TRADE EFFLUENTS 335 



films, the filters gain in purifying power by use as long as they 

 pass the effluent freely. 



Logwood liquor is reported to present special difficulties and 

 to require separate precipitation, " though the product is at 

 present valueless, even for manure, as it generates fungus ; it is 

 simply pressed and burnt." Bichromate liquors in some cases 

 are pumped into the boilers for preventing scale : " no other 

 boiler composition has been found necessary, and the valves 

 have lasted better than previously." It is usually more profit- 

 able to recover bichromate by precipitation. 



The recovery of indigo is now profitably adopted, according 

 to Mr. Tatton, at all large works. In a Mersey print factory 

 about ;£'i,200 a year was recovered on ^^4,000 worth of raw 

 material. Wash waters are precipitated with alumino-ferric 

 and soda (aluminium chloride partially saturated by sodium 

 carbonate is said to be better^), and the solids, with the vat 

 sediments, treated chemically to separate pure indigo 



Considerable saving is effected in materials by the necessity 

 for preventing pollution of rivers. " Formerly, if a mixing 

 was wrong it was sent down the drains into the river, now it 

 has to be taken to the tip and the error is detected." The 

 West Riding Rivers Board report, 1903, states that many 

 manufacturers found their purification works profitable. One 

 paper-maker found that his purification plant resulted in a 

 saving of ^^500 a year, while a colliery manager similarly 

 effected a saving of 300 tons of coal a fortnight, which formerly 

 went into the river. 



One of the most polluting liquids is the water from the kiers 

 in which cloth, rags, esparto, and straw are boiled — a strongly 

 alkaline fluid which alone is difficult to treat. Judicious mixture 

 with acid liquors and precipitation are used, while at one works 

 carbonic acid is forced in to reduce the caustic alkalinity. 



Disposal of Grease. 



The average amount of grease contained in the sewage of 

 various towns is given as, in parts per 100,000 : — Ilkley i*i, 

 Keighley 9-8, Harrogate 13, Huddersfield 15*2, Cassel 18, 

 Hemsw^orth 39*6, Bradford (Frizinghall) 45*3, Pudsey 46*8.- 



Grease from wool is peculiarly intractable, owing to the 

 partial replacement of the glycerin of ordinary fats by choles- 



^ J. Soc. Chem. Ind., May 31, 1902, p. 702. 



- J. Garfield, Sanitary Record, Feb, 2, 1905, p. 87. 



