Wastewater treatment system and process in which the influent is passed through a mixing chamber and then through a series of filters of progressively smaller size to separate the materials carrying the colonized bacteria from finer solids in the influent and separate the materials according to size. The materials from the filters are progressively reduced to a smaller size, the materials of smaller size are returned to the mixing chamber, and new influent is inoculated with the colonized bacteria carried by the materials returned to the mixing chamber.