CHEMICAL PRECIPITATION 



157 



ment of the sludge, and a more or less clear effluent, dissolves 

 a by no means inconsiderable quantity of the offensive matters 

 previously in suspension, and this is apt to render the last state 

 of the liquid worse than the first. The well-known offensive 

 [character of the liquids from sludge-presses when lime has been 

 used is an example of its solvent action." 



If other water than sewage is used for making up the lime 

 [mixtures, the corresponding dilution of the effluent must be 

 •emembered in judging of its quality. 



When lime is used in conjunction with salts like sulphates of 

 .alumina and iron, there will be no free Hme left if the molecular 

 iroportions are observed, thus : 



A1,(S0 4)3 + 3Ca(OH)2 = Al2(OH), + sCaSO^. 



FeSO^ + Ca(OH), = Fe(OH), + CaSO^. 

 Fe2(SO,)3 + 3Ca(OH)2 = Fe2(0H)e + sCaSO^. 



Only sulphate of lime will be left in the liquid, increasing its 

 )ermanent hardness, and sometimes affording a measure of the 

 [sewage when discharged into rivers (p. 40). 



Alumina or Iron Clarification, — The use of aluminium and 

 iron salts as clarifiers and deodorizers has long been known. It 

 [depends on several actions, namely : 



1. Forming in neutral solutions insoluble compounds (called 

 generally "lakes"), with colouring matters and other dissolved 



f^substances. 



2. Antiseptic power of the metallic salts themselves, and also, 

 |in commercial specimens, of the excess of acid, generally sul- 



)huric, with which they are mixed. The latter, when they are 

 [used conjointly with lime, or when the acid is neutralized by 

 [the ammonia or other alkalies of sewage, will, of course, not 



:ount as an antiseptic. 



3. In an alkaline solution the gelatinous precipitate of 

 [hydroxides entangles and carries down suspended matters, in- 

 cluding organisms. The latter, however, rapidly rediffuse in 



'the liquid, as with mechanical agents, so that the precipitate 

 ;must be quickly separated. This separation by deposition and 

 iltration, with subsequent sludge-press, adds a great difficulty 

 [and expense to the method. 



4. Aluminium and iron salts neutralize ammonia and basic 

 (compounds ; iron salts also destroy sulphuretted hydrogen^ 



giving a black sulphide, FeS (plus free sulphur in the case 

 of ferric), eventually oxidized to red-brown ferric sulphate,. 



