The Sanitary Engineer and the Public Health. 
51 
faction, and it has been found that they do their work with amazing 
rapidity under favorable circumstances. In this process — known as the 
septic tank method — the sewage usually passes slowly through a tank, 
occupying several hours in the journey — rarely more than from twenty- 
four to thirty-six — and is drawn off at the further end without disturb- 
ing the portion that remains, after which the effluent is passed rapidly 
through coarse filters, generally of coke or coke breeze, and issues from 
the underdrains in a remarkably pure condition. The sewage in the 
tank becomes covered with a scum-like coating from one to three or four 
inches thick, in which the bacteria are very active. Heavier particles in 
the sewage are precipitated to the bottom and are there acted on by the 
bacteria which break up the organic compounds rendering many of them 
soluble in water, and which pass out with the contents of the tank, so 
that the accumulation of sludge in the bottom of the tank is compara- 
tively slow. Whenever it becomes necessary to draw this off it is passed 
to coarse filters, or sludge beds, where most of the water is drained off. 
The action of the anaerobic bacteria in the tank not only destroys much 
of the organic wastes, but renders those that remain susceptible of puri- 
fication by thin, coarse filters at a very rapid rate, besides avoiding the 
great quantities of sludge that are so troublesome in the chemical meth- 
ods, and which, despite the fertilizing qualities that it possesses to some 
considerable degree, it has been found almost impossible to dispose of to 
farmers, except in some portions of Europe, even for the cost of carting 
away. The first of these septic tanks to be constructed on a scale of con- 
siderable magnitude was, as far as I could learn, at Exeter, England, 
where it has been in successful operation since 1897. Many such tanks 
have since been constructed, however, and have been found to greatly 
facilitate the process of filtration. 
The State College of Iowa, at Ames, built a purification plant in 1898, 
which combines the principle of the septic tank with direct filtration and 
which treats the sewage of about 600 persons in a very efficient manner. 
The tank holds sewage for only about six or seven hours and is auto- 
matically emptied by means of a Miller 8-inch siphon — all of the con- 
tents except the sludge being emptied each time the siphon operates. 
Under these conditions the tank acts more as a settling basin than as a 
septic tank, but, nevertheless, it has been found that the effect upon the 
sewage is highly beneficial, a partial purification taking place within the 
tank, and the tank effluent being left in a condition to be readily acted 
upon by the bacteria in the filter bed. Professor Marston, who designed 
the plant, has kept valuable temperature and discharge records, and finds 
that the effluent from the filter beds can not be told from ordinary spring 
water, either by appearance or odor, it being a clear, limpid looking- 
water. The plant is situated within 1000 feet of the nearest college 
