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 



