28 



SEWAGE AND ITS PURIFICATION 



with a white scum of fatty Hme salts that tends to clog filters 

 and leave a greasy deposit on channels. Fixed alkalinity also 

 appears, with an increase in the sodium salts ; subsequently the 

 sulphuretted odour of vegetable washings is evident, and the 

 liquid may even become temporarily acid. The road detritus 

 and heavier matters are usually caught in a grit chamber, while 

 paper, string, and animal and vegetable fragments are commonly 

 carried forward with the mixture, which rapidly becomes black, 

 alkaline, and putrescent. The effect of mere mechanical strain- 

 ing or filtration is shown by the following table of averages of 

 thirteen hourly samples from 6 a.m. to 6 p.m. from different 

 sewers of a large town on the water-closet system. 



The suspended solids contain about one-third the organic 

 nitrogen and half the carbonaceous matter of the sewage. 



Table showing Variation in Quality of Sewage in Different 

 Sewers of the Same Town. 



