344 SEWAGE AND ITS PURIFICATION 



Tannery liquors in the United States are said to contain 

 such a quantity of arsenic, even after sedimentation, as to 

 hinder bacterial treatment. The Massachusetts State Board 

 (Report, 1898) found that passing through a coke strainer com- 

 pletely removed the arsenic as ferric arsenate, and that by 

 afterwards filtering through beds of sand and gravel at the 

 same rate as ordinary town sewage, a satisfactory effluent, 

 fairly nitrified, could be obtained. The fats, retained by the 

 filter for a time, were destroyed by bacteria. A similar process 

 was used successfully with the waste liquor of paper mills. 

 With that from wool-scouring it was found impossible to filter 

 the heavy liquors, as they quickly clogged the surfaces of either 

 coke or sand. After neutralizing with sulphuric acid to remove 

 the fat, the liquid was mixed with 5 times its volume of city 

 sewage, when the bacterial action became very vigorous, finally 

 high nitrification set in, and a sand filter gave a good result. 



As an instance of an acid effluent, I found that a soap works 

 at Exeter was discharging J-ton of acid liquor daily. Even if 

 this contained i per cent, of sulphuric acid, it would amount 

 on a million gallons of sewage to o'l part per 100,000. But 

 the crude sewage has sufficient alkalinity to neutralize more 

 than this amount of acid provided the latter be not supplied 

 in spurts as when poured direct on a filter. I have already 

 remarked on the beneficial mixing and " smoothing" effect of 

 the septic tank on the great fluctuations that occur at different 

 times in all varieties of sewage. I believe that the same natural 

 neutralization and precipitation would dispose of most metallic 

 admixtures such as iron salts, galvanizing pickle, etc. 



With regard to tanning refuse, the antiseptic power of tannin 

 itself is very small, and, moreover, it does not pay to let much 

 of it escape. At Exeter I estimated the daily quantity from 

 the large tannery in that tow^n as equivalent to that in six fluid 

 ounces of brewed tea per head of population, and it certainly 

 could have no influence. 



Popp and Becker^ found that " liquefying bacteria " were 

 killed by 0*5% of sulphuric acid or by 1% of sodium carbonate, 

 an acidity or alkalinity that would be higher than the ordinary 

 factor)^ runnings, and would be brought down when mixed with 

 the whole of the sewage to an unimportant factor. As an 

 example I ascertained that at a certain paper mill 35 lbs. of 

 soda-ash were used daily : the maximum addition to the 

 ^ Chem. Hyg. Inst., Frankfort, i8g6. 



