FILTRATION AT INDIANAPOLIS

FILTRATION AT INDIANAPOLIS

In a paper read before the convention of the American Water Works association at Boston, F. A. W. Davis, president of the Indianapolis Water company, treated of “Filtration of water —The element with which we have to deal,” he said that his company had adopted the slow sandfiltration method for several reasons—one being that the local conditions were well adapted to slow sand-filtration, by reason of the ownership of a portion of the Indiana Central canal front Broad Ripple to the city of Indianapolis—about nine miles in length—the current being so moderate that much of the sediment is precipitated be fore it reaches the filters. White river is the principal source of supply, the water being impounded at Broad Ripple by a dam built by the State of Indiana and maintained by the Indianapolis Water company. The dam is 8 ft. high and backs the water up the river about four miles. The comb of the dam is 32 ft. above the low water in White river in the city. Front this point the water is diverted to the head-gates of the canal From the head-pates the water flows through the canal (which was constructed by the State and from whom it was purchased) to the aqueduct over Fall creek, and then to the intake house near the filter plant. Before entering the raw water line the water must pass through fine racks and sponges in the intake house, which keep trash and inhabitants of the water off the beds. From the intake house the water flows through a 48-in. cast iron pipe line to the basement of the laboratory, where it flows upwards through large cast iron cylinders having cages filled with sponges, and then on through the conduit by gravity to the filterbeds. There are arranged in the laboratory building a duplicate set of centrifugal pumps, having a capacity of 20,000,000 gals., that can pump water from Fall creek whenever the water is out of the canal. This gives the company three sources of supplynamely. White river. Fall creek and its system of thirty 8-in. and 10-in. wells, with compressors furnishing 8,000 cu. ft. of free air per minute, for lifting the water from the wells, which are used in case of necessity. The aqueduct, which it was necessary to construct to convey the water over Fall creek, is ago ft. long, 36 ft. broad and 5 ft. deep, and is quite a fine structure. This aqueduct and one in France form, it is believed, the only ones of the kind made of concrete. To the water company there were objections to some features of the construction of slow sand filterbeds as ordinarily built. One was the open-joint, round underdrain tile. In this construction it is absolutely necessary for the water to flow from the centre of the tile to the open joint. The tendency of the water to this common opening produces unequal drainage and coursing of the raw water, and the result is an unequal surface of the beds and not an altogether satisfactory effluent at all times. To avoid this difficulty the company adopted a perforated tile having ⅝-in. openings. There are about 1,000,000 of these perforations in each of the filterbeds, being spaced about 2½ in. apart over the entire bottom. The arrangement of the filtering medium is practically the same as at other plants—namely, 1 ft. of graded gravel and 3 ft. of fine sand. The sand used was taken from Fall creek near the filter plant, and, while not as uniform and fine as some cities have, the results have been equally as good, if not better. The beds were originally 200 by 350 ft., and contained about 70,000 sq. ft. Since the original construction a division wall has been put in—making the area of each bed about 35,000 sq. ft. This was done, so as not to have such large units out of service when cleaning, and to furnish a support for the roof. There was no other objection to the size of the bed. The filtration from the large bed was just as good as from the small bed. The action of the wind and sunlight on water is unquestionably beneficial, but the sunlight causes vegetable growths which are very undesirable, and then, again, the ice forms upon the beds in winter and makes it difficult and expensive to clean them. To avoid these troubles the beds were covered with 3-in. concrete and steel rods. Upon this cover is placed 2 ft. of cinders. The supports of the roofs of the beds are 7-in. cast iron columns, with flanges 8 in. apart, of .different depths, so as to prevent the raw water from finding its way down the columns. The walls of the beds are stepped off for the same teason. The advantage of the columns, either iron or cement, is the saving of space in the beds over the groined arch. All the walls of the beds and clear water basins are concrete, with steel reinforcement. In the roof of the beds steel Ibeams were used, so that there are both concrete and steel beams. The concrete, however, are preferable. No advantage has been found in the steel I-beams, covered with cement, over the concrete beams reinforced with steel rods. The bottom of the bed is made of 6-in. concrete laid upon rolled clay. On this is laid, in cement, the tile, with perforated top, the joints being filled with cement. The tile are 2 ft. long and 12 in. wide, with division walls making conduits 3 in. by 5 in. The tile support the material forming the filterbed, and are laid end to end across the bed and through and into the baffle-walls, forming the collecting gallery on the right, and the washout gallery on the left. Hose placed in the tile in the collecting gallery will force any sediment in the tile to the other side of the bed and into the washout gallery. These galleries have manholes, and, when the bed is drained for cleaning, they allow the air to pass under the bed and up through the gravel and sand. The galleries can be entered and the walls washed down and made sweet and clean. All parts of the underdrain system are accessible and easy of examina tion. The construction of the bed with the underdrain tile is an original feature, and, in the company’s opinion, a very great improvement over the round tile construction. It will thus be seen that there must be uniform filtration from every portion of the bed without coursing. The water flows from the collecting galleries into the clear-water storage basins through a 30-in. pipe, which conveys it to the far end of the clear-water basin—a distance of 300 ft.—where it is discharged. The purpose of conveying the water to the far end of the storage basins and then discharging it, is to keep the entire body in motion, thereby preventing any possibility of stagnation. From the filterbeds the water is taken through Venturi meters to the clear-water basins. In the regulating houses, are all the latest devices for measuring the quantity of water discharged per day from each filter bed, showing the friction losses from day to day, and regulating the operation of the beds as to the rate of filtration. The regulating houses are connected with the clear-water basins and the 48-in. conduit line leading to the Riverside pumping station. The 48-in. conduit-line is made of concrete reinforced with steel, and is nearly 6,000 ft. long. From the conduit-line the water is discharged by gravity into a receiving well, from which suction-mains are laid to the pumps. One of the engines that pumps water from the receiving well at the Riverside pumping station is a triple-expansion high-dtity engine. It is required to pump 30,000,000 gals, in twentyfour hours against 150 lbs. water-pressure. The filterbeds, as constructed, have an entrance at each end, each entrance serving for two beds and being located on the dividing walls. This arrangement proves very convenient for cleaning and recharging. The sand, when taken off the beds, is washed and stored in concrete bins, no other handling being necessary than the scraping of the beds. The scraped sand is transported by water under pressure by means of a portable Korting ejector and sand-washer. The sand on the beds is made as level as a floor, and, when the water is drawn off for cleaning, the surface of the bed is just as perfect as before the water was turned on. When filling the beds, after cleaning, the filtered water from the clear-water storage basin is introduced from below, through the perforated tile, and rises up evenly through the sand until the surface of the bed is covered to a depth of a few inches. Raw water is then turned on to the bed to a depth of 4V2 ft. above the sand, when filtration is started at a low rate per day until, by bacteriological examination, the effluent shows the water to be potable. The bed is then put into service at from 3,000,000 to 4×100,000 gals, per day per acre. The water as it comes from the filterbeds is bright and sparkling, as beautiful as spring water, and of a high degree of purity. The beds, inlets, outlets, conduit and receiving wells are entirely free from odor. The clear-water basins are 70 by 350 ft., with a depth of 16 ft. 2 in.; capacity. 2,500,000 gals. They are covered with a concrete roof, supported by concrete columns and concrete girders reinforced with steel. Upon this roof was laid a track upon ties (rails weighing 70 lbs. to the yard) and as many as five dump cars, each hauling 3 yds., have been upon the roof at one time. The valves in the clear-water basin are so arranged that water can be stored and the minimum or maximum amount taken out of the basin. At times during this period the raw water was heavily laden with sediment from heavy rains and the spring thaws. Notwithstanding these conditions the results were highly satisfactory as shown by the table:

AQUEDUCT, INDIANAPOLIS.INTAKE HOUSE, INDIANAPOLIS.
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