[pct-l] Pct-L Digest, Vol 51, Issue 15

[Michael S]

There are a lot of issues that may be problematic for doing this.

Your first point is probably the most important one.  This only works if you maximize the light rays because SODIS depends upon having the UV-A wavelengths penetrating deep into the water and on increasing the temperature of the water.  In countries where this process is recommended, they actually set up corrugated metal which is pointed at an acute angle to the sun.  You just can't do that with a pack, unless for every turn and every step you hunch over at a 90º angle to the sunlight.  Oh, and if you're in trees, or the day is cloudy, you're pretty much screwed.  A cloudy day takes 2-3 days to disinfect the water.
The bottle must be pristine.  Every scratch or piece of paper or anything on the bottle will refract the light, preventing the UV-A from doing it's job.
As someone mentioned, the turbidity of the water matters.  Mountain streams may be clean, so this isn't a problem, but if you're trying to get the water out of a ditch in the Mojave desert, I'd be concerned.
To maximize the effect, you need to fill a bottle only 2/3 full (and then shake vigorously to dissolve oxygen, which is part of the chemical process that disinfects the water).  You also need to use a relatively small bottle, maybe like the 500ml bottles used for bottled water.  So, in essence, if you could actually make this work, each bottle will give you around 350 ml of water, which is around 10% of your daily needs (if it's cool).  So, you'd need a lot of botles hanging off your pack.
To remain disinfected, the bottle needs to remain in the sun.  The process does not destroy 100% of the pathogens, it just reduces the load to a level that keeps the water from being pathogenic.  Once the bottle is placed in the dark (say inside of your backpack), various organisms start multiplying again, and within a day, your water may get you sick quickly.

SODIS is great if you're trying to produce a boatload of water in one static location.  I think that SODIS is one of the great ideas that have come around, and that's not even considering using plastic waste for a noble purpose.  But for backpacking, I'd bring a good water filtration device which will remove both the bugs and reduce the quantity of suspended particulates.  Once you get past the drier regions of the PCT, you can actually carry less water because you can filter water as you need it (though this depends upon a lot of issues, so I'm generalizing).

Your idea was sound, but just difficult to apply to backpacking.  Even in an emergency, and if you did everything perfectly, you'd be waiting 6 hours for water.  I think it would take just a few minutes to boil it and let it cool down.  I think I even saw an episode of Survivor Man on Discovery where Les Stroud boiled water in a plastic bottle (can't remember how, and I was pretty amazed).

Michael
michaels at skepticalraptor.com
http://www.skepticalraptor.com/skepticalraptorblog.php



On Mar 13, 2012, at 10:00 :02PDT, pct-l-request at backcountry.net wrote:


Message: 8
Date: Tue, 13 Mar 2012 15:10:03 +0100
From: "vet at schroll" <vet at schroll.at>
Subject: [pct-l] SODIS - Solar Water Disinfection
To: "'pct-l'" <pct-l at backcountry.net>
Message-ID: <269F79B39C9C405F9756B763D68946A0 at PC1>
Content-Type: text/plain;	charset="us-ascii"


Does anyone have experience with solar water disinfection - SODIS?
Water should be exposed for 6 hours in PET bottles at sunlight - UVA - which
has the same effect as Steripen, only cheaper and no batteries.

I think that it is maybe not very practical to carry water for 6 hours on
top of the backpack, horizontally, but I am interested anyway if someone
tried this method, maybe as backup?