Water preserver.

[grumpy old man]

i think ?
it is the uv light that sterilise like the pens you can buy and the sun's rays and just being warm makes algee grow
i think like a steam that is small fast flowing and been filter though the ground would be safer because the uv rays can go through the stream to the bottom it is moving fast so it's cold and passes through the ground to the water table.
where as a deeper lake that does not move won't let the uv light go to the bottom kill the bacterea and with it being still and the sun's rays beating down the temp rise's and allows bacterea to grow
i am not too sure i could be wrong
but still filter and boil for 10 mins and drink if it taste flat pour from one cup to another a few times to oxegnate the water it wil taste a little better it works i'am still alive