p 



CHEMICAL CHANGES 99 



sung). It seems sufficient to recognise the first as a j^drol^c 

 and the second as an oxidation change. 



Calculating from the discharges (p. 54), an average sewage 

 from a water-closet town with a water-supply of, say, 

 25 gallons per head should, when fresh, contain about 10 parts 

 of organic nitrogen per 100,000 ; yet in most cases the sewage 

 of a town contains only from i to 2 parts of organic nitrogen, 

 and frequently less than this amount. The difference must be 

 due to the very rapid breaking up of the organic matter by the 

 anaerobic changes described, and is accompanied by a corre- 

 sponding increase in the ammonia from mere traces up to 

 8 parts per 100,000, and a loss from the evolution of free 

 nitrogen gas and possibly oxides of nitrogen. 



When faecal and other solid matters are first discharged, the 

 earliest changes must be aerobic, because of the free oxygen 

 dissolved in the water and contained in the air. The effect is 

 mainly the same as the last stage — i.e.^ the organisms acting in a 

 normal manner upon those simpler constituents like ammonia, 

 already present in small quantities, into which the process 

 itself afterwards resolves the main ingredients of the sewage. 

 Nitrates in small quantities are consequently often observed in 

 moderately fresh discharges. 



As soon as the free O has been exhausted, these oxidation 

 changes come to an end, and the bacteria which require air in 

 part disappear and in part remain quiescent, to resume their 

 functions at a later stage. On the other hand, the anaerobic 

 organisms will commence to multiply, the nitrate will be 

 reduced to nitrite, and this to nitrogen, according to reactions 

 we shall explain later, and the liquefaction and hydrolysis 

 changes will proceed. This is the~"usual condition when the 

 sewage arrives at the works, and the first, or anaerobic, stage 

 of the treatment proper commences. 



In the second stage aeration is to be encouraged, so that the 

 aerobic bacteria may act and ammonia and carbonic acid be 

 produced with the help of some of the facultative anaerobes. 



In the third stage, with provision of a still larger quantity 

 of oxygen, the nitrifying group will get rid of the remaining 

 products. 



We may summarize the order of the changes as follows : 



7—2 



