50 SEWAGE AND ITS PURIFICATION 



usually be neglected. When, however, the oxygen is low, the 

 reagents, being presumably saturated with oxygen under 

 atmospheric conditions, will make the result too high. The 

 correction then to be applied is : 



looo^- Rw 



Corrected oxygen in c.c. per litre 



V-w 



where a = c.c. of oxygen found by titration, V = volume of bottle, 

 n that of reagents, while R is the c.c. of oxygen in a litre of 

 saturated water at the temperature of the experiment, which is 

 preferably actually determined, or can be obtained from a table 

 such as Roscoe and Lunt's. 



The approximate " percentage of saturation," or " aeration 

 figure," is calculated by multiplying the dissolved oxygen in 

 the sample by lOO, and dividing by the normal dissolved oxygen 

 at the temperature, as obtained by testing tap water, or from 

 a table. The figure becomes much less reliable if any great 

 changes of temperature (or pressure) have occurred during the 

 sewage treatment. 



The rate of change of the free oxygen and of the nitrates 

 and nitrites, and the *' oxygen consumed," when controlled by 

 a chlorine determination, measure the activity of bacterial 

 processes. 



Dissolved Nitrogen and other Gases. — When these can be 

 separated and measured in a gasometric apparatus, they give 

 further indications, as Adeney has pointed out, of the nature 

 of the changes in sewage and in river and sea water. But his 

 figures show that the proportional range of variation of the 

 nitrogen (usually 13 to 15*5 c.c. per litre) is much less than 

 that of the oxygen (7*5 to nothing), and that the amount of 

 CO2 is largely affected by rain. Dr. Adeney assumes that the 

 N and O are derived entirely from the atmosphere, and should 

 have the ratio of nearly 2 to i in which they are dissolved by 

 pure water exposed to the air. He then calculates the per- 

 centage of saturation with oxygen (taken as an index of the 

 rate at which the liquid is absorbing oxygen, and therefore the 

 degree to which it is polluted) from the dissolved nitrogen 

 found. Xhus : 



Dissolved O found x 100 . . ,, 

 ^ p^ . -T = " percentage 01 saturation. 



This is not safe, because (i) the dissolved nitrogen is increased 

 by bacterial action — in some cases he records more nitrogen 



4 



