216 Introduction to the Study of Science 



and millions to the cubic inch. Some of these dissolve organic 

 matter, converting it into compounds that are not only free from 

 dangerous or offensive elements, but also valuable as fertilizer. 



There are in general three kinds of bacteria in sewage, two of 

 which are harmless to human health and are beneficial in such 

 conditions as the septic tank provides. The third kind is harm- 

 ful, disease-causing, or pathogenic, as it is usually described. 



The bacteria of one of the harmless groups flourish only 

 where the supply of air or oxygen is abundant. They work 

 near the surface of the soil, as in matter spread thinly over the 

 ground or in wood such as fence posts, where one may find evi- 

 dence of their work in the decay near the surface of the ground. 

 These bacteria throw off waste products containing much more 

 oxygen than do the substances upon which they live. For this 

 reason as well as the fact that they depend upon oxygen for 

 their existence, they are called oxidizing or aerobic bacteria. 



The bacteria of the other harmless group which promotes 

 decay of organic matter thrive in the absence of air or oxygen, 

 that is, in just such conditions as are provided by the septic 

 tank. These become firmly established in such a place, and 

 liquefy, putrefy, and purify all its contents. Because they 

 thrive in the apparent absence of oxygen, they are called 

 anaerobic bacteria. The central division of the septic tank is 

 most favorable, for there the sewage is still or at most moving 

 very gently. They reside mostly in the sludge formed on the 

 bottom, although the slow eddying of the liquid brings some to 

 the surface, where they inoculate incoming matter and start 

 its decomposition. 



It is easy now to understand why at least three divisions of 

 the septic tank are necessary. Because sedimentation and 

 agitation of the liquid are unfavorable to these bacteria, the 

 first compartment is provided to allow settling and prevent 

 the disturbance of the second or middle division. In the first 

 section the incoming matter loses its sediment and is slowly 

 and gently transferred into the middle compartment. The 



