CHAPTER X._The filtration of public water-supplies Third edition, revised and enlarged.

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:

-----------+--------------+-----------+--------+---------+----------

-----------+--------------+-----------+--------+---------+----------

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.

------------+----------+--------------------+----------+--------+-----------

of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina

------------+----------+----------+---------+----------+--------+-----------

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.

------------+----------+----------+---------+----------+--------+-----------

of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina

------------+----------+----------+---------+----------+--------+-----------

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.

[Illustration: FIG. 21.—BACTERIAL EFFICIENCIES OF MECHANICAL FILTERS.]

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.

------------+----------+--------------------+----------+--------+-----------

of Days |Turbidity.+----------+---------+remaining.|removed.|used Grains

------------+----------+----------+---------+----------+--------+-----------

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.

------------+----------+--------------------+----------+--------+-----------

of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains

------------+----------+----------+---------+----------+--------+-----------

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.

----------+----------+---------+-----------+----------+--------------

----------+----------+---------+-----------+----------+--------------

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:

------------+----------+--------------------+----------+--------+-----------

of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains

------------+----------+----------+---------+----------+--------+-----------

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.

[Illustration: FIG. 22.—SECTION OF JEWELL MECHANICAL FILTER USED IN

PITTSBURG EXPERIMENTS.]

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.

[Illustration: MECHANICAL FILTERS AT ELMIRA, N. Y. OUTLET TO FILTERS

WITH CONTROLLER AND PURE-WATER FLUME.

[_To face page 174._]

]

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.

[Illustration: PLAN JUST ABOVE COPPER.

SECTION SHOWING FILTER DURING ORDINARY OPERATION.

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

[Illustration: PLAN OF AGITATOR, GUTTER CASTINGS, ETC.

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.

[Illustration: MECHANICAL FILTERS AT ELMIRA, N. Y. UPPER PLATFORM AND

GENERAL ARRANGEMENT OF FILTERS.

[_To face page 178._]

]

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.

The filtration of public water-supplies Third edition, revised and enlarged. · 第 10 / 16 章