118 
The Paris Sewage Irrigation at Gennevilliers. 
effectually a precipitation of the solid matters held in solution 
and suspension by the sewage-water, by means of sulphate of 
alumina, lime, and other reagents, the use of which had been 
recommended in England and other countries ; and to that 
effect new tanks were constructed on a still larger scale on the 
Gennevilliers side of the river. 
The sulphate of alumina process seemed at first to give the 
most favourable results. Sulphate of alumina, after being decom- 
posed by the alkaline elements contained in the sewage, was 
transformed into alumina in the form of granulated gelatine. 
It then merely exercised a kind of mechanical action over the 
solid matters suspended in the sewage, precipitating them, 
along with itself, to the bottom of the tank. 
Under the action of that process the water was entirely clari- 
fied, and was decanted back into the river in a clear state ; but 
it was soon found that the purification was merely mechanical, 
and that the water, clear and pure though it might appear, re- 
tained all its noxious characters. Sulphate of alumina acts only 
on the matter suspended ; the water retains all the matters in 
solution, together with the organic elements liable to fermenta- 
tion. This was clearly demonstrated by chemical analysis, as 
the following Table will show : — 
Sewage-water 
ia its 
natural slate. 
Water clarified 
with sulphate 
of alumina. 
Kilog. 
0-037 
Kilog. 
0-021 
Volatile and combustible matters . . 
0-729 
0-240 
2-038 
0-724 
2-80i 
0-985 
The clarified water was thus found to contain two-thirds of 
the nitrogen of the sewage, and one-third of the volatile and 
combustible matters, which are for the most part organic. 
Besides this unsatisfactory result, the large amount of solid 
matter remained to be dealt with, after it had been removed from 
the bottom of the tanks. This had to be spread over a large area 
to undergo the necessary process of drying ; and when it is con- 
sidered that more than 200,000 tons would have had to be 
treated every year in that manner, it may be imagined what 
a mass of noxious gases would have arisen from such an accu- 
mulation of filth. It was, besides, calculated that the simple 
cost of the cliemicals used would have been equal to the 
manuring value of the mud, and therefore its removal would 
have entailed an additional burden upon the ratepayers. 
