The sump shall be not less than 18 inches (457 mm) in diameter and 24 inches (610 mm) deep, unless otherwise approved. The sump shall be accessible and located so that drainage flows into the sump by gravity. The sump shall be constructed of tile, concrete, steel, plastic or other approved materials. The sump bottom shall be solid and provide permanent support for the pump. The sump shall be fitted with a gas-tight removable cover that is installed not more than 2 inches (51 mm) below grade or floor level. The cover shall be adequate to support anticipated loads in the area of use. The sump shall be vented in accordance with Chapter 31.