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Solar Water Purification

Solar water purification system is a water purification system at household level based on solar radiation treatment and water distillation with additional use of solar heating. It is a combination of two water purification processes, the Solar Water Disinfection System (SODIS)and the solar distillation process. Since SODIS, initiated by Professor Aftim Acra, is only ideal to disinfect small quantities of low turbidity, micro-biologically contaminated water, a solar heated still is added to the system to address the issue of heavily contaminated water( such as sea water, water with high turbidity and water contaminated by heavy metal or pathogenic microorganisms).

For the cases where low turbidity water is not available, contaminated water will be distilled to drinking water using the solar heated still to remove any non-volatile solid impurities such as salts, sediment, heavy metals and microorganisms. Water from some wells or rivulets may be visibly clear (turbidity of less than 30 Nephelometric Turbidity Units), but it may not be drinkable since the water may still contain pathogenic microorganisms. To solve this problem, the contaminated water would be contained in clean, transparent Polyethylene terephthalate (PET) bottles and are exposed to the sunlight for a certain amount time (depending on the intensity of the sunlight) allowing the solar radiation to deactivate any waterborne pathogens in the contaminated water. Solar water disinfection is an effective way to disinfect drinking water as it is recommended by World Health Organization. The solar water purification system uses only solar energy and can be built using recycling materials, thus, the system is environmentally sustainable.

Water distillation is a physical process that filter solid impurities out of fluid based on the difference in the volatility. At a given temperature and pressure, substances with higher volatility (water in this case) vaporizes more readily than the substances(solid impurities) with lower volatility. The water vapor is then directed to a cool region which condenses the water vapor back to liquid state, leaving all the non-volatile solid impurities such as salts, sediment, pathogenic microorganisms and heavy metals behind. However, the distilled water may not be suitable for drinking since it may still contain some volatile organic compounds. The rate of vaporization is proportional to the vapor pressure, fluid surface area and the fluid temperature.

The principle of SODIS is based on Ultraviolet water treatment . It uses two components of the sunlight for the water disinfection process :Ultraviolet radiationand infrared radiation. UV-A radiation(wavelength 320-400 nm) interacts with the DNA, nucleic acids and enzymes of the organic cell, destroys the cell molecular structures which leads to cell deaths. UV-A radiation also reacts with oxygen dissolved in the water producing highly reactive forms of oxygen (oxygen free radicals and Hydrogen peroxide], that can help the germicidal process. Infrared radiation is a long-wave form of sun radiation, it can be felt as heat, as it is responsible for raising the fluid temperature. Studies had proven that 99.9% of microorganisms in the water are eliminated if the water is heated to 50-60°C for one hour. In order to disinfect contaminated water for drinking effectively, it is recommended to expose the contaminated water to full sunlight using clear PET bottles for 6 hours.If water temperatures exceed 50°C, one hour of exposure is sufficient to obtain safe drinking water.When the weather is cloudy for more than 50% , the contaminated water need to be exposed for 2 consecutive days. The treatment efficiency can be improved by raising the fluid temperature and exposing the contaminated water to additional reflecting surfaces such as aluminium or corrugated iron sheets.

The system is consisted of three main components: the solar energy collector, the solar distillation system and the solar water disinfection system. The solar energy collector is a device that collects solar radiation and converts it into thermal energy for the SODIS and the solar distillation process. Solar distillation system is similar to the conventional water distillation system, except it does not vaporize the water at boiling temperature. Solar water disinfection system takes low turbidity, micro-biologically contaminated water and disinfects it to drinkable water with utilization of solar radiation. The process can be summarized in Figure 1. Insulated or thermal resistive piping system is used to connect all three systems and the piping system should be as short as possible to minimized the heat losses. For the water transportation, Polyvinyl chloride(PVC) piping is recommended due to its sufficient chemical resistance.
The idea is first developed by Cansolair Inc., converting solar energy to house heating energy using aluminum can. Solar energy collector is composed of columns of painted black aluminum can, a frame to house the columns and a ventilation for the heat transportation. Before all the cans are glued together to form a collected column, the top and the bottom of aluminum can is needed to be removed. When placed under the sunlight, the columns absorb the solar radiation and heat is convected to the air inside the columns. Due to difference in the air density , warm air would raise to the top of the columns and cool air would be sucked into the columns from the bottom. The warm air flow is then collected at the top of the columns. The columns are painted in black to enhance the radiation absorbability and the size of the columns can be varied for different requirement. Note that the total height of the column is not equal to the sum of exact height of each can since aluminum cans are designed to fit on top of each other with use of groove.

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