Abstract: A filter unit for use in a casing disposed in a septic tank which includes a plurality of stacked disc-dam units to form a cartridge having a longitudinal on each disc-dam unit having a generally circular planar base having a chord edge where the chord edge of each disc-dam is in mutual alignment so that first and second sealing edges are formed at the respective ends of the chord edge where the first and second sealing edges are parallel to the longitudinal axis and where each disc like base includes a continuous elongate dam located on one side of the base and extending outwardly therefrom to terminate at a dam top where the elongate dam has a dam first end adjacent the first sealing edge and extends continuously to a dam second end at said second sealing edge to divide the base into an upstream and downstream side with retaining device to hold the disc-dam units in stacked relation with selected spacing between the dam top of one disc-dam unit and the surface of the next base.