120 
Tlie Paris Sewage Irrigation at Gennevilliers. 
culties in which the Paris sewage question was involved. During- 
the years 1867-1868, sewage irrigation was tried on a piece 
of waste ground at Clichy. The sewage-water was pumped up' 
to that piece of land and made to percolate through 3 feet of 
soil. A drain was provided, and the filtered water collected 
and carefully analysed. It was found that so much as 80 per 
cent, of the carbonate of ammonia was retained by the soil. 
Sewage, containing 43 grammes of nitrogen per ton before its 
application to the land, when analysed after percolation through 
the soil, gave scarcely any trace of it in a decomposable state. 
Only 1'6 gramme of nitrogen in a state of mineral ammonia 
could be found. It was the same with the quantity of soluble 
oxygen. The sewage-water, when laid on the land, scarcely 
contained 2 cubic centimetres of oxygen per quart. On its 
effluence from the soil it was found to contain from 8 to 10, 
which is the unerring characteristic of healthy water. 
These satisfactory results, added to the success achieved in- 
the agricultural part of the experiments, naturally pointed to the 
direct application of sewage to the land as the only effectual and 
practical means of solving the troublesome problem. 
In 1869 an announcement was made that sewage would be 
supplied gratis to any one who felt inclined to use it on the Gen- 
nevilliers territory for agricultural or market-gardening purposes. 
The response to that offer was sufficiently encouraging to in- 
duce the Commissioners to undertake at once the necessary works 
of delivery and distribution. To that effect, it was requisite, 
in the first place, to raise the sewage from the level of its outlet 
to a height of 11 metres, a little over 36 feet, so as to command 
the available area in the plain of Gennevilliers. A powerful 
pump, on the centrifugal principle, was first erected at Clichy. 
By means of this engine, the sewage is now raised so as to flow 
through an iron pipe which is laid underneath the footway 
of the Clichy Ijridge, and is thus carried over to the left bank of 
the river. Thence it is distributed all over the available space 
by means of open ditches, built in bricks and cement ; and, 
owing to the high level attained, it is calculated that at least 
5000 acres can be irrigated in Gennevilliers alone. 
The quantity of sewage annually discharged from the Paris 
sewers cannot be estimated at less than 100,000,000 tons ; this 
gives a daily average of nearly 300,000 tons. 
Calculating 8000 tons as the average quantity which one acre 
of light gravelly soil can effectually absorb, it would require 
12,500 acres to get rid of the Paris sewage ; but up to the end 
of 1875, the owners of only 350 acres had taken advantage of the 
boon so graciously proffered ; and even this small area is 
thn!atened to be greatly diminishetl, if not removed altogether. 
