LAWRENCE FILTER DIVISION WALLS
Specially written for FIRE AND WATER ENGINEERING.
The last United States census on vital statistics issued in July, 1902, certifies to the success of the well known system of filtration as carried on at Lawrence, Mass., and ranks the city as one of the most healthful communities in the country. It comments on the marked decrease in the death rate since the introduction of its filter and attributes this improved condition to the substitution of pure filtered water for the polluted water drawn front the Merrimac prior to 1903 The city government is, therefore, urged to maintain this high standard by appropriating funds for additional filter area, the need of which is apparent to all who have studied the pure water problem for that city. The population to be supplied is estimated at over 68,500 (the figures of the United States census of tyoo were 62,559). The source ot supply is the Merrimac river, and the mode of supply pumping to a storage reservoir ami standpipe. T he estimated population supplied is 65,000. the total consumption for the year was 1,250,280,270 gallonsaveraging 3425.425 gallons, or a per capita consumption of fifty gallons a day to each inhabitant; 52.6 to each consumer: and 557 to each of the 6,575 taps in service.
During the winter of 1902 the ice accumulated to such an extent on the filter as at times to threaten the cutting of the supply from the (lumps. So serious was the situation that for several days the reserve supply was so low. that the slightest accident or any unusual draught from one or more of the large factories would have exhausted the filter supply, ami reduced the water department to pumping impure water from the river. The city should not be limited to so close a margin. It was, therefore, resolved that M. F. Collins, superintendent, and President D. J. Gallagher, of the water hoard, should consider the plans of the former for dividing the filter into three sections, which would not only decrease the cost of operating the filter, but, also, that of pumping. When the filter (which is constructed for sand filtration) was first constructed, it was not expected it would serve the city without any additions. when the population reached its present size. The meter system, however, and the good work put into the pipe system to prevent leaks have been successful in warding off the expense of making any addition. That, however, has become inevitable, atid the State board of health recommended that the new filter bed should have an area of two acres and consist of two separate beds, with provision for further extension up the river in the future. The State board of health also recommended that the old filter he covered at a cost of $47,000. The city government agreed only to an appropriation of $5,000 for the construction of two dividing walls at the filter. On July 25, 1902, contract was awarded to Joseph Wagenbach for $5,522.78. The work was finished on October 8, and the water was turned on to the thirds of the filter capacity can be pumped if the east side of the filter, and has been operated successfully ever since. Under the present system tworeservoir needs repairs, instead of pumping ceasing altogether till repairs were made.
The following is a description of the filter bed division walls, which were constructed in 1902 after the designs of Superintendent Michael F. Collins. Their intention is to prevent the freezing of the whole filter bed in winter, while a portion is being cleaned, as well as to allow two-thirds of the bed to be in use at all times during the entire year. Two walls were constructed last season across the bed, dividing the filtering area into three nearly equal parts. A large iron pipe ranging from thirty inches to sixteen inches in diameter was laid along the river side of the bed in the old conduit and connected with the gate-chamber which supplies the bed with water from the river. This pipe feeds the different sections with the river water through sixteen-inch valves. In the old conduit is also laid an eight-inch iron pipe, which is connected with a pump and assists in removing the water from the surface of the bed when preparing to clean it. This drain pipe also has valves connecting with each section of the bed. The walls are of concrete mixed in the following proportions: One part of Atlas Portland cement, three parts sand, and five parts clean screened gravel. The top of the walls is at elevation 38, and on a level with the driveway and the walk around the bed. The bottom of the foundation is at about elevation 30.75, or about one foot and nine inches above the bottom of the underdrains. They were built as nearly as possible along the summit of the ridges in the bottom of the bed, and about fifteen feet distant from the underdrains that convey the filtered water to the pump well. Being located so far from these underdrains the danger of unfiltered water reaching the pump-well from the disturbance of the bed was reduced to a minimum. The foundation section of the walls is of three different thicknesses, stepping up from four feet in thickness at the bottom to two feet, eight inches in thickness just below the surface of the bed. Each step in the concrete has a rise of nine inches and a thread of four inches, makes an indirect course for the water down the sides of the foundation, and gives a bond for the sand filling. The exposed section of the wall above the surface of the bed is two feet thick, capped with a pointed granite flagging one foot thick, and of a width just equal to the thickness of the wall. The top edge of this flagging is rounded, leaving no sharp edges to he broken off in the process of cleaning the bed of ice. The concrete walls run into the embankment at either end sufficiently to prevent the water leaking around them from one bed into the adjoining bed. The valves on the feed-pipe are located in small concrete chambers built close to the walls in one corner of the different sections. A concrete apron outside the gate chamber receives the first wash of the water in refilling the bed from the river, and prevents the surface of the bed from being disturbed. The prices paid for the work called for under the contract, including the furnishing of all the materials, tools, labor, and all things necessary to complete the walls were as follows: One dollar and twenty cents per cubic yard for the excavation: $9.95 per cubic yard for the concrete: $50 per thousand feet, board measure, for plank foundations, and $2.90 per lineal foot for granite capping. It was anticipated that the foundations would be in river silt, and not very firm, so the plan was made for a plank foundation : but the bottom of the trench at the depth to which it was excavated was found so firm and solid that the only plank used in the foundation was a strip about sixteen feet long over the filtered water conduit under the easterly wall. New distributing channels were constructed in the west and middle sections, or on the south ends of beds running from one to seventeen. These consist of cobble paving, three feet wide, grated with Portland cement mortar. Other improvements and repairs were made during the year. !>etween the times of regular work on the filter Among these was a retaining wall, 237 feet long and an average of six feet nine inches high, on the south and west of the Doyle property. On the retaining wall of the filter roadway, were placed 310 feet of granite capping eighteen inches wide. From the washing machine, cast and west, the edge of the slope was paved a distance of 240 feet three feet wide. The river bank slope paving, west of the intake, was relaid, for a distance of fifty-five feet, with Portland cement mortar. One purchase made on January 1. 1902, has been a great success—namely, the new ice elevator, which has more than paid for itself. The early start of freezing weather and its steady continuance made very thick ice on the filter; and without such a machine—built as it is to carry a thirty-sixinch sauare cake—the progress of ice cutting would have been slow; consequently, the clearing of the beds would have been delayed, and the city’s supply of water would have been diminished.
The original filter gallery at Lawrence was excavated in gravel parallel to and too feet from the river. It was 1,300 feet long and eight feet wide. Its walls were of rubble masonry five feet high supporting a semicircular brick arch. At the bottom were cross braces of granite one foot square placed ten feet apart and filled between with coarse screened cravel. The ground water from a narrow tract of intervale land from 200 to 300 feet wide and underlaid with a mixed stratum of gravel and sand entered the gallerv from the bottom. Water could be drawn directly from the river through a thirty-inch cast iron pipe leading to an inlet chamber seventeen and one-half by thirty-eight and one-half feet, with a wing thirteen and one-half by fifteen and one-half feet wide with solid granite walls laid in cement. The inlet pipe was 220 feet long: it extended fortyfive feet into the river, where it was strapped to a grillage of piles. A screen and gate were placed at the outer end and a second gate 13(1 feet from the chambers. The yield was 150 gallons in twenty-four hours per square foot of bottom area. This became insufficient for the wants of the city, and in 1875 the water was taken directly from the Merrimac, and it was decided to increase the yield by building a filter bed in the gravel between the filter gallery and the river, too by 114 feet at bottom – eight feet below the Pawtucket dam. On the gravel was laid a dry stone drain fifteen inches square at the river end and thirty inches square at the end nearest the gallery, where it terminated in a ten-foot circular chamber of brick, connected with the filter gallery by a thirty inch pipe. From the centre drain twenty-seven lateral stone drains were laid, each eight by twelve inches. All the drains were covered with thirty-six inches of from eight to three-inch cobble stone, then with ten inches of from two inch to one-inch screened gravel, then with six-inch of coarse screened sand, and, finally, with eighteen inches of fine sand.
The filtering apparatus proved a failure and in 1893 Lawrence was visited with a terrible epidemic of typhoid fever. The mortality was very great, and the outbreak was traced to impure water. It was, therefore, resolved to build a sand filtration plant on a large scale. The result has been to make Lawrence one of the healthiest cities in the United States, and, practically, to rid it of typhoid epidemic.