Abstract: A septic tank for storing and treating wastewater may be provided. In one implementation, a septic tank, having a first lengthwise end, an opposing second lengthwise end, a top, a bottom, a horizontal centerplane, and a vertical lengthwise centerplane, and a basic wall thickness T may be provided. The septic tank may include corrugated walls and an oblong flange lying nominally in the horizontal centerplane. The tank may further include opposing endwalls, each endwall integrally connecting an end of a first sidewall to an end of a second sidewall. The endwalls may include an endwall top, an endwall side running from the upper half tank flange to the endwall top along an inward slant, and the endwall side may along a curve which is a semicircle or a portion of a curve.