SEWAGE, the matter which passes through the drains, conduits, or sewers leading away from human habitations singly, or from houses collected into vil lages, towns, and cities. It is made up of excreted matter, solid and liquid, the water necessary to carry such away, and the waste water of domestic operations; but to these are added the liquid waste products of manufacturing operations, and generally much of the surface drain age water of the area in which the con veying sewers are situated.
There can be no doubt that the pesti lence and plagues which at frequent in tervals devastated ancient and mediaeval cities were almost invariably caused, and always intensified, by the entire absence of any system for treating or removing excreta and other decaying organic mat ter; and even yet, notwithstanding the assiduous regard paid to sanitary science in most well-governed towns, many dis eases are directly traceable to the noxious influence of decomposing sewage matter. It is not too much to say that the ef ficient and economical treatment of towns' sewage is the greatest and most urgent social problem of our times.
The question presents itself in a two fold aspect: (1) the necessity for the prompt and complete removal of sewage from the neighborhood of human dwell ings, and its disposal in a way the least of fensive and injurious to health; (2) the desirability of saving, for agricultural purposes, the rich and essential fertiliz ing agents which sewage contains. To the sanitary officer the former is the question of greatest moment, to the agri 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.