A waste water separation vessel is provided having a wedge shaped body with an internal water containing volume. The body is configured to trap FOG and wastes within the body allowing water to pass through the body. At least one top facing access port to allow access to the internal volume of the body is provided and the wedge shaped body tapers outwardly from top to bottom to define a larger bottom area than a top area. The side walls include vertically oriented strengthening ribs which define intervening indentations, and each lower end of the vertically oriented ribs is connected to a generally horizontal beam which defines closed bottom ends on the indentations. When the vessel is buried in the ground the wedge shaped body and the closed bottom indentations interact with the ground to help oppose any buoyancy forces acting on the buried body.