Pennsylvania Anthracite as a Filter H.G.TURNER Anthracite Equipment Corporation, N e w Yo&,
N. Y.
filter anthracite (Anthrafilt) over the past seven years Figure 1shows how this growth has taken place.
Since Pennsylvania anthracite is essentially a carbon and hence more nearly chemically inert than other substances used in filters, it is well suited for many filtration processes. Its conchoidal fracture produces a particle which has the most desirable shape for filter use; its low specific gravity and angular particle shape make possible longer filter runs and more effective filter bed washing at lower backwash rates. Filter capacities have been increased from 25 to 100 per cent through the use of anthracite as a filter medium. Pennsylvania anthracite is used in municipal filters, power plant filters for the hot process softening of waters and for oil removal from boiler condensates, swimming pool filters, filters for the clarification of chemicals, end sewage filters. In the preparation of Anthrafilt, care is given to the selection of a low-ash material with the optimum particle shape; the most important single factor involved is the manufacture of the correct particle size to meet the conditions of filter use and design.
Preparation o f Anthracite
The preparation of anthracite for filters requires more care than does the preparation of anthracite for fuel purposes. The first step is the selection of a coal that will yield a particle free from small fracture cracks and possessing the best shape. The particle must be strong enough to withstand years of backwashing without breaking into smaller grains; its shape should be longer than it is wide but should not be extremely thin. This anthracite must be cleaned to a low ash product, and of course that comes within the province of the coal-breaker engineer. The coal-breaker engineer can also make it to the desired particle size; special knowledge is required, however, to know what size to make. It has been found that three fine sizes fit most cases, but the selection of which size to use in a given filter comes within the province of an engineer thoroughly versed in filter design, treatment methods, and filter operation. Since all Pennsylvania anthracites are not suited for use in filters and since quality, particle shape, and particle size are critical factors in all anthracites that are suitable, the term “Anthrafilt” has been used to designate Pennsylvania anthracite that meets all requirements for successful use in filters. For the sake of brevity and clarification, therefore, it is proposed to use the term Anthrafilt throughout this paper so the reader will know that a sneciallv selected and Drenared anthracite is being described. I
R
ESEARCH on the use of Pennsylvania anthracite as a filter medium was initiated by the Anthracite Institute in 1930. The earlv exneriments were conducted a t ” * Lehigh University under the writer’s direction; later the laboratory was moved to Pennsylvania State College where the experimental work was continued t o its conclusion in 1934. Since that time field studies have been conducted and are still in progress. The use of Pennsylvania anthracite in filters was originated by James H. Fuertes about 1896when he built an experimental filter a t Harrisburg, Penna., using dredged river coal as a filter medium because no local sand was available. Fuertes later built plants a t Dallas, Texas, Cumberland, Md., and Denver, Colo. All these plants used dredged river coal. The Cumberland and Denver plants are still in operation although they have rescreened their original filter medium and added anthracite specially selected and prepared for filter purposes. The filters in these early plants were designed to fit the coal. Dredged river coal is not the right size nor does it have the proper particle shape. This may explain why thirtynine years were to elapse before anthracite was again used in filters. Of course, little was known about filter media in general a t that time and nothing was known about the proper size, shape, and bed depth of anthracite to use in filters. As against three filtration plants in thirty-nine years we find a total of eleven hundred filtration plants of all kinds using
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Properties of Anthracite and Sand
Since the dawn of filtration, silica sand has been the prevailing filter medium in industrial and municipal water filters. It is composed almost entirely of SiOz. It is insoluble in most acids but reacts readily with alkalies. The particle shape is usually rounded. It weighs approximately 100 pounds per cubic foot. Aside from mineral matter associated with it, anthracite is essentially a carbon and hence is neutral in its behavior with acids and alkalies. The mineral matter associated with anthracite is composed of about 60 per cent silica, the remainder being oxides of aluminum with minor amounts of magnesium, calcium, and iron. The particle shape is sharp, angular, and flat. It weighs about 50 pounds per cubic foot. The particle charge on silica sand is negative and so likewise is the particle charge on crushed anthracite. The charge on silica sand is weak, however, compared to the strong charge on anthracite. SIGNIFICANCE OF PROPERTIES. Since silica sand is about the only medium other than Anthrafilt to be used extensively in filters, the properties of these two materials will be discussed from the standpoint of their contrasting behavior in filters. I n the removal of solids from liquids by drainage beds, filters act as entrapping agents rather than as strainers. The 145
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water or other liquid is usually treated with a coagulant which agglomerates the solid particles, many of which are so small that they would pass through an ordinary filter. Since the degree of entrapment is directly proportional to the surface exposure of the filter medium, the sharp angular particle of Anthrafilt is more efficient than the same size sand having
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As compared with a fine filter medium, a coarse one will give longer runs between backwash periods, will not clog so readily with fibrous turbid matter, will release the dirt on grains with greater ease when backwashed, will have less head loss through the filters, and will permit higher filtration rates. Since Anthrafilt of coarse size is as efficient in turbid matter removal as finer sand and weighs half as much, it can obviously be substituted for fine sand without sacrificing quality of effluent and without detracting from a high degree of bed expansion under the same backwash velocity as used with fine sand, Because of the carbon composition of Anthrafilt, it does not react chemically with the turbid matter which envelopes the grains during filtration; hence the filter particles are more readily cleaned under routine backwashing, and with proper filter design the particles will never coat with salts such as those of lime, iron, and manganese. Sand, on the other hand, often coats so badly that it has to be discarded because of filter-bed cementation or because the sand particle becomes so heavy that it can no longer be lifted by the backwash water. Where water is treated with lime in cold-process softening, silica sand becomes badly coated with layers of lime that cannot be removed by mechanical means because of the chemical reaction between the acid silica and the alkaline lime. Where hot-soda process softening is used, as in boiler water treatment, the hot alkaline waters dissolve the silica sand and impart a silica scale to boiler tubes which is difficult to remove. A small amount of silica is leached out of Anthrafilt (8) under these conditions, but after the first few days of operation the effluent is practically free of silica. This advantage of Anthrafilt is so marked that it is rapidly replacing sand in filters of this kind in all parts of the United States and in many foreign countries. Mercerizing plants have also found Anthrafilt a boon in the filtration of caustic soda. It can be seen also that Anthrafilt is advantageous in the filtration of many chemicals which would react with other filter media. Case Histories
Figure 1. Growth in the Number of Filtration Plants of All Kinds Using Anthrafilt during the Years from 1935 through 1941
a spherical shape. I n practice it has been found that Anthrafilt having an effective size of 0.70 mm. and a uniformity coefficient of less than 1.60 removes the same amount of solids as does a rounded silica sand with an effective size of 0.45 mm. and a uniformity coefficient of 1.60. For many years the average sand size used in filters has been one with an effective size of 0.45 mm. and a uniformity coefficient of less than 1.60. Recently, however, there has been a trend toward a coarser particle size. One of the limiting factors has been that most filters are not designed with a wash water velocity high enough to produce the necessary degree of bed expansion n.hen the filter is backwashed. Hulbert (4) showed that 50 per cent expansion of a filter bed is desirable for effective washing of a filter. Because of the light weight of Bnthrafilt a 50 per cent expansion is possible without increasing backwash velocities, even when $he particle is considerably coarser.
Many filtration plants have reported longer filter runs, higher rates, and cleaner filters through the use of Anthrafilt in place of sand. The Passaic Valley Water Commission, Paterson, N. J., reports (2) marked increase in plant capacity through the use of Anthrafilt. The plant capacity of Oklahoma City was increased from 16 million gallons per day to 30 million by substituting Anthrafilt for sand. Anthrafilt increased the capacity of the New Toronto, Canada, plant 50 per cent. The filtration plant of Cornel1 University, Ithaca, N. Y., using Anthrafilt reports (3) experimental runs of 300 hours each, which is considerably longer than runs for the usual sand filters. Swimming pool filters have used Anthrafilt with marked improvement over sand. The turbid matter of swimming pool filters is largely fibrous, being composed chiefly of lint and hair. This kind of turbid matter clogs fine sand in a few hours but does not clog coarse Anthrafilt. Furthermore, swimming pool filters are usually provided with a low washwater velocity which is not very effective with the relatively heavy silica sand. Lake waters are commonly infested with algae during spring and autumn. At such times Anthrafilt filters still give long runs (6), whereas runs through sand are so short as to be serious in the reduction of plant capacity. Buffalo, as well as many other cities along the Great Lakes using Anthrafilt, reports long runs during algae periods.
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I n the removal of emulsified oil and turbidity from boiler feed condensate, Anthrafilt has been found very effective. Coagulants such as alum, ferric sulfate, or ferrous sulfate are added and produce spongy gelatinous masses which are easily removed without clogging the coarse Anthrafilt filter bed. I n the removal of iron and manganese salts from municipal water supplies, Anthrafilt has been more effective (I)than sand in spite of the fact that, it is commonly believed, a sand must be coated with iron or manganese oxides to be effective; Anthrafilt apparently does not coat. Use in Sewage Filters
One of the earliest significant references to the use of anthracite in sewage work is found in a report (6) of the State Board of Health of Massachusetts referring to the Lawrence Experiment Station Report for 1910. I n connection with the preliminary treatment of sewage appears the following paragraphs: “Strainer E , containing 12 inches in depth of Buckwheat coal, was put into operation in 1901, and has been operated at a rate of 800,000 gallons per acre daily throughout 1910. Sixty-six per cent of suspended matter was removed as shown by albuminoid ammonia determinations; and the total removal of organic matter was about 39 per cent, as shown by albuminoid ammonia and oxygen consumed determinations. “The work of this strainer is remarkable and in some respects unexplainable. Year after year it removes large percentages of suspended matter from the sewage, and by some biological process not fully understood appears to destroy this matter with great rapidity. During the entire nine and one half years which this strainer has been receiving sewage at rates from 800,000 to 1,500,000 gallons per acre daily, averaging 1,000,000 gallons per acre daily, sludge has been removed from the surface only twice, and the filtering material has been raked or otherwise disturbed only four times. The two scrapings were made during the first year of operation. Of the other four disturbances of material, three were necessitated by the collapse of the tank, requiring the transfer of the material to a new tank. From September 23, 1905, when the material was last transferred, to January 28, 1910, when the strainer was raked to a depth of three inches, or during a period of four and one third years, no treatment of the surface or removal of sludge was required.” The Borough of State College used Anthrafilt in sewage sludge beds (1) and found that the effluent was about the same as that from sand beds; the Anthrafilt beds did not freeze during winter weather and did not pack when walked on by the man lifting the sludge. The companion sand beds froze shut during the same winter weather. Anthrafilt has been used in rapid mechanical filters employed in the clarification of waters from chemical precipitation of sewage. This installation was in the sewage plant of Oklahoma City. It was reported that three filters were used a t the outset, one provided with New Jersey sand, one with Texas sand, and the third with Anthrafilt. The sand filters became clogged within the first three months, but the Anthrafilt did not clog. The one Anthrafilt filter is now doing the job of the three filters. This rather confirms the findings of the Lawrence Experiment Station. One large canning factory is using Anthrafilt in sludge bed type filters for clarifying canning wastes. They report favorable results. Anthracite is well suited for trickling filters although, as far as I know, it has not been used for this purpose. It should have several advantages over limestone or traprock which are the materials now in common use.
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Combination with Other Filter M e d i a
The first attempt to use Anthrafilt in combination with other filter bed materials (7) was reported in 1935 by the city of Raton, N. Mex., in connection with its domestic water supply. Silica gravel was used as a supporting base for a bed of coarse activated carbon on top of which was a bed of fine Anthrafilt. The theory was that Anthrafilt would remove all turbid matter so that the activated carbon could operate with clear water in taste and odor removal for a long time without becoming clogged. This combination filter is said to produce 88 per cent more filtered water than the sand filter, and the effluent is free from turbidity, taste, and odors. Three municipal filtration plants (Chester and Erie, Penna., and Buffalo, N. Y.) are now using a combination of Anthrafilt and sand. The theory is that a relatively coarse bed’of Anthrafilt on top of a fine bed of sand will provide a roughing and finishing filter in one unit. The coarse Anthrafilt will remove practically all the turbid matter, and the fine sand will catch the almost colloidal particles that may escape the Anthrafilt. The advantages of a very fine sand can be obtained without the usual handicap of short runs, low rates, and clogged beds. Combination Anthrafilt and sand beds are feasible only where the filter is provided with agitation devices to be used during backwashing. It has been found that a layer of the turbid matter forms at the junction of the sand and Anthrafilt, and this can be removed only by agitation during backwashing. Where sand combinations are used, the total filter bed depth should be made up of 50 to 66 per cent Anthrafilt for best results. Relative Cost of Filter Sand and Anthrafilt High-quality silica sand for use in filters is prepared in very few places in the United States. Most of it is prepared a t sandbanks in New Jersey. Practically all foreign countries secure their filter sand from the United States. For this reason good sand for filters is not so inexpensive as one might assume. Anthrafilt is prepared by one of the large anthraciteproducing companies in Pennsylvania. I n most parts of the United States it costs more per ton than does filter sand, but since it takes half as many tons to fill a given space, it is no more expensive than sand, volume for volume. I n distant parts of the country and in foreign countries Anthrafilt has a decided price advantage over sand because in such cases the cost is largely freight. By virtue of its light weight Anthrafilt will require half as many freight cars and half as many ships. From the standpoint of degradation, Anthrafilt and sand have the same length of life in filters. Where renewals of filter medium are made necessary through incrustations on the filter medium, Anthrafilt will last about twice as long as sand. Literature Cited (1) Cleland, R. R., and Scott, G. S., Public Works, 66, No. 9, 13 (1935). (2) Cook, A. T.,Wuter Works Eon., Sept., 1936. (3) Georgia, F. R.,J . Am. Wuter Works Assoc., 34,1055 (July,1942). 14) Hulbert, Roberts, Ibid., 34, 1045 (July,1942). Mass. State Board of Health, Ann. Rept., 42, 864 (1910). Palmer, C. E.,J . Pennu. Water Works Operators” Assoc., 11, 91 (1939). Palmes, G. H., and Jamellier, S. E., Truns. Rocby Mountain Sect., Am. Water W o r k Assoc., Sept., 1935. Turner, H. G.,and Scott, G. S., Combustion, 5, No. 11, 23 (1934).
