an improved aeration device for septic tanks which allows fine bubbles of air to aerate waste material in the tank so that aerobic bacteria may fully perform their decomposing function . air enters the upper end of a shaft and exists adjacent a propeller . the propeller is protected from interfering with its bubble formation action by a guard bushing concentrically positioned in the air tube so that typical non - organic waste adulterants such is plastic and rubber materials , that are often found in septic tanks , are not drawn into the propeller to interfere with its action .