Placing a clear plastic bottle of clear water in direct sunshine for at least 6 hours is a recognized method of water disinfection called Solar Water Disinfection (SODIS), and it does make the water safe to drink from a microbial standpoint. This method is recommended by the World Health Organization (WHO) for household water treatment in developing countries and emergencies. 

How it Works

The process uses two components of sunlight to kill pathogens: 

• UV-A Radiation: The ultraviolet light damages the DNA and other cell structures of bacteria, viruses, and some parasites (like Giardia and Cryptosporidium), causing them to die.

• Heat: The sun's infrared rays heat the water. If the water temperature rises above 45°C (113°F), it creates a synergistic effect with the UV rays, speeding up the disinfection process. 

Conditions for Success

To ensure the method is effective and safe, specific guidelines must be followed:

• Container: Use clear PET plastic bottles (usually 1-2 litre size). Glass bottles can block UV rays, reducing efficiency. The bottles should be in good condition, as scratches can also block UV light.

• Water Clarity: The water must be relatively clear (low turbidity) for the UV light to penetrate effectively. If you can read a newspaper headline through the bottle, the water is clear enough.

• Exposure Time: At least 6 hours of bright, direct sunlight is required. On cloudy days, exposure must be extended to two consecutive days.

• Placement: Place the bottles in a position to receive full, direct sunlight, ideally on a reflective surface (like a corrugated iron roof or aluminum foil) to increase efficiency.

• Storage: Once disinfected, the water should ideally be stored and consumed from the same bottle to minimize the risk of recontamination. 

Important Limitations

• Chemicals: SODIS does not remove toxic chemicals, heavy metals, or industrial pollutants.

• Microplastics: While the amounts of plastic chemicals leached into the water are generally considered safe and below WHO guidelines, leaving plastic bottles in the sun for prolonged periods can increase the release of microplastics and affect the water's taste.

• Fire Risk: A clear bottle of water in a hot car can act as a magnifying glass and pose a rare but real fire hazard. 

In summary, the method works for microbial contamination under the right conditions, but it is not a cure-all for all types of water pollutants.