The tank should generally be placed with its top at the level of the ground surface, and the sewer from the house should enter the end of the tank with its flow line or invert 12 inches below the top of the walls. The house sewer or drain should have a grade or fall of not less than 9 inches in 100 feet. Preferably, the sewer should be laid at the above minimum grade for at least 50 feet or so before it enters the tank in order to prevent excessive velocity in the sewage flow at this point. At the entrance to the tank the sewer should be provided with an elbow so that the sewage will be discharged downward below the surface. Similarly, if an outlet pipe from the tank is used, as shown in Fig. 5, this pipe should pass through the wall at the outlet end of the tank, one foot below the top of the tank, and should also be provided with an elbow which will start from below the surface. Where a siphon is to be used to discharge the effluent from the tank onto a filter or into a system of sub-surface tiling, the separate chamber in which the siphon must be placed may be built as an extension of the settling tank so that the end wall of the settling tank will serve as one of the walls of the siphon chamber. The siphon chamber floor may be placed considerably above the level of the floor of the tank as shown in Figs. 2 and 3, since a sufficiently large quantity of effluent for dosing a filter or a sub-surface irrigation system may be collected in the chamber of reduced depth thus provided. This shallower construction saves excavation and also reduces the operating head or fall, which latter is sometimes hardly equal to the demands of the subsequent treatment. The capacity needed in this chamber for different installations will be given later in the discussion of sewage filters and sub-surface irrigation systems. Having determined upon the dimensions of the tank and selected the site, the construction is commenced by making the excavation about four feet wider and longer than the outside dimensions of the tank and siphon chamber combined, in order to provide room for setting the forms for placing the concrete, provided concrete is to be used in its construction. With brick walls an additional width and length of two feet is needed.