When camping, hiking, backpacking or traveling the Classic 3 with Pre-Filter is the perfect pack to keep you safely hydrated. Classic 3's UV light destroys over 99.9% of waterborne microorganisms that cause illness. With SteriPEN Classic 3, one set of four alkaline batteries will treat 100 half-liters or 50 full liters of water. Or supercharge your SteriPEN with a set of lithium batteries and you'll treat up to 150 liters! Classic 3's enhanced LED indicator panel lets you know treatment size and status of batteries and UV lamp. Classic 3 is sleek and ergonomic with an easy-to-remove twist-off lamp cover. Like all SteriPEN UV water purifiers, Classic 3 is simple to use: remove the lamp cover, press the activation button, submerge in water and stir. In 48 seconds you have 1/2 liter of safe drinking water or purify a full liter in 90 seconds. Compared to standard chemical treatments that can take up to four hours, that's safe water fast without adding chemicals that can leave an unpleasant aftertaste. The 40x40 micron screen Pre-Filter fits perfectly on wide-mouth one liter water bottles and removes particulates that can get in the way of UV light in water. The Pre-Filter is designed to be stored directly on a wide mouth bottle and the SteriPEN Classic 3 fits perfectly in the filter. Just insert Classic 3 in the filter opening, turn the bottle upside down, and gently rock to circulate light throughout bottle. The tapered end also fits snugly in plastic water bottles. Just insert the lamp into the water bottle, turn it upside down and gently rock the bottle.