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.
HUDSON RIVER
NEAR INTAKE
Fig. 1.
Sedimentation-basin, Pumping-station, and Outlets.
Sedimentation-basin, an Outlet, and Laboratory.
[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.
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.
LONGITUDINAL SECTION ON a-b-c-d-e-f-g-h
FILTER BEDS
PLAN AND SECTION OF FILTER NO. 2
Fig. 3.
Outside Wall, ready for Concrete Backing.
Sedimentation-basin: showing Construction of Floor.
[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 21⁄2 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.
Fig. 4.
This sand-run is provided with permanent timber runways and with secure doors.
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.
Fig. 6.
Placing the Floor of a Filter.
Building the Brick Piers.
[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 33⁄8 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.
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 21⁄2 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 21⁄2 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.
Diameters in Millimeters
MECHANICAL COMPOSITION OF FILTER SAND AND GRAVELS.
(ARROWS SHOW REQUIREMENT OF SPECIFICATION)
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.”
Placing the Concrete Vaulting.
General View of Vaulting, under Construction.
[To face page 302.
Fig. 9.
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.
Interior of a Filter: Drain, Gravel and Sand Layers.
Interior of a Filter, Ready for Use.
[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.