152 PROCEEDINGS OF SECTION B. 
Sewage Filtration. 
Very closely allied to the decomposition of manurial 
erganic matter in the heap, or in the soil, is the fermentation 
of sewage matter by the bacterial processes. The only 
difference is that the hygienist endeavours to regulate the 
fermentation, while the farmer leaves it very much to 
chance. Of the many systems, or devices, for oxidising 
sewage matter, there does not appear to be one universally 
applicable, for every sewage requires its own particular 
treatment. In the sewage-meadow the oil is irrigated with 
sewage for a certain time, after which it is turned off, and 
the soil allowed to become aérated. This is generally done 
by dividing the bed into four parts, and running the sewage 
into each part for six hours. Thus each quarter gets a 
rest for 18 hours per diem. In the sewage contact filter the 
sewage is allowed to flow into the bed containing coarse 
material until it is full, and, after standing for two hours, 
the liquid is run off imto another bed containing finer 
material. Here it remains for a similar period, after which 
it is generally clear enough to be discharged into a river. 
The operation is repeated every eight hours, there being 
three fillmmgs in the day of 24 hours. It is but a step to 
arrange a filter of some coarse loose material coated with 
bacterial slime, so that aération and filtration can go on 
simultaneously. This is the principle of the continuous bac- 
terial filter. But before a filter, either contact or con- 
tinuous, is “ripe,” 7.¢., before the fragments of clinkers, 
coal, gravel, &c., become coated with bacterial slime, and 
the filter becomes capable of doing a maximum amount of 
work, a time varying from six weeks must elapse. This is 
a waste; and I see no reason why the “ripening” should 
not be started and accelerated by means of artificial infec- 
tion, combined with feeding the bacteria with appropriate 
salts. 
The aérobic filter is sometimes preceded by the anaérobic 
chamber, in which quantities of the sewage are kept for 
about 24 hours. The chamber may be air-tight, but this 
does not appear to be a necessity, for the fermentation is so 
vigorous that the superincumbent air has little opportunity 
to get into the mass of the sewage. In these chambers the 
disintegration of the raw material, and reduction to simpler 
chemical substances, is very rapid, and (with some kinds of 
sewage) this preliminary treatment is of the greatest help 
to the aérobic filters, which rapidly oxidise the reduced 
(anaérobic) products. The function of the anaérobic treat- 
ment is to ammonise, and of the aérobic filter to nitrify ; 
