Can I use sunlight to purify water without a special container?

Question

According to the answers at Does water in the sun get purified from or polluted by algae, fungi and bacteria? & Can sunlight be used to purify water? water in special containers can be purified in sun light in about 5 hours:

Can I purify water with sunlight and without a special container?

Clarify: can water in a shallow pan be purified by sunlight? There are multiple reasons a person might not have a PET bottle, and need to purify water. As an example; hiking in the back country, I would be more likely to have a cooking pan than a supply of plastic bottles. While fire can be used to boil water, why use a fire if sunlight can do the same thing?

Answer 1

PET (Polyethylene terephthalate) bottles are not "special" containers, they are the most widespread containers holding potable water in the world, with more than 100 million used per day.

Now, you don't necessarily need a bottle made of PET, but even a basic reading and understanding of how the SODIS process works should make it obvious why using anything other than a PET bottle is pointless. SODIS is more than just basic sunlight.

SODIS is performed by filling a bottle about 3/4 full, shaking the water vigorously for at least 20 seconds, then putting the bottle in the sun for a minimum of 5 hours, for up to several days, ideally with a reflective element around the bottle to concentrate as much sunlight into the water as is practical.

The way this works is through 2 methods. First and foremost is the interaction with UV-A and the dissolved oxygen in the water. UV-A is barely ionizing radiation, and it will form free radicals and hydrogen peroxides in the water by interacting with the oxygen. These substances are very nasty to microorganisms. Additionally, the entire sunlight band helps warm the water. The warmer the water, the more effective the free radicals and peroxides are. The less UV and heat that can be delivered to the water, the less effective the process will be. If you're in a cold-weather area far in the north, SODIS may never be suitable.

So, to use something other than PET, you need to provide something that doesn't absorb UV radiation, and is effective at keeping the water warm. Glass and Polycarbonate both absorb UV, so with Glass, PC, and PET out, whatever you are left with is going to be far more "special" and uncommon I'm not even going to offer an alternative.

So no, a shallow pan will not work. It neither provides a means to oxygenate the water, nor will it keep it warm. If you are going to hike in the back country, be prepared, and that means not improvising with critical wellness aspects like safe drinking water when it takes almost no effort or cost to bring the right supplies.