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pure water of Lake Zurich, found by analysis to be purer than spring water, is polluted about 2 miles below the lake by the sewage of Zurich, a city of about 93,000 inhabitants. This sewage amounts to about 0.2 per cent of the total flow of the river, and it contaminates the river to such a degree that nearly a million bacteria, in their various forms, per cubic centimeter (a centimeter is about two-fifths of an inch), are sometimes found in the water just below the outflow of the sewage into the river.

In 1889 a series of weekly bacteriological investigations, extending from January to April (see Appendix 4), was very carefully made by the Hygienic Institute of Zurich to determine from samples of water, taken from the river at measured intervals below the outfall of the sewage, whether the sewage rendered the water below Zurich unfit for domestic purposes and for pisciculture.

It was found that at the point where the sewage enters the river the water of the river contained on an average 296,670 bacteria to the cubic centimeter; that at a point about one-third of a mile below the number of bacteria had decreased to an average of 12,870 per cubic centimeter, a decrease of 96 per cent; that of this number there was found at a point about one third of a mile further down the river 10,892 bacteria per cubic centimeter, a decrease of 15 per cent; that of this number there was found at a point about five-eighths of a mile still further down 5,902 bacteria per cubic centimeter, a decrease of 46 per cent; and so on down the river to a point where the water was found to be as pure as at its issue from the lake, and the conclusion was reached that "under the conditions described, and provided there are no intermediate sources of pollution, a river such as the Limmat, flowing at a mean velocity of about 4 miles per hour, will purify itself within a distance of about 16 miles from the point of pollution." Details of the very full and complete experiments at Zurich will be found in Appendix 4. The entire paper from which they have been taken, which contains in addition very interesting accounts of the thorough system of bacteriological and chemical examinations of the Limmat water, and of the new filtration works of that city, may be found in the Proceedings of the Institution (British) of Civil Engineers, Vol. cxI, 1892-'93, Part 1. The experiments on the water of the Limmat seem to be conclusive in respect of that river, but they should not lead to the taking for domestic purposes of the water of any other river polluted by the drainage from a sewered town, except at such a distance below the point of pollution that there can be no question as to the quality of the water.

It will be seen from the foregoing table that the first town above Great Falls is the small town of Harpers Ferry, containing less than 1,000 inhabitants. It is 44 miles from Great Falls and is not sewered, and while the conditions are thus all favorable to the healthfulness of our Potomac water, I think it would be wise to attach to the next appropriation for the Washington Aqueduct, with a special appropriation for the purpose, if necessary, a provision of law for a systematic monthly chemical and bacteriological examination of it as it is sent to the city from the distributing reservoir, the examinations to be made by the Department of Agriculture or the Smithsonian Institution, and the results to be sent to this office for publication in the annual reports of the Washington Aqueduct.

I will now proceed to describe the two principal systems of filtration and state the cost of each system.

Mechanical or rapid filtration. - "Mechanical filtration is a somewhat broad term used to denote those systems of water purification in which an exceedingly rapid rate of filtration is made possible by means of mechanical devices for frequent, quick, and thorough washings of the filtering materials without removing them from the filter."

The filters of this system, which is called the American system, are cylinders of iron or steel containing filtering material, which is generally sand, and are extensively used by paper-makers, bottlers, brewers, ice-makers, laundrymen, and in other similar occupations. They are used also for the filtration of the public water supplies of several American cities, but so far none of the larger cities of this country have established filtration works for their entire water supplies. The cylinders are made vertical, in sizes up to 12 feet in diameter and 16 feet high, and horizontal, in sizes up to 71⁄2 feet in diameter and 35 feet long. The numbers of filters are regulated by the quantity of water to be filtered. The cleansing of the sand is ordinarily done by means of a reverse current of water during the stirring up of the sand by a rake with long vertical teeth, which is revolved horizontally by machinery through the sand. The cleansing is done daily or oftener, depending on the turbidity of the water.

In the American system the fall in the column of water above the sand is at the rate of 5 inches or more a minute, making a yield of filtered water of 3,000 to 4,000 gallons per square foot of filter surface per diem. This rapid rate is produced either by a great pressure or head of water on the sand by which the water is forced at a rapid rate through it, or by frequent cleansing of the sand for the purpose of removing obstructions to a rapid passage of the water, or by both.

The objection to the first is that fissures or channels are liable to be forced through the sand through which the water may pass without any modification, and the objection to the second is that clean sand alone does not make an effective filter against bacteria contained in the water. This latter objection, it is claimed, has been overcome by the use of alum, which is said to combine with the carbonate of lime, existing to a greater or less degree in all natural waters, and to form a jelly-like substance (hydrate of alumina) on the surface and in the interstices of the sand, which collects and retains the bacteria contained in the water until, after the stopping of the inflow, the reverse current is turned on and the sand is cleaned.

The amount of alum used, which varies with the turbidity of the water, is ordinarily about three-fifths of a grain to a gallon of water. It is one of the principal items of expense in this system. At Atlanta, Ga., 70,032 pounds of alum were used in filtering the 756,762,600 gallons of water used by thatcityin 1889. At this rate there would be required for our Washington supply of about 50,000,000 gallons per diem 5,200 pounds of alum per diem, or 1,905,300 pounds per annum, the cost of which, at 2 cents per pound, would be $38,106.

By some the use of alum is objected to as dangerous to health, but it is claimed by the owners of the patent and makers of the filters that no alum passes through the sand and that none can be found in water filtered by this system when the process is used as directed. Many testimonials to this effect and of the general excellence of this system accompany the advertisements of these filters, but this, it may be remarked, is the case with the advertisements of all patented articles. There is one point in connection with these filters that I have not seen mentioned. After the sand has been cleansed, and the water again passes downward through the sand, a certain time must elapse before the alumina jelly, on which the efficiency of the filter as against the bacteria depends, is again formed on the surface and in the interstices of the sand and replaces that which has been washed away by the reverse current, together with the dirt retained by the filter. Until this time it is obvious that the water passing through the filter should be wasted and not sent into the mains, but as far as I have observed this has not been done in any use of this system for the purification of public water supplies.

In my judgment this system should in no case be applied to our Washington water supply, which would require probably two hundred or more filters of the largest size, before it has been thoroughly tested without expense to the United States or the District of Columbia. This could be done at a cost not exceeding $5,000 by means of a single filter through which is made to pass, for such length of time as may be deemed expedient, water filled with bacteria, cultivated for the purpose, if necessary, and comparing the results with the known results* of the other system of filters about to be described. The testfor color should also be made at the same time, and the water for testing should be taken from the conduit immediately after one of our heavy spring freshets.

Natural or slow filtration. - This is the system that has been in use for the purification of public water supplies of Europe for a very long period. The water of London, Berlin, Hamburg, and other large cities is filtered by this system, the most extensive application of it being at London, where about 200,000,000 gallons are filtered daily. The system has also been in use in several cities of this country, and is not patented.

A filter (filter bed) of this system is a small basin generally of an acre or less in size, with water-tight side walls of masonry or of earth paved with masonry. The number of beds required in any case depends on the daily supply of filtered water required. On the bottom of the bed, which is also water-tight, a large drain extends longitudinally from end to end of the bed and discharges through the wall into a filtered water basin. From this central drain extend, so as to drain every part of the bed, small drains of perforated tile pipes. Above this system of small drains are. placed several layers of filtering material, increasing in fineness to the top. The lower layer is generally formed of small stones or broken stone, then, proceeding upward, there is coarse gravel, then fine gravel, then coarse sand, and lastly, at top, the filtering material proper, which is fine sand. The aggregate depth of the layers and the depth of each layer are not uniform in the different countries and in the different cities of the same country, but they vary with the materials available and the judgment of the engineers. The aggregate thickness of the layers is usually from 6 to 8 feet and the thickness of the sand is from 2 to 4 feet. On this latter depth; the depth or pressure of water on the sand, and the degree of cleanness of the sand, mainly depend the vertical rate of fall of the column of water above the sand, the rate and the duration of the percolation of water through the sand, and the yield of filtered water per square foot of filter surface, and on them, in turn, depends the degree of efficiency of the filter, especially as against bacteria.

It has been stated in the foregoing description of mechanical filters that clean sand alone is not effective as against bacteria and that with the use of these filters it is found necessary to dissolve alum in the water to be filtered; that this forms a jelly-like substance on the surface and in the interstices of the sand, and this, it is claimed, prevents bacteria from passing through these filters.

* A reduction of bacteria of 99.9 per cent at Berlin.

In the European or natural system of slow filtration, the efficiency of the filters depends on the slimy deposit that commences at once to form on the surface of the sand and in its interstices, and which, when in sufficient quantity, not only collects, but consumes and destroys the bacteria.* +

When the deposit forms to such a degree as to impede too much the percolation of water, the upper portion of the sand is removed, and at proper intervals the entire body of sand is replaced by fresh sand.

From time to time, as is found necessary, the water is drained from the sand and the filter is allowed to rest. The air which replaces the water oxidizes any organic matter that remains in the sand.

In order to protect the water from the heat of summer and from freezing in winter filter beds in this climate should be roofed.

When a filter bed is first put in operation and afterwards, after each of these changes, the water passing through the filter is allowed to run to waste for about ten days or two weeks until it is found to be chemically and bacteriologically pure by a chemist and biologist constantly employed at the filtration works.

The vertical rate of fall of water in the European system is not allowed to exceed a rate of about 4 inches an hour, making a yield of filtered water of from 50 to 100 gallons per square foot of filter surface. As has been stated, the rate of vertical fall of the column of water above the sand in the mechanical or American system is 5 inches or more a minute or 25 feet or more an hour, making a yield of 3,000 to 4,000 gallons per diem of filtered water per square foot of filter surface. The interstices in the sand, occupying as they do about one-third of the body of the sand, the downward rate of percolation through the sand in the European

*"It is easy to see how the filters remove the dirt and suspended, matter, but the way in which bacteria were eliminated was a complete mystery until the last four or five years. But few people had ever seen or examined bacteria before that period. It now has been shown that the bacteria remove the bacteria. The bacteria in the waters are comparatively few of a dangerous character; the great bulk of them are our greatest friends. It is through their aid, together with the oxygen of the air, that the filth in the water is destroyed. They feed upon it and they feed upon each other. Since that knowledge has been obtained, the object now is to cultivate the bacteria. In order to make the filter bed do its work effectively it is necessary that the growth of the bacteria shall be facilitated until a filter bed becomes populated with an incredible number of millions of them. As the result of their activity they multiply themselves in vast numbers, and they form, at the top of the filter beds and between particles of sand, a sort of jelly or slime-a bacteria jelly-and it is by the aid of this bacteria jelly that the bacteria in the unfiltered water are removed." (Prof. Leeds, of Stevens Institute.)

†" On examining with the microscope the surfaces of the particles of sand when the filter is in perfect working order, they are found to be coated with a greasy, slimy substance, which is a mass of bacteria jelly. It is to this coating of bacteria jelly that Peifke attributes the efficiency of these filters, and until the jelly forms in sufficient amount to completely envelop each particle of sand the filter works imperfectly. This, then, is his explanation of the fact that minute microorganisms and particles of clay of infinitely smaller size than the channels in the sand are stopped in their passage through it-they are simply caught in this slimy coating and can not get farther."

The latter extract is from a paper by Thomas M. Drown, read before the Boston Society of Civil Engineers and published in the Journal of the Association of Engineering Societies, July, 1890. In the advertisements of the American filters it is stated that the jelly of hydrate of alumina used with them is a far more cleanly agent than the jelly above described and this forms one of the chief claims of excellence of the American system, but it should be remembered that as the deposit of bacteria, etc., from the water commences to form on the surface and in the interstices of the sand in the American system as soon as the filtration commences and constantly increases in quantity until the sand is cleansed, the filtering in the American system is also done through the jelly formed in the natural system, and that this can not be avoided in any system of filteration.

system is therefore about 1 foot an hour, and in the American system is about 75 feet or more an hour. The depth of sand in the European system being on an average, say 3 feet, and in the American system on an average say 5 feet, it follows that water in process of filtering is, in the European system, in contact with the filtering sand about three hours and in the American system about four minutes.

In Europe no subject connected with health has had in recent years so much careful and scientific investigation as the filtration of public water supplies. The Imperial Board of Health of Berlin, of which Prof. Koch, the discoverer of the microbes of consumption and cholera, is a member, has formulated the rules by which filtration should be conducted. In this country also, especially at Lawrence, Mass., the experiments with filtration have been most thorough and conclusive.

On a statement by the State Board of Health of Massachusetts that "no mechanical filter examined by it removed enough bacteria to warrant the board in recommending the city to accept it," the city of Lawrence entered upon the construction of a system of natural filtration works which for efficiency are probably not excelled in any country, and the result has been that typhoid fever, from which the city formerly suffered severely by reason of pollution of its water supply (the river Merrimac) from the sewered city of Lowell a few miles above, has been almost completely eradicated.

COST OF FILTRATION WORKS AND COST OF MAINTENANCE.

The city of Providence, R. I., has very recently had in competition as to cost the two systems of filtration, and it furnishes the basis of close calculations as to the first cost and cost of maintenance of the two systems if applied to the Washington water supply.

I quote the following extracts from a letter dated July 26 last, that I received from Mr. J. Herbert Shedd, city engineer of Providence, in answer to my inquiries:

The proposition to furnish mechanical filters to the city of Providence included the erection of 60 steel filters, 12 feet 8 inches in diameter and 16 feet high, with all suitable appurtenances and piping, housed in a brick building about 52 by 370 feet, with an engine-room annex 51 by 62 feet for $280,896. There was also nacluded $15,000 for earth filling about the building and $1,100 for raising the standpipe to give the necessary increased pressure required by the loss of head through the filters. This provides for the filtration of 15,000,000 gallons per day, but at a rither slower rate than the filter company deemed necessary, they thinking it practicable to do the work with 45 such filter tanks. The estimated cost of maintaining these filters based upon our experiments is as follows:

Sulphate of alumina at 2 cents, including delivery

Engineers, 2 men, at $2.50.

Washing, 2 men, at $2.

Firemen, 2 men, at $2

Irregular washing, 2 men, at $2.

Water for washing, at $10 per 1,000,000 gallons

Water for rewashing.

Caustic soda....

Waste and oil....

$25.70

$5.00

4.00

4.00

4.00

17.00

7.35

4.35

16.35

.50

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