166 



SCIENCE. 



[N. S. Vol. XVI. No. 396. 



weather. The gas wliich escapes from tiie 

 liquid, when closed tanks are used, is con- 

 ducted off, and, containing large amounts 

 of mai-sh gas, often 76 per cent., can be 

 used in the same way as natviral gas for a 

 fuel or for an illuminant. 



This self purification of sewage accom- 

 plishes almost as much as chemical treat- 

 ment, removing about 50 per cent, of the 

 putrescible substances contained in the 

 jsewage, and besides saving the cost of the 

 ■chemicals used in the chemical process, 

 leaves very much less solid matter in the 

 tank to be removed and pressed. The 

 odor, however, from the liquid that leaves 

 the septic tank is often most disagreeable, 

 differing in this waj^ from the liquid from 

 chemical treatment, and with a sewage 

 containing certain kinds of manufacturing 

 wastes, as large amounts of free acid, 20 to 

 25 parts in 100,000 parts, or other sub- 

 stances Avhich act as germicides, the septic 

 tank can not be used. 



The great practical value of the septic 

 tank is that— removing, as it does, half of 

 the impurities— it reduces the amount of 

 sand area required for the purification of 

 sewage. By the intermittent filtration pro- 

 cess, where the anaerobic and the aerobic 

 bacteria are working side by side, only 

 50,000 to 75,000 gallons can be treated per 

 day on one acre of sand area ; by the use of 

 a septic tank, which is providing a sepa- 

 rate work-house for the anaerobes, about 

 five times as much sewage can be treated 

 on the same area. Further, the septic 

 tank has made the English method of con- 

 tact beds possible. 



The value of the septic tank is now fully 

 recognized and the process is used not only 

 in England, but in many places in this 

 country, and a few of the photographs I 

 have taken I will have thrown on the 

 screen. 



Contact Bed Treatment.— This method 

 is the result of experiments made by W. J. 



Dibden on London sewage. It differs from 

 intermittent filtration in that the sewage 

 instead of being applied slowly and al- 

 lowed to drain through a bed of sand, is 

 run rapidly into a water-tight bed, filled 

 with a coai-se material as cinders, coke or 

 broken stone, and retained in the bed for a 

 given number of hours, after which the 

 liquid is quickly run out of the bed. 



An English contact bed is a water-tight 

 bed, one fourth to one half an acre in area, 

 three to four feet deep, thoroughly drained, 

 and filled with almost any hard, jagged 

 material, as burnt clay, coke, cinders, stone, 

 broken to a size that will be rejected by a 

 quarter-inch mesh, but that will just pass 

 through a half -inch mesh. In most places 

 the beds are built on two levels, so that if 

 sufficient purification is not accomplished 

 by the firet bed, the liquid from that bed 

 can be run upon a second bed at a lower 

 level. Such a plant is called a double con- 

 tact system. 



This process is not adapted for treating 

 crude sewage, as the action of the bed is in- 

 tended to be very aerobic and the method 

 of working the process is to fill the bed as 

 quickly as possible, usually in one half 

 hour, with septic tank or chemically 

 treated sewage, allowing the bed to remain 

 full of liquid for two or three hours, and 

 then by opening valves in the drainage 

 system, to empty the bed, in about the 

 same time as was required for filling. 



The same bed can be used only twice, or 

 at most three times, in twenty-four hours, 

 and to keep it in good condition it must 

 be allowed to remain idle one day in seven. 

 AVorking in this way, about 500,000 gallons 

 of sewage can be purified on one acre of 

 contact beds per day, which is certainly 

 over five times the quantity that can be pu- 

 rified by the intermittent filtration process. 



The action of a contact bed depends on 

 the presence of aerobic bacteria. If the 

 filling material of a contact bed in active 



