Abstract: A water purification system including a rectilinear housing having a plurality of vertical baffles spaced therein. The ends of each of the baffles are located a certain distance below the housing upper edge or above the housing floor to allow for the flow of washwater therethrough from a housing inlet to a housing outlet. A rectilinear array of media, e.g. corrugated plastic, is located between the baffles for harboring bacteria, and an air supply means is provided therebeneath. A feeder mechanism is also provided on an exterior wall of the housing for feeding supplemental bacteria into the housing to make up for bacteria depleted through the operation and use of the system.