Chapter 15

Another topic in Mr. Hardy's paper which has interested the writer is that of preliminary filters. The experiments described at length indicate clearly that such devices would prove of little or no benefit under the conditions existing in Washington, and that when the river contains considerable amounts of suspended clay nothing less than chemical coagulation will suffice to treat the water so that the effluent will be perfectly clear. Preliminary filters have been used for a number of years at various places and with varying success. In few instances have they been operated for a sufficient length of time or been studied with sufficient care to determine fully their economy and efficiency as compared with other possible methods of preliminary treatment.

Among other experiments on this matter are those made at Albany, N. Y., and published by Wallace Greenalch, Assoc. M. Am. Soc. C. E., in the Fifty-ninth Annual Report of the Bureau of Water for the year ending September 30th, 1909. 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 the winter, however, the numbers of bacteria did not increase in the effluent from the experimental filter as they did in the effluent from the city filter. This is shown by Table 26, taken from the report mentioned.

Apparently, therefore, at Albany the benefits of the preliminary filter, as far as bacterial efficiency is concerned, would be confined to a short period of three or four months in each year. Under such circumstances it may well be questioned whether the advantages of preliminary filtration justify its cost.

~Table 26--Results of Experiments with Preliminary Filter at Albany, N. Y.~ ==========+==========+==========+============+============= Month, | Bacteria | Bacteria | Bacteria in|Bacteria in 1906. | in raw | in pre- | effluent | effluent | water. | liminary | from | from | | filter |experimental|city filter. | | effluent.|sand filter.| ----------+----------+----------+------------+------------- March | 133,480 | 36,000 | 151 | 706 April | 77,420 | 4,810 | 72 | 155 May | 15,800 | 2,250 | 48 | 37 June | 4,520 | 358 | 38 | 34 July | 2,090 | 163 | 25 | 22 August | 2,740 | 121 | 36 | 22 September | 8,280 | 445 | 20 | 24 October | 38,350 | 4,235 | 67 | 227 November | 67,910 | 15,570 | 337 | 341 December | 645,500 | 25,440 | 144 | 2,783 ----------+----------+----------+------------+------------- 1907. | | | | January | 127,560 | 4,660 | 48 | 443 February | 28,000 | 1,800 | 13 | 116 ==========+==========+==========+============+=============

On the diagram, Figure 11, will be found various data taken from the published records of the Albany filter, from 1899 to 1909. These data include: The numbers of bacteria before and after filtration; the percentage of bacteria remaining in the effluent; the average quantity of water filtered, in millions of gallons per day; the quantities of water filtered between scrapings; the turbidity of the raw water; the cost of filtration, including capital charges and cost of operation; and the typhoid death rates of the city per month. Several points are brought out conspicuously by this diagram. One is the uniformly low death rate from typhoid throughout the entire period. The filter was operated from 1899 until the fall of 1907 with raw water taken from what is known as the "Back Channel." Since then it has been taken from a new intake which extends into the Hudson River itself. Until the fall of 1908 the preliminary treatment consisted merely of sedimentation, but since then the water has received an additional preliminary treatment in mechanical filters operated without coagulant, along the lines of the experiments just mentioned. During this time the average rate of filtration of the sand filter has not changed materially, although it is said that the maximum rate has been increased since the preliminary filters were put in service. The study of the bacteriological analyses shows that the best results were obtained during 1902, 1903, and 1904. Since then the numbers of bacteria in both the raw and filtered water have increased. This was especially noticeable during the winters of 1907 and 1908 when the water was taken from the new intake. It will be interesting to compare the results after the preliminary filters have been operated for a long period to ascertain their normal effect on efficiency and on the increased yield.

Another fact to be drawn from the plotted Albany data is the increase in the cost of filtration, both in capital charges and in operation. From 1899 until 1906 the cost of operation, including the cost of low-lift pumping, was approximately $5 per million gallons of water filtered; and the total cost of filtration, including capital charges, was about $10 per million gallons. During the year ending September 30th, 1909, the cost of operation had increased to $7.63 per million gallons, and the total cost of filtration to $15.92 per million gallons, or approximately 50% in three years.

~Table 27--Results of Bacteriological Analyses of Samples of Water at Peekskill, N. Y., Before and After Filtration.~

~Bacteria per cubic centimeter.~ ==============+=======+=======+========+========+========+========+======== | Raw | Clear |Effluent|Effluent|Effluent|Effluent| Tap in Date. | water.| reser-| No. 1. | No. 2. | No. 3. | No. 4. | city. | | voir.| | | | | --------------+-------+-------+--------+--------+--------+--------+-------- 1909. --------------+-------+-------+--------+--------+--------+--------+-------- December 29th | 190 | 100 | ... | ... | ... | ... | ... --------------+-------+-------+--------+--------+--------+--------+-------- 1910. --------------+-------+-------+--------+--------+--------+--------+-------- February 15th | 135 | 10 | 10 | 30 | 20 | ... | 265 March 31st | 225 | 50 | 25 | 45 | 60 | ... | 35 May 18th | 300 | 29 | 22 | 26 | 35 | 43 | 36 July 6th | 300 | 44 | 9 | 3 | 41 | 10 | 31 August 16th | 60 | 5 | 0 | 4 | 1 | 13 | 15 October 3d | 550 | 14 | 12 | 14 | 38 | ... | ... November 21st | 315 | 22 | 26 | 17 | 6 | ... | ... --------------+-------+-------+--------+--------+--------+--------+-------- 1911. --------------+-------+-------+--------+--------+--------+--------+-------- January 25th | 415 | 7 | 8 | 4 | 6 | ... | 7 --------------+-------+-------+--------+--------+--------+--------+-------- Average | 277 | 30 | 14 | 16 | 26 | 22 | 65 ==============+=======+=======+========+========+========+========+========

~Table 17~--(_Continued._) ~Tests for~ _B. Coli._ ==================+====================== |~Percentage of Samples Quantity of water |Containing~ _B. Coli_. tested. +--------+------------- | Raw. | Filtered. ------------------+--------+------------- 0.1 cu. cm. | 0 | 0 1.0 cu. cm. | 20 | 0 10.0 cu. cm. | 40 | 0 ==================+========+=============

As a matter of record, the results of a series of analyses made at Peekskill, N. Y., during 1910 are presented in Table 27. A sand filter was constructed for the water supply of this city in 1909, and put in operation in December. The filter has a capacity of 4,000,000 gal. per day. The supply is taken from Peekskill Creek, and the water receives about one week's nominal storage before flowing to the filters. An aerator is used before filtration during the summer, when algae are likely to develop in the reservoir. The filter was installed after an epidemic of typhoid which was apparently caused by an infection of the water supply. Normally, the water has been little contaminated, but the supply is subject to accidental contamination at any time, among other possible sources of infection being the camps of workmen now engaged in constructing the Catskill Aqueduct for New York City.

~Table 28--Average Results of Chemical Analysis at Peekskill, N. Y., Made at Intervals of Six Weeks During 1910.~ =====================+=================+================+================= | ~Parts per | | ~Parts per | Million.~ | | Million.~ |--------+--------| |--------+-------- | Raw |Filtered| | Raw |Filtered | water. | water. | | water. | water. ---------------------+--------+--------+----------------+--------+-------- Turbidity | 2. | 0 |Total residue | 70. | 76.00 Color | 25. | 20. |Loss on ignition| 19.00 | 17.00 Nitrogen as albumi- | | |Fixed residue | 50.00 | 59.00 noid ammonia | 0.112 | 0.076 |Iron | 0.17 | 0.13 Nitrogen as free | | |Total hardness | 38.70 | 45.10 ammonia | 0.024 | 0.006 |Alkalinity | 33.90 | 42.60 Nitrogen as nitrites | 0.001 | 0.001 |Incrustants | 4.60 | 4.50 Nitrogen as nitrates | 0.06 | 0.06 |Chlorine | 2.60 | 2.70 =====================+========+========+================+========+========

~F. F. Longley, Assoc. M. Am. Soc. C. E.~ (by letter).--In this paper the author has presented a mass of data which will be welcomed by engineers engaged in water purification work, because complete operating records form a substantial basis for improvement in the art, and are often the inspiration for interesting discussions and the exchange of experiences of different observers whose views are mutually appreciated.

Recent tendencies in filtration engineering have been largely in the direction of reducing the cost of operation. A comparison of the operating costs of the earlier American plants of about a decade ago, with those here presented of the Washington plant, is very gratifying to those who have been intimately connected with the latter work. 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.

Table 14 shows that, from the first year, there has been a progressive increase in the total cost of operation per million gallons filtered, but this has not meant an increase in the annual total expenditure. 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. Obviously, then, the total cost per million gallons would increase.

This decrease in consumption has been brought about by the elimination of waste in the distribution system, which is not in the same department as the filtration plant, but with regard to which a word may not be amiss in connection with this discussion.

The Washington Aqueduct was built half a century ago on lines which at that time were considered extraordinarily generous. Until recently, therefore, there has been no occasion for concern over the high rate of consumption. During recent years, however, the use and waste of water have increased, reaching a climax under unusual conditions in the winter of 1904-05. The maximum capacity of the aqueduct system is about 90,000,000 gal. The maximum daily consumption at the time mentioned arose almost to 100,000,000 gal., with the result that, before normal conditions were restored, the reservoirs of the system were almost depleted.

This had a beneficial effect, as provision was made for an active campaign for reducing the waste of water, which was known to be very large. These investigations, using the pitometer, were begun in July, 1906, and have been pursued continuously since that time, with most excellent results. Up to January, 1909, leaks aggregating about 12,000,000 gal. per day were detected and eliminated, and about half the house services had still to be covered by the pitometer bureau.

Although this reduction in waste has brought about an apparent increase in the cost of filtration, its economical results have been far-reaching. The causes which brought about this investigation also resulted in securing an appropriation for the study of the question of increased supply. The writer was in charge of these studies, and the most significant conclusion was that, owing to the excellent results of the efforts for waste restriction, the total consumption and waste of water in the district during the next few years would be far enough below the safe working capacity of the existing aqueduct system to make it entirely safe to postpone the construction of new works, involving the expenditure of several million dollars, in spite of the threatening conditions of a few years ago.

There has been so much controversy over typhoid fever in the District of Columbia that the writer hesitates to discuss this subject. Viewing the situation through the perspective of several years, however, it does not seem to be as hopeless as the criticisms of four or five years ago would lead one to believe.

In Table 9, showing the typhoid death rates, out of nine years given prior to 1905-06, when the filters were started in operation, only one shows an annual death rate as low as the highest one since that year. Further than this, the annual average typhoid death rate for the period since that year has been one-third lower than for a corresponding period before the filters were started.

The exhaustive researches of the Public Health and Marine Hospital Service into this whole question, covering a period of about four years, have raised the present filtered water supply of the District of Columbia above any well-founded criticism. There has long been a strong and growing feeling that the water supply, before filtration was introduced, had been blamed for more than its share of the typhoid, and this is borne out by much evidence that has been presented from time to time.

It is not an unreasonable conjecture, therefore, that perhaps the reduction of one-third in the total typhoid death rate may represent a much larger reduction in that part of the total which was due to polluted water alone; and that, as the authorities in the District of Columbia and in certain other cities, particularly in the South, are now recognizing, the fight against much of the remaining typhoid must be in the direction of the improvement of milk supplies, precautions against secondary infection, and attention to a large number of details surrounding the individual, which may effectively protect him against the insidious attack of the disease favored by unknown agencies.

~Experiments in Filter Cleaning~.

The author refers to the difficulty encountered during the first two summers in keeping the filters cleaned fast enough to maintain the capacity of the plant. The real seriousness of this may be judged from the following facts. The average increase in loss of head on all the filters for the entire year, July 1st, 1906, to July 1st, 1907, was about 0.053 ft. per day. During the 1906 period of low capacity under discussion, the loss of head on twelve of the filters increased for a period of eight days at the average rate of 0.45 ft. per day, or about nine times the normal rate of increase. This difficulty was caused by the presence of large numbers of micro-organisms in the applied water. During the first summer (1906) this fact was not recognized, but the sudden decrease in capacity was supposed to have been caused by the unusually high and long-continued turbidity which prevailed during that summer in the Potomac River, and persisted in the water supplied to the filters even after about four days of sedimentation in the reservoirs. During the second summer (1907) the same phenomenon of suddenly and rapidly increasing losses of head appeared again, but without any unusual turbidity in the applied water. Investigation, however, showed the presence of large quantities of organisms, particularly _melosira_ and _synedra_, in the applied water, and examinations in subsequent years have shown a periodic recurrence of these forms in quantities sufficient to cause the trouble mentioned. In June, 1907, examination showed repeatedly more than 1,000 and 1,500 standard units of _melosira_ per cu. cm., and one count showed nearly 3,000 standard units.

Several expedients were tried in an effort to restore the rapidly decreasing capacity of the filters. One of the earlier conjectures as to the cause of the trouble was that it might be due to the accumulation of large quantities of air under the surface of the sand, as air had been observed bubbling up through the sand, especially in filters which had been in service for some time. The expedient was tried, therefore, of draining the water out of the sand and then re-filling the filter in the usual manner from below, in the hope of driving out the entrained air. Presumably this treatment got rid of the air, but it did not restore the capacity of the filter, as the point of maximum resistance was in the surface of the sand and not below it.

As the author states, raking the filters was tried and found to give results which were satisfactory enough to meet the emergencies already referred to. When the filters were first put in operation, in the fall of 1905, the method of bringing back the capacity of a filter after the end of a run was to remove all the dirty sand to a depth determined by the marked discoloration caused by the penetration of the clay turbidity. This sometimes necessitated the removal of large quantities of sand at a cleaning, as the turbidity was exceedingly fine, and penetrated at times to a depth of 3 or 4 in.

With the idea of effecting an economy in the cost of cleaning the filters, a schedule of experiments was arranged shortly before July 1st, 1907. The general object of the experiments was to determine, first, the relative costs of all different methods tried; second, whether the removal of only a thin layer of sand, or the mere breaking up of the surface of the sand by thorough raking, would give the filter its proper capacity for the succeeding run; third, whether the filters under these treatments would maintain a high standard of quality in the effluents; fourth, whether the continued application of any less thorough method than the one then in use might materially affect the future capacity of the filters.

To this end the filters were divided into four groups which, during a period of about six months, were subjected to treatments as follows:

Group _A._--Filters scraped deep at the end of each run; Group _B._--Filters scraped light at the end of each run; Group _C._--Filters raked at the end of each run, until raking failed to bring back the proper capacity; then they were scraped light, and at the end of the next run the raking was resumed; Group _D._--Light scrapings and rakings alternate at ends of runs.

The term "deep scraping" means the removal of practically all the discolored sand, in accordance with the usual practice prior to the beginning of these experiments; "light scraping" means the removal of only a thin surface layer of sand. This depth has usually averaged about 3/8 in. "Raking" means the thorough breaking up of the clogged surface of the filter by iron-toothed rakes, to a depth of about 1 or 2 in.

_Results._--A general summary of the results of these experiments is given in Table 29, which also shows the relative costs of the different methods per million gallons of water filtered. A normal period of 9 months just prior to the beginning of these experiments shows a labor cost (corresponding to that in Table 29) of $0.29-1/4 per million gallons filtered.

~Table 29--Average Results.~

Columns: A - Group. B - Number of filters. C - Number of days of service. D - Million gallons filtered. E - Cost of labor per treatment. F - Sand removed in cubic yards. G - Sand removed in cubic yards. H - Cost of labor. I - Bacteria per cu. cm. in effluent. J - Turbidity in effluent.

=========================================+===============+============== | Per Million | | Per Run: | Gallons | | | Filtered | I | J -----+-----+-----+-------+-------+-------+-------+-------+ | A | B | C | D | E | F | G | H | | -----+-----+-----+-------+-------+-------+-------+-------+-------+------ _A_ | 5 | 82 | 221.2 |$68.44 | 215 | 1.11 |$0.309 | 13 | 1 _B_ | 9 | 36 | 101.4 | 29.25 | 84 | 0.83 | 0.288 | 16 | 1 _C_ | 5 | 21 | 60.0 | 10.92 | 24 | 0.40 | 0.182 | 18 | 1 _D_ | 10 | 32 | 86.0 | 20.10 | 46 | 0.54 | 0.234 | 22 | 1 =====+=====+=====+=======+=======+=======+=======+=======+=======+======

_Capacity of Filters._--The capacity of the filters under the different methods of treatment are shown in a general way in Table 29 for days of service and millions of gallons filtered per run. This element by itself is decidedly in favor of the deep scrapings, and least in favor of the repeated rakings.