TRADE EFFLUENTS 345 



alkalinity of the whole daily sewage was 0*3 parts per 100,000 

 or -0003%. 



Gas liquor and the effluents from timber works often contain 

 a large quantity of suspended tar, which clogs up filter beds 

 and presses, and fouls the catch-pits and sewers. Therefore 

 they must usually be excluded. A sample of refuse from a 

 timber yard which I examined in May, 1899, contained, in 

 parts per 100,000 : — 



Heavy petroleum ... ... ... ... ... 1560 



Pieces of wood, straw, leaves ... ... ... ... 210 



Earthy matter and oxide of iron ... ... ... ... 827 



Solids in solution .. ... ... ... ... 33 



This is an example of discharges that are easily dealt with 

 by a catch-basin and straining, as the filtrate was nearly clear, 

 almost inodorous, neutral, and not injurious to bacteria. With- 

 out such treatment the floating tarry film might possibly some- 

 what hinder the activity of the upper bacterial layer of a septic 

 tank, but the aqueous liquid itself in its dilution would not be 

 likely to interfere either by its sulphides, cyanides, ammonia or 

 tar-acids, inasmuch as many bacteria generate and live in a 

 medium impregnated with ammonium sulphide, while cyanogen 

 compounds are far less poisonous to lower organisms than to 

 higher animals, and the strongest of the tar derivatives are not 

 bactericidal under 0*5%, or 500 parts per 100,000, — an impos- 

 sible amount to be present in the mixed sewage. 



In exceptional cases, however, where intense acidity or other 

 strong admixture cannot be avoided, the use of lime and a 

 settling tank would become necessary : in this case a sludge 

 would be created which would not be that of sewage. 



Neutralization of acid sewage containing galvanizing pickle 

 by lime, and subsequent passage through contact beds, has 

 been found satisfactory at Bilston, Staffordshire. 



Kinnicutt and Eddy examined the action of the septic tank 

 on the acid iron sewage of Worcester, Massachusetts, which 

 contained in parts per 100,000, FeS04 1577, Al2(S04)3 0*44, 

 free H-.SO^ 10*32. They concluded that (i) about one-fourth 

 of the total solids was removed by passage through the tank ; 

 (2) about 21 per cent, of the soluble matter, and 25 per cent, of 

 the suspended matter, was removed, the former being greater, 

 and the latter much less, than the proportion usual with 

 sewages ; the result being due to the change of the soluble 

 sulphate of iron into insoluble sulphide of iron, a part of which 



