Fig. 14.
Dortmund Type of Tank.—Where the volume of sewage is fairly large, and it would be convenient to have the sludge outlet at 2 feet to 3 feet below the level of the invert of the outfall sewer, the advantages of designing the detritus tanks on the lines of the Dortmund tank may be considered. An example of this kind of tank is shown in [Fig. 14]. Tanks of this type have the following special features:—Great depth—from 16 feet to 20 feet below water level—and the bottom in the form of an inverted cone, with an outlet at its apex connected to a cast-iron sludge delivery-pipe, which may be carried up either outside the tank, as shown in solid lines, or on the inside of the tank, as shown in dotted lines. In either case this pipe should be continued vertically up to, and finish with, an open end at the level of the top of the wall of the tank, so as to form a means for inspection and rodding in case the pipe should become choked. From this vertical pipe a right-angled branch is arranged at about 2 feet below the top-water level in the tank, and provided with a sluice-valve. Ordinarily this valve is closed. When it is desired to remove the sludge deposited in the cone-shaped bottom of the tank, the sluice-valve on the sludge outlet is opened and the sludge is forced up by the head of water, due to the difference in level between the top-water level in the tank and the invert of the sludge-outlet. It has been stated that this method of sludge removal is subject to difficulties, due to the consolidation of the sludge in the cone, to such an extent that it becomes of too thick a consistence to flow up the vertical pipe. In some cases a special mechanical contrivance is adopted, by means of which the sludge may be stirred up at the apex of the cone-shaped bottom while the sludge-valve is open. Again, in a special form of tank which has been brought into use in Germany, the sludge is stirred up by means of jets of water, under pressure from the main, forced through a ring-shaped perforated pipe laid near the apex of the cone. In both cases it is evidently assumed that the sludge will only be removed at long intervals, and in the author’s opinion the difficulties referred to above may be avoided by the application of the motto “Little and often,” as described in his book on the management of Sewage Disposal Works.
Fig. 15.—Sludge Scraper in Detritus Tank.
Special Apparatus for Sludge Removal.—A further type of detritus tank is illustrated in [Fig. 15], in which the tank is circular in form but has a flat bottom. The sludge is discharged by the same means as that shown in [Fig. 14], but a special scraping machine operated by hand is used to facilitate the removal of the sludge by drawing it towards the inlet to the sludge pipe, which is situated at the centre of the floor. The scraper, which is manufactured by Messrs. Ham, Baker and Co., is helical in form, and is attached to and rigidly supported by a framework mounted on the central shaft, which is rotated by suitable gearing fixed at the side of the tank over the sludge discharge inspection chamber, so that the operator may be able to regulate the rate of the sludge delivery. It will be noticed that the outlet from this tank is by means of cross-channels, described in detail later in connection with sedimentation tanks.