This one's good for travel but is also good for the survival kit and would have been handy back when I was a kid and we had to take a boot full of plastic bottles filled with water from home to the bach because the tap water there wasn't safe to drink.
Water purification tablets
They sell them at Kathmandu and if you are canny (wait for one of the many sales), you never have to pay full price for anything there. I got a packet of 50 for $7. That makes 50 litres of water. Much, much cheaper than buying it by the bottle. An added bonus is the feeling of smugness when others are paying upwards of $5 for a water bottle at Machu Picchu when you can quench your thirst by visiting the sink of a public restroom.