Thus we see that the sewers really form vast filters, as it were, for clearing the air which passes through them of germs of disease, and it is the province of the science of sanitary engineering to make the most of this wonderful provision of nature in man's behalf by co-operating with her methods and increasing to the utmost extent the ventilation of the sewers and drains.
We know, for instance, that, especially in times of epidemics, the air of the streets contains many disease germs. If we could erect in front of our windows and doors continuously moistened filters of such a construction that no particle of air could enter the house without first coming in direct contact with some part of the moist surface of the filter, and if the filter could be provided with a germicide maintained constantly active, it is certain not only that the inmates of these houses would be shielded from the diseases conveyed by germs as long as they remained within the protection of the filters, but also that the germs themselves in the locality would gradually be diminished in numbers until, if the traps were numerous enough, all might ultimately be destroyed.
Now the sewers form precisely such filters, and if copiously ventilated by pure air currents induced by making every house drain a suction pipe, no offensive odor could come from the sewers, and all the air thus drawn in from the streets would be freed from dust and germs.
How short-sighted and foolish it is, then, to legislate against such beneficent ventilation and purification by requiring a barrier to be interposed at every house drain outlet, by which the only really practicable way of thoroughly ventilating the sewers is prevented.
There is no way known of setting up such screens as I have described in front of our windows. But it is easily possible to utilize the sewers already provided for us and equipped with a most marvelously effective dust arrester and germicide.
I do not say that with such perfectly constructed and ventilated sewers as these it would be wise in times of epidemics to nail up our windows and ventilate from the sewers, because the house drains and soil pipes will always contain foul gases, which it will justify the use of our best science to exclude from the house, and because the smallest of the city sewers would have to be enlarged for such service to the size of a Hoosac tunnel. But I do say that any air which might accidentally enter the house through them would be safer to breathe, so far as disease germs are concerned, than that which would enter through the windows, and that no odor would enter the house from the sewers themselves, however much might be generated in the private drains, because the warmth of the sewage and of the house would always create an upward draught in cold weather and fresh air would enter the sewers through the street openings everywhere provided for it. In hot weather citizens could take their choice between disease germs from the windows and an occasional odor from the private drain. Supposing, now, that the houses of an average city average twenty-five feet in width on the street lines, if we were to ventilate the sewers through every house drain we should have suction openings every twelve and one-half feet into the sewers, and these drains being four inches in diameter, we should have about twelve and one-half square inches of suction area for every twelve and one-half feet of sewer, or one square inch for every foot of sewer in a city. For supplying the air to the sewers corresponding street openings would be provided.
If we assume an average of twenty-five miles of streets in every square mile of a city we have for a city containing 100 miles of street about 528,000 square inches or 3,666 square feet area of sewer ventilation, which would be equivalent to a round ventilating flue sixty-eight feet in diameter or to 130 round flues six feet in diameter each.
Supposing the average height of the houses in our city to be fifty feet, and that the quantity of air discharged per minute through the 4-inch ducts of the 42,240 houses in our city of a mile square be estimated. Assuming that these ducts draw up the air at the rate of 419 feet per minute per square foot of ventilating area for an average difference of temperature in winter between the air of the house and that of the street of 300 F. (calculated under the formula V=240 in which V stands for the velocity of the current, h the height of the building, t1 the interior and t the exterior temperature, and allowing 50 per cent for loss by friction), we have a ventilation produced by all the 42,240 ducts of 1,536,054 cubic feet of air per minute.
Allowing five occupants for each house, we have somewhat over 200,000 inhabitants in our city of a mile square and our sewer ventilation would thus filter the air at the rate of over seven cubic feet per minute for every inhabitant, or if we make a much larger allowance for friction and assume a smaller difference of temperature between the inner and outer air, we have a sewer ventilation of say from one to five cubic feet of air per minute for each inhabitant. The warmth of the sewage produced by the introduction of warm water into the drains would under these circumstances always produce suitable ventilation even in the summer months.
Thus the sewer ventilation produced by the omission of the main house trap and its vent would not only be the most perfect that can be devised, but it would reduce the cost of the whole sewerage system by a large percentage, in fact, over half a million dollars in our city of a mile square. In rebuilding the city of San Francisco probably several millions could be saved in this manner.
Fig. 51. Apparatus for determining the degree of retention of bac teria by the moist surface of sewers and drains.
Fig. 51 shows our glass and metal sewer and drain pipe as arranged for the lecture room. The measured pile of dust to be experimented with is shown at the bellows end of the pipe. The first opening provided with a stopper is for the introduction of the dust. The other openings are to permit of tests being made upon short or long pipes and upon straight or bent pipes, as desired.
The arrows indicate the direction of the air currents and the manner in which the dust particles are blown against the moist inner surfaces of the pipes to which they adhere. The bend in the middle of the pipe may be placed either horizontally or vertically. When vertical it becomes a trap and arrests, when full of water, all dust and germs.