Only WC wastes go to the septic tank, and paper is used for anal cleaning. Such a calculation is beyond the scope of this book, and reference should be made to a manual on reinforced concrete or masonry design.ĭesign a septic tank for a household having five occupants in a medium-density housing area in which the houses have full plumbing. This is most likely when the tank is empty. This can be prevented by increasing the mass of the structure (e.g., by increasing the thickness of the floor or walls) or reducing the amount of the tank that is below the water table.įor large tanks, such as for a school or a number of houses, it is necessary to check that the side walls of the tank are not likely to collapse owing to the outside soil and water pressure. If the mass of water displaced is greater than the total mass of the empty septic tank then the tank may float. Multiply the volume by the density of water (1000 kg/m 3). Measure the volume of the tank (outside dimensions) between the highest groundwater level and the bottom of the tank. This will only happen if the groundwater level is higher than the bottom of the tank.Ĭalculate the mass of the walls, floor, roof and any baffle walls (concrete: 2400 kg/m 3 brickwork: 1500 kg/m 3). Flotation will occur if the total mass of the empty septic tank is less than the mass of the water it displaces. Since septic tanks have sealed walls and floor, the design must be checked to make sure that the tank does not float out of the ground. Depth of tank from floor to soffit of cover slab

3 stage septic tank