There is, perhaps, little need for cleaning the tank as often as once in six weeks, but it is generally found and has been affirmed in court testimony that the removing of the sludge from a settling tank once every six weeks will prevent septic action from taking place, and the tank will then be operated as a settling tank and not as a septic tank. This is desirable in view of the fact that royalties have been claimed under certain patents on septic tanks. As explained on p. 11, the important function of the tank is to settle out suspended solids, while the processes that take place in the septic tank but not in the settling tank are of minor importance, and it is advisable therefore to operate these tanks as settling tanks when possible. In no case should the sludge be allowed to accumulate until it fills more than one-quarter of the tank. The sludge may be disposed of by burying in trenches or ploughing under, or it may be spread on the surface at points remote from highways and dwellings or sources of water supply. The depth of accumulated matter in the tank should frequently be tested at the inlet end by using a pole or stick. [Illustration: FIG. 8.—Plan and Longitudinal Section of Modified Imhoff Tank.] [Illustration: FIG. 9.—Vertical Cross-Section of Modified Imhoff Tank.] In reference to the preliminary treatment of sewage in tanks, it should be noted that the most recent development in the design of sewage-disposal plants has been the improved method of sedimentation of sewage represented by the Imhoff or Emscher tank. A modified design of this tank is shown in plan and longitudinal section in Fig. 8, and a cross-section of the tank is shown in Fig. 9. The principle employed is to provide a separate chamber for storing the sludge which results from the sedimentation of the suspended matters in the sewage, this chamber being almost entirely separated from the portion of the tank in which the sedimentation takes place. This separation of the sludge from the flowing sewage is accomplished in the tank shown by inserting in the tank, parallel with the side walls, two inner partitions _AA_, which are vertical for a few feet below the surface of the sewage and then slope toward the centre line of the tank, but, as shown by Fig. 9, do not meet at the centre line, the one passing a few inches under the other. The opening or slot thus formed between the two inner partitions allows the suspended matters which settle out of the sewage flowing through the upper compartment to pass into the lower or sludge compartment and there remain in a quiescent state until removed from the tank. The object of this separation of the sludge from the flowing sewage is to prevent the gas bubbles which emanate from the sludge during its decomposition from rising through the flowing sewage and interfering with the process of sedimentation going on in the upper compartment, and to provide for a more complete decomposition or “digestion” of the sludge. The gas