IOWA ACADEMY OF SCIENCES. 
243 
If no diseases were produced by unpurified sewage, the 
stench arising from it would be sufficient reason for urging 
its purification. In this connection it may be well to state 
that Dr. L. P. Kinnicutt, (15) of Polytechnic, Boston, in a 
paper, “Sewer Air and Mistaken Ideas Regarding It,” main- 
tains with a considerable force of reason that it is not as 
harmful as commonly believed, but even this does not do 
away with the fact that it is decidedly disagreeable. 
Now that we have noticed some of the reasons for sew- 
age purification it may be well to investigate some of the 
various means by which it may be accomplished. In a 
short paper it is impossible to go into details of all the 
various systems or indeed to even consider them all. So 
this paper will be confined to the treatment of the follow- 
ing systems: Natural dilution, sewage farming, chemical 
precipitation, filtration both continuous and intermittent, 
the septic tank, and the combination of several of these 
systems into combined systems. 
The natural dilution of sewage can hardly be called a 
system, and yet it is the only means employed in the vast 
majority of cases. It is nothing more or less than the allow- 
ing of sewage to flow into the natural waterways, seas, etc. 
In this way the concentrated sewage becomes diluted 
(hence the derivation of the name applied) and nature does 
the rest. If it were not for the fact that the majority of 
towns and cities draw their water supply from the rivers 
on which they are situated, in some few cases it might do 
very well. A great many factors must be considered in 
determining the effectiveness of natural dilution, among 
which the most important are the rapidity of the stream 
and the volume of water that it carries. As all the rivers 
in Iowa are relatively small and unimportant this method 
cannot be considered as sufficient in itself in this state. 
The system of sewage farming has been employed quite 
extensively in various places, but is not commonly consid- 
ered as a success. The method employed is similar to that 
used in irrigation. The sewage is allowed to flow through a 
system of trenches provided with flood gates so that the flow 
can be controlled. The theory is, and it is correct, that the 
