IOWA ACADEMY OF SCIENCES. 245 



tation of sewage is considerable, beside^ there is left on 

 hand a sludge which must be disposed of. This would not 

 be a serious drawback if it were valuable as, a fertilizer, 

 but chemical analysis seem to show the contrary to be 

 true. On the whole, chemical precipitation is not re- 

 garded with favor by the majority of experts. 



Filtration is the application of raw or precipitated 

 sewage to beds composed of various substances, either 

 continuously or intermittently. In 1870 the first report of 

 the royal commission on the best means of preventing the 

 pollution of rivers was made. In regard to the filtration 

 method it contained the following: 



"The process of filtration through sand, chalk, or cer- 

 tain kinds of soil, if properly carried out, is the most 

 effective means for the purification of sewage. In con- 

 tinuous filtration the sewage is applied to the beds indef- 

 initely without giving them time to rest. This was found 

 to be unsuccessful so a system of allowing the beds to rest 

 at stated periods was tried and found to be highly success- 

 ful. This latter method is known as the intermittent fil- 

 tration of sewage. This system of filtration recognizes 

 the fact that the active agents in the purification of 

 sewage are minute plants; variously named microbes, mi- 

 cro-organisms, germs, bacteria, etc. Bacteria is the name 

 now commonly accepted and used in scientific writings and 

 discussions. 



Certain species of bacteria have the power of breaking 

 up the complex organic compound of sewage into simpler 

 inorganic harmless compounds. This process is commonly 

 spoken of as nitrification and the bacteria as nitrifying 

 organisms, because the chief inorganic substances formed 

 them are nitrites and nitrates. There are other species of 

 bacteria however that decompose organic materials into 

 various gases, hydrogen (H), carbon dioxide (CO.), marsh 

 gas (CH4), nitrogen (N), ammonia (NHs),etc. Gas-produc- 

 ing bacteria will be spoken of again in connection with 

 the septic tank. 



Filter beds, as those used for filtration of sewage are 

 called, are composed of various materials: sand, gravel, 



