174 SEWAGE AND ITS PURIFICATION 



of sewage are all basic like ammonia, or readily combine with 

 chemical reagents. Acids and many other chemicals, when 

 added to urine, faeces, or vegetable refuse, develop a very 

 unpleasant odour, which may be often noticed in the vicinity 

 of works where organic matters are treated. Substances like 

 indol and skatol, from faeces, are very weak bases, and readily 

 escape in the vapour even from acid solutions. 



In distilling sewages or contaminated waters for ammonia 

 and albuminoid, the distillate will be found to have a peculiar 

 nauseous, somewhat aromatic odour, which is so constant that 

 in waters it points strongly to sewage admixture. When in 

 considerable quantities, the compound causing the smell 

 collects as a greasy white scum on the top of the distillate. 

 On account of its ready volatility, and its not combining with 

 reagents, it is very difficult to separate, but from large volumes 

 of sewage I have obtained it as a white neutral crystalline 

 substance. In small quantities, it floats like a grease on the 

 surface of water : from its odour and general occurrence, though 

 in minute amount, it would seem to be an important cause of 

 the residual sewage odour when ammonia, etc., have been 

 removed. 



The volatile oil giving the chief odour to urine has also been 

 isolated : it is neutral and does not readily combine ; the same 

 would be the case with essential oils from vegetables, hydro- 

 carbons like naphthalene from gas-tar, etc. Among acid 

 compounds, phenylacetic acid, which I have isolated from 

 effluents, has a strong odour. Ethereal salts, like mercaptan, 

 may also be mentioned among the many substances which 

 may render chemical deodorization inefficient. 



2. Chemicals, in the quantities that were applied, did not 

 kill the organisms of putrefaction, and only to a slight extent 

 reduced the organic matters in solution, therefore the effluent 

 soon resumed a condition of turbidity and foulness. Some 

 chemicals render the liquid acid, others unduly alkaline — both 

 objectionable features. We have already spoken of the increase 

 of the sludge by precipitants ; while the difficulty of sterilizing 

 it is well known. 



Where expense is a secondary factor, as, for instance, in 

 some of the effluents from hospitals, metallic salts are of 

 service, notably those of copper, on account of their combining 

 with sulphur and ammonia, and their marked germicidal pro- 

 perties. The easy removal by lime and sand filtration, with 



