PHILADELPHIA BUREAU OF WATER FILTER OPERATION
Popular demand centers upon a clear water service and adverse comment seldom manifests itself when a water works furnishes a sparkling supply throughout its distribution system. Few residents of a large community have any conception of the appearance or harmful content of unfiltered water and a filter superintendent must have his plant in such a state of preparedness that it may cope with one load or several should there be a repetition of unusual conditions during a short period of time. As water passes to a distribution system the task of determining the quality is imposed upon the laboratories. The registering of results in a speedy way gives the plant operator evidence to correct many troubles that might assume serious proportions should warnings be neglected. At the Delaware unit (Torresdale) the raw water has been applied without treatment or sedimentation. Upon occasions when the elements of wind, tide and temperature have entered, the supply of water from the pumping station has been below the demand of the plant. The water on the sand surface decreases in depth and all filters with a high loss of head automatically decrease in rate. In order to keep the plant in operation these filters arc washed; tiie greater the loss of water on the sand the more frequent the washing, resulting in an increased amount of wash water and power. High winds frequently create waves which scour the river banks, causing increased turbidity and the natural sequence is shorter runs and more frequent washings. At the time of writing a basin which will admit of 12 hours’ sedimentation is under construction and with the application of a coagulant it is expected to materially reduce turbidities and bacteria in the water applied to the preliminary filters. These filters are of a rapid sand type operating at an 80 mm. rate through 30 inches of graded gravel and 12 inches of 0.8 to 1.0 mm. sand. Air and water for washing are admitted through a 3-16-inch orifice on under side of pipe so spaced that each wash influence must control 33.1 sq. in. A.s the capped ends of these pipes are spaced 3 inches from those of the adjacent scries the volume of unwashed sand and gravel above will gradually increase until a considerable area of sand throughout the filter gives no service. The demand for water from the Torresdale plant requires an average rate through the final filters of about 4.5 m.g. per acre and the tresent year has given the plant its heaviest oad as the dredging for the new basin has forced a heavily charged water upon the prefilters which unit has not, on account of said deficiency as described above, performed efficiently. Deep cleaning of final filters is routine work at this plant and a “house-cleaning” is necessary at least every two years at which time a layer of sand just sufficient to protect the gravel is left undisturbed. Schuylkill water at Philadelphia readily yields to filter purification. After a severe local storm a water charged with heavy, yellowish, suspended matter is encountered. This soon clarifies itself and has very little effect upon the filters. A severe general storm produces first a heavy, red water followed closely by one charged with coal dust from the mining regions which water is hard to clear, at times carrying a very disagreeable gaseous odor and taste. Aeration and sedimentation reduce the gas. The Schuylkill units are provided with sedimentation basins and have, in addition, the aid of a coagulant at times of heavy turbidities. This river, after a heavy storm, rises rapidly and it is upon the slow or rapid rise and extent of the storm that the superintendent decides whether to start dosing. A high turbidity of short duration will require no coagulant while one much lower covering an extended period of time will necessitate its use. At Queen Lane the preliminary filters are of the same type as those at Torresdale. The wash water is applied through ¾-inch brass strainers 5 inches long containing 3-16-inch holes discharging horizontally from the top. These strainers are inserted in slabs of reinforced concrete at 9-inch centers above the main and lateral collectors. Around and above these strainers is placed 15 inches of graded gravel and 12 inches of 0.8 to 1.0 mm. sand. The air is applied through 1-inch brass piping between the second and third layer of gravel or about 23 inches below the normal sand surface. This piping is tapped on the under side with ⅛-inch holes spaced 6 inches apart and the pipe is set on 9-inch centers, making an area of 54 sq. in. under the control of each orifice. Air and water can be used jointly or separately. The filters at this station operate at about 50 M rate and run about five or six days between washings, at 4.5 foot loss of head marking the limit of run. More frequent cleanings have now been resorted to, 1-3 of the plant being washed each day, giving each filter a 3-day run with a maximum loss of head of about two feet. Considerable trouble has been experienced with the sticking of rate controllers at Torresdale. This is caused by the pressure of sand grains between the side of the piston and the piston chamber. These pistons are now being removed and turned down 1-16 of an inch, decreasing their diameter. This permits the sand grains to pass and prevents the piston from sticking in one position. At the present time all the controllers so treated arc working satisfactorily. The Philadelphia filters are located at a considerable distance from the residence section of the city and often, during times of bad travel, the work of operating filters is seriously interfered with by shortage of labor. At times this shortage is overcome by working overtime but, as city labor receives one and half time or double time for such work, economy of operation does not result. Constant attention to the depth and condition of the filtering materials is most important. Thorough inspection of each filter unit should be a matter of routine work. Upon one occasion after a long period of dry weather a storm produced a heavily charged river, the final filters at one of the stations went out of service rapidly and after being cleaned, short runs ensued. Inspection showed sub-surface clogging of the sand. The lower two to five inches of sand above the gravel was compact and dirty, above which was found a layer of clean sand and in order layers of clean and dirty sand alternated every few inches until the sand surface was reached. In order to remove the subsurface layers the entire body of sand extending to the gravel was washed. This is the third time it has been found necessary to wash the full depth of sand in the filters at this plant. Ibis subsurface clogging is caused by placing clean sand upon the old unwashed content. The undisturbed sand appears clean but the voids are partly filled with the gelatinous matter removed from the water in past runs and as the unwashed sand is more compact due to this foreign matter, and also to the workmen walking upon it, it offers greater resistance to the water passing through than the newly washed sand above. The suspended matter passing through the upper layer of the clean, loose grains will be caught and retained by the more compact layer beneath. Sometimes during periods of low turbidity a superintendent finds his filters of the slow sand type producing long runs, the loss of head increasing slowly or at times decreasing. With a sudden rise in turbidity of the applied water the loss of head leaps and the plant’s capacity is so reduced that every means is adopted to suppty the demand for water. One experience of this kind will be sufficient to teach a superintendent to keep enough units of his plant in condition to meet any and all requirements. Deep cleanings of slow sand filters should be avoided as much as possible in the winter months. Experience has proven that a filter will give much better service when the sand is washed before cold weather sets in than it will when washed deeply in the winter. Chlorine apparatus is attached to eacli plant and a low degree of dosing (J/⅞ pound per m.g.) is applied upon the Schuylkill plants throughout the greater portion of the year. At times when general rains have scoured the watershed contributing to the respective rivers, a heavier dosing is applied more particularly as a precautionary measure. The maximum quantity used at the Schuylkill plant is one pound of chlorine per m.g. of water. On account of the disturbance by dredges the dosing at Torresdale has been at the rate of 1¾ pounds per m.g. The point of chlorine application at all stations is at the inlet gate house of the filtered water basins. No extra labor is required for this service and the character of water at remote locations in the distribution system many miles from the filter plant is highly satisfactory bearing the endorsement of the State and City Health Dements.
Abstract of paper read at December meeting of New York Section of American Water Works Association.