Abstract: A sub-soil apparatus for treatment of domestic wastewater and sewage, such as that generated from single family residences, small apartment buildings, small offices and the like. The apparatus includes two contiguous chambers, the first chamber being a mixing/aeration chamber, and the second being a settling/separation chamber. A transverse wall with lower lateral passages divides the two chambers but allows liquid and solid exchange between the two chambers. The mixing/aeration chamber accepts the sewage to be treated, and turbulent mixing and aeration is achieved therein by a transverse aeration bar. After mixing and aeration, which results in biodegradation of the sewage, the biodegraded liquid, containing fine solids, passes into the settling/separation chamber where quiescent conditions promote solid settling. Three inclined walls of the settling/separation chamber direct settled solids back into the mixing/aeration chamber for further biodegradation.