Practical preventive medicine . wage is retained from one to twelve hours. The sludge isremoved frequently and is not permitted to undergo decomposi-tion in the tank (Fig. 13). 3. Septic Tanks.—In construction these tanks are similar tothe foregoing and differ only in the method of their low velocity of the sewage in passing through the tankresults in its detention for 8 to 24 hours, so that practicallyall of the suspended matter is deposited. This is permitted no PRACTICAL PREVENTIVE MEDICINE to accumulate at the bottom of the tank for long periods, where,in the absence of free
Practical preventive medicine . wage is retained from one to twelve hours. The sludge isremoved frequently and is not permitted to undergo decomposi-tion in the tank (Fig. 13). 3. Septic Tanks.—In construction these tanks are similar tothe foregoing and differ only in the method of their low velocity of the sewage in passing through the tankresults in its detention for 8 to 24 hours, so that practicallyall of the suspended matter is deposited. This is permitted no PRACTICAL PREVENTIVE MEDICINE 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 thesludge is very much reduced. Bubbles of gas from the decom-position changes in the sludge, may bring extensive masses ofsludge to the surface where it floats as a scum several feet of the sludge is generally only undertaken when sucha volume has accumulated so as to seriously reduce the capacityof the tank (Fig. 14).. FlG. 14.—Septic tanks, Columbus, Ohio. Twenty million gallons of sewage aday can be treated in this recently constructed plant. (Am. Mus. Nat. Hist.,Guide Leaflet 33.) 4. Two Story Digestion Tanks.—Several different designs ofdigestion tanks belong to this group, of which the best knownare the Imhoff tanks. From their construction the sludge ispermitted to settle in a lower chamber, where it undergoesanaerobic decomposition out of contact with the fresh flowingsewage. As a consequence the effluent of these tanks is lessoffensive, and the digestion of the sludge in the lower chamberis more complete (Figs. 15, 16). 5. Chemical Precipitation.—This method is of very limitedapplication and its use is practically limited to those communi-ties where iron wastes in the sewage render the accomplish- EXCRETA DISPOSAL III ment of a precipitating reaction easy. The floculi thus formedenmesh the fine suspended particles and hasten their the react
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectprevent, bookyear1920
