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The foul water discharge of kitchen sinks, or of whatever form of slop-sink is used for the water of bedrooms, should discharge into a flush-tank, and should be led from this by a tightly cemented four-inch drain to a tight settling basin in the ground beyond. If water-closets are used, the soil-pipe should deliver into the drain between the flush-tank and the settling basin.

If there is an opportunity for the free admission of air from the well-ventilated sewer to feed the upward current almost constantly prevailing in a soil-pipe open at both ends, the gases resulting from the decomposition will be of a different and less injurious character than where the air is confined, and by the mere volume of air passing through the pipe they will be so diluted that even were they originally poisonous their power for harm will be lessened.

The arrangement of the soil-pipe here indicated, although excellent and efficient, is susceptible of further improvement by the use of a ventilating cowl or hood at its top.

Then, too, the sealing-water of the trap readily absorbs any foul gases presented at its outer end, toward the soil-pipe, and gives it off in an unchanged condition at the inner or house end. Such traps retard, but do not prevent, the entrance of sewer gases into the house.

The soil-pipe should be of cast iron, carefully jointed with lead, not less than four inches in diameter, and carried by the straightest course possible up through the roof and generally higher than the ridge-pole. Its open top must not be near any window, and if within ten feet of a chimney it should be at least one foot below the level of the top of that chimney.

If the soil-pipe is closed at its top, or if it is obstructed by a trap in the lower part of its course, there can be no such circulation of air as safety requires.

If the soil-pipe is open at its upper end this expansion will be at once relieved; but if the top of the pipe be closed there will always be danger of the forcing of the feeble barrier offered by the ordinary water-seal trap of a branch pipe leading from a wash-basin or sink.

Water-seal traps which are unused for any considerable time are emptied by evaporation, and thus open a channel through which the air of the soil-pipe may find its way into the house. It is usual in modern plumbing to relieve the pressure of gas in the soil-pipe by what is called a "stench-pipe."

The soil-pipe, carrying the discharge of water-closets, should not be delivered into the flush-tank, but at a point farther down the drain, so that any solid matter it may deposit shall be swept forward by the next action of the flush-tank. The more often the flush-tank is filled, and the greater the proportion of its water to its impurities, the more efficient will be its action.

The gases formed by the decomposition of organic matter in the sewer itself, or in the soil-pipe, have a certain expansive force which is greatly increased by the elevation of temperature, caused, for example, by the discharge of hot water into the pipe or sewer.