Immunity from sewer gas in the house is largely dependent upon the flushing and ventilation of the drain and the soil pipe. In the case of a drain which is trapped as described, there is an air connection through the vent before the trap; then the soil pipe which is in the house should continue upward through the roof. Thus there is a fresh air inlet through the drain, and upward through the soil pipe of the house. Such a connection prevents the possibility of siphoning the traps, as it gives an outward air connection. The water passing through the drain or soil pipe can draw its supply of air from the upward soil vent, rather than through the traps which contain water. When there is no upward vent of the soil or drain, the water in the traps which connect therewith will be drawn out by the passage of water through the drain where fixtures are used.
There are those who maintain that there should be no trap in the yard or adjacent to the house, but that there should be a straight run from the soil pipe to the sewer or vault, and upward through the roof and above the house. It is good practice to use the trap as described for sewer connections, but not for open vault connections.
A grease sink is frequently placed in the drain to intercept the passage of grease into the vault. It is so placed and connected that only the water from the kitchen sink, or other fixtures where the water contains grease, may enter it. It is made of brick, and is usually of six or eight barrels capacity. A four-inch pipe connects it with the kitchen waste, and if the grease sink is placed adjacent to the main drain, there can be a similar connection between it and the main drain. It should be a siphon connection, so that the sink will become nearly full before it discharges. When it discharges through the siphon the water will go out with a rush and leave the grease in the sink. This makes an intermittent discharge into the main drain, which flushes or cleanses it thoroughly and is much better than a constant small flow of water. This grease sink must be cleaned from time to time. Small cast-iron grease sinks are sometimes placed under kitchen sinks in very large dwellings or hotels.
Nothing particular need be said in regard to wash-stands more than has been said, excepting, possibly, that the drain should be trapped, ventilated, and connected with the soil pipe; also that there should be a lead safe or safety pan on the floor under the wash-stand when they are enclosed; it is preferable that they should remain unenclosed. It has been common to connect this safe with the soil pipe. It is only intended that it should be useful in cases of accidental overflow; but, notwithstanding the fact that there be a trap in the safe waste or drain, it would be empty most of the time, because of the evaporation of the water. It is proper to make direct connection with the cellar or kitchen sink.