TRADE EFFLUENTS 337 



excellent results. They consist of * Sap-tanks " (in which the 

 water from the scouring processes, which contains most of the 

 grease, is treated with acid and the grease recovered), also 

 precipitation tanks and filter-beds. 



Dr. Beckhold's^ report on the sewage sludge in the pre- 

 cipitation basins at Frankfort, Germany, states that (i) the 

 sludge contains an easily saponified mixture of fats and fatty 

 acids (27*8 per cent, of the latter being combined with bases), 

 the total fatty matters varying from 3 to 27 per cent. ; (2) the 

 scum contains 80 per cent, of mixed fats; (3) the annual 

 amount of fat lost in the sewage is over 1^ million pounds, or 

 about 8 lbs. per head; (4) the amount of sulphuric acid required 

 for complete decomposition is very large, 35 to 50% by weight ; 

 (5) the iron in the sludge is wholly in the ferrous state ; (6) the 

 fat collected in the basins is reduced in a few months by 

 bacteria to a small fraction of the original, more rapidly in the 

 dark and at summer temperature. 



Several special processes are used in connection with wool. 

 See also p. 198. 



I. Degreasing. Treatment with volatile solvents, such as 

 bisulphide of carbon, benzene, or light petroleum in an 

 apparatus similar to Leuner's, used for degreasing bones ; the 

 fat is thus extracted almost unaltered, and the solvent used 

 again. At present this method has not been found commer- 

 cially successful. The Delattre Process treats first with sulphuric 

 acid, then with steam and benzene : it is said that it can be 

 applied economically to wool-suds or to a very dense sludge. 

 The apparatus is an inclined cylinder, 4 ft. diam. and 200 ft. 

 long, in three horizontal sections. The wet sludge is pumped 

 in at the top, and in flowing down the incline, meets the 

 benzene or other volatile solvent pumped in from below, 

 mixture being attained by revolving paddles at short intervals. 

 The solvent charged with grease is separated and distilled, the 

 distillate being used again, and the residue of grease run into 

 barrels. The extracted sludge is steamed to recover remaining 

 solvent, and pressed hot, resulting in a dry cake, which can be 

 sold as manure. The method has been applied at the sewage 

 works, Roubaix, France, and was tried in 1900 on a ferric- 

 sulphate sludge sent from Bradford, but it was found that it 

 would be necessary to reduce the bulk to about one-fifth in the 

 process of precipitation. The grease was valued at £g per ton, 



^ Zeits. angeiv. Chemie, xxxvi., 849. 



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