PURIFICATION OF SEWAGE-POLLUTED WATERS BY SAND FILTRATION.
AS to the purification of sewagepolluted waters, the matter may be stated briefly, something like this: Why do we attempt to purify sewage-polluted water? For two reasons: First, because such water is a nuisance; and, second, because it is apt to be dangerous to the public health. It certainly is dangerous to drink. How shall we go to work to purify it? Well, how does nature go to work? If the fanner puts upon his field barnyard manure year after year, giving it a heavy dressing of the material, this is easily taken care of by the mother earth. No one knows or thinks that the farmer is creating a nuisance; the odor is not objected to by any one. On the contrary, every one of us feels a sort of primitive agricultural delight in the smell of good barnyard manure thus utilized, and we have no objection to living in the neighborhood. I have seen the lawns on Commonwealth avenue in Boston so covered with stable manure that one had almost to hold his nose in passing; and yet the same people who do not object to that odor would complain most bitterly of the slightest odor coming from a garbage pail or anything else of equally unsavory nature or origin. We know that the earth takes care of this organic matter. And the same is true of the conditions of the ordinary untidy country house or the tenement house, where the housewife, to get rid of her dish water or slops, throws them out of the window. Unless this is done in excess, or for too long a time and under unfavorable cirumstances, the earth takes care of all the stuff thus thrown out, and little or no trouble ensues. Here is a hint for the doing of the whole thing. The earth is capable of taking care of the organic matter, provided it does not have too much of it or have it for a long time without charge.
We have arrived at this conclusion by experimentation. Back in 1839 London began to filter the highly-polluted water of the river Thames through sand filters. It was supposed that by simply straining out the suspended matters and some of the dissolved matters the water would do no harm. Bacteria had not then been heard of. But time has gone on, and it hits been found by making a study of the affair that, by putting sewage-polluted water on the land, with the right kind of soil, you can dispose of sewage on land as the farmer disposes of his barnyard manure; or, on the other hand, in the case of sewage-polluted water, that a larger volume of water can be treated, and then recovered through an underdrain and used for drinking.
As an example of the first case—the city of Berlin, a city of a million and six or seven hundred thousand population, disposes of its sewage upon the land. The river Spree runs through the city, but does not receive the sewage except in times of storms, when the overflow goes into the river. The sewage ordinarily is disposed of upon the land; and the same principle is applied as here in Brockton—namely, the putting of a thin layer of sewage upon the soil and letting the earth take care of it. In the case of Berlin they make use of the sewage-impregnated soil to raise crops; but, when you commence to raise crops at the place where the filtering is going on, you must remember that you cease to operate the filter to its highest capacity. In Berlin they raise cabbages and some other vegetables, where the sewage of sixteen or seventeen hundred thousand people is disposed of on the land; and the effluent water, as it comes outbelow, is used for drinking. Here is an instance of the disposition of sewage in this way on a large scale. And if any one says that the plan of Brockton, Mass., is only an experiment, and that no one can say how long it is going to continue satisfactorily, or how it will be in fifteen or twenty years from now, we may reply that Berlin has been doing the same thing for years, and there is no uncertainty about it; thatthe mother earth, when the place is rightly chosen and the operation is properly managed, will take care of any amount of organic matter which you may wish to put upon it.
When you come to water purification, there is the same problem with certain modifications. Sewage is water gone bad, water with excess of foul matters really—sewagepolluted water. And that is what sewage always is. The sewage of Berlin is so polluted that it becomes almost as black as ink, and is as thick as thin mud. I have seen it of that character on the filter fields of Berlin, like thin black mud or thick ink. Now in such a case, ordinaiily, there is one unfavorable condition: the sewage does not contain any oxygen. These filters are not strainers, they are not mere heaps of sand; but, when in good working order, they are rather like living organisms, for they are filled with bacteria which breathe and feed, taking up the. organic matters of the sewage. Oxygen being available, the bacteria resident in the sand feed upon the organic matters, and reduce them to similar matters of a mineral sort, and their organic character is lost. It is as if those filter beds were a gigantic living sponge. You know the sponge before itis gathered is a living mass permeated by narrow channels. And, if we imagine one of these filters to be a heap of sand, swarming with living bacteria, we can see how easy it is for the sewage-polluted water to flow into this great sponge, and for the living contents of the latter to feed upon the matters it contains. When the filters are water liters they work more freely, because ordinary water is saturated with oxygen.
The theory becomes comparatively simple when looking at these filters, if one does not regard them as mere sand, but rather as something which would remind us of London and Paris with their dense populations, great centres teeming with multitudes of individuals. The sewage that arrives they receive with open arms, taking from it the organic matters which are their food, and working them over into animal matters, which, in solution, pass off below. The purification of sewage-polluted water is pretty much the same thing as the purification of sewage, only the work is done faster because of the oxygenin the water. You have an example of this in the city _of Lawrence, Mass., where you have heard of the municipal filter designed by Mr. H. F. Mills, of the State Board of Health. That city is now supplied with water fit for drinking, purified by simple sand filtration in a bed two and a half acres in extent. It is not merely a bed of sand, but contains a vast multitude of bacteria, feeding there upon the impure water, which, after it is filtered, goes into a reservoir pure and clear. We may see how the the thing works, as far as can be seen with the naked eye, but for scientific purposes we must take a microscope; we shall then find every grain of sand in this great filter coated with bacterial jelly, and bacteria actively at work doing their part in the process. If we wish to see what has been accomplished in this way,we can compare the death rate in Lawrence as it is now with the death rate which prevailed before the filter went into operation. Lawrence, instead of standing higher than any other city in the Commonwealth in deaths from typhoid fever, as it did, stands now on a par with those cities which are furnished with a good water supply and having similar industries and climate; and its bad reputation as a breeder of typhoid fever has disappeared. Typhoid fever is no longer “ epidemic” in Lawrence; it is only occasional ami sporadic, as it is in all cities with good water supplies. There are a few cases arising from the use of water taken unfiltered from the river, and a few imported cases, and also cases from using bad milk, and from other sources; but the death rate ofI.awrencs from typhoid fever now compares favorably with that of other cities of its own size, character, and sit, uation having perfect water supplies. In other words, sewage-polluted water that is purified is not objectionable, but safe. The same thing is true of London, which for many years has been supplied with sewage-polluted water thus purified.
In looking at these so-called intermittent filters, then, one should take out of his mind any idea that they are mere heaps of sand. Every grain of that sand has on it thousands of bacteria which have taken up their residence there in a jelly-like form, so that the total effect is softness of the soil, because of this great aggregate of bacterial life. It Is bacterial life which takes care of the farmer’s manure spread upon the earth and of the dishwater thrown out of the window by the careless housewife and of the sewage which is spread on fields.