THE PURIFICATION OF SEWER-WATERS. 809 



forest of St. Germain would be as efficacious, if their action merely 

 as an absorbing medium is contemplated, as 40,000 or 60,000 hectares 

 would be in a scheme for irrigation. The demand for the water un- 

 der the operations of irrigation would necessarily be very fluctuating. 

 None would be wanted in the winter or in hot weather. While green 

 crops might require much during the whole season of their growth, 

 grain-crops must be carefully protected against it as the season of their 

 ripening approaches. What is to be done with the constantly accu- 

 mulating supplies of waste water during these seasons when it can not 

 be used ? 



The whole difficulty arises from the fact that the fertilizing matter 

 which we wish to make useful is drowned in an excess of water. It is 

 this excess which renders sewage unfit for the fertilization of cereals, 

 crops which are too often injured by the superabundance of rain-water. 

 And it is the same excess which makes it impossible to find sufficient 

 surfaces on which it can all be employed in the cultivation of kitchen- 

 garden vegetables, with whose demands its organic constituents well 

 agree. Were this matter extracted in a dry state, it would furnish a 

 precious element of fertilization to a large agricultural interest ; and we 

 might preserve it without difficulty and without loss, and transport it 

 at will to apply it to crops of all kinds. At the same time, the water 

 in which it is held could be returned clear and wholesome to the riv- 

 ers, whose salubrity it is now destroying. Clearly, there can be no 

 discrepancy in the conditions required for the accomplishment of this 

 double object ; whatever favors one side must be equally favorable to 

 the other. The two results are absolutely concordant, and may be 

 produced at the same time by one and the same operation, an opera- 

 tion which we may call decantation. The practicability of this process 

 is established by the fact that it has been adopted and is employed 

 with complete success in a large factory in the neighborhood of Paris. 



The paper-mill of Essonnes has to deal with 10,000 cubic metres 

 of foul water a day. For two years it has returned to the river 

 Essonnes these 10,000 cubic metres of water clarified, while it has at 

 the same time extracted the mud which they held, and delivered it 

 in a solid state as manure to the agriculturists of the neighborhood. 



The apparatus emjjloyed at this establishment is composed of two 

 parts, corresponding with two very distinct phases of the operations : 



First, is a series of water-tight basins, or tanks, which are used in 

 the decantation, properly so called, of the foul waters. 



Second, is arranged a series of tanks having permeable bottoms, 

 constructed parallel with the former tanks, but on a lower level ; these 

 are destined for the drainage of the mud which is deposited in the de- 

 canting-vats. 



The process is as follows : The foul waters from the factory are 

 drawn into a single conduit from about twenty inches to two feet 

 wide, along and over which is disposed a series of circular bucking- 



