336 SEWAGE AND ITS PURIFICATION 



terin, Cc>^^4^^{Oll), a solid insoluble alcohol with a distinct 

 affinity for water, insoluble in alkalies and in acids except 

 concentrated sulphuric. The cholesterides show a remarkable 

 adhesion to water, not rising to the surface like ordinary fats, 

 but remaining suspended, so as not to be separated by centri- 

 fugal machines without concentration. Ferric sulphate, with 

 lime, carries down the greater part, giving a sludge difficult to 

 press (p. i66), and containing about 25% of grease; moreover, 

 an inordinate amount of chemicals is required. 



Wool-grease has a special commercial value, when purified, 

 as "lanoline "; it also contains considerable quantities oi potash ^ 

 which pays for separation along with tlie alkali from the potash 

 or soft soaps used for scouring. Grease is now recovered at 

 some of the larger factories, and if smaller firms combined and 

 conveyed their suds by pipe sewers to one or more centres, 

 they could be dealt with collectively.^ This suggestion of 

 co-operative treatment applies to other trades, and economy 

 of neutralization might sometimes be effected by mixing alkalies 

 with acid waste. 



At a flannel and dye works near Rochdale the following 

 treatment is pronounced to be very effective : — The liquid first 

 passes through a fine copper sieve to remove wool fibres : the 

 sieve is cleaned by a revolving brush, and the flocks of value 

 collected. It is then mixed by a water-wheel with milk of 

 lime, afterwards with a solution of ferric chloride 12 feet further 

 on, next passes through five settling tanks (arranged so that 

 any one of the first three can be thrown out for cleansing, and 

 each provided with scum-boards), on to two filters of fine 

 ashes, used alternately, thence into the streams. 



The sludge is treated with sulphuric acid to break up lime 

 soaps, and pressed through cocoanut matting, the acid filtrate 

 being used again. The sludge cake is further pressed to 

 recover oil, various products from which are made and sold ; 

 the refuse is burnt. 



At a print works on the Mersey, soap liquor is treated with 

 lime and iron alum, pressed, the cake steamed with acid, the 

 grease separated, and the acid and alum from the press-filtrate 

 used again. The grease is stated to be worth at least £j per 

 ton, but even at £^ los. it pays the whole of the expenses of 

 recovery, the quantity being 15 to 20 tons a year. 



In a large number of woollen mills, simpler works have given 

 1 Borough Surveyor's Report, Bradford, September, 1896. 



