sunlight) over 6 months. ”Ubomba-Jaswa, E., Fernánez-Ibáñez, P., McGuigan, K.G. (2010). A preliminary Ames fluctuation assay assessment of the genotoxicity of drinking water that has been solar disinfected in polyethylene terephthalate (PET) bottles. Journal of Water and Health, 08.4, 712-719.

    “[…] data demonstrate that the SODIS procedure is safe with respect to human exposure to DEHA and DEHP.“ Schmid, P., Kohler, M., Meierhofer, R., Luzi, S., & Wegelin, M. (2008). Does the reuse of PET bottles during solar water disinfection pose a health risk due to the migration of plasticisers and other chemicals into the water? Water Research, 42(20), 5054-5060.

    “These [DEHA and DEHP] levels are distinctly below the WHO guidelines for drinking water quality […].”

    Kohler, M. (2003). Migration of organic components from polyethylene terephthalate (PET) bottles into water.” Report 429670. Swiss Federal Laboratories for Materials Testing and Research (EMPA)

    „No indication for migration of possible photoproducts or additives from PET bottles into water was observed […].“

    Wegelin, M., Canonica, S., Alder, A. C., Marazuela, D., Suter, M. J. F., Bucheli, T. D., et al. (2001). Does sunlight change the material and content of polyethylene terephthalate (PET) bottles? Journal of Water Supply Research and Technology-Aqua, 50(3), 125-133.

    PET-bottles used for SODIS shall not exceed volumes of three liters (100 oz.). This is because only a part of the sun’s UV-radiation will reach the bottom of the bottle if the water depth exceeds 10 cm (3.9 in). The bottles must be in horizontal position when using SODIS. They must be transparent, thoroughly cleaned and unscratched. Labels must be removed from the bottles before exposing them to sunlight. The bottles may be used for a period of up to six months. Theoretically, larger PET-vessels fulfilling the above criteria may be used, provided that they are flat and do not exceed the stated height of around 10 cm (3.9 in.). Glass bottles shall not be used.

  • Which kind of water can be used for SODIS?

    SODIS does not have any effect on metal or chemical pollution of water. Therefore, water polluted with metals or harmful chemicals must not be used for SODIS. Apart from this, SODIS requires relatively clear water. This is because particles in the water could shield the water from receiving sufficient UV-radiation. In order to find out whether the water is clear enough, one may conduct a simple test by holding the filled PET-bottle in one’s hand. The spread fingers on the other side of the PET-bottle must be clearly visible.

    If this test turns out unsuccessful, the WADI™ funnel may be applied to reduce the turbidity of the water. The funnel was designed to get rid of the coarsest sand and mud particles, in case the water is too turbid for SODIS. Simply put a cloth, a piece of clothing or cotton into the funnel and let the water you want to disinfect run through it. Attach the PET-bottle you want to use to the bottom of the funnel to catch the water. This process may be repeated a number of times. If the test keeps turning out unsuccessful, the water is not suitable for SODIS.This process may be repeated a number of times. If the test keeps turning out unsuccessful, the water is not suitable for SODIS

    This process may be repeated a number of times. If the test keeps turning out unsuccessful, the water is not suitable for SODIS.

  • How can I improve the efficiency of SODIS?

    By increasing the water temperature and by raising the UV radiation that hits the bottle, the efficiency of SODIS can be improved. Both the increase of temperature and UV can be reached by placing the bottles on a reflecting surface, f.e. a corrugated iron sheet. Note that WADI does not measure temperature, and does not measure UV radiation coming from a direction other than that of the sun. However, using a corrugated iron sheet for SODIS is highly recommended, and will result in even higher pathogen inactivation rates at the point in time at which the WADI™ display indicates its happy smiley face.

  • Does WADI treat the water?

    No, WADI™ does not treat the water. Please refer to the question “What is WADI™?” for better understanding.

  • Does SODIS or WADI change the appearance or taste of the water?

    No, neither SODIS nor WADI™ changes the appearance or taste of the treated water. The SODIS method improves the water quality without changing its taste or appearance.

  • Are there any toxic substances in WADI?

    No, WADI™ is free from harmful and toxic substances.

  • How long after the disinfection process can the water be used for?

    HELIOZ recommends the consumption of all solar-disinfected water within 24 hours after the disinfection process. This is because the very few remaining harmful pathogens in the water may regrow over time.

  • Does using WADI require any batteries?

    No, WADI™ works without batteries. It contains a built-in solar cell and works energy-self-sufficient via exposure to the sun.

  • Does WADI require maintenance or spare parts?

    No, WADI™ neither requires maintenance nor spare parts. HELIOZ guarantees a minimum WADI™ lifetime of two years, based on normal handling as expected for any electronics equipment. Regular cleaning of the solar cell and of the UV sensor is required to guarantee adequate energy supply and correct measurements.

  • How long can one WADI be used for?

    HELIOZ provides a two-year guarantee on WADI™, and expects WADI™ to function for a longer period of time.

  • How many liters of water can be disinfected during WADI’s minimum lifetime of two years?

    Let us assume that a user would like to disinfect ten 2-liter (68 oz.) bottles of water every day, simultaneously. In doing so, the user would be able to disinfect around 15,000 liters (4,000 gallons) of water during the first two years of operation. Please note that the period of two years refers to the guaranteed minimum lifetime of WADI™. HELIOZ expects WADI™ to function for a longer period of time.

  • Is there proof confirming the functionality of WADI?

    Yes, there is. WADI™ has been tested in various independent laboratories across Europe, Africa and Asia. All of these tests confirm the functionality of WADI™. Our goal of at least a 3log10 (99.9%) reduction of common coliform bacteria in the water was reached in all tests, while in all but one even at least a 4log10 (99.99%) reduction of common coliform bacteria was reached. For detailed test results, please contact us at [email protected].

  • Which pathogens are inactivated if SODIS and WADI are applied correctly?

    Functionality of WADI™ has been confirmed by various field and laboratory tests which have shown that the use of SODIS in combination with WADI™ results in at least a 3log10 (99.9%) reduction of common coliform bacteria in the water, meaning it is very effective against this kind of pathogens. In all but one test a common coliform bacteria reduction of 4log10 (99.99%) was reached.

    Laboratory tests on the SODIS/WADI™ combination were done in:

    •  Austria (05/2013)
    •  Ethiopia (03/2014)
    •  Uganda (05/2014)
    •  South Africa (05/2014)
    •  Thailand (08/2014)

    Scientific research (WHO, 2011) provides evidence that SODIS is also reducing viruses and protozoa. The according verification for SODIS in combination with WADI™ has not yet taken place, but is planned for the near future.

  • Does WADI have an “end of life” indicator?

    Yes, it does. Once the solar cell does no longer produce sufficient energy, the face and the progress bars on the WADI™ display stop flashing and are permanently visible, or not visible at all. In such case, WADI™ must no longer be used.

  • Where can I receive further information on WADI and SODIS?

    For additional information on WADI™ or SODIS, please refer to or If you have any specific questions for the HELIOZ team, please contact [email protected].

  • How is WADI applied correctly?

    • (1) Fill one or more PET-bottle(s) with water.
    • (2) Place WADI™ directly next to the PET-bottle(s), or screw WADI™ onto one of the PET-bottles (like a bottle top).
    • (3) Expose the set of PET-bottle(s) with WADI™ to the sun.
    • (4) Press the reset button.
    • (5) Wait for a happy smiley face to appear on the WADI™ display.
    • (6) Water is ready for cooling-down and/or consumption.
  • Do you have any recommendation on how to use WADI in one’s daily life?

    Yes, HELIOZ recommends the inclusion of WADI™ into its users’ daily routine. The best way to ensure efficient use of WADI™ is to start the disinfection process of one or several bottles of water each morning. As soon as the water is ready, verified by the WADI™ smiley, users may drink the water, take the bottles to a cool storage place or use it for other purposes. While doing so, users may already disinfect several more bottles of water, after resetting WADI™ by pressing the reset button. This process may be repeated until dark. If preferred, users may even let the water cool off over night, in order to have fresh and safe drinking water by the next morning. If applied daily, this routine will ensure continuous availability of fresh and safe drinking water throughout each day. 

    For better understanding, let us provide a numerical example: Let us assume that a user would like to disinfect ten 2-liter (68 oz.) bottles of water every day, simultaneously. In doing so, the user would be able to disinfect around 15,000 liters (4,000 gallons) of water during the first two years of operation. Please note that the period of two years refers to the guaranteed minimum lifetime of WADI™. HELIOZ expects WADI™ to function for a longer period of time.