SEWAGE DISPOSAL. The problem of the disposal of sewage becomes one of immense importance in all thickly populated districts. In the country, a properly constructed cesspool in favorable soil forms a simple and satisfactory means of disposing of domestic waste, but in cities the cesspool becomes utterly inade quate and some means of removing sew age wholesale has to be devised. No sys tern at present in common use can be considered perfect. Sewage contains much material of value, and its destruc tion involves a great economic waste. Owing to the large quantities of humus and nitrogenous matter which it contains, its fertilizing properties are great, and there are in it considerable amounts of grease and fat which only need extract ing and purifying to find industrial ap plication. At the present time, however, most cities are concerned only with the disposal of the sewage in an economical, sanitary and inoffensive manner and do not concern themselves with the recov ery of by-products. There are several methods of sewage disposal, but one fea ture, common to them all, is the sewer. These sewers almost invariably carry off rain as well as domestic and industrial sewage, but in some cities an arrange ment is made by which the ordinary rain fall goes into the sewer, while heavier rains are diverted into special channels. The purpose of this is to avoid having to deal with large volumes of dilute sewage, a point which is of importance in some systems, as will be seen later.
It is clear that the sewer does not solve the problem of sewage disposal. It re moves the sewage from the city, but sooner or later there must arise the prob lem of dealing with the effluent from the sewer. In cities located near the coast,
it is common to lead the sewer into the sea to a point well beyond low tide. Those with a river running through, or near, them, frequently turn their sewage into the river—a practice with many obvious objections and dangers. Inland towns far from rivers or large bodies of water have no such easy solution of their difficulties. So-called "sewage farms" are frequently resorted to. The sewage is treated in settling tanks with lime, or with mixtures of lime and sulphate of iron or alumina, by which means the solids are precipi tated, leaving the supernatant liquor com paratively pure. The sludge is spread over the land which is cultivated. This system comes nearest to Nature's method of purification, and it has the good fea ture of utilizing the fertilizing proper ties of the sewage. It is, however, sel dom entirely inoffensive. Sometimes the sludge is treated in filter presses, and the comparatively dry cake dug into the earth or even burned. The septic tank treatment is one which has met with suc cess in some cases. The sewage is caused to flow into tanks where it receives suc cessive treatment with anaerobic and aerobic bacteria. Theoretically, solid matter should become liquified and the final effluent be harmless and inoffensive. Many industrial effluents, however, con tain matter which poisons the bacteria, when, of course, the whole system breaks down.