In survival, we have the rules of 3:
3 minutes without air
3 hours without shelter
3 days without water
3 weeks without food
Depending on the terain, season and weather, shelter and water can change places. Dependably clean water is a basic human need. Go without it for just a few days and see what happens. In a previous article about eh Steripen UV water sanitizing system (link) , I wrote this story
Even bottled water may not be safe. I once caught a vendor in Thailand who was extracting the clean water from the bottles [by making a small incision in the bottom of the plastic bottles], replacing it with local tap water and then resealing the bottom with a dab of crazy glue. Bottled water can be bad because of foul play or simply because the contents weren’t treated as thoroughly as you would like.
“An estimated 10 million people develop Traveler’s Diarrhea each year. High-risk destinations include developing countries of Latin America, Asia, Africa and the Middle East.” - U.S. Center for Disease Control
Those who have travelled with me (business or pleasure) know that my Steripen never leaves my sights. As much as I want to travel light and size down my travel (or camp/survival) gear, water purification is one thing that can't be squezed or shrunk down. There are in-field alternatives if you are stuck and need clean water (like boiling for 10 minutes) but these are much more demanding than simply being prepared.
Now the Steripen means I can drink most clear water without too many worries (I have never gotten sick even when travelling to dangerously remote locations with no sanitation), it doesn't remove particulate matter from the water. You could carry a large and heavy water filter (which I know you won't) or you can use a LifeStraw.
The LifeStraw is small and weighs next to nothing (less than 2 OZ).
Using the LifeStraw is as simple as putting one end into the water and sucking on the other end. At first, I thought I would have to become the human sunction pump but after a few normal sucks, water starts to flow (relatively easily). It's so easy my 4 year old daughter was able to use it. And the resulting water tastes clean, fresh and pure. It many cases it tastes better than the crappy bottle water you spend $3 on.
Each straw gives you about 1000 liters of pure fresh water. It is a 0.2 micron filter that elimites up to 99.99% of disease causing bacteria and protozoa. for what its worth, this meets all EPA standards for clean and safe drinking water so it's probably cleaner than your tap water.
This is a piece of kit I highly recommend. It is now a permanent tenant in my travel, camping and survival kits next to my 2 Steripen sanitizers. It's cheap (roughly $US20), no maintenance and reliable.
I have tried the Lifestraw with river water, pond water, stagnant water and have never had a problem.