Portable water filtration system replacement filter for the ND 28oz and 38oz portable water bottle and canteen. When you are travelling in areas where water quality is suspect, the unique ND water filtration system makes life easy. It gives you a convenient and low-cost alternative to boiling, purification drops and bottled water, and is suitable for travel throughout the world. Simply fill your ND bottle from any tap, stream, river or lake, even in third world countries. Enjoy safe, clean drinking water straight away.