SEWAGE DISPOSAL 357 SEWAGE DISPOSAL culturist the latter; while the mass of the population has an equal interest in both. The composition of sewage as it passes outward, varies greatly, as regards amount and condition of organic matter it contains, and the season of the year — hot or cold — affects very considerably the activity of chemical action. But at all times sewer gases are given off which are fetid and offensive to smell, containing sulphuretted hydrogen, light carburetted hydrogen, free nitrogen, and carbonic acid. To prevent sewer gases from pene- trating into houses by means of the drains and pipes, and to keep the sewage from saturating the surrounding soil, and from contaminating any water supply, are objects of prime importance. To ac- complish these, the pipes leading from dwellings into main drains and sewers require to be trapped, and the sewers themselves ventilated, drain pipes and tubes made of glazed earthenware or other non-absorbent material must be carefully fitted, and the main sewers made either of pipes of large diameter or of hard bricks bedded in Portland ce- ment. Sewers are properly built in cross section; they should have a uniform gradient, and be as far as possible built free of curves, so that the sewage matter may be carried forward without any tendency to silt or deposit at particular spots. It is also essential that there be ready access to the sewers, as well as to the smaller drains leading into them. The entrance of sewer gas into houses is provided against by external ventila- tion of the pipes and by trapping the pipes either by a syphon arrangement, a midfeather, or a flap trap. The mid- feather is a modified kind of syphon, con- sisting of a trough having an inlet and discharge pipe at the same level on op- posite sides, but between them a partition passes down into the water with which the trough is always filled up to the level of the two pipes. Such traps are usually put upon sinks. The flap trap consists simply of a hinged valve which opens out- ward to allow the escape of sewage, etc., but which closes against the resurge of water or the inward pressure of gases. Modified forms of these varieties are numerous. 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- tem 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 gewer. 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,
