A method for obtaining grey water excreta includes reducing ammonia content of the liquid fraction in a first porous medium in which the ammonia content is decomposed, and reducing the byproducts in a second batch of similar porous medium. The byproduct may be decomposed into nitrogen. A system for obtaining grey water from a liquid fraction of human or animal excreta includes: a first bio-filter for reducing an ammonia content of the liquid fraction, the first bio-filter including a first vessel and a first porous medium in which the ammonia content is at least in part decomposed into at least one byproduct; and a second bio-filter for reducing the at least one byproduct, the second bio-filter including a second vessel and a second batch of similar porous medium within the second vessel and in which the at least one byproduct is at least in part decomposed.