Hollow Fiber membrane are tiny hollow tubes that look like straws. These tubes have a porous membrane wall and work through size exclusion. Clean water is allowed to go through the membrane while larger contaminants cannot fit through the pores. Blocks bacteria, E. Coli, Cholera, Typhoid, protozoa, Cryptosporidium, and parasites (Giardia). Filters down to 0.1 microns and up to 100,000 gallons. Boxed.