342 SEWAGE AND ITS PURIFICATION 



ing acids, and partly by competitive growth, of almost com- 

 pletely preventing souring of liquors containing starch or other 

 carbohydrates. The outflow from the tank is delivered by a 

 revolving sprinkler on a bed of furnace-ashes or coal, and after 

 passing through a small sand filter is as satisfactory as the one 

 last mentioned. The sewage has a coagulating effect in 

 removing turbidity from difficult liquors such as those from 

 esparto, china clay, etc. 



Slack-washing, practised at some collieries for separating small 

 coal, gives a water which readily clears in settling tanks, and 

 can be used again and again. As to neglect of this precaution, 

 the Royal Commission on Salmon Fisheries, igo2, says of the 

 river Rhymney, that on account of the coal washing from 

 collieries the bed is in a filthy state and the beach is covered 

 with coal dust. 



Chemical works give waste water of so various a character 

 that each case must be decided separately. 



In a number of experiments on trade effluents, Meade-King^ 

 found that the addition of salt water greatly helped the pre- 

 cipitation, either by iron alum, which he considers the most 

 useful precipitant, or by tannin (from oak bark, leaves or galls), 

 which is specially useful in gelatinous effluents like those from 

 print works, but is rather an expensive precipitant. 



Gas Liquors. At Rotherham the waste water from sulphate 

 of ammonia plant occasioned trouble, as it has in other places 

 when not controlled. The suspended matter can very easily 

 be removed, but the purification of the liquid portion is quite 

 another thing. When much of this refuse is present in the 

 sewage, there is no process that can be adopted on a large scale 

 that will precipitate the cyanogen compounds. H. W. Crow- 

 ther (patent 11,964 of 1893) precipitates and recovers them by 

 cuprous oxide, which has been replaced by a solution of cupric 

 and ferrous sulphates followed by lime, but the process is hardly 

 applicable to sewage. Kershaw found at times as much as 

 no parts NH4SCN per 100,000 in these liquors. This class of 

 refuse is present in the sewage of many towns, and if there is 

 any place where it is satisfactorily dealt with it is largely due 

 to the fact that its strength is greatly reduced by the volume 

 of sewage. In this condition it can be purified biologically to 

 a considerable extent. He further pointed out in his evidence 

 to the Commission, that very often sulphocyanides in filtrates, 



^ Proc. Inst. Civ. Engineers, Jan. 9, 1900. 



