Abstract: The invention provides a septic tank system of greatly improved effectiveness in maintaining optimum bacterial action for a maximum length of time by utilizing a plurality of series connected tanks with the water level in each successive tank being lower than that in the first tank so that gravity flow is established between the successive tanks. A unique transfer means is provided between each of the successive tanks comprising one or more horizontal pipes having their inlet ends closed, and each horizontal pipe having one or more horizontal slots formed in the wall thereof at a level above the bottom of the pipe bore to permit the removal of scum through said slots by entrainment and/or suction action of the fluid flowing through the transfer pipe, thereby permitting gases to freely escape through the scum from the effluent surface of each tank and preventing the deleterious effects of trapped gases upon the biodegradation action normally produced in each successive septic tank.