Chapter XXVII: Appendix: XI

THE ALBANY WATER-FILTRATION PLANT.

(Abridged from Proceedings American Society of Civil Engineers, Nov. 1899.)

Albany, N. Y., was originally supplied with water by gravity from certain reservoirs on small streams west and north of the city. In time, with increasing consumption, the supply obtained from these sources became inadequate, and an additional supply from the Hudson River was introduced. The water was obtained from the river through a tunnel under the Erie Basin, and a pumping-station was erected in Quackenbush Street to pump it to reservoirs, one of which served also as the distributing point for one of the gravity supplies. The intake, which was used first in 1873, drew water from the river opposite the heart of the city. In recent years, the amount of water drawn from this source has greatly exceeded that obtained from the gravity sources.

The Hudson River, at the point of intake, has a drainage area of 8240 square miles. Of this, 4541 square miles are tributary to the Hudson above Troy, 3493 are tributary to the Mohawk, and 168 are tributary to the Hudson below the Mohawk.

The minimum flow may be estimated at 1657 cubic feet per second, or 1,060,000,000 gallons per 24 hours, or at least fifty times the maximum consumption.

The cities and larger towns upon the river above the intake, with estimated populations and distances, are as follows:

MOST IMPORTANT CITIES, TOWNS, AND VILLAGES ON THE WATERSHED OF THE HUDSON ABOVE ALBANY.

------------+-----------+-----------+-----------------------------
| |Approximate| Population in
Place. | County. | Distance +--------+-------+------------
| | above | 1880. | 1890. | 1900.
| | Intake, | | |
| | Miles. | | |(Estimated.)
------------+-----------+-----------+--------+-------+------------
Troy |Rensselaer | 4 | 56,747 | 60,956| 65,470
Watervliet |Albany | 4 | 8,820 | 12,967| 19,040
Green Island|Rensselaer | 5 | 4,160 | 4,463| 4,788
Cohoes |Albany | 8 | 19,416 | 22,509| 26,450
Lansingburg |Rensselaer | 8 | 7,432 | 10,550| 14,980
Waterford |Saratoga | 9 | (1,822)| 1,822| (1,822)
Schenectady |Schenectady| 28 | 13,655 | 19,002| 26,450
Hoosic Falls|Rensselaer | 44 | 4,530 | 7,014| 10,860
Amsterdam |Montgomery | 44 | 9,466 | 17,336| 31,730
Glens Falls |Warren | 49 | 4,900 | 9,509| 18,450
Saratoga |Saratoga | 51 | 8,421 | 11,975| 17,010
Springs | | | | |
Johnstown |Fulton | 56 | 5,013 | 7,768| 12,040
Gloversville|Fulton | 58 | 7,133 | 13,864| 26,930
North Adams,| | | | |
Mass. |Berkshire | 68 | 10,191 | 16,074| 25,340
Adams, Mass.|Berkshire | 75 | 5,591 | 9,213| 15,181
Little Falls|Herkimer | 82 | 6,910 | 8,783| 11,160
Utica |Oneida | 107 | 33,914 | 44,007| 57,090
Rome |Oneida | 127 | 12,194 | 14,991| 18,430
32 villages | | | 52,523 | 61,869| 76,194
------------+-----------+-----------+--------+-------+------------
Total, not including rural | | |
population |272,838 |354,672| 479,415
Per square mile | 33 | 43| 59
------------------------------------+--------+-------+------------

Without entering into a detailed discussion, it may be said that the amount of sewage, with reference to the size of the river and the volume of flow, is a fraction less than that at Lawrence, Mass., where a filter-plant has also been constructed, but the pollution is much greater than that of most American rivers from which municipal water-supplies are taken.

The filtration-plant completed in 1899 takes the water from a point about two miles above the old intake. Pumps lift the water to the sedimentation-basin, from which it flows to the filters and thence through a conduit to the pumping-station previously used.

DESCRIPTION OF PLANT.

=Intake.=—The intake consists of a simple concrete structure in the form of a box, having an open top covered with rails 6 inches apart, and connected below, through a 36-inch pipe, with a well in the pumping-station. Before going to the pumps the water passes through a screen with bars 2 inches apart, so arranged as to be raked readily. The rails over the intake and this screen are intended to stop matters which might obstruct the passageways of the pumps, but no attempt is made to stop fish, leaves, or other floating matters which may be in the water. The arrangement, in this respect, is like that of the filter at Lawrence, Mass., where the raw water is not subjected to close screening. There is room, however, to place finer screens in the pump-well, should they be found desirable.

NEAR INTAKE

FIG. 1.]

[_To face page 290._]
]

=Pumping-station.=—The centrifugal pumps have a guaranteed capacity of 16,000,000 gallons per 24 hours against a lift of 18 feet, or 12,000,000 gallons per 24 hours against a lift of 24 feet. The ordinary pumping at low water is against the higher lift, and under these conditions either pump can supply the ordinary consumption, the other pump being held in reserve.

The pumping-station building, to a point above the highest flood-level, is of massive concrete construction, without openings. Nearly all the machinery is necessarily below this level, and in high water the sluice-gates are closed, and the machinery is thus protected from flooding. The superstructure is of pressed brick, with granite trimmings.

=Meter for Raw Water.=—Upon leaving the pumping-station the water passes through a 36-inch Venturi meter having a throat diameter of 17 inches, the throat area being two ninths of the area of the pipe. The meter records the quantity of water pumped, and is also arranged to show on gauges in the pumping-station the rate of pumping.

=Aeration.=—After leaving the meter, the water passes to the sedimentation-basin through eleven outlets. These outlets consist of 12-inch pipes on end, the tops of which are 4 feet above the nominal flow-line of the sedimentation-basin. Each of these outlet-pipes is pierced with 296 3/8-inch holes extending from 0.5 to 3.5 feet below the top of the pipe. These holes are computed so that when 11,000,000 gallons of water per day are pumped all the water will pass through the holes, the water in the pipes standing flush with the tops. The water is thus thrown out in 3256 small streams, and becomes aerated. When more than the above amount is pumped, the excess flows over the tops of the outlet-pipes in thin sheets, which are broken by the jets.

GENERAL PLAN

FIG. 2.]

Regarding the necessity for aeration, no observations have been taken upon the Hudson River, but, judging from experience with the Merrimac at Lawrence, where the conditions are in many respects similar, the water is at all times more or less aerated, and, for the greater part of the year, it is nearly saturated with oxygen, and aeration is not necessary. During low water in summer, however, there is much less oxygen in the water, and at these times aeration is a distinct advantage. Further, the river-water will often have a slight odor, and aeration will tend to remove it. The outlets are arranged so that they can be removed readily in winter if they are not found necessary at that season.

=Sedimentation-basin.=—The sedimentation-basin has an area of 5 acres and is 9 feet deep. To the overflow it has a capacity of 14,600,000 gallons, and to the flow-line of the filters 8,900,000 gallons. There is thus a reserve capacity of 5,700,000 gallons between these limits, and this amount can be drawn upon, without inconvenience, for maintaining the filters in service while the pumps are shut down. This allows a freedom in the operation of the pumps which would not exist with the water supplied direct to the filters.

The water enters the sedimentation-basin from eleven inlets along one side, and is withdrawn from eleven outlets directly opposite. The inlets and aerating devices described previously bring the water into the basin without current and evenly distributed along one side. Both inlets and outlets are controlled by gates, so that any irregularities in distribution can be avoided. The concrete floor of the sedimentation-basin is built with even slopes from the toe of each embankment to a sump, the heights of these slopes being 1 foot, whatever their lengths. The sump is connected with a 24-inch pipe leading to a large manhole in which there is a gate through which water can be drawn to empty the basin. There is an overflow from the basin to this manhole which makes it impossible to fill the basin above the intended level.

FILTER BEDS

PLAN AND SECTION OF FILTER NO. 2

FIG. 3.]

[_To face page 294._]
]

=Filters.=—The filters are of masonry, and are covered to protect them against the winters, which are quite severe in Albany. The piers, cross-walls, and linings of the outside walls, entrances, etc., are of vitrified brick. All other masonry is concrete. The average depth of excavation for the filters was 4 feet, and the material at the bottom was usually blue or yellow clay. In some places shale was encountered. In one place soft clay was found, and there the foundations were made deeper. The floors consisted of inverted, groined, concrete arches, arranged to distribute the weight of the walls and vaulting over the whole area of the bottom.

The groined arch-vaulting is of concrete with a clear span of 11 feet 11 inches, a rise of 2-1/2 feet, and a thickness of 6 inches at the crown. It was put in in squares, the joints being on the crowns of the arches parallel with the lines of the piers, and each pier being the centre of one square. The manholes are in alternate sections, and are of concrete, built in steel forms with castings at the tops, securely jointed to the concrete.

Above the vaulting there are 2 feet of earth and soil, grassed on top. The tops of the manholes are 6 inches above the soil to prevent rain-water from entering them. The drainage of the soil is effected by a depression of the vaulting over each pier, partially filled with gravel and sand, from which water is removed by a 2-inch tile-drain going down the centre of the pier and discharging through its side just above the top of the sand in the filter.

In order to provide ready access to each filter, a part of the vaulting near one side is elevated and made cylindrical in shape, making an inclined runway from the sand-level to a door the threshold of which is 6 inches above the level of the overflow.

FILTER BEDS

SECTION OF FILTERS

FIG. 4.]

This sand-run is provided with permanent timber runways and with secure doors.

Elevation.

FIG. 5.—ENTRANCE TO A FILTER.]

The manholes of the filters are provided with double covers of steel plates to exclude the cold. The covers also exclude light. When cleaning the filters, light can be admitted by removing the covers. Supports for electric lights are placed in the vaulting, so that the filters can be lighted by electricity and the work of cleaning can be done at night, and in winter under heavy snow, without removing the covers. The electric lights have not yet been installed.

The regulator-houses, the entrances to the sand-runs, and all exposed work are of pressed brick with Milford granite trimmings and slate roofs. The regulator-houses have double walls and double windows and a tight ceiling in the roof, to make them as warm as possible and to avoid the necessity of artificial heat to prevent freezing.

STANDARD GRAVEL SECTIONS.

FIG. 6.]

[_To face page 298._]
]

The main underdrains for removing the filtered water are of vitrified pipe surrounded by concrete and are entirely below the floors of the filters.

Connections with the main drain are made through thirty-eight 6-inch outlets in each filter, passing through the floor and connected with 6-inch lateral drains running through the whole width of the filter. These drains were made with pipes having one side of the bell cut off so that they would lie flat on the floor and make concentric joints, without support and without having to be wedged. They were laid with a space of about 1 inch between the barrels, leaving a large opening for the admission of water from the gravel.

The underdrainage system is so designed that, when starting a filter after cleaning, the friction of the sand is about 50 mm. at a rate of 3,000,000 gallons per acre daily, and the friction of the underdrainage system is estimated at 10 mm. This very low friction, which is necessary, is obtained by the use of ample sizes for the underdrains and low velocities in them. In the outlet and measuring devices moderate losses of head are not objectionable, and the sizes of the pipes and connections are, therefore, smaller than the main underdrains.

The gravel surrounding the underdrains is of three grades. The material was obtained from the river-bed by dredging, and was of the same stock as that used for preparing ballast for the concrete. It was separated and cleaned by a special, cylindrical, revolving screen. The coarsest grade of gravel was that which would not pass round holes 1 inch in diameter, and free from stones more than about 2 inches in diameter. At first it was required to pass a screen with holes 2 inches in diameter, but this screen removed many stones which it was desired to retain, and the screen was afterward changed to have holes 3 inches in diameter. The intermediate grades of gravel passed the 1-inch holes, and were retained by a screen with round holes 3/8 inch in diameter. The finest gravel passed the above screens and was retained by a screen with round holes 3/16 inch in diameter. The gravel was washed, until free from sand and dirt, by water played upon it during the process of screening, and it was afterward taken over screens in the chutes, where it was separated from the dirty water, and, when necessary, further quantities of water were played upon it at these points.

INLET TO FILTER BED. SECTION ON M-N-O

REGULATOR CHAMBER
LONGITUDINAL SECTION ON A-B-C-D

REGULATOR CHAMBER
SECTION ON K-L

FILTER BEDS
INLET VALVES AND REGULATOR CHAMBER

INLET TO FILTER BED
SECTIONAL PLAN ON P-Q

ORIFICE INDICATOR MARKER
DETAILS OF APPARATUS IN REGULATOR HOUSE

REGULATOR CHAMBER
SECTIONAL PLAN ON E-F-G-H-J

FIG. 7.]

The average mechanical analyses of the three grades of gravel are shown by Fig. 8. Their effective sizes were 23, 8, and 3 mm. respectively, and for convenience they are designated by these numbers. The average uniformity coefficient for each grade was about 1.8.

The 23-mm. gravel entirely surrounded the 6-inch pipe-drains, and was carried slightly above their tops. In some cases it was used to cover nearly the whole of the floor, but this was not insisted upon.

The 8-mm. gravel was obtained in larger quantity than the other sizes, and was used to fill all spaces up to a plane 2-1/2 inches below the finished surface of the gravel, this layer being about 2 inches thick over the tops of the drains, and somewhat thicker elsewhere.

The 3-mm. gravel was then applied in a layer 2-1/2 inches deep, and the surface levelled.

The preliminary estimates of cost were based upon the use of filter-sand from a bank near the filter-site. Further examination showed that this sand contained a considerable quantity of lime, and it was found by experiment with a small filter constructed for that purpose that the use of this sand would harden the water by about 2 parts in 100,000, and the amount of lime contained in the sand, namely, about 7 per cent, was sufficient to continue this hardening action for a considerable number of years. This was regarded as a serious objection to its use, and the specifications were drawn limiting the amount of lime in the sand. This excluded all of the local bank sands. The river-sands which were used were nearly free from lime, and in the end the sand as secured was probably not only free from lime, but more satisfactory in other ways, and also cheaper than the bank-sand would have been.

(ARROWS SHOW REQUIREMENT OF SPECIFICATION)

_Diameters in Millimeters_

FIG. 8.]

The specifications of the filter-sand require that “The filter-sand shall be clean river-, beach-, or bank-sand, with either sharp or rounded grains. It shall be entirely free from clay, dust, or organic impurities, and shall, if necessary, be washed to remove such materials from it. The grains shall, all of them, be of hard material which will not disintegrate, and shall be of the following diameters: Not more than 1 per cent, by weight, less than 0.13 mm., nor more than 10 per cent less than 0.27 mm.; at least 10 per cent, by weight, shall be less than 0.36 mm., and at least 70 per cent, by weight, shall be less than 1 mm., and no particles shall be more than 5 mm. in diameter. The diameters of the sand-grains will be computed as the diameters of spheres of equal volume. The sand shall not contain more than 2 per cent, by weight, of lime and magnesia taken together and calculated as carbonates.”

[_To face page 302._]
]

The sand was obtained from the river at various places by dredging. It was first taken up by dipper-dredges, and brought in scows to a point in the back channel a little north of the filter-plant. It was there dumped in a specially prepared place in the bottom of the river, from which it was lifted by a hydraulic dredge and pumped through a 15-inch pipe an average distance of 525 feet to points selected, and varied from time to time, on the flats north of the filters. The water containing the sand was then put through screens having meshes which excluded all stones 5 mm. in diameter and over, and was then taken into basins where the sand was deposited and afterward carted to the filters.

Two ejector sand-washing machines, shown in Fig. 9, are provided at convenient places between the filters. In them the dirty sand is mixed with water, and is thrown up by an ejector, after which it runs through a chute into a receptacle, from which it is again lifted by another ejector. It passes in all through five ejectors, part of the dirty water being wasted each time. The sand is finally collected from the last ejector, where it is allowed to deposit from the water.

Water is admitted to each filter through a 20-inch pipe from a pipe system connecting with the sedimentation-basin. Just inside of the filter-wall is placed a standard gate, and beyond that a balanced valve connected with an adjustable float to shut off the water when it reaches the desired height on the filter. These valves and floats were constructed from special designs, and are similar in principle to valves used for the same purpose in the Berlin water-filters.

Each filter is provided with an overflow, so arranged that it cannot be closed, which prevents the water-level from exceeding a fixed limit in case the balanced valve fails to act. An outlet is also provided near the sand-run, so that unfiltered water can be removed quickly from the surface of the filter, should it be necessary, to facilitate cleaning.

The outlet of each filter is through a 20-inch gate controlled by a standard graduated to show the exact distance the gate is open. The water rises in a chamber and flows through an orifice in a brass plate 4 by 24 inches, the centre of which is 1 foot below the level of the sand-line. At the nominal rate of filtration, 3,000,000 gallons per acre daily, 1 foot of head is required to force the water through the orifice. With other rates the head increases or decreases approximately as the square of the rate and forms a measure of it. With water standing in the lower chamber, so that the orifice is submerged, it is assumed that the same rates will be obtained with a given difference in level between the water on the two sides of the orifice as from an equal head above the centre of the orifice when discharging into air.

=Measurement of Effluent.=—In order to show the rate of filtration two floats are connected with the water on the two sides of the orifice. These floats are counterbalanced; one carries a graduated scale, and the other a marker which moves in front of the scale and shows the rate of filtration corresponding to the difference in level of the water on the two sides. When the water in the lower chamber falls below the centre of the orifice, the water in the float-chamber is nevertheless maintained at this level. This is accomplished by making the lower part of the tube water-tight, with openings just at the desired level, so that when the water falls below this point in the outer chamber it does not fall in the float-chamber.

To prevent the loss of water in the float-chamber by evaporation or from other causes, a lead pipe is brought from the other chamber and supplies a driblet of water to it constantly; this overflows through the openings, and maintains the water-level at precisely the desired point. The floats thus indicate the difference in water-level on the two sides of the orifice whenever the water in the lower chamber is above the centre of the orifice; otherwise they indicate the height of water in the upper chamber above the centre of the orifice, regardless of the water-level in the lower chamber. The scale is graduated to show the rates of filtration in millions of gallons per acre of filtering area. In computing this scale the area of the filters is taken as 0.7 acre, and the coefficient of discharge as 0.61.

At the ordinary rates of filtration the errors introduced by the different conditions under which the orifice operates will rarely amount to as much as 100,000 gallons per acre daily, or one thirtieth of the ordinary rate of filtration. Usually they are much less than this. The apparatus thus shows directly, and with substantial accuracy, the rate of filtration under all conditions.

=Measurement of Loss of Head.=—Two other floats with similar connections show the difference in level between the water standing on the filter and the water in the main drain-pipe back of the gate, or, in other words, the frictional resistance of the filter, including the drains. This is commonly called the loss of head, and increases from 0.2 foot or less, with a perfectly clean filter, to 4 feet with the filter ready for cleaning. When the loss of head exceeds 4 feet the rate of filtration cannot be maintained at 3,000,000 gallons per acre daily with the outlet devices provided, and, in order to maintain the rate, the filter must be cleaned.

=Adjustment of Gauges.=—The adjustment of the gauges showing the rate of filtration and loss of head is extremely simple. When a filter is put in service the gates from the lower chamber to the pure-water reservoir and to the drain are closed, the outlet of the filter opened, and both chambers allowed to fill to the level of the water on the filter. The length of the wire carrying the gauge is then adjusted so that the gauge will make the desired run without hitting at either end, and then the marker is adjusted. As both the rate of filtration and loss of head are zero under these conditions, it is only necessary to set the markers to read zero on the gauges to adjust them. The gates can then be opened for regular operation, and the readings on the gauges will be correct.

[_To face page 306._]
]

It is necessary to use wires which are light, flexible, and which will not stretch. At first piano-wire, No. 27 B. & S. gauge, was used, and was well adapted to the purpose, except that it rusted rapidly. Because of the rusting it was found necessary to substitute another wire, and cold-drawn copper wire, No. 24 B. & S. gauge, was used with fair results. Stretching is less serious than it would otherwise be, as the correctness of the adjustment can be observed and corrected readily every time a filter is out of service.

From the lower chambers in the regulator-houses the water flows through gates to the pipe system leading to the pure-water reservoir. Drain-pipes are also provided which allow the water to be entirely drawn out of each filter, should that be necessary for any reason, and without interfering with the other filters or with the pure-water reservoir.

The outlets of the filters are connected in pairs, so that filtered water can be used for filling the underdrains and sand of the filters from below prior to starting, thus avoiding the disturbance which results from bringing dirty water upon the sand of a filter not filled with water.

=Laboratory Building.=—The scientific control of filters is regarded as one of the essentials to the best results, and to provide for this there is a laboratory building at one end of the central court between the filters and close to the sedimentation-basin, supplied with the necessary equipment for full bacterial examinations, and also with facilities for observing the colors and turbidities of raw and filtered waters, and for making such chemical examinations as may be necessary. This building also provides a comfortable office, dark room, and storage room for tools, etc., used in the work.

=Pure-water Reservoir.=—A small pure-water reservoir, 94 feet square and holding about 600,000 gallons, is provided at the filter-plant. The construction is similar to that of the filters, but the shapes of the piers and vaulting were changed slightly, as there was no necessity for the ledges about the bottoms of the piers and walls; while provision is made for taking the rain-water, falling upon the vaulting above, to the nearest filters instead of allowing it to enter the reservoir. The floor and roof of the reservoir are at the same levels as those of the filters.

CAPACITY OF PLANT AND MEANS OF REGULATION.

The various filters have effective filtering areas of from 0.702 to 0.704 acre, depending upon slight differences in the thickness of the walls in different places. For the purpose of computation, the area of each filter is taken at 0.7 acre. The nominal rate of filtration is taken as 3,000,000 gallons per acre daily, at which rate each filter will yield 2,100,000 gallons daily, and, with one filter out of use for the purpose of being cleaned, seven filters normally in use will yield 14,700,000 gallons. The entrances and outlets are all made of sufficient size, so that rates 50 per cent greater than the foregoing are possible. The capacities of the intake, pumping-station, and piping are such as to supply any quantity of water which the filters can take, up to an extreme maximum of 25,000,000 gallons in 24 hours. The pure-water conduit from the filters to Quackenbush Street is nominally rated at 25,000,000 gallons per 24 hours, after it has become old and somewhat tuberculated. In its present excellent condition it will carry a larger quantity,

At the pumping-station at Quackenbush Street there are three Allis pumps, each capable of pumping 5,000,000 gallons per 24 hours. In addition to the above there are the old reserve pumps with a nominal capacity of 10,000,000 gallons per 24 hours, which can be used if necessary, but which require so much coal that they are seldom used. For practical purposes the 15,000,000 gallons represents the pumping capacity of this station and also the capacity of the filters, but the arrangements are such that in case of emergency the supply can be increased to 20,000,000 or even 25,000,000 gallons for a short time.

The water is pumped through rising mains to reservoirs holding 37,000,000 gallons, not including the Tivoli low-service reservoir, which is usually supplied from gravity sources. The reservoir capacity is such that the pumping can be suspended at Quackenbush Street for considerable periods if necessary, and in practice it has been suspended at certain times, especially on Sundays. The amount of water required is also somewhat irregular. The drainage areas supplying the gravity reservoirs are much larger, relatively, than the reservoirs, and at flood periods the volume of the gravity supply is much greater than that which can be drawn in dry weather. Thus it happens that, at certain seasons of the year, the amount of water to be pumped is but a fraction of the nominal capacity of the pumps, and at these times it is possible to shut the pumps down for greater lengths of time.

=Capacity of Pure-water Reservoir.=—The storage capacity provided between the filters and the Quackenbush Street pumps is comparatively small, namely, 600,000 gallons, or one hour’s supply at the full nominal rate. A larger basin, holding as much as one third or one half of a day’s supply, would be in many respects desirable in this position, but the conditions were such as to make it practically impossible. The bottom of the reservoir could not be put lower without deepening and increasing greatly the expense of the conduit-line. On the other hand, the flow-line of the reservoir could not be raised without raising the level of the filters, which was hardly possible upon the site selected. The available depth of the reservoir was thus limited between very narrow bounds, and to secure a large capacity would have necessitated a very large area, and consequently a great expense. Under these circumstances, and especially in view of the abundant storage capacity for filtered water in the distributing reservoirs, it was not deemed necessary to provide a large storage, and only so much was provided as would allow the pumps to be started at the convenience of the engineer, and give a reasonable length of time for the filters to be brought into operation. For this the pure-water reservoir is ample, but it is not enough to balance any continued fluctuations in the rate of pumping.

=Method of Regulating and Changing the Rate of Filtration.=—With all the Allis pumps running at their nominal capacity, the quantity of water required will just about equal the nominal capacity of the filters. When only one or two pumps are running, the rate of filtration can be reduced. With the plant operating up to its full capacity, the water-level in the pure-water reservoir will be below the level of the standard orifices in the filter outlets. When the rate of pumping is reduced, if no change is made in the gates controlling the filter outlets, the water will gradually rise in the pure-water reservoir and in the various regulator chambers, and will submerge the orifices and gradually reduce the head on the filters, and consequently the rates of filtration, until those rates equal the quantity pumped. In case the pumping is stopped altogether, the filters will keep on delivering at gradually reduced rates until the water-level in the pure-water reservoir reaches that of the water on the filters.

When the pumps are started up, after such stoppage or reduced rate of pumping, the water-levels in the pure-water reservoir and in the gate-chambers will be lowered gradually, and the filters will start to operate it first with extremely low rates, which will increase gradually until the water is depressed below the orifices, when they will again reach the rates at which they were last set. The regulators during all this time will show the rate of filtration on each filter, and, if any inequalities occur which demand correction, the gates on the various outlets can be adjusted accordingly.

[_To face page 310._]
]

The arrangement, in this respect, combines some of the features of the English and German plants. In the English plants the filters are usually connected directly with the clear-water basin, and that in turn with the pumps, and the speed of filtration is required to respond to the speed of the pumps, increasing and decreasing with it, being regulated at all times by the height of water in the pure-water reservoir. This arrangement has been subject to severe criticism, because the rate of filtration fluctuates with the consumption, and especially because the rates of filtration obtained simultaneously in different filters may be different. There was no way to determine at what rate any individual filter was working, and there was always a tendency for a freshly scraped filter to operate much more rapidly than those which had not been scraped for some time.

This led to the procedure, first formulated by the Commission of German Water-works Engineers in 1894, and provided for in most of the German works built or remodelled since that time, of providing pure-water storage sufficient in amount to make the rate of filtration entirely independent of the operation of the pumps. Each filter was to be controlled by itself, be independent of the others, and deliver its water into a pure-water reservoir lower than itself, so that it could never be affected by back-water, and so large that there would never be a demand for sudden changes in the rate of filtration.

This procedure has given excellent results in the German works; but it leads oftentimes to expensive construction. It involves, in the first place, a much greater loss of head in passing through the works, because the pure-water reservoir must be lower than the filters, and the cost of the pure-water reservoir is increased greatly because of its large size. The regulation of the filters is put upon the attendants entirely, or upon automatic devices, and regulation by what is known as “responding to the pumps” is eliminated.

More recently, the German authorities have shown less disposition to insist rigidly upon the principles advanced in 1894. In a compilation of the results of several years’ experience with German water-filters, Dr. Pannwiz[66] makes a statement of particular interest, of which a free translation is as follows:

“Most of the German works have sufficient pure-water reservoir capacity to balance the normal fluctuations in consumption,

so that the rate of filtration is at least independent of the hourly fluctuations in consumption. Of especial importance is the superficial area of the pure-water reservoir. If it is sufficiently large, there is no objection to allowing the water-level in it to rise to that of the water upon the filters. With very low rates of consumption during the night the filters may work slowly and even stop, without damage to the sediment layers when the stopping and starting take place slowly and regularly, because of the ample reservoir area.”

“The very considerable fluctuations from day to day, especially those arising from unusual and unforeseen occurrences, are not provided for entirely by even very large and well-arranged reservoirs. To provide for these without causing shock, the rate of filtration must be changed carefully and gradually, and the first essential to success is a good regulation apparatus.”

“Responding to the pumps” has a great deal to recommend it. It allows the pure-water reservoir to be put at the highest possible level, it reduces to a minimum the loss of head in the plant, and yet provides automatically, and without the slightest trouble on the part of the attendants, for the delivery of the required quantity of water by the filters at all times. If the filters are connected directly to the pumps there is a tendency for the pulsations of the pumps to disturb their operation, which is highly objectionable, even if the pumps are far removed; and this exists where filters are connected directly to the pumps, and a pure-water reservoir is attached to them indirectly. By taking all the water through the pure-water reservoir and having no connection except through it, this condition is absolutely avoided, and the pull on the filters is at all times perfectly steady.

Much has been said as to the effect of variation in the rate of filtration upon the efficiency of filters. Experiments have been made at Lawrence and elsewhere which have shown that, as long as the maximum rate does not exceed a proper one, and under reasonable regulations, and with the filter in all respects in good order, no marked decrease in efficiency results from moderate fluctuations in rate. There is probably a greater decrease in efficiency by stopping the filter altogether, especially if it is done suddenly, than by simply reducing the rate. The former sometimes results in loosening air-bubbles in the sand, which rise to the surface and cause disturbances, but this is not often caused by simple change in rate.

On the whole, there is little evidence to show that, within reasonable limits, fluctuations in rate are objectionable, or should be excluded entirely, especially in such cases as at Albany, where arrangements to prevent them would have resulted in very greatly increased first cost. The inferior results sometimes obtained with the system of “responding to the pumps” as it existed in earlier works, and still exists in many important places, undoubtedly arises from the fact that there is no means of knowing and controlling the simultaneous rate of filtration in different filters, and that one filter may be filtering two or three times as fast as another, with nothing to indicate it.

This contingency is fully provided for in the Albany plant. The orifices are of such size that even with a filter just scraped and put in service, with the minimum loss of head, with the outlet-gate wide open, and with the water-level in the pure-water reservoir clear down—that is, with the most unfavorable conditions which could possibly exist—the rate of filtration cannot exceed 5,000,000 or 6,000,000 gallons per acre daily, or double the nominal rate. This rate, while much too high for a filter which has just been cleaned, is not nearly as high as was possible, and in fact actually occurred in the old Stralau filters at Berlin, and in many English works; and, further, such a condition could only occur through the gross negligence of the attendants, because the rate of filtration is indicated clearly at all times by the gauges. These regulating-devices have been specially designed to show the rate with unmistakable clearness, so that no attendant, however stupid, can make an error by an incorrect computation from the gauge heights. It is believed that the advantage of clearness by this procedure is much more important than any increased accuracy which might be secured by refinements in the method of computation, which should take into account variations in the value of the coefficient of discharge, but which would render direct readings impossible.

In designing the Albany plant the object has been to combine the best features of German regulation with the economical and convenient features of the older English system, and filters are allowed to respond to the pumps within certain limits, while guarding against the dangers ordinarily incident thereto.

RESULTS OF OPERATION.

The filters were designed to remove from the water the bacteria which cause disease. They have already reached a bacterial efficiency of over 99 per cent, and it is expected that their use will result in a great reduction in the death-rate from water-borne diseases in the city. They also remove a part of the color and all of the suspended matters and turbidity, so that the water is satisfactory in its physical properties.

The filters have reached with perfect ease their rated capacity, and on several occasions have been operated to deliver one third more than this amount; that is to say, at a rate of 4,000,000 gallons per acre, daily.

COST OF CONSTRUCTION.

The approximate cost of the filtration-plant complete was as follows:

Land $8,290
Pumping-station and intake 49,745
Filters and sedimentation-basin, with piping 323,960
Pure-water conduit and connection with Quackenbush
Street pumping-station 86,638
Engineering and minor expenses 28,000
————————
Total $496,633

The filters, sedimentation-basin, and pure-water reservoir are connected in such a way as to make an exact separation of their costs impossible; but, approximately, the sedimentation-basin cost $60,000, the pure-water reservoir $9,000, and the filters $255,000. The sedimentation-basin thus cost $4,100 per million gallons capacity; and the filters complete cost $45,600 per acre of net filtering area, including all piping, office and laboratory building, but exclusive of land and engineering.

ACKNOWLEDGMENT.

The general plan and location of the plant were first conceived by the Superintendent of Water-works, George I. Bailey, M. Am. Soc. C. E., and the successful execution is largely due to his efforts. The members of the Water Board, and especially the Construction Committee, have followed the work in detail closely and personally, and their interest and support have been essential factors in the results accomplished. In the designs and specifications for the pure-water conduit the author is greatly indebted to Emil Kuichling, M. Am. Soc. C. E., and also for most valuable suggestions relative to the performance of this part of the work. To William Wheeler, M. Am. Soc. C. E., of Boston, the author is indebted for advice upon the vaulting and cross-sections of the walls, and these matters were submitted to him before the plans were put in final shape. All the architectural designs have been supplied by Mr. A. W. Fuller, of Albany. W. B. Fuller, M. Am. Soc. C. E., as Resident Engineer, has been in direct charge of the work, and its success is largely due to his interest in it and the close attention which he and the assistant engineers have given it.

FOOTNOTES:

[1] The American gallon is 231 cubic inches or 0.8333 of the imperial gallon. In this work American gallons are always used, and English quantities are stated in American, not imperial, gallons.

[2] Filtration of River Waters. Van Nostrand & Co., 1869.

[3] Annual Report of Albert F. Noyes, City Engineer for 1891.

[4] Rept. Mass. State Board of Health, 1892, p. 541. See Appendix III.

[5] The method of calculating the size is given in Appendix III.

[6] A full table of frictions with various velocities and gravels was given in the Rept. of Mass. State Board of Health, 1892, p. 555.

[7] Frühling, Handbuch der Ingenieurwissenschaften, II. Band, VI. Kapitel.

[8] The American gallon is used throughout this book; the English gallon is one fifth larger.

[9] Piefke, _Zeitschrift für Hygiene_, 1894, p. 177.

[10] _Zeitschrift für Hygiene_, 1891, page 38.

[11] _Journal für Gas- u. Wasserversorgung_, 1891, 208 and 228.

[12] _Journal für Gas- u. Wasserversorgung_, 1893, 161.

[13] Samuelson’s translation of Kirkwood’s “Filtration of River-waters;” Lindley, Die Nutzbarmachung des Flusswassers, _Journal für Gas- u. Wasserversorgung_, 1890, 501; Kaiserlichen Gesundheitsamt, Grundsätze für die Reinigung von Oberflächenwasser durch Sandfiltration; _Journal für Gas- u. Wasserversorgung_, 1894, Appendix I.

[14] Lindley, _Journal für Gas- u. Wasserversorgung_, 1890, 501; Grahn, _Journal für Gas- u. Wasserversorgung_, 1890, 511; Halbertsma, _Journal für Gas- u. Wasserversorgung_, 1892, 686; Piefke, _Zeitschrift für Hygiene_, 1894, 151; and others.

[15] Appendix I.

[16] The Water Supply of Towns. London, 1894.

[17] A special species of bacteria artificially added to secure more precise information in regard to the passage of germs through the filter.

[18] _Zeitschrift für Hygiene_, 1894, p. 173.

[19] Report Mass. State Board of Health for 1891, p. 438; 1892, page 409.

[20] Appendix IV.

[21] Piefke, _Zeitschrift für Hygiene_, 1894, p, 177.

[22] _Journal für Gas- und Wasserversorgung_, 1887, p. 595.

[23] _Zeitschrift für Hygiene_, 1894, p. 172.

[24] Appendix IV.

[25] Appendix I.

[26] _Glaser’s Annalen_, 1886, p. 48; _Zeit. f. Hygiene_, 1889, p. 128.

[27] _Vierteljahresschrift für öffentliche Gesundheitspflege_, 1891, p. 59.

[28] _Journal für Gas- und Wasserversorgung_, 1891, 108.

[29] _Zeitschrift für Hygiene_, 1894, 182.

[30] I am informed that several other filters upon the same principle have been more recently built.

[31] Report on Water Purification at Cincinnati, page 378.

[32] Translation in German in Dingler’s Polytechnical Journal, 1832, 386.

[33] Water Purification at Louisville, page 378.

[34] Special Report Mass. State Board of Health 1890, Purification of Sewage and Water, page 747.

[35] Water Purification at Cincinnati, p. 485.

[36] Jour. of the New England Water Works Assoc., Vol. VIII, page 183.

[37] Report of the Pittsburg Filtration Commission, 1899, page 55.

[38] Rhode Island State Board of Health Report for 1894.

[39] Report of the Rhode Island State Board of Health for 1894.

[40] Report on the Investigations into the Purification of the Ohio River Water at Louisville, Kentucky. D. Van Nostrand & Co., 1898.

[41] Ohio State Board of Health Report, 1897, page 154.

[42] Report of the Pittsburg Filtration Commission, City Document, 1899.

[43] Fuller, Water Purification at Louisville, page 425.

[44] Warren, Feb. 9; June 1; July 6. Jewell, July 1; Feb. 9, 16, 17.

[45] “Removal of Iron from Ground Waters,” Journal of the New England Water Works Association, Vol. xi, 1897, page 277.

[46] Journal of the New England Water Works Association, Vol. ii, page 294. Description of plant by Supt. Lewis M. Bancroft.

[47] This number was the result of numerous counts made from fæces from persons suffering with typhoid fever in the Lawrence City Hospital in 1891 and 1892. Mr. G. W. Fuller afterward made at the Lawrence Experiment Station some further investigation of fæces from healthy people in which the numbers were considerably lower, usually less than 200,000,000, per gram and sometimes as low as 10,000,000 per gram.

[48] These experiments, so far as they have come to the notice of the author, have been made with water sterilized by heating, usually in small tubes stoppered with cotton-wool or other organic matter. In this case the water, no matter how carefully purified in the first place, becomes an infusion of organic matters capable of supporting bacterial growths, and not at all to be compared to natural waters.

In experiments often repeated under my direction, carefully distilled water in bottles, _most scrupulously clean_, with glass stoppers, and protected from dust, but _not sterilized_, has uniformly refused to support bacterial growths even when cautiously seeded at the start, and the same is usually true of pure natural waters. Some further experiments showed hardly any bacterial growth even of the most hardy water bacteria in a solution 1 part of peptone in 1,000,000,000 parts of distilled water, and solutions ten times as strong only gave moderate growths.

[49] The Water-supply of Chicago: Its Source and Sanitary Aspects. By Arthur R. Reynolds, M.D., Commissioner of Health of Chicago, and Allen Hazen. _American Public Health Association_, 1893. Page 146.

[50] _Journal für Gas- u. Wasserversorgung_, 1893, 694.

[51] _Journal für Gas- u. Wasserversorgung_, 1894, 185.

[52] The method of making this determination was given in the _American Chemical Journal_, vol. 12, p. 427.

[53] Some of the companies secure some ground water which they mix with the filtered water, and this is included in the quantities for the separate companies, but is excluded from the totals for all the companies by years.

[54] Exclusive of gravity supplies.

[55] Not in use.

[56] Under construction.

[57] Not in use.

[58] Under construction.

[59] Not in use.

[60] Under construction.

[61] Not in use.

[62] Under construction.

[63] In the _Centralblatt für Bakteriologie_, 1895, page 881, Reinsch discusses at length the cause of the inferior results at Altona in winter, and has apparently discovered a new factor in producing them. Owing to defective construction of the outlets for the sedimentation-basins they have failed to act properly in presence of excessive quantities of ice, and the sediment from the basins has been discharged in large quantity upon the filters, and a small fraction of the many millions of bacteria in it have passed through the filters. He has experimented with this sediment applied to small filters, and has become convinced that to secure good work under all conditions a much deeper layer of sand than that generally considered necessary must be used, and his work emphasizes the importance of the action of the sand in distinction from the action of the sediment layer, which has often been thought to be the sole, or at least the principal, requirement of good filtration.

[64] Licht- u. Wasserwerke, Zürich, 1892, page 32.

[65] Descriptions of some of the leading European ground-water supplies were given by the author in the Jour. Asso. Eng. Soc., Feb. 1895, p. 113.

[66] “_Arbeiten aus dem Kaiserlichen Gesundheitsamte_,” vol. xiv. p. 260.

INDEX.

Albany, N. Y., filters at, 254, 288.

Alkalinity, 155.

Altona, double filtration at, 198.
filters at, 265.

Alum, use of, in filtration, 92, 144.

American cities, water-supplies of, and typhoid fever in, 211.

Amsterdam, filters at, 272.
iron removal at, 192.

Anderson process, 147.

Antwerp, filters at, 272.

Asbestos as filtering material, 181.

Asbury Park, iron removal at, 192.

Ashland, Wis., filters at, 252.

Area of filters to be provided, 47.

Bacteria, apparent and actual removal of, by filters, 87.
from underdrains, 87.
in Elbe at Altona, 228.
in fæces, 215.
in water, 84.
number to be allowed in filtered water, 222.
of cholera in river water, 231.
of typhoid fever, life of, in water, 216.
of special kinds to test efficiency of filtration, 86.
to be determined daily, 222.

Bacterial examination of water, 93.

Berlin, regulation of depth of water, 59.
cholera infantum from water, 229.
friction in underdrains, 44.
regulation of rate, 53, 55.
water works, 261.

Berwyn, Penn., filters at, 253.

Boston, protection of purity of water-supply, 110.
experimental filters at, 73.

Bremen, double filtration at, 198.

Breslau, filters at, 274.

Brussels, ground-water, supply of, 276.

Budapest, filters at, 274.

Burton, regulation of rate at Tokyo, Japan, 58.

Carpenter, Prof. L. G., 24.

Chemnitz, intermittent filtration at, 107.

Chicago, reduced death-rate with new intake, 217.

Cholera infantum from impure water, 226.

Cholera, in Hamburg from water, 230.
caused by water, 214.

Clarification, definition of, 113.

Clark, H. W., 24, 190.

Clark’s process for softening water, 92, 145.

Clay particles, size of, 123.

Cleaning filters, 68.

Coagulant, absorption of, by suspended matters, 154.
successive applications of, 154.

Coagulants used in practice, 150.

Coagulation of waters, 144.

Cologne, water-supply of, from wells, 276.

Color, 113.
amount of coagulant required to remove, 153.
amount of, in various waters, 115.
measurement of, 114.

Color, removal of, 117.

Continuous filters, 5.
filtration, nature of, 83, 92.

Cost of filters and filtration, 4, 48, 102, 200, 314.

Covered filters, efficiency of, 17.

Covers for filters, 12, 15.
at Albany, 295.
in the United States, 17.
omitted at Lawrence, 101.

Crenothrix, 105, 186.

Diarrhœa from impure water, 226.

Dibden, W. J., 129.

Disease from water, 210.

Double filtration at Schiedam, 273.

Drainage areas of a number of rivers, 133.

Dresden, water-supply of, from filter-gallery, 276.

Drown, Dr. Thomas M., 150, 191.

Effective size of sand, 21, 238.
European sands, 25.

Efficiency of filtration, 83, 88, 91.
effect of rate upon, 50.
effect of size of sand-grain upon, 30.
effect of thickness of sand layer upon, 34.
at Lawrence, 106.
European filters, 91, 260.

Effluents, wasting after scraping, 74.

Fæces, number of bacteria in, 215.

Far Rockaway, L. I., filters at, 193, 253.

Filling sand with water from below, 68, 307.

Filter beds, bottoms of, must be water-tight, 12.
covers for, 12.
form of, 11.
size of, 10.

Filters, aggregate capacity of, 254.
depths of waters on, 45.
list of cities using, 244.
reserve area required, 47.
first constructed at London, 83.
for household use, 183.
general arrangement of, 6.

Filters, statistics of, at various cities, 241.

Filtration, cost of, 200.
degree of purification required, 5.
general nature of, 92.

Fischer tile system, 181.

FitzGerald, Desmond, 73, 111, 196.

Flood flows not taken for supply, 10.

Fränkel and Piefke, experiments on removal of disease germs, 86.

Frankfort on Main, water supply of, from springs, 276.

Frankland, Dr. Percy, 84.

Friction of filtered water in pipes, 264.
water in gravel, 37.
water in sand, 22.
water in underdrains, 40.

Frost, effect of, upon filters, 12, 229, 266.

Frühling, on the heating of water by sunshine, 16.
underdraining at Königsberg, 39.

Fuller, G. W., 118, 123, 131, 139, 140, 145, 152, 154, 161, 165.

German Imperial Board of Health, 34, 51, 54, 75, 95.
regulations in regard to filtration, 221.

Gill, apparatus for regulation, 55.

Glasgow, water-supply of, from Loch Katrine, 275.

Gravel at Albany, 299.
layers, 35.
friction of water in, 37.
screening of, for filters, 37.

Grand Forks, N. D., filters at, 252.

Ground-water supplies, 3.
the use of, in Europe, 276.

Halbertsma, H. P. N., 54, 59.

Hamburg, apparatus for regulating depth of water, 59.
health of, 226, 271.
regulation of rate of filtration, 56.
underdrains of filters at, 42.
water-supply of, 269.

Hamilton, N. Y., filters at, 253.

Hardness, removal of, 92, 145.

Harrisburg, Penn., filters at, 253.

Hermany, Charles, 161.

High rates of filtration without coagulant, 182.

Household filters, 183.

Hudson, N. Y., filters at, 251.

Ice on filters, 13.

Inlet regulators, 59.

Impounding reservoirs, 2.

Intermittent filtration, 97.
application of, 111, 197.
at Chemnitz, 107.
at Lawrence, 100.
of Pegan Brook, 110.

Iron, compounds of, as coagulants, 146.
in ground-waters, 186.
in ground-water at Lawrence, 105.
metallic, the Anderson process, 147.
present as ferrous sulphate, 191.
removal plants in operation, 192.

Iron waters, treatment of, 189.

Jewel filter, 151, 161, 162, 172, 173.

Kirkwood, James P., 8, 36, 47, 51, 55, 61, 63, 67.

Kümmel, 50, 51, 86.

Lambertsville, N. J., filters at, 252.

Lawrence City filter, description of, 100.

Lawrence Experiment Station, 97.
air in water filtered in winter at, 46.
depth of sand removed at, 70.
depth of water on filters, 46.
effect of loss of head upon efficiency, 61.
effect of size of sand-grain upon efficiency, 32.
effect of size of sand-grain upon frequency of scraping, 32.
efficiency of filters at various rates, 50.
efficiency of filtration at, 86, 89.
experiments with continuous filtration, 110.
filters of fine sand, 31.
filters of various sand-grain sizes, 32.
gravel for filters at, 39.
growth of bacteria in sterilized sand at, 85.
intermittent filtration investigated, 97.

Lawrence Experiment Station, method of sand analysis at, 20.
quantities of water filtered at various losses of head, 66.
wasting effluents not necessary, 75.

Lawrence, typhoid fever at, 102.

Leipzig, water-supply of, from wells, 276.

Lime in sand, 29.
sterilizing effect of, 146.
as a coagulant, 145.
application of, to water, 157.

Lindley, 43, 51, 54, 57, 81.

Literature on filtration, 277, 285.

Little Falls, N. Y., filters at, 253.

Loam in filters, 35.

London, cost of operating filters at, 202.
water-supply of, 255.

Long, Prof., 131.

Lorain, tests of mechanical filters, 161.

Loss of head, 52.
limit to, 60, 67.
reasons for allowing high, 65.

Louisville, mechanical filters at, 161.

Magdeburg, filters at, 273.

Maignen system, 181.

Manchester, water-supply of, 275.

Manganese, compounds of, as coagulants, 148.
in ground-waters, 188.

Massachusetts State Board of Health, see Lawrence Experiment
Station.

Mechanical filters, 159.
application of, 199.
efficiency of, 179.
list of, 247.
pressure filters, 180.
rates of filtration used, 175.
types of, 172.
wasting effluent after washing, 163.

Millford, Mass., filters at, 252.

Mills, H. F., 97, 99, 102.

Mount Vernon, N. Y., filters at, 252.

Mud, see turbidity.

Muddy waters, 113.

Munich, water-supply of, from springs, 275.

Nichols, Prof., suspended matters in European streams, 131.

Nitrification, effect of, upon bacteria, 98.

Odors, removal of, by filtration, 112.

Organic matters in water, 83.
removed by intermittent filters, 98.

Paper manufacturing, filtration of water for, 5.

Paris, ground-water supply of, 276.

Palmer, Prof., 131.

Passages through the sand in filters, 67.

Pegan Brook, purification of, 110.

Period, how computed and length of, 72.
length of, dependent upon turbidity, 137.

Piefke, 48, 50, 54, 63, 69, 73, 74, 75, 80, 84, 85, 90.

Pittsburgh, experiments with mechanical filters, 162.

Plägge and Proskauer, 84.

Plymouth, Penn., typhoid fever at, 208.

Pollution of European water-supplies, 93.

Polluted waters, utilization of excessively, 111.

Porcelain filters for household use, 183.

Poughkeepsie, N. Y., filters at, 251.

Pressure filters, 180.

Providence, mechanical filters at, 159.

Rate of filtration, 47, 224.
at various places, 241.
effect of, upon cost, 48.
effect of, upon efficiency, 50.
lower after scraping, 76.
regulation of, 52.

Red Bank, N. J., filters at, 193, 253.

Regulation of filters, 52.
old forms of regulators, 52.
modern forms of regulators, 54.
at Albany, 305, 308, 310.
of mechanical filters, 178.

Reincke, Dr., report on health of Hamburg for 1892, 226.

Reinsch on the cause of poor filtration at Altona, 267.

Reserve area required in case of ice, 18.

Reservoirs, purposes served by, 133.

Rock Island, Ill., filters at, 254.

Roofs for filters, 16.

Rotterdam, filters at, 272.

St. Johnsbury, Vt., filters at, 251.

St. Louis, regulators for proposed filters, 55.

St. Petersburg, filters at, 275.

Samuelson, 51.

Sand, 20.
at Albany, 301.
analysis of European, 25.
analysis of, from leading works, 28.
appliances for moving, 68.
compactness of, in natural banks, 61.
depth of, in filters, 34.
depth to be removed from filters, 69.
dune, 26.
dune, washing of, impossible, 82.
effect of grain-size upon frequency of scraping, 32.
effect of grain-size upon the efficiency, 30.
effective size of, 21, 238.
extra scraping before replacing fresh, 71.
for filtration, 20, 33.
for mechanical filters, 175.
friction of water in, 22.
grain-size of, 20, 233.
in European filters, 24.
in Lawrence filters, two sizes of, 100.
lime in, 29.
method of analysis of, 233.
quantity to be removed by scraping, 74.
replacing, 71.
selection of, 33.
size of passages between grains of, 6.
sterilized, experiments with, 85.
thickness of layer, 34.
uniformity coefficient, 21, 238.

Sand washing, 26, 76, 304.
cost of, 81.
water for, 80.

Sandstone filters for household use, 183.

Schiedam, double filtration at, 273.

Scraping filters, 7, 68.

Scraping filters, amount of labor required for, 81.
depth of sand removed, 33, 66, 69.
frequency of, 49, 72, 241.

Sedgwick, Prof. W. T., 86.

Sediment, removal of, 92, 133.

Sediment layer, 6, 31.
influence of, upon bacterial purification, 84.
thickness of, 33, 66, 69.

Sedimentation basins, 8, 133, 293.
effect of, 134.

Sewage, number of bacteria in, 215.

Simpson, James, 83.

Soda-ash, application of, 157.

Somersworth, N. H., filters at, 253.

Storage for raw water, 136.

Subsidence, limits to the use of, 142.

Sulphate of alumina, action of, upon waters, 144.

Surface-waters, use of, unfiltered, 275.

Suspended matters, 113, 117.
in relation to turbidities, 122.
in various waters, 129.

The Hague, iron removal at, 192.

Tokyo, regulation of rate at, 58.

Trenched bottoms for filters, 36, 40, 100.

Turbidity, 92, 113.
amount which is noticeable, 121.
amount in several streams, 124.
duration of, 128.
in relation to suspended matters, 122.
measurement of, 117.
power of sand filters to remove, 139.
preliminary processes to remove, 133.
source of, 123.

Typhoid fever in Berlin and Altona, 12, 85, 267.
in American cities, 211.

Typhoid fever in Hamburg, 271.
in Lawrence, 102.
in London, 259.
in Zürich, 275.

Typhoid-fever germs, life of, in water, 216.

Underdrains, 35, 39.
bacteria from, 87.
friction of, at Albany, 299.
size of, 41.
ventilators for, 44.

Uniformity coefficient of sand, 21, 238.

Ventilators for underdrains, 44.

Vienna, water-supply of, from springs, 276.

Warren filter, 151, 161, 162, 172, 176, 177.

Warsaw, filters at, 275.
friction in underdrains, 43.
regulation of rate at, 57.

Wasting effluents, 74.

Water, depth of, on filters, 45, 59.
heating of, in filters, 45.
organic matters in, 83.

Water-supplies of American cities, 211.

Water-supply and disease, 210.

Waters, what require filtration, 207.

Weston, E. B., 153, 154, 159.

Weston, R. S., 153, 189.

West Superior, iron in ground-water at, 189.

Winter, effect of, upon filtration, 12.
temperatures of places having open and covered filters, 15.

Worms tile system, 181.

Zürich, filters at, 274.

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