The water-carriage system of sewage is, as the late Dr. Parkes put it, “the cleanest, the readiest, the quickest, and in many cases the most inexpensive method.” But when the sewage is conveyed to the outfall of the sewer, its ultimate disposal is still one of the most difficult problems of the present day. Various plans have been adopted, of which the following are the chief:—
3. Separation of solid and liquid parts { By settlement. { By precipitation.
4. Filtration through various artificial media or through land.
=Discharge at once into running water= was formerly the favourite plan, as it was certainly the most convenient. The sewage was turned into the nearest water-course, regardless of the facts that this might have to supply the drinking water of people at a lower point, that the mouth of the river tended to become obstructed by sewage mud, that valuable stocks of fish were destroyed, and that the river which had practically become a sewer was a source of annoyance and danger to all on it or near it. The enforcement of the Rivers Pollution Prevention Act of 1876 has not been followed by as great improvement as is desirable.
The sewage entering rivers undergoes considerable purification by subsidence, by oxidation, by the influence of water plants, and still more by the active work of microbes, causing nitrification of nitrogenous matter. The vitality of the typhoid bacillus and of the cholera vibrio when discharged by sewage into a large river is probably not very protracted; but water from such a river would form a very dangerous source of domestic supply.
=Discharge into the Sea= is resorted to in seaboard towns. The outfall must be carried well below the lowest low-water mark, and to such a point that the incoming tide or wind will not bring the sewage back upon the shore, or on the shore of neighbouring places.
=Discharge into an Estuary= is only justifiable when the flow of the river is rapid, when the volume of water passing out to sea is very greatly in excess of the volume of sewage, and when there is no possibility of contaminating oyster-layings or beds of mussels or other molluscs.
Objection has been taken to the above method on the ground of waste of manure; but modern sewage is so dilute that its profitable utilization on land still remains a dream.