2. Plain Settling Tanks.—These are large basins in which the sewage is retained from one to twelve hours. The sludge' is removed frequently and is not permitted to undergo decomposi tion in the tank (Fig. 13).
3. Septic Tanks.—In construction these tanks are similar to the foregoing and differ only in the method of their operation. The low velocity of the sewage in passing through the tank results in its detention for 8 to 24 hours, so that practically all of the suspended matter is deposited. This is permitted to accumulate at the bottom of the tank for long periods, where, in the absence of free oxygen, it undergoes anaerobic decomposi tion. As a result of the decomposition the volume of the sludge is very much reduced. Bubbles of gas from the decom position changes in the sludge, may bring extensive masses of sludge to the surface where it floats as a scum several feet thick. Removal of the sludge is generally only undertaken when such a volume has accumulated so as to seriously reduce the capacity of the tank (Fig. ILO.
4. Two Story Digestion Tanks.—Several different designs of digestion tanks belong to this group, of which the best known are the Imhoff tanks. From their construction the sludge is permitted to settle in a lower chamber, where it undergoes anaerobic decomposition out of contact with the fresh flowing sewage. As a consequence the effluent of these tanks is less offensive, and the digestion of the sludge in the lower chamber is more complete (Figs. 15, 16).
5. Chemical Precipitation.—This method is of very limited application and its use is practically limited to those communi ties where iron wastes in the sewage render the accomplish ment of a precipitating reaction easy. The floculi thus formed enmesh the fine suspended particles and hasten their removal. After the reaction of precipitation has occured, the sewage is run into large open tanks for sedimentation to take place. Precipitation is accomplished by the employment of lime, rous sulphate or alum. The principles are similar to the em
ployment of coagulants in water purification which will be spoken of later in more detail.
6. Activation.—In the sedimentation chambers previously described, the decomposition of the sludge is anaerobic in character, offensive gases are produced and the stable sludge finally produced has little value. In the activation process, compressed air is admitted to the tanks through porous plates in the bottom, and as a consequence aerobic processes of decom position are facilitated. The sludge is rapidly oxidized, is non offensive and has considerable value as a fertilizer (Fig. 17).
7. Roughing Filters.—These are large vats or tanks filled with coke, crushed stone, or similar material, and filled with sewage. As a consequence of the rock or other inert material present, sedimentation is facilitated, since the vertical distance a sus pended particle has to pass to reach a firm support is very much reduced.
7. Sludge Disposal.—The amount of suspended matter collected in the tanks is considerable and since (except in that collected in the activation process), its value is slight or nothing, the disposal of sludge is usually considerable of a task and a problem. Digestion measures considerably reduce the volume to be disposed of, but do not altogether dispose of the sludge. The following additional measures are employed: (a) Pressing the sludge in filter presses to remove the surplus moisture. This is necessary to permit its ready handling. After a drier sludge is secured it is either: (b) Used for filling low areas of land, or (c). Loaded onto cars, or barges and dumped at sea.
(d) Some of the more stable sludges, for example those secured from the Imhoff tanks, are rim off into drying beds. When dry the sludge is of loamy consistency and is then scraped off and buried in trenches.