246 
IOWA ACADEMY OF SCIENCES: 
coke breeze, chalk, clinkers, clay, cinders, ballast, etc. 
Experiments with different materials have been tried at 
various places. The Massachusetts State Board of Health 
has probably done the most work along this line in 
America. 
Dibden and Thudicum of England, however, are the 
pioneers in this line of investigation. There is no small 
amount of discussion as to the relative merits of the vari- 
ous substances used as fillers in filter beds. But no matter 
what the material, the object to be obtained in all cases is 
the same, namely, a substance that will serve as a resting 
place for the gelatinous masses of bacteria. Any substance 
that will do this and still be porous enough to admit of 
complete aeration may be termed a successful filler. 
For plans of beds, materials used, dimensions, etc., no 
better information can be obtained than that in the Mas- 
sachusetts State Board of Health reports, and for the plans 
and specifications of the Iowa State College Sewage Plant 
by Prof. Marston (19). 
There remains yet the septic tank. It is a tank in which 
the sewage is retained for a limited time in order to allow 
the anaerobic bacteria to work. Two kinds have been 
employed, the open and the closed. Most experimenters 
along these lines are now of the opinion that one is as 
effective as the other, on account of the scum (composed 
essentially of bacteria) that covers the sewage in the tank. 
According to L. P. Kinnicutt (16) the following changes are 
due to anaerobic bacteria in a septic tank. First, the decom- 
position of cellulose and allied substances, and the formation 
of marsh gas. Second, the decomposition of complex 
nitrogenous organic matter, with the production of am- 
monia, hydrogen and odoriferous substances. Third, the 
removal of oxygen from nitrates with simultaneous oxida- 
tion of organic matter. 
As has been stated before, the filter bed gives an excel- 
lent opportunity for the action of aerobic bacteria, to 
which, according to Kinnicutt, the following changes are 
due: The conversion of urea, and similar substances into 
ammonium salts, and the conversion of ammonium salts 
