Part 13

~Table 20--Summary of Results of Experimental Filters.~ ==================+========+========+========+=========+=========+======== Filter number.....| 1 | 2 | 3 | 4 | 5 | 6 | | | | | | Number of runs....| 3 | 6 | 11 | 12 | 25 | 28 ------------------+--------+--------+--------+---------+---------+-------- Rate, million gallons per acre per day: Maximum.......| 1.35 | 3.95 | 7.96 | 12.60 | 37.5 | 118.9 Minimum.......| 0.62 | 2.30 | 3.73 | 5.77 | 6.68 | 7.1 Average.......| 1.06 | 3.26 | 6.69 | 10.17 | 26.1 | 38.54 ------------------+--------+--------+--------+---------+---------+-------- Length of run, in days: Maximum.......| 233.5 | 150.5 | 75.2 | 90.9 | 48.71 | 39.83 Minimum.......| 181.7 | 42.0 | 14.5 | 10.1 | 0.67 | 0.62 Average.......| 206.4 | 109.6 | 48.89 | 40.5 | 14.41 | 12.61 ------------------+--------+--------+--------+---------+---------+-------- Million gallons filtered per acre per run: Maximum.......| 242.61 | 484.46 | 534.67 | 960.72 |1,463.35 |1,022.27 Minimum.......| 202.60 | 135.66 | 93.79 | 92.57 | 19.53 | 53.32 Average.......| 218.58 | 302.82 | 326.76 | 417.23 | 374.14 | 361.92 ------------------+--------+--------+--------+---------+---------+-------- Cubic yards of sand removed per acre at end of each run: Maximum.......| 269 | 269 | 672 |1,612 |2,420 |3,360 Minimum.......| 269 | 134 | 101 | 134 | 134 | 101 Average.......| 269 | 213 | 272 | 392 | 583 | 635 ------------------+--------+--------+--------+---------+---------+-------- Cubic yards of | | | | | | sand removed | | | | | | per acre per | | | | | | million gallons | | | | | | filtered........| 1.23 | 0.70 | 0.83 | 0.94 | 1.55 | 1.72 ------------------+--------+--------+--------+---------+---------+-------- Average initial | | | | | | loss of head....| 0.07 | 0.19 | 0.51 | 0.78 | 3.88 | 5.38 ------------------+--------+--------+--------+---------+---------+-------- Turbidity, influent: Maximum.......| 120 | 120 | 120 | 120 | 90 | 100 Minimum.......| 2 | 2 | 2 | 2 | 2 | 2 Average.......| 20 | 20 | 21 | 22 | 18 | 19 ------------------+--------+--------+--------+---------+---------+-------- Turbidity, effluent: Maximum.......| 11 | 13 | 17 | 18 | 30 | 30 Minimum.......| 0 | 0 | 0 | 0 | 0 | 0 Average.......| 1 | 1 | 2 | 2 | 4 | 3 ------------------+--------+--------+--------+---------+---------+-------- Percentage | | | | | | reduction | 95.0 | 95.0 | 90.5 | 90.9 | 77.8 | 84.3 ------------------+--------+--------+--------+---------+---------+-------- Bacteria, influent: Maximum.......|180,000 |180,000 |180,000 | 110,000 | 180,000 | 37,500 Minimum.......| 22 | 20 | 22 | 20 | 25 | 24 Average.......| 4,800 | 5,100 | 4,500 | 4,200 | 6,900 | 5,900 ------------------+--------+--------+--------+---------+---------+-------- Bacteria, effluent: Maximum.......| 4,000 | 1,300 | 3,200 | 5,400 | 12,800 | 2,400 Minimum.......| 2 | 3 | 1 | 1 | 2 | 2 Average.......| 160 | 85 | 110 | 120 | 190 | 180 ------------------+--------+--------+--------+---------+---------+-------- Percentage, | | | | | | Reduction.......| 96.7 | 98.3 | 97.6 | 97.3 | 97.3 | 97.0 ------------------+--------+--------+--------+---------+---------+-------- Number of samples examined for _bacillus coli_ in influent: 10 c.c........| 549 | 478 | 476 | 436 | 325 | 336 1 c.c........| 560 | 492 | 486 | 445 | 335 | 342 0.1 c.c......| 525 | 459 | 452 | 413 | 318 | 317 0.01 c.c.....| 511 | 443 | 439 | 405 | 308 | 304 0.001 c.c....| 500 | 434 | 429 | 394 | 299 | 294 ------------------+--------+--------+--------+---------+---------+-------- Number of samples examined for _bacillus coli_ in effluent: 10 c.c........| 512 | 452 | 454 | 404 | 296 | 309 1 c.c........| 513 | 454 | 457 | 406 | 299 | 311 0.1 c.c......| 480 | 419 | 426 | 383 | 271 | 286 0.01 c.c.....| 478 | 406 | 410 | 367 | 261 | 276 0.001 c.c....| 478 | 406 | 410 | 367 | 261 | 276 ------------------+--------+--------+--------+---------+---------+-------- Number samples positive, influent: 10 c.c........| 226 | 211 | 201 | 258 | 136 | 152 1 c.c........| 127 | 123 | 116 | 108 | 81 | 93 0.1 c.c......| 55 | 59 | 54 | 51 | 43 | 42 0.01 c.c.....| 26 | 34 | 33 | 33 | 27 | 25 0.001 c.c....| 6 | 6 | 5 | 6 | 3 | 3 ------------------+--------+--------+--------+---------+---------+-------- Number samples positive, effluent: 10 c.c........| 100 | 109 | 134 | 98 | 94 | 106 1 c.c........| 51 | 61 | 55 | 56 | 46 | 50 0.1 c.c......| 9 | 13 | 16 | 16 | 4 | 13 0.01 c.c.....| 0 | 0 | 0 | 0 | 0 | 0 0.001 c.c....| 0 | 0 | 0 | 0 | 0 | 0 ------------------+--------+--------+--------+---------+---------+-------- Percentage of samples showing _bacillus coli_ in influent: 10 c.c........| 41.2 | 44.2 | 42.2 | 59.2 | 41.9 | 45.2 1 c.c........| 22.7 | 25.0 | 23.9 | 24.3 | 24.2 | 27.2 0.1 c.c......| 10.5 | 12.8 | 11.9 | 12.3 | 13.5 | 13.2 0.01 c.c.....| 5.1 | 7.7 | 7.5 | 8.2 | 8.8 | 8.2 0.001 c.c....| 1.2 | 1.4 | 1.2 | 1.5 | 1.0 | 1.0 ------------------+--------+--------+--------+---------+---------+-------- Percentage of samples showing _bacillus coli_ in effluent: 10 c.c........| 19.5 | 24.1 | 29.5 | 24.2 | 31.7 | 34.3 1 c.c........| 10.0 | 13.4 | 12.0 | 13.8 | 15.4 | 16.1 0.1 c.c......| 1.9 | 3.1 | 3.8 | 4.2 | 1.5 | 4.5 0.01 c.c.....| 0 | 0 | 0 | 0 | 0 | 0 0.001 c.c....| 0 | 0 | 0 | 0 | 0 | 0 ------------------+--------+--------+--------+---------+---------+-------- Cost per million | | | | | | gallons for | | | | | | sand handling.....| $0.43 | $0.25 | $0.29 | $0.33 | $0.54 | $0.60 ------------------+--------+--------+--------+---------+---------+-------- Interest charges | | | | | | at 3%...........| 6.85 | 2.25 | 1.12 | 0.73 | 0.32 | 0.22 ------------------+--------+--------+--------+---------+---------+-------- Total.........| 7.28 | 2.50 | 1.41 | 1.06 | 0.86 | .82 ==================+========+========+========+=========+=========+========= _Coli_ tests presumptive.

DISCUSSION

~Allen Hazen, M. Am. Soc. C. E.~ (by letter).--This paper contains a most interesting and instructive record of the actual operation of a large filter plant, and also a record of a number of experiments. The author has described some useful arrangements for improving the efficiency or reducing the cost.

The utility of raking, as an intermediate treatment between scrapings, seems to have been clearly demonstrated. Its practical effect is to allow a greater quantity of water to be passed between scrapings, thereby saturating--if the term may be used--the surface layer with clay and other fine matter before removing it, instead of taking it off when only a thin surface layer of it has been thus saturated.

The large proportion of the total purification that takes place in passing through three reservoirs successively, holding in the aggregate a quantity of water equal to about 7 days' use, is very striking. Taking all the records, the percentage remaining after passing through these reservoirs, is as follows:

Sediment for the year, 1909-1910, Table 2....................17% Turbidities, 5-year average, Table 3.........................25% Bacteria, 5-year average, Table 4............................24% Bacteria, selected winter months with high numbers in the raw water...................................20% Bacteria, selected summer months with high numbers in the raw water................................... 2.5%

There is considerable seasonal fluctuation in the results of settling and filtering, as is shown in Table 21.

~Table 21--Average Removal of Turbidity and Bacteria by Washington Filters for Whole Period, Arranged by Seasons~. ===========================+========+========+========+========+======== | Winter.| Spring.| Summer.| Fall. | Year. ----------------+----------+--------+--------+--------+--------+-------- Turbidity, in | raw | 135 | 96 | 144 | 42 | 105 parts per | settled | 33 | 28 | 27 | 15 | 26 million: | filtered | 4 | 3 | 1 | 0.5 | 2 ----------------+----------+--------+--------+--------+--------+-------- Percentage | settling | 24 | 29 | 19 | 36 | 25 left from: | filtering| 12 | 10 | 4 | 3 | 8 | both | 3 | 1 | 0.3 | 1 | 2 ----------------+----------+--------+--------+--------+--------+-------- Bacteria per | raw | 16,600 | 4,150 | 4,100 | 1,960 | 6,700 cubic | settled | 6,300 | 980 | 160 | 270 | 1,940 centimeter: | filtered | 149 | 29 | 18 | 22 | 54 ----------------+----------+--------+--------+--------+--------+-------- Percentage | settling | 38 | 24 | 4 | 14 | 29 left from: | filtering| 2.4 | 3.0 | 11.2 | 8.2 | 2.8 | both | 0.90 | 0.79 | 0.44 | 1.12 | 0.81 ================+==========+========+========+========+========+========

The fluctuation in the efficiency of the plant as a whole by seasons is greater with the turbidity than with the bacteria. During the winter the effluent contains 3% of the turbidity of the raw water, and in summer only 0.3 per cent. Most of this difference is represented by the increased efficiency of the filters in summer, and only a little of it by the increased efficiency of settling. With bacteria, on the other hand, the seasonal fluctuation of the plant as a whole is comparatively small, but the settling and storage processes are much more efficient in summer than in winter, the filters being apparently less efficient. The writer believes that they are only apparently less efficient, and not really so, the explanation being that some bacteria always grow in the under-drains and lower parts of the filter, and are washed away by the effluent. The average number of bacteria in summer in the settled water is 160 per cu. cm. and in the filtered water 18. These are very low numbers. It is the writer's view that nearly all of these 18 represent under-drain bacteria, and practically bear no relation to those in the applied water, and, if this view is correct, the number of bacteria actually passing through the various processes is at all times less than the figures indicate. 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 sewage pollution of the Potomac is much less than that of the Merrimac and the Hudson, and it is perhaps not surprising that this relatively small amount of pollution was less potent in causing typhoid fever than the greater pollution of rivers draining more densely populated areas.

The method of replacing the washed sand hydraulically seems to have worked better than could have been reasonably anticipated, and the writer believes that this was due, in part, to the excellent method of manipulation described in the paper. It is his feeling, however, that part of the success is attributable to the very low uniformity coefficient of the sand. In other words, the sand grains are nearly all of the same size, due to the character of the stock from which the filter sand was prepared; and, therefore, there is much less opportunity for separation of the sand according to grain sizes than there would be with the filter sand which has been available in most other cases. 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. The capacity of the whole Washington Aqueduct system is reduced by whatever quantity is used in this way, and, in calculating the cost of sand handling, the value of the water used should be calculated on a basis which will cover the whole cost of the water, including all capital charges, depreciation, operating expenses, and all costs of every description. On this basis the water used in the sand-handling operations would probably be worth five or more times the sum mentioned by the author.

The cost of operation of the plant has come within the estimates made in advance, and has certainly been most reasonable. The cost of filter operations has averaged only about 50 cents per million gallons, and is so low that it is obvious that the savings which may be made by introducing further labor-saving appliances would be relatively small. It will be remembered that ten or fifteen years ago the cost of operating such filters under American conditions was commonly from $2 to $5 per million gallons.

The experiments represented by Tables 17 to 19, inclusive, serve to show that preliminary filtration, or multiple filtration, or any system of mechanical separation is incapable of entirely removing the finer clay particles which cause the residual turbidity in the effluent. They also show that this turbidity may be easily and certainly removed by the application of coagulant to the raw water during the occasional periods when its character is such as to require it.

These general propositions were understood by those responsible for the original design of the plant, as is shown by the author's quotations. These experiments, however, were necessary in order to demonstrate and bring home the conditions to those who thought differently, and who believed that full purification could be obtained by filtration alone, or by double filtration, without recourse to the occasional use of coagulant.

The experiments briefly summarized in Table 20 are of the greatest interest and importance. Six small filters, otherwise alike and like the large filters, all received the same raw water and were operated at different rates to determine the effect of rate on efficiency.

That the experimental results from the filter operating at the same rate as the large filters were on the whole somewhat inferior to those from the large filters for approximately the same period, may be attributed to the fact that the experimental filter was new while the large filters had been in service for some time and had thereby gained in efficiency. The greatest difference was in the _coli_ results in Table 20, where it is shown that 24% of the 10-cu. cm. effluent samples from the experimental filter contained _coli_, in comparison with only from 1 to 3% of such samples from the main filters.

The results from the experimental filter operating at a rate of 1,000,000 gal. per acre daily may fairly be excluded, as the effluent probably contained more under-drain bacteria in proportion than filters operated at higher rates. The number of bacteria in the filter operating at a 3,000,000-gal. rate were 1.7% of those in the applied water; for the filter operating twice as fast, the percentage was 2.4; and, for the one operating more than ten times as fast, was only 3.0; thus indicating a surprisingly small increase in the number of bacteria with increase in rate.

Further and more detailed study by the writer of the unpublished individual results, briefly summarized in Table 20, confirms the substantial accuracy of the comparison based on the average figures as stated in that table.

It must be kept in mind, in considering these results, that the number of bacteria in each case is made up of two parts, namely, those coming through the filter--which number is presumably greater as the rate is greater--and, second, those coming from harmless growths in the under-drains and lower parts of the filter--the numbers of which per cubic centimeter are presumably less as the rate is greater--and these two parts, varying in opposite directions, may balance each other, as they seem to do in this case, through a considerable range. It may thus be that the number of bacteria really passing the filter varies much more with the rate than is indicated by the gross results.

It is also of interest to note that the sand filter (called a preliminary filter) in Table 18, filled with the same kind of sand, when operated at an average rate of 50,000,000 gal. per acre daily for a year, allowed 18% of the applied bacteria to pass, in comparison with 3% found in Filter No. 6 of Table 20, operated at an average rate of 38,000,000 gal. per acre daily.

There was one point of difference in the manipulation: the preliminary filter was washed by a reversed current of water, as mechanical filters are washed, while Filter No. 6 was cleaned by scraping off the surface layer, as is usual with sand filters. Whether the great difference in bacterial results with a relatively small difference in rate is to be attributed to this difference in manipulation the writer will not undertake to state.

If the experimental results of Table 20 indicate correctly the conditions which obtain in filtering Potomac water, then increasing the rate of filtration so as to double it, or more than double it, would make but little difference in the quality of the effluent as measured by the usual bacterial methods. If the increase in rate were accompanied by the preliminary filtration of the water, then, presumably, there would be little change in the quality of the effluent, and the maintenance of excellent results might be incorrectly attributed to the influence of the preliminary filter.

It would also seem that the apparatus which is sometimes used for determining and controlling the rate with more than the ordinary degree of precision is hardly justified by such experimental results as those presented by the author.

In contrast to these results may be mentioned those obtained by Mr. H. W. Clark,[1] for experimental filters operated with Merrimac River water, at rates ranging from 3,000,000 to 16,000,000 gal. per acre daily. The results are the average of nearly two years of experimental work, the period having been nearly coincident with that covered by the author's experiments, and of many hundreds of bacterial analyses of each effluent, and form, with the author's experiments, the most thorough-going studies of the effect of rate on efficiency that have come to the writer's attention.

Mr. Clark's results are given in Table 22.

~Table 22.~ ===========+============+===========+=============+===========+============ | | | | | _B. Coli_ | | | | |in 1 cu.cm. Effective | | Rate | Bacteria per| |(percentage size of | | in gallons| cubic | Bacterial |of positive sand. | Filter No. |acre daily.|centimeter in|efficiency.| tests). -----------+------------+-----------+-------------+-----------+------------ 0.28 | A | 3,000,000 | 48 | 99.1 | 5.0 0.25 | B | 5,000,000 | 85 | 98.4 | 24.0 0.22 | C | 7,500,000 | 105 | 98.1 | 25.0 0.22 | D |10,000,000 | 110 | 98.0 | 25.0 0.22 | E |16,000,000 | 280 | 95.0 | 38.0 ===========+============+===========+=============+===========+============

It will be seen that the number of bacteria passing increases rapidly with the rate, and whether the total number of bacteria is considered or the _B. coli_ results, the number passing is approximately in proportion to the rate. In other words, doubling the rate substantially doubles the number of bacteria in the effluent.

This is entirely in harmony with all the Lawrence experimental results extending over a period of 20 years. There have been occasional apparent exceptions, but, on the whole, experience with Merrimac River water has uniformly been that more bacteria pass as the rates are higher.

The theory sometimes advanced, that the efficiency of filtration is controlled to a certain extent by gelatinous films, and that, as far as thus controlled, is less dependent on rate, would not seem to be borne out by these results. The Merrimac River water, carrying large amounts of organic matter, would certainly seem better adapted to the formation of such films than the clay-bearing Potomac water, comparatively free from organic matter; but it is the Potomac water which seems to show the least influence of rate on efficiency.

[Footnote 1: _Journal_, New England Water-Works Association, Vol. 24, p. 589.]

The experiments show that turbidity passes more freely at the higher rates with the Potomac water, as has also been found to be the case with other clay-bearing waters.

In the last lines of Table 20 are given cost per million gallons for filtering at various rates. There is no discussion of these figures, and as they differ considerably from those which the writer has been accustomed to use, the calculation in Table 23, made three years ago for a particular case, may be of interest.