American Society of Civil Engineers

“ When one knows how to secure a good and safe result, it is unwise to depart therefrom for a mere whim, or to secure a supposedly lessened expense, unless other facts be also determined favorably. The desire for economy must be tempered by good sense, which means that one should be willing to change a method only when the wisdom of such has been clearly demonstrated. ”

“ Who can say that in some subtle and, at present, unknown manner, the failure in some places, where filtration is practiced, to reduce the death rate from typhoid fever may not be due to the introduction of radical departures from the older, slower, safer, and more efficient methods which have produced such excellent results, both in America and in Europe? ”

“ The full value of surface raking may not be generally appreciated, but the results which have followed a trial of this procedure at Washington, Philadelphia, and Pittsburg have shown that the output of filtered water between scrapings may be doubled or trebled thereby, with no injury to the filter itself or to the quality of the filtered water. The cost of raking over the surface of a 1‐acre slow sand filter unit is less than $10 at all the above‐mentioned places, which fact in itself shows the great saving in money and time effected by periodically substituting surface raking for scraping. ”

“ As this plan involved such a complete departure from those principles which had been well tried and had proven successful, it was believed that it was not safe to adopt such a method on the municipal filtration works, from which the people were to derive their drinking water. There is more to be considered in such a problem than mere economy of operation; the economy of human life, the effect on which requires far longer than a few months of trial to determine, is a much more important factor. ”

“ Efficient service can only be secured by strict discipline, accompanied by fair dealing. This means employing no more men than are actually necessary, paying them on the basis of the standard of service and output produced, taking an interest in the working conditions, and providing for their health and welfare.
About twelve years ago, the writer made some investigations of the efficiency of laboring gangs in scraping and handling sand at filter beds, 1 and found that ten men was the most economical number to use in scraping the surface of the Lawrence filter, as then built and operated. ”

“ The cost of the management of water‐works, one of the largest factors of public enterprise, has never been investigated extensively and thoroughly. There is much possibility in planning for greater efficiency and in determining what can be accomplished under economical administration. Every one is aware of the multiplicity of men in municipal service. ”

“ It may be questioned whether a certain amount of segregation of the materials will make any practical difference in the efficiency of a filter. In all probability this depends on the degree of the segregation, the quantity of pollution in the water to be filtered, the rate of filtration, and the uniformity of methods followed in the operation, etc. For an applied water as excellent in quality as that of the Washington City Reservoir during favorable summer conditions, a considerable degree of segregation might exist without producing any diminution in efficiency. ”

“ In the purification of water containing only insignificant quantities of suspended matter, but a relatively large amount of unstable organic matter, it will be conceded at once that the chief factor in the purification is the nitrification produced by the bacteria in the upper layers of the sand. On the other hand, the purification by sand filters of a hypothetical water containing no organic matter, but only finely‐divided mineral matter in suspension, could take place only by the physical deposition of the particles upon the sand grains. ”

“ Filter sand with a uniformity coefficient as low as that obtained at Washington has been rarely available for the construction of sand filters, and while the method of hydraulic return should certainly be considered, it will not be safe to assume that equally favorable results may be obtained with it with sands of high uniformity coefficients until actual favorable experience is obtained.
The writer believes that in calculating the cost of the water used in the plant itself the price chosen by the author, covering only the actual operating expenses of pumping and filtering, is too low. ”

“ During winter some of the external destroying agencies are less potent, such as the sterilizing effect of sunlight, and the presence and activity of some of the larger forms of microscopic organisms which prey on the bacteria. Another factor may be the greater amount of dissolved oxygen normally present in water during cold weather, as experiments have shown that dissolved oxygen favors longevity.
Still another reason for the larger numbers of bacteria that pass through a water filter during cold weather may be the effect that the low temperature has on the size of the bacteria themselves. ”

“ Under ordinary conditions it has been found that a filter can be raked to advantage at least twice between scrapings.
In the case of filters thus raked, a deeper penetration of suspended matter into the sand layer is inevitable, but at Pittsburg, as at Washington, such penetration does not extend more than about 2 in. below the filter surface. When the filter is finally scraped, a deeper layer is removed, of course, but it is clearly more economical to remove a deep layer at one operation than to remove separately several thinner layers of an equal total thickness. ”

“ Engineers have been taught to believe, by the German school of thought, that the film of organic matter on the surface of the sand plays a very important role in filtration. This Schmutzdecke, as it is called, has been considered so precious that stress has been placed on treating it with great care. It was not to be wholly removed at the time of cleaning, and it was not to be walked on, or indented, or in any other way consolidated or destroyed. ”

“ The method of dry raking does not remove the dirty material, but loosens up the pores of the surface, and through this porosity permits clogging to penetrate deeper into the filter. The method of raking with water on the bed, although it removes some of the organic dirt, also permits deeper penetration of the remainder. The latest devised system of washing the sand in place, by upward spraying with water, called the "Blaisdell method," thoroughly destroys the Schmutzdecke above, and, at the same time, must permit the formation of a subsidiary one below. ”

“ The lost‐time element is an important one, and at Washington this was the main reason for trying surface raking. It became necessary to increase the output of the filters, and the ordinary scraping consumed so much time that the sand‐handling force was increased, working day and night. The raking expedient introduced at this time overcame this, and Mr. Hardy states that it is still followed when the work is at all pressing. The speaker has found at Pittsburg, as Mr. Hardy has found at Washington, that raking is nearly if not quite as effective as scraping in restoring the filter capacity. ”

“ It is frequently the case, however, that a succession of waves of high turbidity will appear so close together that the effect of one has not disappeared before that of another is felt.
The clarification of the water supply begins at the dam at Great Falls. Here it is a clarification by exclusion, for when an excessive quantity of mud appears in the river water, the gates are closed, and the muddy water is allowed to flow over the dam and form mud‐bars in the Lower Potomac, while the city is supplied from the water stored in the three settling reservoirs. ”

“ The Hudson River water used at Albany is quite different in character from the Potomac River water used at Washington, as it is less turbid and contains rather more organic matter. The results obtained in these experiments showed that during the summer the number of bacteria in the effluent from the experimental sand filter used in connection with a preliminary filter did not differ widely from the number found in the effluent of the city filter where there was no other preliminary treatment than sedimentation. ”

“ In any section of the sand layer there are areas through which the water passes with a velocity much greater than its mean velocity through the total area of voids, while there are other areas in which the velocity is very much less, perhaps in an almost quiescent state from time to time, greatly favoring the deposition of particles, but with a gentle intermittent circulation, displacing the settled or partly‐settled water and supplying from the main currents water containing more suspended matter particles to be removed. ”

“ Another way of viewing the matter is that cold retards the growth of bacteria on the filter, thus reducing the effect of the Schmutzdecke. Still another view of the greater danger from bacterial contamination in winter is the theory that cold prolongs the life of the bacteria by merely preventing them from living through their life cycle and reaching natural old age and death as rapidly as in warm weather. ”

“ The largest percentage of increase in any item has been in "Care of Grounds and Parking," and covers much‐desired landscape improvements. Aside from this, the principal factor affecting the table of costs has been the reduction in water consumption in the District of Columbia. Nothing pertaining to this reduction has produced any corresponding reduction in the force required for the maintenance and operation of the filtration plant, office and laboratory, and pumping station, though probably there has been some reduction in filter cleaning. ”

“ The method of washing is also well adapted to this method of re‐sanding, as the sand is made very clean in its passage through the washers and storage bins. The hydraulic method of replacing sand tends to make it cleaner still, because any clay which may be left in the sand is constantly being carried away over the weir and out of the bed, to the sewer. Sand replaced by the hydraulic method is much more compact than when replaced by other methods, and consequently the depth of penetration of mud in a filter thus re‐sanded is less. ”

“ Through perfection in design and reasonable care in operation, the cost of filter cleaning, which is a very considerable part of the total cost, has been reduced to an unusually low figure, without any sacrifice in efficiency, and in the interests of the public health. ”

“ In other words, there is additional insurance in long storage against the faulty and careless operation of incompetent filter employees. The addition of coagulant, especially the fact that only a very small investment of capital is required for the necessary apparatus for dosing the water, and that the cost of the coagulating materials has to be met only when used, seems to give the process, in a most satisfactory manner, the requirement for economical management and thoroughness in preparing the water for final filtration. ”

“ Each of the centrifugal pumps has a nominal capacity of 40,000,000 gal. per day when pumping against a head of 21 ft., and each sand‐washer pump has a capacity of 2,500,000 gal. when pumping against a head of 250 ft. The electric light engines and generators supply the current for lighting the pumping station, the office and laboratory and other buildings, and also the courts and interior of the filter beds, and for operating a machine‐shop. ”

“ The diagram for Group A shows that the variations in the period of service of the filters scraped each time to clean sand follow a more or less definite curve from year to year. For the period covered by this curve, the tendency seems to be toward a slight decrease in capacity from year to year, as shown by the lower average maximum and minimum in the second year than in the first. Group B shows a sudden decrease in capacity following the first light scrapings and, since that time, a low but quite constant capacity. Group C shows a constantly decreasing capacity with successive rakings. ”

“ In preparing water for the use of the people, the realms of the unknown are so much larger than those which have been investigated and developed that there may be many undiscovered factors affecting the public health, and many ways in which it is dangerous to depart from well‐known and surely safe methods. ”

“ Carelessness is engendered by the thought that such work can be handled in a rough and rapid way, and, further, by the ridicule of all these things, which we have learned to be careful about, as old‐fogyish, out‐of‐fashion, and archaic. ”

“ In the warmer part of the year the difference is a wide one, and the hygienic efficiency of the process is much greater than is indicated by the gross numbers of bacteria.
The reduction of the typhoid death rate has not been as great with the change in water supply as was the case at Lawrence, Albany, and other cities, apparently because the Potomac water before it was filtered was not the cause of a large part of the typhoid fever. ”

“ The writer has a firm opinion that when an investment is made for public works, it costs but little in addition to construct buildings along appropriate architectural lines, to treat the grounds in a pleasing manner, and to make the entire works a credit to the municipality from an artistic standpoint. ”

“ Washington Methods.—In Washington, it is stated that the filters are still cleaned by the old‐fashioned method of scraping with shovels, throwing the sand into piles, and afterward removing it with a movable ejector. Between scrapings there is also an occasional mid‐period action of raking the unwatered sand surface, for the purpose of stirring up the dirty film. This process does not remove any of the clogging material from the bed, but it is said that no injurious effects are produced, and that it is economical. ”

“ The data available in published reports and papers are usually more or less fragmentary, and unexplained local conditions with reference to the character of the raw water, the cost of labor and supplies, and methods of apportioning these costs, introduce variables so wide as frequently to render the published figures almost useless for purposes of comparison. ”

“ Assuming the value of this water to be the total cost for pumping, filtering, etc., or $3.80 per 1,000,000 gal., the cost per cubic yard of sand replaced would be about $0.02 when one ejector is used, and $0.01 when two are in operation.
It is not considered absolutely necessary to have an upward flow of water in the filter which is being re‐sanded, and it is not always done. It was used, however, as an additional safeguard against the formation of a stratum of mud between the old and new layers of sand while the hydraulic method was in an experimental stage. ”