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 



