It is evident that sewage must be removed in a thorough manner, otherwise it would endanger the lives and health of the people.
(1) From its offensive odors, which, while not always directly dangerous to health, often produce headaches, nausea, etc.
(2) The organic matter contained in sewage decomposes and eliminates gases and other products of decomposition.
(3) Sewage may contain a large number of pathogenic bacteria (typhoid, dysentery, cholera, etc.).
(4) Contamination of the soil, ground water, and air by percolation of sewage.
The problem of sewage disposal is twofold: (1) immediate, viz., the need of not allowing sewage to remain too long on the premises, and its immediate removal beyond the limits of the city; and (2) the final disposition of the sewage, after its removal from the cities, etc.
=Modes of Ultimate Disposal of Sewage.=--The chief constituents of sewage are organic matter, mineral salts, nitrogenous substances, potash, and phosphoric acid. Fresh-mixed excrementitious matter has an acid reaction, but within twelve to twenty hours it becomes alkaline, because of the free ammonia formed in it. Sewage rapidly decomposes, evolving organic and fetid matters, ammonium sulphide, sulphureted and carbureted hydrogen, etc., besides teeming with animal and bacterial life. A great many of the substances contained in sewage are valuable as fertilizers of soil.
The systems of final disposal of sewage are as follows:
(1) Discharge into seas, lakes, and rivers. (2) Cremation. (3) Physical and chemical precipitation. (4) Intermittent filtration. (5) Land irrigation. (6) "Bacterial" methods.
_Discharge into Waters._--The easiest way to dispose of sewage is to let it flow into the sea or other running water course. The objections to sewage discharging into the rivers and lakes near cities, and especially such lakes and rivers as supply water to the municipalities, are obvious. But as water can purify a great amount of sewage, this method is still in vogue in certain places, although it is to be hoped that it will in the near future be superseded by more proper methods. The objection against discharging into seas is the operation of the tides, which cause a backflow and overflow of sewage from the pipes. This backflow is remedied by the following methods: (1) providing tidal flap valves, permitting the outflow of sewage, but preventing the inflow of sea water; (2) discharging the sewage intermittently, only during low tide; and (3) providing a constant outflow by means of steam-power pressure.