IOWA ACADEMY OF SCIENCES. 
245 
tation of sewage is considerable, besides 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 (CH 4 ), nitrogen (N), ammonia (NH 3 ),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. 
