Chapter X: Mechanical Filters

The term mechanical filters is used to designate a general class of filters differing in many respects quite radically from the sand filters previously described. They had their origin in the United States, and consisted originally of iron or wooden cylinders filled with sand through which the water was forced at rates of one to two hundred million gallons per acre daily, or from fifty to one hundred times the rates usually employed with sand filters. These filters were first used in paper-mills to remove from the large volumes of water required the comparatively large particles, which would otherwise affect the appearance and texture of the paper; and in their earlier forms they were entirely inadequate to remove the finer particles, such as the bacteria, and the clay particles which constitute the turbidity of river waters. Various improvements in construction have since been made, and, in connection with the use of coagulants, much more satisfactory results can now be obtained with filters of this class; and their use has been extended from manufacturing operations to municipal supplies, in many cases with most satisfactory results.

The information gathered in regard to the conditions essential to the successful design and operation of these filters in the last few years is very great, and may be briefly reviewed.

PROVIDENCE EXPERIMENTS.[39]

The first data of importance were secured from a series of experiments conducted by Mr. Edmund B. Weston of Providence, R. I., in 1893 and 1894, upon the Pawtuxet river water used by

that city. The experimental filter was 30 inches in diameter, and had a layer of sand 2 feet 10 inches deep. The sand was washed by the use of a reverse current, the sand being stirred by a revolving rake at the same time. The amount of coagulant employed was about 0.7 of a grain per gallon. The raw water was practically free from turbidity, and the filter was operated to remove color and bacteria.

The removal of color, as stated in Mr. Weston’s report, amounted to from 70 to 90 per cent. The experiments extended over a period of ten months. The rate of filtration employed was about 128 million gallons per acre daily. The bacterial results of the first six months’ operations were rejected by Mr. Weston on account of defective methods of manipulation.

During the period from November 17, 1893, to January 30, 1894, the average bacterial efficiency of filtration was about 95 per cent, and the manipulation was considered to be in every respect satisfactory. The efficiency was occasionally below 90 per cent, but for four selected weeks was as high as 98.6 per cent. The average amount of sulphate of alumina used, as calculated from Mr. Weston’s tables, was two thirds of a grain per gallon. The highest efficiency followed the application of a solution of caustic soda to the filtering material. The first day following this treatment the bacterial efficiency was above 99 per cent. Afterwards it decreased until January 30, when the experiments were stopped. The high bacterial efficiency following the use of caustic soda was of such short duration as to suggest very grave doubts as to its practical value. It is extremely unfortunate that the experiments stopped only a week after this experiment, and the results were never repeated. I consider that the average bacterial efficiency of about 95 per cent obtained for the period of October 17 to January 30, when the manipulation was considered to be in every way satisfactory, more nearly represents what can be obtained under these conditions than the results for certain periods, particularly after the use of the caustic soda.

LOUISVILLE EXPERIMENTS.[40]

These experiments were inaugurated by the Louisville Water Company in connection with the manufacturers of certain patented filters. Mr. Charles Hermany, Chief Engineer of the Company, had general charge of the experiments. Mr. George W. Fuller was Chief Chemist and Bacteriologist and had direct charge of the work and has made a most elaborate report upon the same. In these examinations many devices were investigated; but the two which particularly deserve our attention are the filters known as the Warren Filter and the Jewell Filter.

These filters were operated for two periods, namely, from October 18, 1895, to July 30, 1896, and from April 5 to July 24, 1897. The investigations were directed toward the clarification of the river water from the mud, and to the removal of bacteria. The water was substantially free from color. The character of the water at this point was such that in its best condition at least three fourths of a grain of sulphate of alumina were necessary for its coagulation, and with this and with larger quantities of coagulant fair bacterial purification was nearly always obtained. The problem studied therefore was principally that of clarification from mud. The average efficiencies, as shown by the total averages, (page 248,) were as follows: Warren filter, bacterial efficiency, 96.7 per cent; Jewell filter, 96.0 per cent.

LORAIN TESTS.[41]

These tests were made by the author of a set of Jewell filters at Lorain, Ohio. The filters were six in number, each 17 feet in diameter, having an effective filtering area of 226 square feet each, or 1356 square feet in all. The construction of the filters was in all respects similar to the Jewell filter used at Louisville. The raw water was from Lake Erie, and during the examination was

always comparatively clear, but contained considerable numbers of bacteria. The problem was thus entirely one of bacterial efficiency. The question of clarification hardly presented itself. Although the water became turbid at times it did not approach in muddiness the condition of the Ohio River water, and an amount of coagulant sufficient for a tolerable bacterial efficiency in all cases was more than sufficient for clarification.

A summary of the results obtained is as follows:

-----------+--------------+-----------+--------+---------+----------
| Average Rate |Sulphate of| | |Bacterial
Week Ending|of Filtration,| Alumina, |Bacteria|Bacteria |Efficiency
6:00 P.M. | Gallons per |Grains per | in Lake| in |per cent.
| Sq. Ft. Min. |Gallon. | Water. |Effluent.|
-----------+--------------+-----------+--------+---------+----------
June 19 | 1.06 | 2.58 | 1441 | 16 | 98.9
26 | 1.10 | 2.50 | 385 | 6 | 98.4
July 3 | 1.11 | 2.27 | 367 | 9 | 97.5
10 | 1.28 | 1.07 | 154 | 14 | 90.9
17 | 1.14 | 0.94 | 189 | 26 | 86.3
+--------------+-----------+--------+---------+----------
Average | 1.14 | 1.83 | 507 | 14 | 96.4
-----------+--------------+-----------+--------+---------+----------

The average bacterial efficiency was 96.4 per cent with 1.83 grains of sulphate of alumina per gallon.

PITTSBURG EXPERIMENTS.[42]

The Pittsburg experiments were inaugurated by the Pittsburg Filtration Commission. The operation of the filters extended from January to August, 1898. A Jewell and a Warren filter were used similar in design to those used at Louisville. The raw water contained large numbers of bacteria, and was also often very turbid, although less turbid than at Louisville. At times more coagulant was necessary for clarification than was required for bacterial efficiency; while as a rule more was required for satisfactory bacterial purification than was necessary for clarification. The opportunities were therefore favorable for the study of both of these conditions. The amount of coagulant necessary for clarification has been mentioned in connection with coagulation.

The results secured upon the relation of the quantity of

coagulant to the number of bacteria in the effluent were more complete than any other experiments available, and are therefore here reproduced from the Pittsburg report nearly in full.

It was found that the amount of sulphate of alumina employed was more important than any other factor in determining the bacterial efficiency, and special experiments were made to establish the effect of more and of less coagulant than used in the ordinary work. These experiments were made upon the Warren filter during May, and with the Jewell filter during June. The monthly averages for these months are thus abnormal and are not to be considered. The remaining six months for each filter may be taken as normal and as representing approximately the work of these filters under ordinary careful working conditions.

During the six months when the Warren filter was in normal order the raw water contained 11,531 bacteria and the effluent 201, the average bacterial efficiency being 98.26 per cent. The bacterial efficiency was very constant, ranging only, by months, from 97.48 to 98.96 per cent. During the same period a sand filter receiving the same water yielded an effluent having an average of 105 bacteria per cubic centimeter.

The Jewell filter, for the six months in which it was in normal order, received raw water containing an average of 11,481 bacteria and yielded an effluent containing an average of 293, the bacterial efficiency being 97.45 per cent, and ranging, in different months, from 93.23 to 98.61 per cent.

WASTING EFFLUENT AFTER WASHING FILTERS.

After washing a mechanical filter the effluent for the first few minutes is often inferior in quality to that obtained at other times, and if samples are taken at these times and averaged with other samples taken during the run, an apparent efficiency may be obtained inferior to the true efficiency. To guard against this source of error, whenever samples have been taken at such times, the average work for the day has been taken, not as the numerical average of the results, but each sample has been given weight in proportion to the amount of time which it could be taken as representing; so that the results represent as nearly as possible the average number of bacteria in the effluent for the whole run. As a matter of fact, however, comparatively few samples were taken during these periods of reduced efficiency, and thus most of the results represent the normal efficiency exclusive of this period. A study has been made, however, of the results of examinations of samples taken directly after washing, somewhat in detail. The following is a tabular statement of the average results obtained from each filter by months, including only the results obtained on those days when samples were taken within twenty minutes after washing, the results of other days being excluded.

AVERAGE NUMBER OF BACTERIA IN EFFLUENT.

--------------+------------+-------------+-------------+-------------
| Shown by | Within Ten | 11 to 20 | More than
| Record |Minutes after|Minutes after| Twenty
| Sheets. | Washing. | Washing. |Minutes after
| | | | Washing.
--------------+------------+-------------+-------------+-------------
WARREN FILTER.| | | |
February | 115 | | 118 | 114
March | 316 | 50 | 515 | 301
April | 79 | 417 | 207 | 75
May | (Special experiments, omitted.)
June | 197 | 493 | 272 | 170
July | 300 | | 546 | 207
August | 174 | 356 | 601 | 223
| | | |
JEWELL FILTER.| | | |
February | 2453 | 2425 | | 2099
March | 455 | 657 | 958 | 354
April | 99 | 665 | 462 | 165
May | 144 | 998 | 346 | 127
June | (Special experiments, omitted.)
July | 279 | 1330 | 272 | 274
August | 344 | 612 | 323 | 376
--------------+------------+-------------+-------------+-------------

The time of inferior work very rarely exceeded twenty minutes. It will be seen from the tables that the results as shown by the record sheets are never very much higher, and are occasionally lower than the results of samples taken on corresponding days more than twenty minutes after washing; and thus while a decrease in bacterial efficiency was noted after washing, no material increase in the average bacterial efficiency of the mechanical filters would have been obtained if these results had been excluded. The results for the whole time would be affected much less than is indicated by the table, because the table includes only results of those days when samples were taken just after washing, while the much larger number of days when no such samples were taken would show no change whatever.

It has been suggested that these inferior effluents after washing should be wasted. Such a procedure would mean wasting probably on an average two per cent of the water filtered, and a corresponding increase in the cost of filtering. Mr. Fuller[43] in his Louisville report comes to the conclusion that with adequate washing and coagulation it is unnecessary to waste any effluent, and that inferior results after washing usually indicate incomplete washing. While our experiments certainly indicate a reduction in efficiency after washing so regular and persistent as to make it doubtful whether incomplete washing can be the cause of it, it may be questioned whether or not wasting the effluent would be necessary or desirable in actual operation. At any rate the results as given in this report are not materially influenced by this factor.

INFLUENCE OF AMOUNT OF SULPHATE OF ALUMINA ON BACTERIAL EFFICIENCY OF MECHANICAL FILTERS.

The number of bacteria passing a mechanical filter is dependent principally upon the amount of sulphate of alumina used; and by using a larger quantity of sulphate of alumina than was actually used in the experiments the bacterial efficiency could be considerably increased. To investigate this point, the results obtained each day with each of the mechanical filters were arranged in the order of the sulphate of alumina quantities used, and averaged by classes. In this and the following tables a few abnormal results were omitted.[44] A summary of the results is as follows:

SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING TO SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
Number | | Bacteria. | | |Sulphate of
of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina
Represented.| |Raw Water.|Effluent.|remaining.|removed.|used Grains
| | | | | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
7 | 0.05 | 4,773 | 1713 | 35.89 | 64.11 | 0.00
2 | 0.08 | 2,785 | 850 | 30.52 | 69.48 | 0.12
4 | 0.10 | 5,109 | 726 | 14.21 | 85.79 | 0.26
2 | 0.20 | 8,713 | 214 | 2.45 | 97.55 | 0.36
8 | 0.06 | 3,224 | 112 | 3.47 | 96.53 | 0.44
19 | 0.06 | 3,488 | 123 | 3.53 | 96.47 | 0.55
11 | 0.06 | 5,673 | 154 | 2.71 | 97.29 | 0.64
10 | 0.10 | 6,100 | 112 | 1.84 | 98.16 | 0.74
8 | 0.09 | 8,647 | 148 | 1.71 | 98.29 | 0.85
5 | 0.16 | 5,645 | 142 | 2.52 | 97.48 | 0.93
13 | 0.12 | 10,397 | 200 | 1.92 | 98.08 | 1.07
10 | 0.08 | 12,778 | 121 | 0.95 | 99.05 | 1.13
13 | 0.14 | 13,397 | 164 | 1.22 | 98.78 | 1.25
19 | 0.13 | 10,462 | 160 | 1.53 | 98.47 | 1.34
10 | 0.12 | 12,851 | 107 | 0.83 | 99.17 | 1.46
4 | 0.27 | 16,015 | 77 | 0.48 | 99.52 | 1.57
7 | 0.53 | 12,262 | 191 | 1.18 | 98.82 | 1.64
4 | 0.58 | 26,950 | 347 | 1.29 | 98.71 | 1.74
5 | 0.29 | 14,570 | 86 | 0.59 | 99.41 | 1.84
3 | 0.23 | 13,833 | 153 | 1.11 | 98.89 | 1.92
19 | 0.40 | 18,222 | 92 | 0.50 | 99.50 | 2.48
5 | 0.45 | 29,300 | 1119 | 3.82 | 96.18 | 3.37
5 | 1.06 | 33,030 | 535 | 1.62 | 98.38 | 8.06
------------+----------+----------+---------+----------+--------+-----------

SUMMARY OF RESULTS WITH JEWELL MECHANICAL FILTER, ARRANGED ACCORDING TO SULPHATE OF ALUMINA QUANTITIES.

------------+----------+----------+---------+----------+--------+-----------
Number | | Bacteria. | | |Sulphate of
of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina
Represented.| |Raw Water.|Effluent.|remaining.|removed.|used Grains
| | | | | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
6 | 0.03 | 14,037 | 6217 | 44.29 | 55.71 | 0.00
5 | 0.07 | 4,267 | 680 | 15.93 | 84.07 | 0.24
14 | 0.06 | 2,613 | 170 | 6.50 | 93.50 | 0.35
10 | 0.06 | 2,446 | 113 | 4.62 | 95.38 | 0.44
9 | 0.11 | 7,303 | 234 | 3.20 | 96.80 | 0.55
20 | 0.09 | 6,979 | 220 | 3.15 | 96.85 | 0.65
9 | 0.08 | 5,191 | 130 | 2.50 | 97.50 | 0.75
16 | 0.12 | 8,504 | 242 | 2.84 | 97.16 | 0.83
22 | 0.16 | 8,506 | 99 | 1.16 | 98.84 | 0.96
12 | 0.11 | 11,998 | 246 | 2.05 | 97.95 | 1.05
14 | 0.18 | 18,982 | 423 | 2.23 | 97.77 | 1.16
5 | 0.14 | 13,981 | 224 | 1.60 | 98.40 | 1.23
9 | 0.27 | 19,806 | 325 | 1.64 | 98.36 | 1.34
14 | 0.27 | 16,549 | 324 | 1.96 | 98.04 | 1.45
9 | 0.29 | 12,194 | 96 | 0.79 | 99.21 | 1.54
6 | 0.25 | 13,483 | 51 | 0.38 | 99.62 | 1.65
7 | 0.53 | 24,243 | 220 | 0.91 | 99.09 | 1.72
3 | 0.90 | 20,953 | 602 | 2.88 | 97.12 | 1.90
5 | 0.43 | 25,958 | 307 | 1.19 | 98.81 | 2.19
4 | 0.84 | 21,017 | 228 | 1.09 | 98.91 | 3.71
------------+----------+----------+---------+----------+--------+-----------

These results are shown graphically by Fig. 21.

INFLUENCE OF DEGREE OF TURBIDITY UPON BACTERIAL EFFICIENCY OF MECHANICAL FILTERS.

It will be noticed by referring to the tables that as the sulphate of alumina quantities increased the turbidities increased and the numbers of bacteria increased, as well as the bacterial efficiencies. That is to say, with the less turbid waters, small sulphate of alumina quantities have been used, the numbers of bacteria in the raw water have been low, and the bacterial efficiencies have also been low. With turbid waters much larger quantities of sulphate of alumina have been used, the raw water has contained more bacteria, and the bacterial efficiencies have been higher. It may be then that the increased efficiencies with increased quantities of sulphate of alumina are not due alone to the increased sulphate of alumina, but in part also to other conditions. Thus it may be easier to remove a large percentage of bacteria from a water containing many than from a water containing only a few.

To investigate this matter and eliminate the influence of turbidity and numbers of bacteria in the raw water, the results were first classified with reference to turbidity. The results with waters having turbidities of 0.10 or less, and called for convenience turbid waters, are arranged by alum quantities as before. Afterwards the results obtained with turbidities from 0.11 to 0.50, and called for convenience muddy waters, are grouped; and finally the results with turbid water having turbidities of 0.51 and over, and called for convenience thick waters. The results thus arranged are as follows:

SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING TO TURBIDITIES AND SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate of
Number | | Bacteria. | Per cent |Per cent| Alumina
of Days |Turbidity.+----------+---------+remaining.|removed.|used Grains
Represented.| |Raw Water.|Effluent.| | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
7 | 0.05 | 4,773 | 1713 | 35.89 | 64.11 | 0.00
2 | 0.07 | 2,785 | 850 | 30.52 | 69.48 | 0.12
12 | 0.06 | 3,209 | 224 | 7.00 | 93.00 | 0.42
31 | 0.06 | 4,238 | 119 | 2.81 | 97.19 | 0.60
9 | 0.06 | 7,953 | 130 | 1.64 | 98.36 | 0.84
16 | 0.04 | 11,265 | 137 | 1.22 | 98.78 | 1.11
29 | 0.06 | 11,500 | 158 | 1.37 | 98.63 | 1.58
| | | | | |
5 | 0.17 | 8,783 | 416 | 4.73 | 95.27 | 0.36
10 | 0.16 | 6,535 | 165 | 2.54 | 97.46 | 0.85
13 | 0.19 | 13,253 | 186 | 1.40 | 98.60 | 1.13
15 | 0.22 | 10,944 | 93 | 0.85 | 99.15 | 1.36
13 | 0.29 | 14,089 | 112 | 0.80 | 99.20 | 1.73
10 | 0.35 | 18,088 | 102 | 0.57 | 99.43 | 2.38
5 | 0.29 | 25,580 | 540 | 2.11 | 97.89 | 4.30
| | | | | |
6 | 0.87 | 25,433 | 369 | 1.45 | 98.55 | 1.74
6 | 0.73 | 26,566 | 79 | 0.30 | 99.70 | 2.64
4 | 1.35 | 42,037 | 1388 | 3.30 | 96.70 | 8.16
------------+----------+----------+---------+----------+--------+-----------

SUMMARY OF RESULTS WITH JEWELL MECHANICAL FILTER, ARRANGED ACCORDING TO TURBIDITIES AND SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate of
Number | | Bacteria. | | | Alumina
of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains
Represented.| |Raw Water.|Effluent.|remaining.|removed.|per Gallon.
------------+----------+----------+---------+----------+--------+-----------
6 | 0.03 | 14,037 | 6217 | 44.29 | 55.71 | 0.00
3 | 0.07 | 5,170 | 991 | 19.15 | 80.85 | 0.21
25 | 0.05 | 2,403 | 143 | 5.95 | 94.05 | 0.38
20 | 0.06 | 6,531 | 185 | 2.84 | 97.16 | 0.64
27 | 0.06 | 5,811 | 122 | 2.10 | 97.90 | 0.88
14 | 0.06 | 14,978 | 412 | 2.75 | 97.25 | 1.11
10 | 0.06 | 15,787 | 390 | 2.47 | 97.53 | 1.37
10 | 0.05 | 10,847 | 47 | 0.43 | 99.57 | 2.17
| | | | | |
14 | 0.16 | 7,525 | 256 | 3.40 | 96.60 | 0.60
17 | 0.24 | 11,310 | 208 | 1.84 | 98.16 | 0.91
15 | 0.24 | 15,441 | 262 | 1.70 | 98.30 | 1.13
10 | 0.28 | 17,842 | 232 | 1.30 | 98.70 | 1.43
8 | 0.29 | 9,556 | 59 | 0.62 | 99.38 | 1.59
4 | 0.29 | 20,212 | 135 | 0.67 | 99.33 | 2.00
| | | | | |
5 | 0.66 | 23,680 | 336 | 1.42 | 98.58 | 1.42
7 | 0.96 | 30,200 | 475 | 1.57 | 98.43 | 1.74
4 | 1.25 | 37,587 | 496 | 1.32 | 98.68 | 2.81
------------+----------+----------+---------+----------+--------+-----------

The following table shows the bacterial efficiencies with turbid, muddy, and thick waters, with substantially equal quantities of sulphate of alumina:

-------------------------------+-------------------------------------
Grains of Sulphate of Alumina.|Corresponding Bacterial Efficiencies.
----------+----------+---------+-----------+----------+--------------
Turbid. | Muddy. | Thick. | Turbid. | Muddy. | Thick.
----------+----------+---------+-----------+----------+--------------
WARREN FILTER.
0.42 | 0.36 | | 93.00 | 95.27 |
0.84 | 0.85 | | 98.36 | 97.46 |
1.11 | 1.13 | | 98.78 | 98.60 |
1.58 | 1.73 | 1.74 | 98.63 | 99.20 | 98.55
| 2.38 | 2.64 | | 99.43 | 99.70
| 4.30 | 8.16 | | 97.89 | 96.70

JEWELL FILTER.
0.64 | 0.60 | | 97.16 | 96.60 |
0.88 | 0.91 | | 97.90 | 98.16 |
1.11 | 1.13 | | 97.25 | 98.30 |
1.37 | 1.43 | 1.42 | 97.53 | 98.70 | 98.58
2.17 | 1.59 | 1.74 | 99.57 | 99.38 | 98.43
| 2.00 | 2.81 | | 99.33 | 98.68
----------+----------+---------+-----------+-------------------------

It appears from this table that waters of various degrees of turbidity give substantially equal bacterial efficiencies with equal quantities of sulphate of alumina, the results varying as often in one direction as the other. Within certain limits it may thus be said that turbidity is without influence upon the bacterial efficiency obtained in mechanical filtration.

It must be borne in mind, however, that the quantities of sulphate of alumina, with very few exceptions, were sufficient to produce full coagulation. Mr. Fuller has shown in his Louisville report that considerable quantities of sulphate of alumina may be added to turbid waters without producing appreciable coagulation; and therefore if a quantity of sulphate of alumina sufficient to produce a certain bacterial efficiency in a clear water should be added to a water so turbid that it was unable to coagulate it, scarcely any effect would be produced. The above statement therefore only applies in those cases where sufficient sulphate of alumina is used to adequately coagulate the water.

As the numbers of bacteria often vary with the turbidity, the variation in the numbers of bacteria in the different classes is much less than in the first tables; but to further investigate the question of whether the numbers of bacteria in the raw water have an important influence upon the bacterial efficiencies, each of the two largest classes in the foregoing tables was divided into two parts, according to the bacterial numbers in the raw water, namely, the results from the Jewell filter with turbid waters and with sulphate of alumina quantities ranging from 0.75 to 1.00 grain per gallon, and the results from the Warren filter with turbid waters and with sulphate of alumina quantities of 1.25 grains per gallon and upward. The results are as follows:

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate
Number | | Bacteria. | | |of Alumina
of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains
Represented.| |Raw Water.|Effluent.|remaining.|removed.|per Gallon.
------------+----------+----------+---------+----------+--------+-----------
JEWELL FILTER.
14 | 0.05 | 3,938 | 81 | 2.06 | 97.94 | 0.88
13 | 0.07 | 7,827 | 167 | 2.13 | 97.87 | 0.87
WARREN FILTER.
15 | 0.06 | 3,545 | 59 | 1.66 | 98.34 | 1.67
14 | 0.06 | 20,022 | 265 | 1.32 | 98.68 | 1.48
------------+----------+----------+---------+----------+--------+-----------

It will be observed that the bacterial efficiencies are substantially the same, with the lower and with the higher numbers of bacteria in the raw water. That is to say, other things being equal, as the number of bacteria increase in the raw water the number of bacteria in the effluent increase in the same ratio. A further analysis of other groups of results would perhaps show variations in one direction or the other, but on the whole it is believed that the comparison is a fair one, and that there is no well-marked tendency for bacterial efficiencies of mechanical filters to increase or decrease with increasing numbers of bacteria.

AVERAGE RESULTS OBTAINED WITH VARIOUS QUANTITIES OF SULPHATE OF ALUMINA.

As it appears that neither the turbidity nor the number of bacteria in the raw water has a material influence upon the percentage bacterial efficiency obtained, we can take the results given above, which include all the results obtained (except a very few abnormal ones) for computing the various efficiencies obtained with various quantities of sulphate of alumina. These results are graphically shown by Fig. 21, p. 167, on which lines have been drawn indicating the normal efficiencies from various quantities of sulphate of alumina as deduced from our experiments.

In computing the amount of sulphate of alumina which it would be necessary to use in operating a plant at a given place to give these efficiencies, the quantities of sulphate of alumina shown by the diagram can be taken as those which it would be necessary to use during those days in the year when the raw water was clear, or sufficiently clear, so that the amounts of sulphate of alumina mentioned would suffice to properly coagulate it.

TYPES OF MECHANICAL FILTERS.

Sections of the Warren and Jewell filters used at Pittsburg are presented herewith. The filters here shown are practically identical with those used at Lorain and Louisville, and nearly all the exact information regarding mechanical filters relates to filters of these types. These sections show clearly the constructions used at Pittsburg and Louisville, but there are some points in connection with the designs of these filters which require to be considered more in detail.

The simplest idea of a mechanical filter is a tub, with sand in the bottom and some form of drainage system. Water is run over the sand, passes through it, and is collected by the drainage system. When the sand becomes clogged it is washed by the use of a reverse current of water. This reverse current of water is so rapid as to preclude the use of a drainage system consisting of gravel, tile-drains, etc., such as are used in sand filters operated at lower rates, and instead metallic strainers in some form are used. The sand comes directly against these strainers, which are made as coarse as it is possible to have them, without allowing the sand to pass.

The rate of washing is usually from five to seven gallons per square foot per minute. In the Warren filter the openings in the strainers at the bottom are 6 to 8 per cent of the total area, and during washing the water has an average velocity of 0.20 foot per second upward through them. This velocity is so slow that the friction of the water in passing through the openings in the screen is practically nothing. A result of this is that if there is any unequal resistance of the sand to the water, the bulk of the water goes up at the points of least resistance in the sand.

This tendency would be fatal were it not for the revolving rake which loosens and mixes the sand and largely corrects it. The correction, however, is imperfect, and some parts of the filter are washed more than others.

The rake is also necessary to prevent the separation of sand into coarser and finer particles. It is practically impossible to get filter sand the grains of which are all of the same size. When a filter is washed the tendency is for the wash water to go up in limited areas. The larger sand grains tend to collect at these points while the finer grains collect in places where there is no upward current, or where it is less rapid. In many filters this tendency is very strong. The revolving rake is necessary to correct it, and to keep the sand thoroughly mixed, otherwise when a filter is put in operation after washing, the frictional resistance through the coarse sand being less, the bulk of the water goes through it, with the result that a part of the area, and the part which is least efficient as a filter, passes nearly all of the water, and with inferior results.

In the Jewell filter provision is made for the distribution of the wash water over the whole area in another way. The strainers have areas at the surface amounting to 1.2 to 1.4 per cent of the whole area, but the water before reaching them passes through throats much smaller in size than the strainer outlets, and amounting in the aggregate to only about 0.07 per cent of the filter area. When washing at a rate of seven gallons per square foot per minute, water passes through these necks at a velocity of 22 feet per second. The friction and velocity head in passing these necks is estimated to be about 30 vertical feet, and is so much greater than the friction of the outlets proper, and of the sand, that the water passes through each strainer with approximately the same velocity, and the wash water is equally distributed over the whole area of the bottom of the filter.

This result is accomplished, however, at a great loss of head in the wash water. When a filter is washed from the pressure-mains without separate pumping, the pressure is usually sufficient and there is no disadvantage in the arrangement. When, however, the water is specially pumped for washing, the required head is much greater than would otherwise be necessary.

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It would not be possible to increase the size of the necks, thereby decreasing the friction, without increasing very largely the size of the pipes in the underdrainage system into which the strainers are fastened. These pipes are so small that during washing the velocity in them is about 13 feet per second, and if the throats of the necks were increased without also enlarging these pipes, the friction would be so reduced that most of the water would go through the necks nearest the supply, thus failing to reach the object to be attained.

A more rational system would be to increase the sizes of all the waterways in the outlet and wash-water system. The Jewell filter is also provided with a rake to keep the sand mixed during washing, as this is necessary even with the complete distribution of wash-water over the area of the filter.

Both the Warren and the Jewell filters are provided with receptacles through which the water passes after receiving the coagulant, and before entering the filter. In the Jewell filter the receptacle, called a sedimentation-basin, is of such size as to hold as much water as is filtered in 15 minutes. In the Warren filter the receptacle is entirely independent and larger, holding about an hour’s supply.

The rates of filtration used in the experiments have ranged from less than 100 to about 130 million gallons per acre daily. To employ a rate much higher than this involves the use of a much coarser sand, or an increase in the height of water upon the filter to an impracticable extent. There would seem to be no material advantage in the use of lower rates within certain limits, while the cost of filters would be greatly increased.

The sand used in the Warren filters has been crushed quartz. In the Jewell filters a silicious sand from Red Wing, Minn., with rounded grains has been used. These sands are somewhat coarser than are commonly used in sand filters, and the uniformity coefficients are very low. It is necessary to use sand with the very lowest uniformity coefficients to avoid the separation of sand particles according to sizes as mentioned above, and for this reason the sand must be selected with much greater care than is required for sand filters.

SECTION SHOWING FILTER DURING ORDINARY OPERATION.

FIG. 23.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 1.]

SECTION SHOWING FILTER DURING OPERATION OF WASHING.

FIG. 24.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 2.]

The round-grained sand is more readily and completely washed than the angular crushed quartz. It has been claimed that the crushed quartz is more efficient as a filtering material, but the evidence of this is not very clear.

The amount of water filtered by a filter between washings is, in a general way, about the same as that filtered by a sand filter between scrapings, in relation to its area. The amount of water required for washing is, on an average, about equal to a vertical column 5 or 6 feet high equal in area to the area of the filter, exclusive of water on the top of the filter wasted before the current is reversed. With clear waters, as for instance, the Allegheny at low water, the amount of washing is almost directly proportional to the amount of sulphate of alumina used. With muddy waters the sulphate of alumina required is proportional to the mud, and the frequency of washing and the amount of wash-water are proportional to both. The amount of wash-water required averages about five per cent; with very muddy waters more is required. At Louisville, with the worst waters, the per cents of wash-water rose at times to 30 per cent of the total quantity of water filtered.

The rate of filtration with mechanical filters should be kept as constant as possible, and can be regulated by devices similar to those described in connection with sand filters. Owing to the smaller areas and capacities, the amounts of water to be handled in the units are smaller, and the regulating devices are thus smaller, and have always been made of metal, either cast iron or copper. None of the devices employed in the above-mentioned experiments has been entirely satisfactory in this respect. The devices employed have been too small, and the water has gone through at too high velocities to allow close adjustment.

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As between the two types of filters, the Jewell filter requires a large loss of head. The water has to be pumped at a sufficient elevation to reach the top of a tank about 18 feet high, while the effluent must be drawn off at the extreme bottom. The Warren filter is much more economical in head, the plants at Pittsburg and Louisville only requiring about 9 feet from the inlet to the outlet.

The earlier mechanical filters were usually constructed of wrought iron or steel plates. More recently wooden tanks have been commonly employed, although steel is regarded as preferable. Concrete or masonry tanks have been suggested, but they have not as yet been employed.

EFFICIENCY OF MECHANICAL FILTERS.

The efficiency of mechanical filters depends entirely upon the use of coagulants. Without coagulants they can only be used to remove very large particles. The efficiency of the filtration depends much more upon the kind, and amount, and method of application of coagulant than upon the arrangement of the filter. In fact, the arrangements of the filter are more directed to the convenience and economy of operation and washing than towards the efficiency of the results.

The conditions which control the efficiency of mechanical filters have been discussed in connection with coagulation. With sufficient coagulant the removal of turbidity or mud is complete. Color also can be removed with these filters. The bacterial efficiencies secured with them have been discussed at length in connection with the Pittsburg experiments.

With careful coagulation and manipulation it is possible to get 98 per cent bacterial efficiency without difficulty. The results are somewhat irregular, for reasons not as yet fully understood. On some occasions higher bacterial efficiencies are secured with smaller quantities of coagulant, while at other times the efficiencies are less without apparent reason. There seems to be a limit to the bacterial efficiency which can be secured with any amount of sulphate of alumina and rapid filtration, and it is doubtful if a plant could be operated to regularly secure as high a bacterial efficiency as 99 per cent with any amount of sulphate of alumina.

PRESSURE FILTERS.

Pressure mechanical filters are constructed in entirely closed receptacles, through which the water is forced under pressure and not by gravity. Many of the earlier mechanical filters were of this type. In small plants this system has the distinct advantage that the water can be pumped from a river or other source of supply through a filter direct to the reservoir or into the mains, while any other system would involve a second pumping. Pressure filters are extensively used for hotel supplies, etc., where, from the conditions, gravity filters are impossible. The practical objections to this system have been found to be so great that it is rarely used under other conditions. Some experiments were made at Louisville with a filter of this type, but they were not long continued, and aside from them there is no precise information as to what can be accomplished with filters of this type.