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Solar water purification

Objective: 
Technology group: 
Definition: 
Solar water disinfection is a type of portable water purification that uses solar energy to make biologically-contaminated water clean.
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It is well documented that solar energy can be an effective means of cleaning contaminated water. This is because ultraviolet (UV) light destroys the formation of DNA linkages in microorganisms, thereby preventing them from reproducing and thus rendering them harmless. The World Health Organization  lists solar disinfection in clear bottles by the combined action of UV radiation, as well as thermal disinfection (pasteurization) in opaque vessels with sunlight from solar cookers or reflectors and combination systems employing chemical coagulation-flocculation as some of the most promising and accessible technologies for household water treatment.

There are various products available on the market for water purification using solar energy. One method for solar water disinfection (also called SoDis) uses solar energy to make water contaminated with bacteria, viruses, protozoa and worms safe to drink. Water contaminated with non-biological agents such as toxic chemicals or heavy metals require additional steps to make the water safe to drink. It is a simple water treatment method using solar radiation (UV-A light and temperature) to destroy pathogenic bacteria and viruses present in the water. It’s efficiency to kill Protozoa is dependent on the water temperature reached during solar exposure and on the climatic and weather conditions. Microbiologically contaminated water is filled into transparent containers and exposed to full sunlight during 6 hours. Very turbid water with a turbidity of more than 30 NTU cannot be used for SODIS. SODIS is a water treatment method that:

  • improves the microbiological quality of drinking water
  • does not change the taste of water,
  • is applicable at household level,  
  • is simple in application,
  • relies on local resources and renewable energy, 
  • is replicable with low investment costs

Relevant Technology Needs Assessment

How it works

SODIS uses two components of the sunlight for the water disinfection. The first, UV-A radiation has a germicidal effect. The second component, infrared radiation, raises the water temperature and is known as pasteurisation when the water temperature is raised to 70°C- 75°C. The combined use of both UV-A radiation and heat produce a synergetic effect enhancing the efficiency of the process.

Effects of UV-radiation

Solar radiation can be divided into three ranges of wavelength: UV radiation, visible light and infrared radiation. UV radiation cannot be perceived by the human eye. It is a very aggressive radiation that can cause severe damage to the skin and eyes and destroys living cells. Luckily most of the UV-C and UV-B light in the range of 200 to 320 nm is absorbed by the ozone (O3 ) layer in the atmosphere which protects the earth from radiation coming from space. Only a higher fraction of UV-A radiation in the wavelength range of 320nm – 400nm, near the visible violet light, reaches the surface of the earth. UV-A light has a lethal effect on human pathogens present in water. These pathogens are not well adapted to aggressive environmental conditions as they find their specific living conditions in the human gastrointestinal tract. Therefore, they are more sensitive to sunlight than organisms commonly abundant in the environment. UV-A radiation directly interacts with the DNA, nucleic acids and enzymes of the living cells, changes the molecular structure and leads to cell death. UV radiation also reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides). These reactive molecules also interfere with cell structures and kill the pathogens.

Effects of temperature (infrared radiation)

Another aspect of the sunlight is the long-wave radiation called infrared. Also this radiation cannot be seen by the human eye, but we can feel the heat produced by light of the wavelength beyond 700nm . The infrared radiation absorbed by the water is responsible for heating it up. Microorganisms are sensitive to heat. The following table lists the temperature and exposure time required to eliminate microorganisms. It can be seen that water does not have to be boiled in order to kill 99.9% of the microorganisms. Heating up the water to 50-60°C for one hour has the same effect.

Advantages of solar water disinfection (SoDis)

  • SODIS improves the microbiological quality of drinking water.
  • SODIS improves the family health.  
  • SODIS can serve as an entry point for health and hygiene education.
  • Public water supply systems in developing countries often fail to provide water safe for consumption. SODIS provides individual users a simple method that can be applied at household level under their own control and responsibility.
  • SODIS is easy to understand.
  • Everybody can afford SODIS, as the only resources required are sunlight, which is cost free and plastic bottles.
  • SODIS does not require a large and costly infrastructure and therefore easily is replicable in self-help projects.
  • SODIS reduces the need for traditional energy sources such as firewood and kerosene/gas.
  • Consequently the use of SODIS reduces deforestation, a major environmental problem in most developing countries, and SODIS decreases airpollution created by burning conventional energy sources.
  • Women and children often spend much of their time and energy collecting firewood. SODIS reduces this workload as less firewood needs to be collected.
  • Financial advantages: Household expenditures can be reduced when the user’s family health is improved: less financial resources are required for medical care. In addition, expenses for traditional energy sources such as gas, kerosene and firewood are reduced. Only limited resources are required for the procurement of transparent plastic bottles. Therefore even the poorest can afford SODIS.

Limitations of solar water disinfection

  • SODIS requires sufficient solar radiation. Therefore it depends on the weather and climatic conditions. S
  • SODIS requires clear water.
  • SODIS does not change the chemical water quality.
  • SODIS is not useful to treat large volumes of water.

Product examples

[Disclaimer: Products listed here are provided by Climate Technology Network members and represent examples of products available within this technology field. The CTCN does not take responsibility for this product information and cannot guarantee its suitability in specific contexts or regions].

  • Solvatten [Sweden]: Solvatten is a combined portable water treatment and water heater system that has been designed for off-grid households use in the developing world. It is an easy, innovative solution that gives access to clean, hot water to impoverished people throughout the world. The unit has an average lifespan of 7-10 years, meaning it withstands those early years in which children are at their most susceptible to waterborne diseases. Solvatten also creates hot water — up to 75°C. Water at this temperature is perfect for a number of household and hygiene purposes, such as cooking, hand washing, bathing and domestic cleaning. Users that want cooler water can simply place their unit in the shade once the water has been purified. Solvatten can be used on partly cloudy days but not during heavy rain or thick cloud cover.

    How it works:

    1. Opened like a book: Each unit contains two five-litre containers, each of which can be filled with water via an opening that houses a fabric filter of 35 microns.

    2. Fueled by the sun: The unit is then placed in direct sunlight, which simultaneously heats the water and exposes it to ultraviolet radiation.

    3. Purified by radiation: The combination of heat and UV light is a highly effective means of purifying water and, depending on conditions, the water will be free of pathogenic material in 2-6 hours.

    4. Cleared to use: An indicator tells the user when the treatment process is complete - a sad red face changes to a green and happy one.

  • ViroBac™ [Solar Wagon, Water One World Solutions]:  ViroBacTM kills waterborne bacteria and viruses instantly on contact and on demand. The proprietary compound is ionically bonded to a carrier media and works on demand, only if microorganisms are present allowing for extended use up to three years. Tested extensively for over twenty eight years by independent laboratories such as Loyola University Medical School using EPA and ANSI/NSF testing protocol, ViroBacTM has been proven to kill waterborne bacteria and viruses under both continuous and repeat conditions. As a result, Water One purifiers have been used by all branches of the US Military since 1998.

Case studies

Another example of a technology that user solar for water purification is provided by CTCN network member Solvatten. Their product is a combined portable water treatment and water heater system that has been designed for off-grid households use in the developing world:

solvatten_unfccc

More videos from CTCN network member Solvatten:

References

World Health Organization

Solvatten

Swiss Federal Institute of Environmental Science and Technology (EAWAG)

Department of Water and Sanitation in Developing Countries (SANDEC)