By far the smallest, lightest and most convenient is a Steripen which will kill everything but wont remove heavy metals or floating contaminants. The large particles can easily be filtered with a coffee filter. If you're worried about metals or other contaminants get the cheapest, smallest, lightest ceramic filter you can and couple it with a steripen for bacteria and viruses.

http://www.steripen.com/

Whatever you decide - get a filter with a good backflush system. Field cleaning is a recipe for lost/broken/contaminated parts.