SEPTIC TANK

PROBLEM TO BE SOLVED: To thin the wall of an approximately rectangular septic tank made from FRP, so as to make the tank lightweight, by providing rounded recesses on both sides while leaving a flat part behind at the center of the bottom surface of the tank, for reducing stresses applied to the corners of the bottom. SOLUTION: A septic tank 1 made from FRP and having an approximately rectangular shape on a projected plan shape has a rounded recess 6 formed in the center of its curved corner part 5 rising from its bottom surface 1B and leading to a side face 4. The recesses 6 are formed on both sides while leaving the flat part of the bottom surface 1B behind, with the depth and width of each recess made as large as possible. The recesses 6 may be provided in plural portions of the septic tank 1. Therefore, stresses generated at the corners of the bottom can be reduced by reinforcing the bottom of the septic tank 1 where earth pressure is highest, without impeding flow along the bottom of the septic tank 1. Therefore, the tank wall can be thinned for weight reduction. Also, the recess 6 of the septic tank 1 can be used as a hooking part for a hanging rope or the like for safe transportation and construction work.