158 SEWAGE AND ITS PURIFICATION 



F'e^(S04)3, forming an ochreous deposit which acts as a further 

 purifier. This red deposit often occurs from iron naturally 

 present, and shows generally that the liquid has been so far 

 oxidized. 



Aluminiiun Sulphate is made from bauxite or clay, by treat- 

 ment with sulphuric acid. As sold, it often contains excess of 

 the acid, and samples should always be tested, as the more 

 neutral it is the better. Freedom from iron is not requisite 

 for this purpose ; in fact, "Spence's Alumino-ferric " is a mixture 

 of the crude sulphates of iron and alumina, made in blocks 

 which slowly dissolve. The amount of iron is generally small. 

 At Glasgow 5 grains per gallon of alumino-ferric were used with 

 7 grains of lime, or 2j grains (or more) of sulphate of alumina 

 with 5 grains (or more) of lime, resulting in a production of 

 over a million gallons of wet sludge from 200 million gallons 

 daily flow of sewage. 



Alum, the double sulphate of aluminium and potash, or 

 ammonium, has the advantage of a definite composition, so 

 that an exact quantity can be used, but is precluded by its 

 cost, and also by its leaving behind the alkaline sulphates. 

 In local purifications on the small scale it has often been of 

 use. 



Iron Salts. — When chemical precipitation was prevalent 

 there was much controversy as to whether ferrous or ferric 

 salts should be used. The former were cheaper, in the form of 

 ferrous sulphate or "green copperas," but had the disadvantage 

 of being reducing. Copperas with lime was largely used for 

 London sewage. The precipitate of ferrous hydroxide, Fe(0H)2, 

 absorbed oxygen from the air, and to a certain extent commu- 

 nicated it to the organic matter, acting as a carrier. 



Ferric Salts not only possess a higher power of clarification, 

 but also act as direct oxidizers. A solution of ferric sulphate 

 has been used in several systems of purification, and the small 

 quantity present in ''alumino-ferric" may consequently be ad- 

 vantageous. Ferric chloride with lime was formerly employed, 

 especially at Northampton. The presence of arsenic in it 

 was commented on by Letheby, Hofmann, and Frankland. 

 " Clarine " was a basic ferric chloride. '' Ferrozone," at one 

 time much used as a precipitant, is a mixture of ferrous, ferric, 

 and some aluminium salt, with a less quantity of calcium and 

 magnesium sulphates. 



An important difference between the behaviour of ferrous 



