IN order to obtain a direct comparison of the relative value of a thorough water and of the greatest possible air flushing, the same pipes, tested as already described under the air current, and containing identically the same deposits, were next tested under a good water flush. They were attached to a properly constructed lavatory, as shown in Fig. 275, and cold water was discharged through them in the usual manner. Although the deposits were dry and hard, they were almost entirely washed away after ten discharges. After fifteen discharges the amount of deposit left on both pipes was less than half a grain. When the substances were soft on the application of the test they were removed at once and entirely by a single discharge.

From these investigations we have found that the water flushing was infinitely more rapid and thorough in its cleansing power than the air flush. Now there is nothing to prevent every lavatory from being so constructed as to properly flush the waste pipes at each discharge. In fact, there are a great many reasons why it should be looked upon as an absolute necessity in good plumbing.

Hence special trap and branch waste vent piping is, for the purpose of removing solid deposits, not only inefficient, but also entirely unnecessary. We come now to the fourth consideration:

Fig. 275. Experiments on Water Flushing.

Fig. 275. Experiments on Water Flushing.

Self-Ventilation of Traps and Branch Waste Pipes.

But supposing it had been shown that special trap ventilation were necessary instead of the reverse, it would still be superfluous to apply the special vent pipe, because the ventilation in proper plumbing is thoroughly accomplished without it, and in several ways.

If our main stacks of pipes are open above and below, as they should be, and thus thoroughly aired, the branch wastes will be ventilated in the first place by the well known law of the diffusion of gases.

In the second place, a movement of fluids up or down the main stack creates in the branches suction strong enough sometimes even to destroy the seal of ordinary traps. This suction, be it strong or feeble, always produces an interchange of air in the branches.

Finally, a third and still more important way in which natural aeration is produced is by the usage of the fixture itself. Every time the water is discharged a column of pure air is drawn from, the room into the waste pipe after the water column. Everyone has observed how the air follows the water, and is drawn through it in the form of an inverted cone or funnel, generally with a loud sucking noise. When the fixture is properly constructed, with an outlet large enough to fill the waste pipe "full bore," a column of air equal to the size of the water column is drawn after it, completely filling the waste pipe with pure air from the room. In short, ample air follows every discharge to accomplish all that the soil pipe air of the trap vent could do in the interval between the usages of the fixture. The pure air from the room could not possibly be rendered so foul in the interval as the soil pipe air would be, as they are constructed to-day, before it entered. This is equally true whether the fixture be used often or seldom, provided it be properly constructed and set, and whether the branch waste be long or short.

Thus the special trap vent is superfluous for scouring, not only because the traps may be fully vented without it, but also because a good water flushing accomplishes all and infinitely more than the air could do.

Removal of Gaseous Impurities.

The chief difference between the main soil pipe and the small branch wastes in relation to venting is that the foul air in the former cannot, and in the latter case can, in good plumbing, be thoroughly changed by flushing and diffusion. Hence, in the main wastes, special venting is necessary to remove gaseous impurities and in the small branch wastes it is not. What has already been said in regard to the capacity for the removal of solid impurities from the smaller waste pipes of a good water flush holds with still greater force in relation to gaseous impurity. The lighter gases are instantly removed by the water stream and replaced by pure air from the room, and this substitution is as much more desirable than the substitution of soil pipe air as the former is richer in oxygen and freer from injurious elements than the latter.

Back Pressure.

"Back pressure" in plumbing is a force acting in a direction precisely opposite to that of siphonage. It indicates that the air in the drains is under compression, where with siphonage it is under rarification. Hence it tends to force the water of traps from the drains outwards into the house where siphonage tends to force it inwards from the house into the drains. This compression of air in the drains may be caused either by the movement of fluids in the house pipes themselves or by external influences acting upon the air of the sewers, such as the pressure of wind and tide at the sewer openings or a change in the temperature or volume of sewage within the pipes. With properly ventilated soil pipes the expansion or contraction of the air or wind or other pressure in the sewers can have no influence on the seals of interior fixture traps, but where an intercepting house trap is used, it is clear that a sewer vent must also be furnished at the trap to protect it from such influences.

The influences which act within the house pipes to create back pressure are: First, the compression of the air in the main soil pipe by waste water passing through it; second, the pressure of the wind; and third, the suction of open fires and ventilating outlets throughout the house.

If a large body of water is thrown suddenly into the soil pipes from one of the upper fixtures in a house, it drives the air in advance of it as it falls like a plug through the pipe. Were there no resistance to the passage of the air, such as is caused by friction, or a sudden bend in the pipe, the air would pass through a properly ventilated pipe in front of the water without compression, but the rough interior of the soil and waste pipes, and sudden bends in their course, causes considerable resistance to the escape of the air in advance of the water, causing condensation of the air, and giving rise to the phenomenon we are discussing, and this "back pressure" is sometimes strong enough to drive the water out of the traps in a sudden jet or fountain. I have completely emptied a 4-inch pot trap, having a seal four inches deep, by this action, even though the soil pipe was properly vented at both top and bottom. Fortunately, however, a very simple remedy exists for back pressure. This force never exceeds a few ounces to the square inch in properly arranged plumbing, and may easily be resisted by a column of water from 12 to 18 inches in height. Hence a trap which would be completely emptied when standing alone, as shown in Fig. 252, 276 or 278, will easily resist the pressure when attached to and placed some little distance below a fixture or when the inlet arm is simply lengthened as shown in Fig. 277. With a common S trap the resistance to back pressure in this figure is twice as great as in the first. The limit of resistance of an S trap is the weight of a column of water twice as high as the depth of its seal. But though the soil pipe air may be blown through the trap when it surpasses the limit of resistance of the seal, yet the fixture above the trap will catch the water thrown up and restore it to the trap.