TRADE EFFLUENTS 339 



principles. The screened sludge is acidified with sulphuric 

 acid, heated to ioo° C. and pressed (the pressing requires 

 12 hours) ; dried by steam till the moisture is reduced to 15 per 

 cent., and afterwards extracted three times by benzene. 300 to 

 450 tons of liquid sludge, containing about go per cent, moisture, 

 are dealt with per week. From each charge of cake, some 

 800 kilos, of crude grease are obtained by the use of 1320 

 gallons of benzene (or petroleum spirit), gg per cent, of which 

 is recovered by distillation. The dry extracted residue con- 

 tains 3 percent, of nitrogen, and fetches 30s. per ton as manure. 

 The crude grease is treated by vacuum distillation, at a cost of 

 7s. to los. per ton : 7 tons crude grease yield i ton of water, 

 5 tons of distillate (valued for soap and candle making at £i.g 

 per ton), i ton of tar, and a little gas, which is burned under 

 the boilers.^ 



X. The Smith-Leach Process, used at Bradford, consists in first 

 driving off about four-fifths of the water in a Yaryan multiple 

 evaporator, when the concentrated suds can be centrifuged into 

 (i) mud, (2) concentrated potash soap solution, and (3) crude 

 lanoline. At the Fieldhead Mills the potash soap is then 

 further concentrated, and finally burnt in a revolving cylindrical 

 incinerator to crude carbonate of potash, which is either sold, 

 or used again for washing wool. No effluent remains, and the 

 distilled water from the evaporator is utilized for wool-washing, 

 and from its softness effects a saving of 15 to 30% in the 

 quantity of soap required. The trials show a considerable 

 profit, as the grease sells at nearly £20 per ton and the potash 

 at £2^. 



The Bradford sewage at the Frizinghall works is at present 

 passed through a detritus tank of 260,000 gallons capacity and 

 then treated by a modification of the Cassel process. The 

 press-cake contains so much grease as to be unsuitable for 

 manure. At Frizinghall a plant is being erected for extracting 

 the grease by distillation, as it has been found that in this way 

 practically all the grease can be extracted, and the cake left in 

 a dry powder containing all the nitrogen. About 25 tons of 

 sulphuric acid are required per day for 12,000,000 gallons of 

 sewage, and 300 tons of sludge are pressed per day, producing 

 60 tons of cake. 



Paper Works. The process must vary with the materials 

 used. Caustic liquors from esparto or straw are evaporated 



1 For details of cost, and analyses of products, see Garfield's paper, loc. cit. 



22 — 2 



