THE PURIFICATION OF SEWER-WATERS 811 



culty of drying the mud fast enough to enable it to be taken away at 

 a profit, which has prevented the success of all previous efforts at puri- 

 fication by deposition. The mud which is spread on the permeable 

 bottom does not soil the scoria, but leaves it perfectly clean, while the 

 Avater flovvs clear from the end of the drains. After two or three 

 days, according to the weather, the mud will appear cracked at some 

 points, and finally over the whole surface. After a week it will have 

 acquired consistency enough to be cut with a shovel. A cart is then 

 brought into the basin, and after a few hours it is emptied and is 

 ready to receive a new charge of mud. The scoria not having been 

 soiled, requires no cleaning, and will be as ready for use even after 

 the end of ten operations as at the beginning. 



The drained mud is carried in the shape of large lumps to an open 

 yard, where it is dried in the air without giving forth any odor. It 

 contains about seventy-five, per cent, in weight of water at the time it 

 leaves the basin, but the amount of Avater is reduced after two or three 

 months of exposure to not more than fifteen or twenty per cent. 



These operations are of the simplest character, and involve nothing 

 cumbrous. The Avhole system, with its decanting and drainage tanks, 

 its open yard and the necessary roads, occupies a surface of not more 

 than two hectares, or five acres, for the effective purification of 10,000 

 cubic metres of Avater every tAventy-four hours. Paris has to get rid 

 of thirty times as much foul water as the Essonnes paper-mill, or 

 300,000 instead of 10,000 cubic metres a day. The system practiced 

 at Essonnes would, therefore, have to be applied on a scale thirty times 

 as large to be adapted to the needs of Paris. Can it be made to suc- 

 ceed on such a scale ? What is there to prevent it ? 



No difficulty is offered by the composition of the sewer-waters. 

 We have procured a quantity of water from the great collector of As- 

 nieres, and' have subjected it to the same treatment that is given the 

 Avaste water at Essonnes. On adding to it lime-water in the propor- 

 tion of 250 grammes of lime yes, even in the smaller proportions of 

 200 and 175 grammes to a cubic metre of Avater, a complete pre- 

 cipitation was promptly produced. At the end of four or five hours 

 the water became clear and limpid. 



The extent of land required to conduct the operations of decanta- 

 tion, drainage, and drying, on the scale demanded by the city of Paris, 

 seems formidable at the first sight, but it is not really so. . As we have 

 seen, the whole system at Essonnes occupies only two hectares, or five 

 acres, of land. For the city of Paris thirty times as much land, sixty 

 hectares, only 150 acres, would be needed to give room for all the 

 apparatus and all the manipulations ; that would be a small tract 

 compared Avith the 1,500 hectares, or 3,750 acres, on which it is pro- 

 posed to establish a nuisance in the forest of St. Germain. 



The scheme Avill compare favorably with any other that has been 

 proposed, in the cost of constructing and operating the works. The 



