The SteriPEN Pre-Filter removes debris from water so the UV light can effectively destroy the microorganisms in it. The SteriPEN Pre-Filter filters fits on Nalgene and other 32 ounce wide-mouth water bottles and filters out particulates and debris. When in use, pour water through the durable 40×40 micron screen filter cartridge to fill the bottle. Then you’re ready to purify the water with a SteriPEN UV water purifier. Remove the filter cartridge from the Pre-Filter, insert the SteriPEN into the center of the filter, flip the bottle, and agitate the water until the SteriPEN indicates the treatment is complete.