Primarily I prefer to boil but my go to emergency method when that is not an option is chlorine dioxide tablets. It will kill viral, bacterial and cyst based threats. I do have a water filter straw but that alone won't eliminate all threats (viruses are too small and will get through). Its primary advantage is allowing me to drink water treated with chlorine dioxide in 15-20 min (after bacteria ane viruses have been killed) instead of having to wait 4 hours for it to kill the cysts too, and for taste. Which is why I don't invest in the super expensive pump type filters. Hell a Millbank bag would work if you are just trying to keep the chunks out.

It seems to me steripen is a decent setup but it doesnt fill a niche for me at the moment. I would imagine because it depends on exposure to UV light that water would have to be pretty clear for it to be effective and might need to be used in conjunction with basoc filtration (even handkerchief) for best results.

Miox makes a water purification setup that uses batteries and salt to make an ionized solution that is supposed to work well and IIRC you can tweak it to purify different amounts..