SteriPen Water Bottle Pre-Filter SteriPEN Pre-Fiter removes particulates from water. Use alone or to ready water for treatment with a SteriPEN handheld, UV water purifier. SteriPEN Pre-Filter fits the openings of Nalgene and comparable wide-mouth water bottles and can be stored on bottle. Features a durable 40-micron filter cartridge to remove particulates from water when filling bottle by submerging or pouringwater into filter. Makes purification easy - insert SteriPEN handheld UV water purifier into center of Pre-Filter, turn bottle upside down and agitate to treat water. SteriPEN Pre-Filter stores easily the your Nalgene bottle when you're on the go.