A filter unit for use in a casing disposed in a septic tank comprising a plurality of stacked filter plates. The individual filter plate is preferably substantially "D" shaped in circumference and has a substantially cone-shaped floor, tapering upward. The filter plates are preferably stacked in a superposed, spaced-apart manner with a particular diameter in relationship to the inner diameter of the case. The filter plates are preferably inserted into the filter case to define a seal horizontally around the bottom opening in the case, thereby allowing wastewater to enter into the filter plates to begin the filtering process. Solids preferably flow upward into the filter plates, where they are separated by size with a predetermined gap in the stacked filter plates.