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The Process
Pollutants in the water, especially if organic, will absorb the ultraviolet rays and infrared radiation, resulting in a slow-diminishing process. It is an example of the basic oxidation reaction, or corrosion. The light rays work the same way UV rays work on skin cells. UV rays have been found to kill skin cells when exposed in great quantity or for long periods of time, thus killing the cells and aging the skin faster. In the same way, the light breaks down the pollutants.
Water Treatment
Water-treatment plants often use this method to get rid of pollutants. They shine ultraviolet light on the water. Usually mercury lamps are installed on the side of the tanks -- so as the water passes through, it is fully exposed for the duration of its travel through the tank. This is sought as a replacement for chlorine in the disinfection process. Visible light, UV and infrared radiation are effective in eradicating pollutants.
Sea Water and Sunlight
Because different plastics are made of different materials, the rate at which they degrade can vary. That is why it is frowned on to litter the ocean with plastics, especially. The plastic rings that usually hold cans of soda or beer together are dangerous because they get can strangle fish, but the rate they degrade is an additional factor. Objects in the water may not always be in direct sunlight, which is the main reason photodegradation in the water is slower.
Adding Catalysts
In some treatment plants, catalysts are added to target specific pollutants or generally speed up the degradation process. The most popular one currently is TiO2, also known as titanium dioxide. It is known to be an efficient combatant of viruses, bacteria, and air and water pollutants. It is odorless and is even used in cleaning solutions because of its effectiveness, and because it is nontoxic.