SOME RECENT FILTRATION PLANTS
Purification of Surface Water Rapidly Coming to be the Established Practice the W orld Over
THE mechanical filter plant, developed and perfected in this country during the past twenty years, is now being successfully used all over the world for the purification of surface water supplies. The well-known principles of subsidence and filtration through sand and gravel beds govern the design and operation of all plants of this kind but the general design of the individual plant is so affected by variable conditions of water supply, capacity and topography that it may be said no two plants are alike. Most of the changes in details of design have been for the advancement of efficiency as regards both cost of operation and quality of the filtered water. Automatic control has been the aim of the designer and many devices for automatically governing the various filter plant operations have been perfected and put into service in recent installations. Some of these are briefly described here.
The three million gallon plant installed at the city of Batavia, N. Y., was finished in the spring of 1918. The raw water supply, which is taken from a small stream, is treated with alumina sulphate by the so-called “wet feed” method in which coagulant is added in the form of a solution. The water is high in alkalinity during norma! conditions but provision for artificial alkalinity treatment has been made by the installation of a dry feed lime machine. The plant is equipped with a mixing chamber of about thirty minutes period and a sedimentation basin with a period of about six hours. There are six filter units of half a million gallons daily capacity each. The filter beds are composed of 30-inch layers of sand on 18-inch layers of gravel. The filtered water is collected by a false bottom strainer system and conducted through rate controllers into a clear water reservoir beneath the filters. Washing of the filters, which is necessary at intertills of from six to ten hours in a plant of this type, is accomplished by means of a supply of filtered water, under pressure, applied through the strainer system in a direction reverse to the filter flow, the waste water being carried away through a special drainage system. The reduction of bacteria is from 90 to 99 per cent. Sterilization of the filtered water is effected with automatic liquid chlorine apparatus.
Ihe filter plant at East Liverpool, Ohio, completed in the summer of 1918, consists of six one-million gallon units. Ohio river water, of comparatively high turbidity and low alkalinity, is treated with lime by manually controlled dry feed machines and with coagulant by means of an automatically controlled solution feed system. The alkalinity of the raw water is so low that treatment with the lime is a necessity, practically at all times. The plant is equipped with a small vertical baffle plant mixing chamber and a sedimentation basin of eight hours’ retention period. The settled water flows by gravity through the filters, is collected by a false bottom strainer system and piped through venturi type rate controllers into the clear water reservoir situated under the filters. Operation of the filters is controlled by hydraulic valves from control tables on the main floor. The filtered water is sterilized with liquid chlorine.
At Fort Worth, Texas, four new one and a quarter million gallon units were constructed and equipped during the year, bringing the total present daily capacity of the plant to ten million gallons. The improvement of the Ft. Worth plant was a war measure of considerable importance by reason of the fact that in addition to the city three military camps are supplied with the filtered water. I’he raw water supply, taken from a small stream, is treated with alumina sulphate and lime and settled for 3 1/2 hours. Dry chemical feed machines have been installed in the remodeled plant to replace the old wet feed apparatus formerly used. Hypochlorite of lime was formerly used for sterilization of the filtered water but the new plant has been equipped with an automatic chlorine sterilization device.
Extensive additions and improvements in the water purification works at Flint, Mich., were completed during 1918, giving the plant a total normal capacity of 16 million gallons per day. The old filter plant built in 1912 has been doubled in capacity and new sedimentation basins, mixing chamber, and chemical equipment have been built, making the Flint plant one of the best equipped in the country. Raw water is supplied through a 48-inch intake line from the Flint river and after the addition of chemicals, flows through a mixing chamber of 35 minutes’ period and a sedimentation basin of three hours’ period. The new filters consist of eight units of one million gallons capacity each, equipped with brass strainers and cast iron filtered water collecting systems and air manifolds for agitation of the sand and gravel layers by compressed air during washing. The rate of filtration is controlled by effluent controllers of the venturi type. A master controller is connected with all of the individual filter controllers so that the total output of the plant is regulated automatically to meet any demands of the water consumption. In brief, all of the filters can be operated all of the time irrespective of the amount of filtered water needed and the whole plant can be operated as a single unit.
The chemical plant at Flint is equipped with a concrete runway for an auto truck which handles the chemicals from the railroad siding to the hydraulic elevator in the chemical house. Chemicals are stored in concrete bin 16 1/2 feet square by 20 feet deep which discharge at the bottom directly into the steel hoppers of automatic drv feed machines. The plant is equipped for the use of lime, alumina sulphate or iron sulphate.
A distinctive feature of the more recent water purification works is the dry feed machine used for adding chemicals in the dry state to the raw water at any predetermined rate. The dry feed machine may be briefly described as a hopper for containing the chemical in granular or powdered form, discharging on a revolving drum, driven by a water motor, and an adjustable orifice, through which the chemical is driven by the drum. The machine is built in two styles, the manually controlled and the automatically controlled. The manually controlled machine can be set to discharge dry chemicals at any desired rate but requires setting by hand when the rate is changed. The automatic machine is controlled by a venturi tube, installed in the raw water line, and discharges the chemical into the raw water in any desired proportions, the variations in the rate being automatically made in proportion to changes in the rate of flow of the raw water. As brought out by recent installations there are four decided advantages that the automatic dry feed machine has over the wet feed process. These are (1) accuracy of dosing, (2) automatic maintainance of the chemical-water ratio. (3) economy of space and labor, (4) certainty of operation and minimum attendance. By the use of the dry feed machine variation of dosing due to change of solution strength, sticking of floats, or clogging of orifices is avoided. After the machine has been set the ratio of the amount of dose and the rate of discharge is constant. The compact machine requires but little space and no labor is necessary for its operation except that of filling the hoppers. The operation of the machine can easily be checked by weighing the quantity of dry chemical discharged during a given interval of time. Xo attendance is necessary except for tilling the hoppers and changing the rate or checking the feed.
Filtration plants constructed and equipped during the year of 1918 by the writer’s company total in the neighborhood of 75 million gallons normal daily capacity. Fifteen million gallons is supplied to training camps, the balance being public supply, largely municipally owned. A list of the larger plants put in operation by the company during the year is as follows: