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stices 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 test for 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:

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The proposed constrution for natural filtration, provided for six basins having an effective filtration area each of 150 by 300 feet, making a little over an acre for each bed with necessary inlets, with racks, screens, channels, pipes, gates, etc., to take the water from the river as now existing through the filters and into the clear well now existing. These beds would filter 15,000,000 gallons per day, passing a vertical depth of about 3 inches per hour and at the rate of about 2,000,000 imperial gallons per acre. My estimate of the cost of doing this work was $208,000. A bid has been received from responsible contractors offering to do the work for $200,000.

From the best information I am able to obtain as to the cost of maintaining and cleaning these filters, the cost ought not to exceed $1 per million gallons per day. If we add to this $5 per day for biological assistant to make it comparable with our estimate of cost of maintaining the mechanical filter, we should have relatively $20 per day for natural filtration and about $94 per day for filtration with alum.

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The color of our water, which is not high, is reduced about one-half by slow filtration. It is reduced a little more, but not to a marked degree, by the use of alum. The color of the Boston water supply is reduced about one half by slow filtration without alum. The color of the Hudson river water at Poughkeepsie is not much reduced by slow filtration. It can be nearly removed by the use of alum, but to do this requires an excessive amount of the chemical. The Massachusetts board of health found at Brockton that quite an inadmissible amount of alum was necessary to clarify the water by that process.

The first cost and the annual cost of maintenance of the works required for the filtration of the water now consumed and wasted in Washington, about 50,000,000 gallons per diem, and also for the filtration of 75,000,000 gallons per diem (to which amount our consumption and waste will probably attain considerably within ten years) at the above rates (after deducting the cost of earth works and raising the standpipe from the price of the mechanical filters at Providence) would be as follows:

First cost (50,000,000 gallons per diem):

For natural or slow filtration...
For mechanical or rapid filtration.
First cost (75,000,000 gallons per diem):
For natural or slow filtration....
For mechanical or rapid filtration.

$666, 667 883, 000

1, 000, 000

1, 324, 500

These amounts do not include the cost of the land required for the filtration works, the cost of filtered water basins, or the cost of the changes that would be required at the reservoirs.

Annual cost of maintenance (50,000,000 gallons per diem):
For natural or slow filtration.

For mechanical or rapid filtration

Annual cost of maintenance (75,000,000 gallons per diem):
For natural or slow filtration..

For mechanical or rapid filtration.

$24, 333.33 114, 366. 67

36,500.00 171, 550.00

CONCLUSION.

Under present conditions there appears to be no cause for apprehension respecting the healthfulness of Potomac water as delivered by the river into the intake of the aqueduct at Great Falls.

It appears that without the use of alum, or with this use if the alum be used in quantities not prejudicial to health, the color of the water that exists after freshets in the Potomac can only be partially removed by filtration.

For these reasons, and for the reason that of the 200 gallons of water or more per capita per diem that are consumed and wasted, it is probable that not more than an average of 50 gallons, or one-quarter, is used for strictly domestic purposes, and that this portion in its delivery through the mains can not be separated from the remainder, it seems to me beyond doubt that as long as the present conditions continue the great expenditures that would be required for the first cost of filtration

works for our water supply, and the annual cost of maintenance of these works would not be justifiable, and that, for the present at least, reliance should be had on sedimentation.

It is expected that when the works of improvement required for restoring to use the Dalecarlia receiving reservoir shall have been completed, at the end of this fiscal year, the time during which the water may be "settled" before it is sent to the city from the distributing reservoir will be so much increased (it will be doubled) that the condition of the water as to color after high-water and freshets in the Potomac and its tributaries will be much improved, and that when the new reservoir near Howard University, which is to contain 300,000,000 gallons of water, shall have been finished and brought into use, there will be but little to be desired in respect of the quality of our water supply.

In Appendix 5 will be found a copy of a useful and interesting paper on the subject of natural or slow filtration by Prof. William T. Sedgwick of the Massachusetts Institute of Technology.

MEASUREMENT OF DAILY AND HOURLY CONSUMPTION AND WASTE OF WATER. [Hourly and total flow from the distributing reservoir for the twenty-four hours ending at 8 a. m. June 28, 1894. City temperature in the shade at 2 p. m. June 27, 86°.]

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Measurements of daily consumption and waste of water in the city in the last fiscal year.

Date.

Gallons.

Thursday, March 29, 1894..

Thursday, June 28, 1894

43, 505, 274 49, 162, 357

By reason of leaks in the gates in the dam at waste weir No. 2 which interfered with the work of excavation of the main drainage tunnel at the Dalecarlia receiving reservoir, and which would be increased by lowering and raising the water in the conduit, and by reason also that the falling off of the supply from Great Falls rendered it unwise to diminish the head of water in the distributing reservoir when it could be avoided, I measured the daily consumption and waste but twice in the fiscal year.

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