August 1, 1902.] 



SCIENCE. 



165 



quired by the intei-mittent filtration 

 method be so reduced that the construction 

 of artiiicial bacteria beds will be a practi- 

 cal possibility? 



The results of the investigations started 

 by this problem seem to show that by allot- 

 ting distinct abodes to the anaerobic and 

 aerobic bacteria, the work of purification 

 can be more rapidly carried on. The abode 

 allotted to the anaerobic bacteria is called 

 the septic tank, that allotted to the aerobic 

 bacteria, contact beds or continuous filters. 



The Septic Tank.— The septic tank, 

 about which we have all heard so much, is 

 only a modified cesspool, and the changes 

 brought about in the septic tank are only 

 the changes that occurred in the old-fash- 

 ioned cesspool of our forefathers, and it is 

 most interesting, as well as somewhat amus- 

 ing, to see how the old-fashioned cesspool, 

 which only a few years ago was regarded 

 as a breeder of all manner of ills, is now 

 regarded by sanitarians as a most valuable 

 adjunct in the disposal of filth. 



A septic tank is only an open or closed 

 tank through which the sewage is run con- 

 tinuously, b\it at so slow a rate that it re- 

 quires from twelve to twenty-four hours 

 for it to pass through the tank. The sew- 

 age itself, as we have seen, contains anae- 

 robic bacteria, and by allowing the sewage 

 to remain in the tank out of contact of air, 

 these bacteria increase immensely in num- 

 ber, and acting upon the solid and liquid 

 substances in the tank, bring about those 

 changes which are grouped under the name 

 of putrefaction. In other words, the sew- 

 age is purifying itself, or, as Professor 

 Sedgwick has recently said, we have sew- 

 age in the septic tank 'working,' as it were, 

 like apple juice when the latter is being fer- 

 mented in a cask by wild yeasts. In cider- 

 making, sweet apple juice, charged with 

 microorganisms derived from the dust on 

 the sldn of the apples, or from the atmos- 

 phere, or from the sides of the cask, is 



slowly worked over by these organisms 

 and turned into hard cider and event- 

 ually into vinegar. In a similar way, the 

 sewage in a septic tank, charged with anae- 

 robic bacteria, is worked over, or fer- 

 mented, by these bacteria, which in this 

 way remove or completely change a large 

 amount of the polluting substances, and 

 give a product which is no longer crude 

 sewage, but a sewage in which a large 

 amount of the solid matter has been lique- 

 fied or changed into gas. 



The change which sewage undergoes in 

 this fermentation process is very marked. 



The carbohydrates, substances like 

 starch, sugar, wood fiber, paper, are broken 

 down into simpler substances and partially 

 liquefied; the nitrogenous substances, the 

 so-called proteids, are liquefied, much in 

 the same way that the albumen of the egg 

 is changed when an egg goes bad; and the 

 fats, though not so quickly acted upon, are 

 also partially decomposed. In this process, 

 as in all processes of fermentation, gases 

 are evolved, and the amount given off when 

 the process is in the most active state is 

 not small, equaling about one cubic foot 

 for every hundred gallons of sewage passed 

 through the tank. 



The appearance of a septic tank in ac- 

 tion is very interesting. The liquid in the 

 tank is very dark and opaque, the surface 

 being, as a rule, coated over with a thick 

 layer or crust of solid matter, through 

 which thousands of bubbles of gas are es- 

 caping, and the energy of the action that is 

 taking place in the tank is shown by the 

 continual rising of large fragments of the 

 solid matter from the bottom of the tank, 

 and sometimes with such force that they 

 break through the crust. The gas which 

 these masses contain escapes and they 

 sink again to the bottom of the tank. The 

 rising and sinking of solid matter, with 

 the escape of gas, give almost a boiling 

 appearance to the tank in hot summer 



