Abstract: A sub-soil wastewater treatment vessel, especially for wastewater produced by single family residences and small businesses and the like. The vessel is divided into at least three contiguous chambers by at least two transverse walls, forming at least two contiguous aeration chambers and a clarifier chamber. Wastewater flows into the first aeration chamber via an influent line, thence through an opening in the first transverse wall into the second aeration chamber. From there, wastewater flows through passages at the bottom of the second transverse wall into the clarifier chamber. Air is supplied to the first and second aeration chambers via low pressure diffuser lines. Three inclined surfaces within the clarifier chamber direct solids through passages in the second transverse wall back into the second aeration chamber for further processing. An effluent line connected to the clarifier chamber permits wastewater outflow.