An interesting paper on the above subject was read by Harold C. Stevens at the 1916 Convention of the American Water Works Association and created considerable discussion as shown in a synopsis below. Mr. Stevens said in part:
The use of pressure filters is as old as the practice of rapid sand filtration; in fact, the first rapid sand or mechanical filter for treating a municipal supply, installed at Somerville, N. J., in 1885, was of the pressure type. At present, the aggregate capacity of pressure filters used for supplying potable water is not less than 265,000,000 gallons per day, which is about 13 per cent, of the total capacity of all the rapid sand filters which have been installed on the North American continent. Plants range in size from very small capacities up to 21,000,000 gallons per day. Despite their extensive use pressure filters have received but indifferent attention from sanitary engineers in general. This may perhaps be accounted for by the lack of the structural features that always involve special design in the case of gravity filters. The unsightly appearance of the pressure filter as compared with the neatness of a gravity plant of concrete construction may also be a contributing factor. The fact that the filter bed is invisible has occasioned frequent and unduly severe criticism. The engineers of filter manufacturing companies have, however, through the years of slow development of rapid sand filtration, appreciated the value of the pressure filter as an efficient and economical device, and its practical development has teen mainly due to their experience and efforts. Extensive and careful studies of the gravity type of rapid sand filter have been made by many engineers, full records of operation have been pretty generally kept, and experience with them has been turned to good account. Much of the knowledge so gained is applicable to the pressure filter, since it is the same in principle as a positive head gravity filter. There is, however, little specific data with regard to pressure filters to aid in a close comparison with other types as to cost, efficiency and general merit. This paucity of data is doubtless due to the small size of plants, to limited operating force, to the omission of automatic recorders and controlling devices in most cases, to technically unskilled supervision, and to the lack of incentive, which the activity of engineers could create, to systematic keeping of records. This condition is unfortunate, because there is no reason in the world why the pressure filter should not be made, by proper equipment, a thoroughly reliable and efficient means of purifying water, practically on a par with gravity filters and even superior to them under some conditions. There are a few installations that are well equipped; a good many are operated with much care, and, so long as vigilance and intelligence are unfailingly exercised, are efficient and reliable. but most are, through lack of automatic controlling devices and through varying degrees of attention to operation, open to question as to their reliability in furnishing wholesome water at all times. Many small gravity filters are equally open to this same criticism. Sterilization, as applied to water purification in recent years, has done much to palliate the uncertainties of filters of all types, slow sand as well as rapid sand. Nevertheless, constant efforts are still being made for the betterment of the gravity type, and why, then, should not the pressure type be equally improved? Certainly the pressure filter cannot otherwise be brought to the high state of usefulness to which it is entitled. The contention may be made that the pressure filter is essentially a comparatively small affair, not suitable for the treatment of large water supplies. This can hardly be admitted as sound reasoning; because in matters of public health, perfect safety is the object to be attained in all cases. Moreover, it is not true that pressure filters arc unsuitable for large supplies. Atlanta, Georgia, has a plant of 21,000,000 gallons daily capacity. Filter units are most frequently of about half a million gallons daily capacity. They can readily be combined in groups, each operated by one set of devices, as many groups can be assembled in one plant as may be desired, and they can even be arranged in tiers if ground space is limited. The writer believes that pressure filters, fully equipped and carefully designed, are suitable for the largest water supplies, and that they will compare well with gravity filters as regards efficiency and cost of construction and of operation. Regarding cost, it is a significant fact that water supply companies, whose primary’ object is to make profits, usually adopt pressure filters in preference to gravity filters. Standardization is one of the factors tending to keep down the cost of construction. It also favors rapid installation. The smaller sizes are actually kept in stock by filler companies, and for larger sizes designs are largely standard and many parts are slocked, so that it only requires a short time to effect the delivery of a filter of any ordinary size. The only deficiency is in the matter of refinements in controlling devices to a point of equality with gravity filters; and this will, of course, be remedied to keep pace with demand, if not in advance of it. There does not seem to be any special difficulty in the way of applying refinements to the pressure filter, sucli as improved strainer systems and filter bottoms, loss of head indicators, rate and wash water controllers, and hydraulically operated gates. The question of chemical application has already been solved. The pressure filter is essentially a positive head filter contained in a closed tank interposed in a pipe line, and provided with valves to permit reversal of flow for washing and the discharge of wash water and with a device for applying coagulant. In principle it docs not differ from the positive head gravity filter, but it has a higher bead of water over the sand, equal to the pressure in the pipe line, and it can be operated with a greater maximum loss of head. The loss of head allowed is sometimes as high as 15 pounds, and is frequently 10 pounds. Probably no definite limit can be stated as generally applicable, as it undoubtedly varies with the character of the raw water, but in general it is cither the point above which there begins to be danger of breaking the sediment layer through sudden changes in rate of filtration combined with excessive filtering pressure, or else it is the point above which further yield costs more on account of increased pumping pressure than is gained by lengthening the run. Pressure filters are made in two characteristic forms, horizontal and vertical, depending mainly upon the size of units, unless available space happens to be the controlling factor. Horizontal filter units, based on a filtering rate of two gallons per square foot per minute, are made for capacities ranging from 240,000 to 350,000 gallons per day; vertical filters are generally more suitable for lower capacities, ranging from 3,000 to 230,000 gallons per day. Cost of construction is the principal determining feature as to form. In the horizontal filter, the sand surface is taken as the area of a plane through the axis and in the vertical filter it is the area of a plane perpendicular to the axis; so it is easy to see that for a given area of sand surface the vertical tank will be of larger diameter and will require a disproportionate amount of metal both in the sides and heads and be correspondingly more expensive. It is not generally economical to build the vertical filters over 10 feet in diameter. Coagulating and settling basins are in many instances not provided for pressure filters, the coagulant being injected into the influent main. All of the coagulum must, therefore, be deposited on the sand bed and more frequent washing would be necessary were it not for the pressure available to force the water through the filter. Sometimes coagulating basins, consisting cither of a separate closed cylinder, or of a compartment in the filter shell, arc provided, hut these are of small capacity as compared with the outside basins provided for gravity filters. Their value for purposes of sedimentation is only slight, but they are sometimes useful in providing a longer period, necessary with some waters, for thorough coagulation. In a number of instances coagulating and settling basins of ample size have been provided, or else existing reservoirs have been utilized as such. Settling basins are a practical necessity in treating muddy waters, but are generally omitted where comparatively clear water is to be filtered, unless a storage or distributing reservoir happens to he available. The ability of the pressure filter to handle unsettled water is one of its distinct advantages, especially where a low first cost is important, Pres _____re filters have certain apparent advantages. They can be installed quickly; they are entirely above ground, and usually fit into some available space within a building, settling basins and clear well an frequently be eliminated; auxiliary pumping may often be avoided; less precision in operation is demanded, by present practice; the cost of installation is low, and the rate of filtration can be increased materially above the usually accepted rate of 2 gallons per square foot per minute. The last feature is especially important as it makes possible, without providing an excessively large nominal capacity, the practice of connecting the filter directly with the distribution system, the filter accommodating itself to the fluctuating demand without special attention. Some of these advantages are not so real as they appear. The omission of a settling basin of ample capacity of course means a low installation cost, but the greater quantity of suspended matter which the filter alone must remove necessitates more frequent washing and correspondingly increases the percentage of filtered water used for that purpose. The greater amount of wash water required tends in some measure to offset the saving otherwise effected by eliminating the settling basin. With very muddy water, unless the plant has an excessive filtering capacity, the clogging of the filter may he so rapid that it will be difficult to deliver a sufficient quantity of water and the loss of head may range very high, to the possible detriment of the effluent.’ The elimination of low lift pumping, which the omission of a large settling basin may permit, is a definite saving in first cost, but does not materially affect the cost of operation, since the total lift of the water remains the same. Less precise operation means an economic waste, sacrificed for the sake of convenience. It is very simple to wash a filter once a day whether it needs it or not, and provide for coagulation merely by filling the alum tank, as needed, letting the dosing device deliver a constant quantity of solution regardless of changes in the character of the raw water, but there can be no doubt that this practice results in the use of unnecessarily large amounts of wash water and alum. The high rate of filtration that is attainable may or may not be a real advantage. So long as the effluent is not impaired, it is an advantage, but there is a limit somewhere. Above a certain point, further increase in loss of head will cost more on account of pumping pressure than it will save by the lengthening of runs and consequent reduction in wash water. There is also a critical point in the loss of head range at which the filter is likely to break, and also another point, varying with the character of the raw water, above which the filter will pass sus~ pended matter without actually breaking. The rate of 2 gallons per square foot per minute, which has, until very recently, been credulously accepted as standard for all rapid sand filters, is really without foundation as a criterion for general practice. There are filters operating on colored, but practically clear water that cannot be made to produce a good effluent at such a high rate, and there are also filters treating turbid waters that can be operated at much higher rate and still give perfectly good results. The success of pressure filters, with their widely varying rates and rule of thumb operation, gives evidence of the possibility of safely utilizing in many cases much higher rates than the normal rate heretofore accepted. While it is manifestly proper to take advantage of higher rates it should be done carefully and with due regard to limitations. The feature that deserves the most careful consideration is the utilization of higher rates of filtration within safe limits, making use of devices and appurtenances similar in purposes to those recognized as essential for gravity filters. One of the troubles frequently experienced with pressure filters is the necessity of adding sand to replace that lost in washing or of renewing all the sand occasionally, on account of accumulated mud within the bed. The loss of sand is plainly due to washing at too high a rate or to uneven washing. The accumulation of mud within the filter bed can only be due, if wash water is applied in the proper quantity, to imperfect distribution of wash water, just as is the case with gravity filters. Suitable wash water controlling devices and proper arrangement of the strainer system will obviate the trouble. Only in a few instances is air used in the washing process. Surely if it is necessary for gravity filters it is equally important for pressure filters, since the conditions are exactly the same. Sometimes the interior of a pressure filter gets into a very foul smelling condition. Naturally the lack of ventilation would make odors particularly noticeable, but only an improperly washed filter would get into seriously bad condition in this respect. It would seem that the air wash should have a marked effect in preventing bad odors and that it may for this reason be especially desirable in connection with the pressure filter. One kind of service for which the pressure filter is particularly well adapted is the treatment of water exclusively for industrial use, where the elimination of bacteria is not essential and where in some instances even complete clarification is not necessary. For this service the pressure filter in its usual form proves cheap and suitable. Good washing is of full importance, and a combined settling and coagulating basin may be a necessity in the case of a muddy water supply, but precise rate control and recording devices are not so important and direct service to the distribution system, allowing demand to control the rate of filtration, is often entirely satisfactory. The cost of a thoroughly up-to-date installation of 1,000,000 gallons capacity would be about as follows. Two 8-foot x 20-foot filters equipped in the ordinary way, including erection and foundations, $5,000; controlling devices, air wash equipment and general improvements, $1,300; settling basin of two hours’ capacity, $3,500; low-lift pumps, $1,200; superstructure, $1,500. Total, $12,500. A first class gravity plant of the same capacity costs in the neighborhood of $20,000. For large plants the cost of construction per million gallons daily capacity would be about $12,500 for gravity filters and about $9,000 for pressure filters. These figures are not offered as being exact and there will, of course, be considerable variation depending upon the character of the site and other conditions attending construction, and upon prices of labor and materials, but they will serve to indicate in a general way the relative cost of installation. The cost of operation of pressure filters should be about the same as is the case with gravity filters, except that the cost of pumping may be a little greater, on account of the greater head utilized in the filter. The cost of upkeep will be a little higher for pressure filters owing to the greater amount of steel work to be kept painted. Depreciation will also be a little more rapid, but the fact that certain pressure filters are still in use after twentyfive to thirty years of service shows this item to be almost insignificant. The conclusions of the writer, summarized briefly, arc as follows: The pressure filter, as thus far constructed, is in some instances a very inferior means of purifying water, hygienically, and in other cases an excellent means, but on the average it is not entirely reliable. There are no great obstacles in the way of developing it to the point of equality with the best gravity filters as regards efficiency and reliability, for any capacity and for most waters. It is better adapted to the treatment of some waters than the gravity filter. It is especially suited to very small water supplies and to the clarification of water for industrial uses. The cost of construction of the highly developed pressure filter is materially less than that of the gravity filter, and low enough to more than offset a somewhat greater expense for operation and maintenance. The improved pressure filter deserves the most careful consideration by sanitary engineers, filter manufacturers and public health officials.
In the discussion that followed the reading of the paper, Dr. W. P. Mason said while he favored the gravity type because he liked to see the wheels go around, he had seen excellent results from pressure filters. W. J. Ledoux said that the main advantage of pressure filters is their applicability in many cases where by their use low service pumps can be eliminated, with a consequent material reduction in cost. He w’as of the opinion that the gravity type of mechanical filters is generally to be preferred : 1st, because they are cheaper to construct; 2nd, because they are more readily accessible for repairs, and 3rd, because they are adaptable to the economical utilization of floor space. On the other hand he said the water purification results arc exactly as good with the pressure type, and where the avoidance of double pumping is an important desideratum, engineers should not hesitate to use pressure filters. In regard to the coagulant and the difficulty about the pump destroying its efficiency, I am positive there is nothing in that at all. The results are equally good so far as the sulphate of aluminum is concerned. I know of one plant that has a pressure capacity of 5 or 6 mill. gals, wherein the sedimenation basin was of about 10 mill. gals, capacity. It is fed into that basin directly, and the water discharged into the sedimentation basin and afterwards pumped through the pressure filters. Very accurate statistics are kept of that plant, as to the bacteria and everything pertaining to the filter plant. Still the water shows about an average, we will say, of 1,000 per c.c.; and the final filtered water shows from ten to fifteen, an average probably of about fifteen; and that is the result of probably only four or five years’ operation.
Mr. J. N. Chester said he never refused to use or to utilize pressure filters where he found them, nor did he deny that good results with careful operation cannot be obtained; in fact, with proper care as good results, as Mr. Ledoux has shown us, as may be attained from the other type; but I have found that the difficulties of operating, especially on western waters, are greatly multiplied over that of the gravity filter; and this is the principal objection. In the western waters we must have a coagulant basin and two hours’ quiescence one day.
Mr. George W. Fuller said it seems to me that the essential proposition involved in this question is, whether all water must be subjected to the process of sedimentatiou before it is applied to the filters, if not, then mechanical filters under many circumstances are more advantageous than gravity filters. That is the exception, not the rule. When you come to the condition where you have to deal with very turhid waters, where for ten or twenty per cent, of the time throughout the year a filter staggers under its load, then you have to reckon with what is the preliminary treatment necessary which will allow the filters to work most advantageously, and if the local conditions are such that preliminary treatment Is not essential, then pressure filters under many circumstances present themselves for consideration. In my own practice I see once or twice every year cases where pressure filters are strongly advocated as an advantageous means for purifying water; hut they are the exception, not the rule. I think the line which separates the exception from the rule is the need for preliminary sedimentation in advance of filtration. There arc many times when you can afford to pump a su ply of water for high service; but that is rather an unusual condition. At the risk of repeating and taking up the time, let me say again, that other features being the same, a pressure filter will give as good service as a gravity filter. I will admit that you have to watch it under rather disadvantageous circumstances as to the point of observation; but granting that you have the ability to observe what the accomplishments are, the same degree of coagulation, the same degree of turbidity, the same kind of sand, and the same efficiency in washing, the time will be about the same, and then as I look at it, gentlemen, the real question is, have you got a water which will or will not filtrate without preliminary sedimentation in advance of filtration. If you have that kind of water, then you can use pressure filters.
Mr. F. B. Leopold: It seems to me that there is one phase, which is the most dangerous one, that has not been spoken of here. There is no question, as the previous speakers have stated, but that pressure filters under proper conditions will give equally good results. The great danger of proclaiming that pressure filters are a satisfactory proposition, and I think this has been unquestionably proven in the plants that have been installed in the past and the number of them that have been abandoned, is the fact that the average man, the average engineer in a small town, has no conception of the conditions under which any kind of a filter plant should be designed or installed; and if that idea generally obtains support that a pressure filter is just as satisfactory as a gravity filter and much cheaper, the temptation to install pressure filter plants under improper conditions, due to the low cost of installation, is entirely too dangerous a proposition to allow it to become prevalent or receive support. Pressure filters from the manufacturer’s point of view are more advantageous. There is no question about that; and under proper conditions, and under proper engineering supervision, they will give results if they are put in as they should be and under conditionwhich they are suitable for. But it seems to me that the danger or temptation to install them uuder improper conditions, and with the character of supervision that a small plant affords, makes them a great source of danger. It is not a question of what the filter can do, but it is the question of what it is doing and the way it is put in A large proportion of the plants of that character installed under improper conditions and under improper supervision; and that is where the great danger lies. This alone, it seems to me, is sufficient cause to prevent any universal or widespread idea advanced or proclaimed as to their usefulness on that account.
Mr. Dow R. Gwinn, Terra Haute, In. I would say that, like Dr. Mason, I was once prejudiced against pressure filters. From 1892 to 1901 I lived with a gravity plant on the Mississippi, and I came to the conviction that a mechanical gravity plant was the only kind, and when 1 was transferred over to Terre Haute and found a pressure filter there, I had to overcome my prejudice. 1 saw that the pressure filter was there and the thing to do was to get results out of it. Now “the proof of the pudding is in the eating,” and the actual results that you get from the pressure filter are what you must go by. I am not here to say that a pressure filter will answer in all cases; it depends entirely upon the character of the waters that ymu have to handle. After all, when it comes to the last analysis, the success of a filtration plant depends upon the kind of water that is applied to the filter bed, that is the condition of it when it is applied. When they first began building pressure filters a great many years ago, they did not know much about sedimentation. That also applied to the gravity filters. They did not provide sufficient sedimentation for the western waters. The plant in Terre Haute was first put in in 1889, with no provision then for sedimentation, and the results were not always satisfactory, because at times the waters became very turbid and would run up as high its hut the most of the year the turbidity would only be about 25, so that one pump was all that was (necessary. In 1891 we put in a large sedimentation basin in order to get rid of the heavy suspended matter present at times when the river was turbid. About twenty-five per cent, of the time we had to use that basin. One year that ran up to about fifty per cent. Of course, as you gentlemen know, we saveu a great deal of fuel on account of that single instead of double pump. Now since the first of the year we have been making examinations of 10 c.c. samples: and out of 152 samples only 8 of them showed positive. During the same period there was only 1 positive B coli, in the 1 c.c. samples,—certainly very remarkable results. On the other hand, the question of typhoid fever mortality would come in there, and I find by examining the official records that the average for the past five years was at the rate of about 25 per 100,000 population. During that period there were only a very small number of fatal cases on premises supplied with filtered water; leaving out the imported cases, and the cases where they used water other than city water, I found there was only 39. per 100,000 population; so that as far as the results are concerned we can get them with a pressure filter plant. There are some reasons why the gravity plant is better. I, like Dr. Mason, “want to see the wheels go ’round.” I like to see the wash water coming up through the filter bed. But in many cases a pressure filter plant can be used to very good advantage, Mr. Gwinn presented the following operating results:
Mr, Robert E. Milligan: I believe this subject is one that the American Water Works Association and the Water Works world will appreciate very much when they get down to the value of the paper. As most of you probably know, I have had some experience in the filtration of water; and while I do so with regret, in a way, I have to endorse Mr. Stevens’ conclusions in the matter. After all is said and done, it is a matter of common sense, it is a question of cleaning water in a practical way. In the old days bacteriology was a very important feature, and I think I can safely say that I had a great deal to do with the days when we had to strain a little bit to get 99 per cent, reduction, and the matter of regulation was one of very serious consequences. I do not want to commit myself at all; but there were times when it was very difficult to get 99 per cent. When you figure at 99 per cent., you know it is considerable to get in the way of efficiency. To-day, with the idea of sterilization we come back to the practical proposition, which is fundamentally the cleansing of the water beforehand by sterilization. There is the practical side of it. Now the pressure filter has this advantage over the gravity filter, that you can get as much water through it as the pipe that leads to it will pass. It may not be 99 per cent., it may be 90 per cent.; but with sterilizing you can get the 99 per cent, that is demanded as a punfving proposition; consequently, as a practical proposition pressure filters must necessarily loom up in the minds of the water works man as a very desirable means of cleansing the water. There can lie no doubt about that. Outside of that, and not putting aside the question of whether the gravity filter or pressure filter is the better, to my mind, I am perfectly willing to go on record that there is very little difference; the fact that one is closed and the other open cuts very little ice, the process being substantially the same. We have the same sand-bed, the same strainer system, the same conditions under which purification is secured Outside of that there are conditions which are largely in favor of the pressure filter. Again, we do not need any clear water well, which from the standpoint of construction is a very serious cost to the water works man. We can pass the water directly through the pressure filter and through the supply in the case of color. I think our President understands that as well as I do. There arc some advantages in treating the color distinctly and directly with a chemical that deSirovs colors, and without preliminary treatment at all If, when I make that statement, it seems to be iconoclastic, I am perfectly willing to go on record on it. There is no doubt in my mind that in destroying the color it is a mere matter of chemical destruction as between the applied chemical, and preliminary sedimentation has no place in the matter whatever. So that any cost for coagulation in advance, so far as color removal is concerned, is to my mind inexcusable. Now, on the question of muddy waters, we will all agree, of course, that it is necessary to have a certain amount of subsidence; that, of course, is regulated by the amount of suspended matter in the water, and there is plenty of evidence that so far as preliminary treatment is concerned it may be had just as well with pressure filters as it can be had with gravity filters. I think, in fact, that the processes involved are exactly the same in the gravity filter as they are in the pressure filter. Now, there has been a lot of uneasiness about the control of the pressure filter, and that is easily understood. But any one who has spent many years in the filter business knows that that is due to the fact that originally the pressure filter was in great demand because it was the cheapest means of cleansing water. You have to consider this, gentlemen, that the first people to cleanse water were the water companies, and they looked at it from the practical side, the question with them being the cheapest means to accomplish the purpose sought; and then, as today, the pressure filter was the cheapest means of doing it, and they abused it; there is no question about that. If I wished to take up your time I could give you instance after instance where they took a 1,000,000-gallon plant and forced 5,000,000 gallons through it. Naturally such methods led to abuse to such an extent that there came a time when that particular means of purifying water fell into disrepute. But we find that exactly the same thing happened to the gravity plant. That has nothing to do with the relative merits of the two, it simply flappers that that was the method that they used, and it was abused, that is all; just exactly as they used a six-inch main to push water through where they should have a large 12-inch main, nothing more or less. Finally, however, it got down to this, that the abuse of the pressure filter relegated it to the rear, and the gravity filter, with its open work and all that, demanded the attention of the engineer, and very properly concrete construction especially came in with its rectangular shape. The pressure filter is naturally limited to 8 x 20 or 8 x 28, which is about all that yon can do in a steel shell. It has its limits, there is no question about that; and there is exactly where its limits are. In a large plant, such as we find in Toronto, with a 22,000,000gallon plant, one has to admit that further extensions may reasonably take the place of the open gravity concrete construction; for the reason that any steel construction must necessarily be limited to the size of the unit. One thing that has occurred to me which I would like to call your attention is, that as a result of these years of development exactly the same controls and regulation can be applied to the pressure filter on the inlet and on the discharge, on the influent and on the effluent, as can be applied to the gravity filter. We have to take our hats off to the Venturi people on that end. There was undoubtedly a lack of regulation on that. To-day there is not. To-day I think all the controls both as to influent and effluent and the control of the chemical requirements can be had to the same extent with the pressure filter as with the gravity filter.
Mr. George P. Catlett, Wilmington, N. C.: I think when you bring up the pressure filter proposition that you will meet with little opposition from us. I am very strong in the belief that even with a 50,000-gallon plant that you oughf to have a man that understands thoroughly all the details of the purification of water. According to Mr. Fuller’s discussion the basic treatment is the thing; and I know that it is the thing which requires real judgment and real skill in any water treatment. I beg to differ with the last speaker as to the color removal being something that we do not have to bother with; and that it is very little trouble, and all that. I believe that color removal is one of the biggest problems that we have to deal with. I can cite quite a number of instances. I have talked with some of the other members, and they agree with me that the color proposition is one that requires great consideration. I believe that the basic treatment of the water is really the biggest problem straight through. For that reason, even in the smallest plants, I believe it is necessary’ and advisable, economically speaking, to have competent men in your laboratory. One of the chief requisites with the pressure filter is to have it fool-proof. I cannot see, provided you have a competent laboratory man and bacteriological man in control of a gravity filter, what the advantage can be of a pressure filter. I would like to have anybody explain that to me. The only advantage I have found that has been advanced to me is, that it is fool-proof. They say they could fill up that little tank with alum and then turn it loose, and that is all, they tell us, there is to it. That is where I have noticed in our testing that pressure filters have been applicable; but I think that the matter of being fool-proof is largely imaginary.
Mr. Stevens: I do not think the pressure filter has any advantage over the gravity filter for purifying water; but I still believe that a pressure filter plant can .be constructed more cheaply, and can deliver perfectly pure water. My figures are pretty general, but I have considered them pretty carefully and think that they are on the right side.