while my first line is a katadyn pocket pump, my back-up is a $2 mini squeeze bottle of TINCTURE OF IODINE 2% (5 drops/qt on warm clear water then wait 30 minutes, or 10 drop/qt if water is cold or dirty, wait one hour). bad taste in water can be removed by adding vitamin C pills/powder after water is ready to use.

available at any Rx or big-box store...