Part 14

~Table 23--Relative Cost of Filtering at Different Rates.~ ======================+=================================================== |Nominal rate, in millions of gallons per acre daily: +------------+------------+------------+------------ | 3 | 5 | 10 | 20 ----------------------+------------+------------+------------+------------ Percentage which | | | | average yield is of | | | | nominal rate..........| 85 | 80 | 75 | 65 ----------------------+------------+------------+------------+------------ Average output per | | | | acre, in millions of | | | | gallons per day.......| 2.55 | 4.00 | 7.5 | 13.0 ----------------------+------------+------------+------------+------------ Cost of that part of | | | | filters per acre | | | | dependent on rate.....| $12,000 | $20,000 | $40,000 | $80,000 ----------------------+------------+------------+------------+------------ Cost of that part of | | | | filters per acre not | | | | dependent on rate.....| 50,000 | 50,000 | 50,000 | 50,000 ----------------------+------------+------------+------------+------------ Total cost of filters | | | | per acre..............| 60,000 | 70,000 | 90,000 | 130,000 ----------------------+------------+------------+------------+------------ Cost per million | | | | gallons of capacity...| 20,600 | 14,000 | 9,000 | 6,500 ----------------------+------------+------------+------------+------------ Cost per million | | | | gallons of average | | | | daily output..........| 24,400 | 17,500 | 12,000 | 10,000 ----------------------+------------+------------+------------+------------ Capital charges and | | | | depreciation at 6% on | | | | cost per million | | | | gallons...............| 4.00 | 2.87 | 1.97 | 1.64 ----------------------+------------+------------+------------+------------ Operating expenses, | | | | the same at all | | | | rates.................| 1.00 | 1.00 | 1.00 | 1.00 ----------------------+------------+------------+------------+------------ Total cost of | | | | filtering, excluding | | | | pumping, storage, and | | | | all auxiliaries.......| 5.00 | 3.87 | 2.97 | 2.64 ----------------------+------------+------------+------------+------------ Relative cost.........| 1.29 | 1.00 | 0.77 | 0.68 ======================+============+============+============+============

When the costs of pumping, pure-water reservoirs usually necessary, etc., are taken into account (which add equally to the cost at all rates), the cost of filtering will vary less with the rate than is indicated.

The effect of rate on cost, as calculated in Table 23, and also the percentages of the bacteria of the raw water found in the effluents by the author and by Mr. Clark, are shown on Figure 10.

Considering all these results together, and also all the other evidence known to the writer bearing on this point, it seems clear that filters are not as sensitive to changes in rate, within reasonable limits, as has been frequently assumed; but, on the other hand, there is usually a substantial increase in the percentage of bacteria passing through a filter with increased rate.

Filters furnish relative, not absolute, protection against infectious matter in the raw water. The higher the bacterial efficiency, the more complete is this relative protection.

The cost of filtering does not decrease in inverse ratio to the rate, but at a much slower rate. This is especially true with rates of more than 5,000,000 or 6,000,000 gal. per acre daily.

In general, a rate of filtration may rationally be selected at which the value of the possible danger resulting from an increase in rate is equal to the saving that may be made in cost by its use. This point must be a matter of individual judgment. The tendency of the last few years has been to use higher rates, or, in other words, to cheapen the process and to tolerate a larger proportion of bacteria in the effluent. The use of auxiliary processes has been favorable to this, especially the use of chloride of lime, in connection with either the raw water or the effluent.

By the judicious use of this substance, efficiency may be maintained while using higher rates than would otherwise have been desirable.

The writer believes that there will be many cases where the added risk of using too high a rate is not worth the relatively small saving in cost that accompanies it.

~George A. Johnson, Assoc. M. Am. Soc. C. E~.--This paper contains information of an exceedingly interesting nature. There is comparatively little difficulty in obtaining accurate figures on the cost of construction of water purification works, but, with costs of operation of such works, it is different. 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.

Mr. Hardy's paper is noteworthy in that it presents certain relatively new features of slow sand filter operation which have been only lightly touched on in water purification literature up to the present time. These refer particularly to means whereby a filter may be continued in service without removing a portion of the surface layer of the filter surface itself when the available head has become exhausted, and to methods whereby washed sand may be expeditiously and more economically restored to the filter than has been the case hitherto.

Sand handling is the most important item of expense in the operation of a slow sand filter. Quite recently a charge of $1.50 per cu. yd. for sand scraping, transportation to sand washers, washing, and restoring to the filter, was not considered exorbitant, but the improved methods developed during recent years at Washington, Philadelphia, Albany, and more recently at Pittsburg (at all of which places hydraulic ejection plays an important part), have shown the feasibility of reducing this figure by nearly, if not quite, two-thirds.

The practice observed at Washington of raking over the surface of the sand layer when the available head becomes exhausted, in order to avoid the cost and loss of time necessitated by shutting down the filter and scraping off the surface layer, is unquestionably one of the most striking advances in slow sand filter operation in recent years. In rapid sand filter operation, to prolong the period of service between washings, agitation of the filter surface has been used to advantage for many years. 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. 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.

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.

Eleven years ago the speaker was connected with the preliminary investigations into the best methods of purifying the Potomac River water for Washington. It then appeared that while for the greater part of the time during an average year the Potomac River could be classed among the clear waters of the East, there were periods when excessive turbidity made it necessary to consider carefully methods of preparatory treatment before this water could be filtered effectively and economically. As Mr. Hardy has said, considerable prejudice existed against the use of a coagulating chemical, and the expedient was therefore adopted of giving the water a long period of sedimentation in order to remove enough of the suspended matter to allow the clarified water to be treated on slow sand filters. The expert commission, consisting of Messrs. Hering, Fuller, and Hazen, recommended the occasional use of a coagulating chemical, but this recommendation was not carried out.

The Potomac River is somewhat peculiar, in that the turbidity of its waters, as shown by the results presented in Mr. Hardy's paper, ranges from 3,000 to practically nothing. The bacterial content also varies widely, and Mr. Hardy's tables show this variation to be from 76,000 to 325 per cu. cm. Such a water as this requires particularly careful preparatory treatment. The Dalecarlia Reservoir has a capacity of something like 2 days' storage, the Georgetown Reservoir the same, and the McMillan Park Reservoir nearly 3 days, making a total sedimentation of more than 7 days. Without the use of a coagulant, it is significant that during a period of five years, even with 7 days' sedimentation, the average maximum turbidity of the water delivered to the filters was 106 parts per million, and the maximum average turbidity in one month was 250 parts per million. The water filtration engineer can readily understand that waters as turbid as this cannot be treated economically and efficiently in slow sand filters. It would appear that coagulating works might advantageously have been installed at the entrance to the Dalecarlia Reservoir. If this had been done, and coagulant had been added to the water at times when it was excessively turbid, a considerably shorter period of subsequent sedimentation than now exists would in all probability have rendered the water at all times amenable to efficient and economical slow sand filter treatment.

The prejudice in Washington against the use of coagulants has also manifested itself in other localities, but the results which have been obtained during the past twenty years from rapid sand filters and from slow sand filters, treating waters previously coagulated with salts of iron or alumina, have shown how thoroughly unreasonable were these objections. In this connection it is interesting to note that there are in the United States more than 350 rapid sand filter plants, and that nearly 12% of the urban population of Continental United States is being supplied with water filtered through rapid sand filters, in connection with all of which a coagulating chemical is used in the preparatory treatment.

~Table 24--Typhoid Fever Death Rates in Cities of the United States with Populations in 1910 of 100,000, or More.~

Statistics gathered by correspondence and from Reports of the Bureau of the Census, Department of Commerce and Labor, Mortality Statistics.

~Note~.--Statistics from Birmingham, Ala., Dayton, Ohio, Fall River, Mass., Louisville, Ky., Memphis, Tenn., Oakland, Cal., and Providence, R. I., are not included, as they are incomplete.

Columns: A - Average for six years, 1900-05, inclusive. B - Average for five years, 1906-10, inclusive. C - Average for 11 years, 1900-11, inclusive.

====================+=============================================== | ~ Typhoid Fever Death Rate City. | per 100,000 Population~. +-----+-----+-----+-----+-----+-----+-----+----- | 1906| 1907| 1908| 1909| 1910| A | B | C --------------------+-----+-----+-----+-----+-----+-----+-----+----- Albany, N. Y. | 20 | 20 | 11 | 19 | 15 | 25 | 17 | 21 Atlanta, Ga. | 50 | 64 | 47 | 44 | 43 | 65 | 50 | 58 Baltimore, Md. | 34 | 41 | 31 | 23 | 41 | 36 | 34 | 35 Boston, Mass. | 22 | 10 | 26 | 14 | 11 | 23 | 16 | 20 Bridgeport, Conn. | 10 | 13 | 13 | 13 | 9 | 15 | 12 | 14 Buffalo, N. Y. | 24 | 29 | 21 | 23 | 20 | 29 | 23 | 26 Cambridge, Mass. | 18 | 10 | 10 | 9 | 12 | 18 | 12 | 15 Chicago, Ill. | 18 | 18 | 15 | 12 | 14 | 27 | 16 | 22 Cincinnati, Ohio | 71 | 46 | 19 | 13 | 6 | 54 | 31 | 44 Cleveland, Ohio | 20 | 19 | 13 | 12 | 19 | 51 | 17 | 36 Columbus, Ohio | 45 | 38 | 110 | 17 | 13 | 61 | 45 | 54 Denver, Colo. | 68 | 67 | 58 | 24 | 30 | 37 | 49 | 42 Detroit, Mich. | 22 | 28 | 22 | 19 | 16 | 17 | 22 | 19 Grand Rapids, Mich. | 39 | 30 | 30 | 17 | 27 | 34 | 28 | 31 Indianapolis, Ind. | 39 | 29 | 26 | 22 | 31 | 76 | 30 | 55 Jersey City, N. J. | 20 | 14 | 10 | 8 | 10 | 19 | 12 | 16 Kansas City, Mo. | 38 | 40 | 35 | 23 | 38 | 48 | 35 | 42 Los Angeles, Cal. | 18 | 23 | 19 | 18 | 12 | 35 | 18 | 27 Lowell, Mass. | 7 | 9 | 24 | 11 | 21 | 19 | 14 | 17 Milwaukee, Wis. | 31 | 26 | 17 | 21 | 45 | 19 | 28 | 23 Minneapolis, Minn. | 33 | 26 | 18 | 20 | 58 | 38 | 29 | 34 Nashville, Tenn. | 66 | 85 | 62 | 53 | 48 | 54 | 58 | 56 Newark, N. J. | 18 | 24 | 12 | 11 | 13 | 17 | 16 | 17 New Haven, Conn. | 54 | 30 | 34 | 20 | 17 | 44 | 31 | 38 New York, N. Y. | 15 | 17 | 12 | 12 | 12 | 19 | 14 | 17 New Orleans, La. | 30 | 56 | 31 | 25 | 28 | 40 | 34 | 37 Omaha, Nebr. | 28 | 24 | 22 | 31 | 75 | 20 | 36 | 27 Paterson, N. J. | 4 | 11 | 10 | 5 | 7 | 25 | 7 | 17 Philadelphia, Pa. | 74 | 60 | 36 | 22 | 17 | 47 | 42 | 45 Pittsburg, Pa. | 141 | 135 |53[1]|13[1]|12[1]| 132 | 71 | 104 Richmond, Va. | 44 | 41 | 50 | 24 | 22 | 66 | 36 | 53 Rochester, N. Y. | 17 | 16 | 12 | 9 | 13 | 15 | 13 | 14 St Louis, Mo. | 18 | 16 | 15 | 15 | 14 | 33 | 16 | 25 St Paul, Minn. | 21 | 17 | 12 | 20 | 20 | 14 | 18 | 16 San Francisco, Cal. | ... | 57 | 27 | 17 | 15 | 20 | 29 | 24 Scranton, Pa. | 11 | 76 | 11 | 11 | 14 | 18 | 35 | 26 Syracuse, N. Y. | 10 | 16 | 15 | 12 | 30 | 14 | 17 | 15 Toledo, Ohio | 45 | 36 | 40 | 31 | 32 | 36 | 37 | 36 Worcester, Mass. | 12 | 14 | 10 | 8 | 16 | 17 | 12 | 15 Washington, D. C. | 52 | 36 | 39 | 33 | 23 | 59 | 37 | 49 ====================+=====+=====+=====+=====+=====+=====+=====+=====

[Footnote 1: Filtered water section. Allegheny District not included.]

Attention has repeatedly been called to the fact that the relatively high typhoid death rate in Washington, since the filter plant was installed, was a possible indication that the filters were inefficient. It is true that there has not been the marked reduction in the typhoid death rate in Washington, following the installation of the water filtration works, that has been observed in other cities in America. For the six years prior to the date on which filtered water was supplied to the citizens of Washington, the average typhoid fever death rate was 59 per 100,000 population, as against 37 per 100,000 for the five years following, a reduction of 37 per cent. At Albany, N. Y., where the first modern slow sand filter was built in 1899, the typhoid death rate has been reduced by 75 per cent. At Cincinnati, Ohio, the average death rate from typhoid ranged around 50 per 100,000 for years, but since the installation of the filtration plant it has been reduced to a point which places that city, with respect to freedom from typhoid fever, at the head of all the large cities in America; in 1910 the death rate from typhoid in Cincinnati was 6 per 100,000. Similarly, at Columbus, Ohio, where the typhoid death rate before the installation of the filtration plant in 1906 was even higher than at Cincinnati, it was reduced to less than 13 per 100,000 in 1910, whereas, for the previous five years, it was 61 per 100,000. Philadelphia, before the installation of the filtration works, had a typhoid death rate of 60 or more per 100,000, and in 1910 the death rate from this disease was 17. Pittsburg, at least that part of it now supplied with filtered water, for years had a typhoid death rate of more than 130 per 100,000, but the present rate is about 12 per 100,000.

~Table 25--Average Monthly Results for the Period, 1905-1910.~

Columns: A - Period of sedimentation in days. B - Turbidity in parts per million. C - Bacteria per cubic centimeter. ============+=====+=====+=======+===================== | | | |~Percentage Removed~ Reservoirs.| A | B | C |----------+---------- | | | | Turbidity| Bacteria ------------+-----+-----+-------+----------+---------- River | ... | 106 | 6,400 | ... | ... Dalecarlia | 2.2 | 50 | 5,000 | 53 | 22 Georgetown | 2.2 | 38 | 3,400 | 24 | 32 McMillan | 2.8 | 26 | 2,000 | 31 | 41 ------------+-----+-----+-------+----------+---------- Totals and | | | | | averages | 7.2 | ... | ... | 75 | 69 ============+=====+=====+=======+==========+==========

While it may perhaps seem unreasonable to single out Washington as a particular sufferer in this respect, it is highly probable that a large share of the typhoid is still caused by secondary infection, flies, impure milk, and private and public wells. The speaker remembers distinctly that ten years ago, when he made an investigation into the purity of the water of about 100 public wells in that city, a large number of them showed unmistakable evidence of being polluted with sewagic matter. Conclusive evidence would be secured to dispel any doubt as to the sanitary quality of the filtered product if hypochlorite of lime were added to the filtered water throughout one year or throughout the typhoid months. It seems strange to the speaker, that for this, if for no other reason, this safe and non-injurious germicide has not as yet been used at Washington, in view of the fact that at the present time it is being used continuously or intermittently in the treatment of the water supplies of scores of the most important cities of this country, among which may be mentioned New York, Philadelphia, Cincinnati, Pittsburg, St. Louis, and Minneapolis.

~Morris Knowles, M. Am. Soc. C. E~. (by letter).--This description of the operation of the Washington Filtration Works is timely and of great interest. It is ten years since the writer, in collaboration with Charles Gilman Hyde, M. ~Am. Soc~. C. E., presented a similar record for the Lawrence, Mass., filter. That paper was the first complete, detailed, and continuous history of the actions and results obtained for a long period of time with such a purification works.[1] Since then, the art of filtration has advanced in many ways, particularly in regard to the methods of cleaning slow sand filters and in the accompanying processes. It is well, therefore, again to take account of stock and see really what progress has been made. Therefore, Mr. Hardy's paper, giving a description of the operations of a system thoughtfully designed, after long consideration of the problem, and of operations carried on under efficient and economical administration, with thorough record of all details, should furnish a groundwork for the careful consideration of the question stated above.

The writer, using as a text some of the ideas given in the paper, but more particularly some of those becoming prevalent elsewhere, desires to discuss methods and costs of operation, especially in relation to sand handling; and to offer suggestions looking toward greater efficiency, as well as economy, in carrying out the standard and well-tried methods.

_Theory of Slow Sand Filtration._--First, what is the process of slow sand filtration? The answer to this question involves many factors, some of which are even yet but imperfectly understood. In the early history of filtration, at the time of the construction of the London filters, only the straining capacity of the sand bed, to remove gross particles, was known. Later, when the organic contents of water had become better understood, the chemical or oxidizing powers of the process were recognized as performing an important part. Finally, co-existent with the discovery of the so-called "germ theory of disease," a study of the bacterial action of filters resulted in the recognition of its importance. It is now universally thought that each of these factors performs its useful function; that the size of the sand, the amount of organic matter remaining on the surface of the bed, the turbidity of the applied water, and the bacterial content of the influent, are some of the things on which depends the determination of the relative importance of each.

[Footnote 1: _Transactions_, Am. Soc. C. E., Vol. XLVI, p. 258.]

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. In fact, in some cases, the wasting of the first water after cleaning has been advocated, for the reason that not a sufficient amount of this organic film would be left on top of the sand to begin the filtration process properly immediately after the cleaning.