GENERAL MANUEES ANIMAL AND VEGETABLE 299 



gallon of nitrogen, from 2 to 3 of phosphoric acid, and 

 from 3 to 4 of potash. These quantities amount to 

 about 10 Ibs. of nitrogen, 4 Ibs. of phosphoric acid, and 

 5 Ibs. of potash annually, dissolved in from 40 to 50 tons 

 of water. The total quantity of fertilising ingredients 

 allowed to go to waste in this way, it will thus be seen, 

 is very large, but the sewage is so dilute that it cannot 

 be profitably employed for manurial purposes. If applied 

 in the ordinary way the return for the quantity of plant 

 food it contains would not pay for the cost of distribution. 

 When applied in large quantities by a system of irriga- 

 tion, heavy crops of coarse, rank herbage may be pro- 

 duced from very poor or almost worthless sandy soils. 

 Such a system is, however, of more importance as a 

 means of disposing of sewage than of manuring agricul- 

 tural lands. In any case it is too large a question to be 

 fully discussed here. 



Sludge Manures. The dried sludge resulting from the 

 purification of sewage contains a certain amount of 

 plant food, and is often sold as manure. Most of the 

 processes employed consist essentially in adding various 

 substances to the sewage so as to produce a voluminous 

 precipitate ; this is allowed to settle slowly in tanks, and 

 carries down with it practically all the suspended matter, 

 and usually, also some of the dissolved ingredients. In 

 some cases the effluent is afterwards filtered and is 

 generally rendered quite clear. The principal object is to 

 prevent pollution of the rivers, into which it is finally 

 discharged. All the processes hitherto employed fail to 

 recover much of the soluble nitrogen which is by far the 

 largest and most valuable fertilising constituent. The 

 manurial value of the sludge which remains in the set- 

 tling tanks depends very largely upon the nature and 

 efficiency of the precipitant employed. At the Dalmar- 

 nock works, hear Glasgow, the precipitate is produced 



