CHEMICAL CHANGES 



121 



by the carbonaceous matter, which was thereby oxidized, and 

 the nitrous again absorbing oxygen and re-forming nitric. In 

 Nos. I and 3 the nitrogenous organic matter has taken part ; 

 but the change, as a rule, is not a Gayon and Dupetit one (see 

 p. 126), as no loss of total N as gas has occurred. It is simply 

 an oxidation of carbon. 



The Third Stage, that of Complete Aeration, 



comprises the final oxidation of the nitrogenous and carbon- 

 aceous residues, and includes the formation of nitrates, or 

 nitrification. 



The amount of oxygen required for the processes of nitrifica- 

 tion and nitrosification is shown in the following table : 



One Gramme of Nitrogen requires- 



For produc- 

 tion of — 



N,0, 

 N,05 



n;o 



Grammes of 

 Oxygen. 



Litres of 

 Oxygen. 



2-85 



17 

 1-13 



0-57 



2'0 

 1-2 



0-8 

 0-4 



Litres of Air. 



Litres of Oxygen- 

 saturated Water 

 at 7 c.c. per Litre. 



IQ-Q 

 6-0 



4-0 



2*0 



286 

 170 

 114 



57 



So that to nitrify in an effluent 5 parts of nitrogen per 

 100,000 (i gramme in 20 litres) will demand about half its 

 volume of air, or about 15 volumes of fully aerated water. 

 This explains the comparative failure and frequent collapse of 

 filter-beds in large masses, especially if the fluid is a raw sewage- 

 or a merely screened or precipitated effluent without preliminary 

 hydrolytic change, as with every 100,000 gallons of sewage 

 about 50,000 gallons of air must be continuously supplied. 



Contrivances like fountains, cascades, and weirs can only 

 raise the dissolved oxygen to the saturation of about 7 c.c. per 

 litre, or 700 gallons per 100,000 ; although useful, if simple, 

 like the aerator at Exeter,^ they are quite inadequate. 



At Manchester in 1897 Mr. Fowler investigated the puri- 

 fication of chemically precipitated effluents by simple exposure 

 to air. He found that shallow layers and a considerable period 

 of time were necessary, and that " it is not probable that 

 aeration sufficient to at all adequately oxidize an effluent could 



^ In November, 1896, I found that while the septic tank effluent contained no 

 oxygen, after passing over this aerator it had dissolved 0-56 c.c. per litre. 



