We live at 27° latitude, about 200 metres above sea level and drink water from PET bottles laying around on the soil (SODIS method). They are filled with fresh water from a well and reused over and over again. We drink the water directly from the bottles (without glasses, mugs or cups). We only refill them with water from the well when they are all empty – about once a week. Thus the oldest water is one week old. But also exposed one week to the sun.

A visitor told us, that this is bad idea, because water in non-moving water in the sun becomes full of algae where bacteria can grow. He told us that we just have to think of ponds or lakes which are very green, which seamed reasonable to us.

So, is it true that the sun’s radiation is not enough to kill all the germs, but it’s the other way round and the sun even helps bacteria and fungi to grow?

  • 1
    Your visitor is right and wrong. UV from the sun in prolonged doses (because the UV is weak) can kill many harmful things. He is right that the warmth can also make many other organisms grow. But he is wrong in that most things like algae which grow under these conditions are harmless to drink.
    – user6972
    Commented Jul 6, 2014 at 8:08
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    It's hard to answer your question without understanding something about what you're trying to do. Are you hiking in the Himalayas? Living in a village in Baja California that has sewage running in the streets?
    – user2169
    Commented Jul 7, 2014 at 3:54
  • @BenCrowell: I have added the height above sea level if that helps. We want to not get ill living here without a connection to drinkable water.
    – erik
    Commented Jul 7, 2014 at 11:23
  • @Erik: The point is that we have no understanding of what your water source is, whether it's likely to be contaminated, and if so, with what. If you're hiking in an unpopulated backcountry area, then the answer is probably that your water wasn't contaminated in the first place, didn't need to be treated in any way, and isn't going to be improved by leaving it in the sun. Microorganisms don't just spontaneously arise in uncontaminated water, and they don't all behave in the same way.
    – user2169
    Commented Jul 7, 2014 at 16:22
  • @BenCrowell: Ok, I have improved my question with some details. Hope this helps in answering my question definitively and unambigiously. Or should I better reask this question in the biology.stackexchange?
    – erik
    Commented Jul 8, 2014 at 0:16

1 Answer 1


I'm going to assume that you are employing the SODIS method of water sterilization.

To sum up the details of the process, this method is where you fill plastic pop bottles, (PET), up with rather clear water 3/4 full, shake them up, and let them sit in the bright sun for 5+ hours.

The mechanism this works by is by utilizing the UV radiation in sunlight. The UV itself blasts the microorganisms in the water. The shaking up dissolves oxygen into the water, which then produces ozone, hydrogen perioxide, and other free radicals from the UV light. These dissolved ozone is especially nasty to microorganisms, but they break down quickly and our bodies are not bothered by the amounts remaining.

This method of water sterilization depends on the correct bottle composition, sufficient sunlight, and clear water. If the water is turbid and has a lot of particulate matter, it will prevent the UV radiation from doing its magic.

As far as the still water being a breeding ground for microorganisms, you'd have to leave the bottles sealed for quite some time to have any effect, not the 6-2day timeframe this sterilization procedure requires.

Lastly, microorganisms can grow and multiply in rapidly moving water just as quickly as in a still pond. The reason sourcing water in the backcountry from streams is recommended is because typically, the raging river water has been delivered to the river via rain more recently, than a lake where the water could have been in the lake for years. The motion of the water makes little to no difference. A river flowing out of a lake through a field of horse farms will be a Giardia breeding ground regardless its speed and sourcing that would be far riskier than from a mirror calm lake in the remote mountain wilderness slowly fed by glaciers.

  • @erik: If the UV radiation sterilizes the water, what is going to make it green and ugly? Commented Jul 6, 2014 at 4:25
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    @whatsisname SODIS reduces bacteria, viruses and protozoa, but algae can bloom. "The SODIS method is used to kill germs in the water. While the bottle is being exposed to the sunlight, other, harmless bacteria and organisms that occur naturally in the environment can grow, for example algae or naturally occurring coliform bacteria. However, these organisms do not represent a threat to human health." sodis.ch/methode/faqs/index_EN
    – user6972
    Commented Jul 6, 2014 at 8:02

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