After the preliminary treatment above described, the sewage requires to be passed over finer filtering beds, in which aerobic microbes complete the purification by changing the dissolved organic matter into inert inorganic compounds, by the process known as nitrification. The two processes run into one another, to some extent going on together.
Hitherto the Local Government Board have required filtration of sewage through land before any sewage effluent is allowed to pass into a stream. In view of the successful results now obtainable by bacterial processes this requirement will be occasionally waived. It is unsafe to assume, however, that the clear effluent obtained is free from all disease-producing microbes; and drinking water should not be obtained from even a very large river below the point of discharge of such an effluent, without the most efficient sand filtration.
[CHAPTER XXIX.]
CONSERVANCY METHODS.
The refuse to be removed from a house consists of fouled water, which is at least equal in quantity to the water-supply of the house; the excreta of the inhabitants; and “dust,” which contains, besides ashes, considerable kitchen refuse; consisting of both vegetable and animal matters.
In dry methods of removing refuse, the “dust” is often added to the excreta, and the two removed together; or the “dust” may be separately removed. In either case the foul water, and to a large extent the urine, remain to be dealt with, and require special drains for their removal. Thus in large towns, whether dry or wet methods of removing sewage are adopted, drains for the removal of foul-water and rain-water will be required, and it is found that they are practically as foul as if they contained the solid excreta.
The dry methods of removing sewage involve a certain amount of retention about the house; hence the general name of conservancy methods. Of these the most important are—
1. The pail system.
2. The dry-earth system.
3. The midden or privy system.