Transactions of the American Society of Civil Engineers, vol. LXXII, June, 1911

Chapter II: Part 2

Text size

~Table 12--Rates of Sand Handling.~
=========+================================+============================
| ~Sand removed from filters.~ |~Sand replaced in filters.~
+-----------+--------+-----------+---------+--------+---------
| Ejector | Cubic | Average | Ejector | Cubic |Average
Date. | hours. |yards of| rate in | hours. |yards of|rate in
| | sand |cubic yards| | sand | cubic
| |removed | per hour | |removed | yards
| | | | | |per hour
---------+-----------+--------+-----------+---------+--------+---------
1906.
---------+-----------+--------+-----------+---------+--------+---------
April | 49 | 253 | 5.2 | ... | ... | ...
May | 380 | 2,511 | 6.6 | ... | ... | ...
June | 567 | 3,280 | 5.8 | ... | ... | ...
July | 931 | 5,376 | 5.8 | ... | ... | ...
August | 105 | 533 | 5.1 | ... | ... | ...
September| 315 | 1,892 | 6.0 | ... | ... | ...
October | 1,067 | 5,173 | 5.8 | ... | ... | ...
November | 168 | 935 | 5.6 | ... | ... | ...
December | 203 | 1,073 | 5.3 | ... | ... | ...
---------+-----------+--------+-----------+---------+--------+---------
1907.
---------+-----------+--------+-----------+---------+--------+---------
January | 399 | 2,974 | 7.3 | ... | ... | ...
February | 140 | 1,139 | 8.1 | ... | ... | ...
March | 115 | 878 | 7.6 | ... | ... | ...
April | 427 | 3,103 | 7.3 | ... | ... | ...
May | 133 | 939 | 7.0 | ... | ... | ...
June | 105 | 674 | 6.4 | ... | ... | ...
July | 7 | 46 | 6.6 | ... | ... | ...
August | 90 | 574 | 6.4 | ... | ... | ...
September| 306 | 1,396 | 6.5 | ... | ... | ...
October | 273 | 1,701 | 6.2 | ... | ... | ...
November | 202 | 1,258 | 6.8 | ... | ... | ...
December | 304 | 2,138 | 5.9 | ... | ... | ...
---------+-----------+--------+-----------+---------+--------+---------
1908.
---------+-----------+--------+-----------+---------+--------+---------
January | 546 | 3,708 | 6.8 | ... | ... | ...
February | 98 | 776 | 7.9 | ... | ... | ...
March | 315 | 2,832 | 9.0 | ... | ... | ...
April | 469 | 3,775 | 8.1 | ... | ... | ...
May | 182 | 1,414 | 7.8 | ... | ... | ...
June | 280 | 2,057 | 7.4 | ... | ... | ...
July | 280-1/2 | 2,683 | 9.6 | ... | ... | ...
August | 327-1/2 | 2,808 | 8.6 | ... | ... | ...
September| 402 | 3,371 | 8.4 | ... | ... | ...
October | 308 | 2,696 | 8.7 | ... | ... | ...
November | 47-1/2 | 333 | 7.0 | ... | ... | ...
December | 153-3/4 | 1,268 | 8.3 | ... | ... | ...
---------+-----------+--------+-----------+---------+--------+---------
1909.
---------+-----------+--------+-----------+---------+--------+---------
January | 119-1/2 | 1,055 | 8.8 | ... | ... | ...
February | 161-1/2 | 1,479 | 9.2 | ... | ... | ...
March | 144 | 1,465 | 10.2 | ... | ... | ...
April | 214-3/4 | 2,260 | 10.5 | 188 | 2,405 | 12.8
May | 219-3/4 | 2,223 | 10.1 | 190 | 2,196 | 11.5
June | 355 | 3,096 | 8.7 | 243 | 3,054 | 12.6
July | 312-1/4 | 2,707 | 8.7 | 425-1/2 | 4,050 | 9.5
August | 218-3/4 | 1,955 | 9.0 | 64-1/2 | 620 | 9.6
September| 172-1/2 | 1,360 | 7.9 | 408 | 2,842 | 7.0
October | 203 | 1,870 | 9.2 | 261-1/4 | 2,350 | 9.0
November | 54 | 397 | 7.4 | 0 | 0 | ...
December | 62 | 382 | 6.2 | 0 | 0 | ...
---------+-----------+--------+-----------+---------+--------+---------
1910.
---------+-----------+--------+-----------+---------+--------+---------
January | 104 | 703 | 6.8 | 0 | 0 | ...
February | 106-1/2 | 1,058 | 9.9 | 28-1/4 | 371 | 13.1
March | 98 | 985 | 10.0 | 72 | 1,008 | 14.0
April | 268-3/4 | 2,852 | 10.7 | 134-1/4 | 2,159 | 16.1
May | 58-3/4 | 693 | 11.8 | 171-3/4 | 3,042 | 17.7
June | 58-3/4 | 642 | 10.9 | 9-3/4 | 166 | 17.0
=========+===========+========+===========+=========+========+========

The cost for pumping water for sand handling, including all labor, materials, and repairs, amounts to $0.06 per cu. yd. of sand ejected and washed, and $0.03 per cu. yd. for replacing.

In addition to the water used for carrying the sand which is being replaced, it is customary to keep a slight upward flow in the filter, thus using about 500,000 gal. of filtered water per day for this purpose. Assuming the value of this water to be the total cost for pumping, filtering, etc., or $3.80 per 1,000,000 gal., the cost per cubic yard of sand replaced would be about $0.02 when one ejector is used, and $0.01 when two are in operation.

It is not considered absolutely necessary to have an upward flow of water in the filter which is being re-sanded, and it is not always done. It was used, however, as an additional safeguard against the formation of a stratum of mud between the old and new layers of sand while the hydraulic method was in an experimental stage.

The quantities of sand removed from the filters per scraping and the rates of sand handling are shown in Tables 11 and 12.

_Cost of Operation._--It is frequently difficult to compare the relative cost of corresponding items for different plants, because of the different methods of dividing the cost and the varying opinions of the officials as to what should properly be charged to each item.

In order that the data may be in sufficient detail to permit it to be rearranged to compare with other plants, a list of employees and charges for supplies is given in Table 13. This list accounts for the entire appropriation for the care and maintenance of the filtration plant, including pumping the water to the filters, parking and caring for the grounds, buildings, roads, sidewalks, etc. The cost for the various items per million gallons pumped to the filters is shown in Table 14, and the cost per cubic yard of sand handled in Table 15.

_Preliminary Treatment._--Before the present filtration plant was designed, Rudolph Hering, George W. Fuller, and Allen Hazen, Members, Am. Soc. C. E., made an investigation and report. This report was dated February 18th, 1901, and contained the following paragraph:

"In consideration of the full evidence, we recommend the construction
of a complete system of slow or sand filters, with such auxiliary
works as may be necessary for preliminary sedimentation, and the use
of a coagulant for part of the time. There is no reason to believe
that the use of this coagulant will in any degree affect the
wholesomeness of the water."

Notwithstanding this opinion, considerable prejudice existed among the citizens of Washington against the use of a coagulant, and, as finally passed, the bill providing for the construction of the filters did not include an appropriation for the coagulant.

The results obtained from operating the filters being such as to justify the conclusions in the report referred to, an experimental plant was constructed for the purpose of studying the efficiency of various methods of preliminary treatment of the water. This plant consisted of three cylindrical concrete filter tanks, each 10 ft. in diameter. These tanks were filled with the layers of gravel and sand necessary to make them represent as accurately as possible the large slow sand units of the main filtration plant. Means were also provided for giving a preliminary treatment to the water supplying each of these experimental slow sand filters. In two cases, the preliminary treatment was rapid filtration, while the third consisted of sedimentation and coagulation. The sedimentation tank was of sufficient size, when compared with the area of the experimental slow sand filter, to represent the Georgetown and McMillan Park Reservoirs when used in connection with the large filters. The first preliminary filter was very similar in construction and operation to a mechanical filter. The sand for this filter was taken from the main filters, and, consequently, was finer than is generally used in mechanical filters. The second preliminary filter was a Maignen scrubber. It consisted of a cylindrical concrete tank, 4 ft. in diameter and 8-1/2 ft. deep, which contained 12 in. of cobble-stones on the bottom, then, successively, 12 in. of egg-size coke, 12 in. of stove-size coke, 24 in. of nut-size coke, and 24 in. of sponge clippings as the final or top layer.

~Table 14--Cost Per Million Gallons Filtered.~
(_A_) ~Labor~.
=============+===========+==========+==================
| Office | | ~Filter~
| and | Pumping | ~operations~:
Month. |laboratory.| station. +------------------
| | | Sand |Repairs
| | |handling.| etc.
-------------+-----------+----------+---------+--------
1909.
-------------+-----------+----------+---------+--------
July | $0.73 | $0.57 | $0.86 | ...
August | 0.75 | 0.64 | 0.59 | ...
September | 0.83 | 0.67 | 0.80 | ...
October | 0.72 | 0.66 | 0.73 | ...
November | 0.87 | 0.76 | 0.42 | ...
December | 0.90 | 0.69 | 0.27 | ...
-------------+-----------+----------+---------+--------
1910.
-------------+-----------+----------+---------+--------
January | 0.81 | 0.63 | 0.33 | ...
February | 0.94 | 0.74 | 0.35 | $0.07
March | 0.92 | 0.81 | 0.30 | 0.07
April | 0.93 | 0.83 | 0.49 | 0.03
May | 0.86 | 0.72 | 0.36 | 0.03
June | 0.88 | 0.67 | 0.38 | ...
-------------+-----------+----------+---------+--------
Average | 0.84 | 0.70 | 0.27 |[1]0.25
-------------+-----------+----------+---------+--------
Fiscal years:
-------------+-----------+----------+---------+--------
1905-1906 | 0.45 | 0.45 | 0.47 | 0.02
1906-1907 | 0.57 | 0.57 | 0.58 | 0.21
1907-1908 | 0.70 | 0.56 | 0.42 | 0.32
1908-1909 | 0.72 | 0.61 | 0.41 | 0.34
-------------+-----------+----------+---------+--------

=============+===========+==============+==========+=========
| Parking | | |
Month. | (care of | Experimental | Main | Total.
| grounds). | filters. | Office. |
-------------+-----------+--------------+----------+---------
1909.
-------------+-----------+--------------+----------+---------
July | $0.31 | ... | $0.15 | $2.62
August | 0.71 | ... | 0.14 | 2.83
September | 0.51 | ... | 0.17 | 2.98
October | 0.34 | ... | 0.08 | 2.53
November | 0.38 | ... | 0.18 | 2.61
December | 0.40 | ... | 0.12 | 2.38
-------------+-----------+--------------+----------+---------
1910.
-------------+-----------+--------------+----------+---------
January | 0.14 | ... | 0.10 | 2.01
February | 0.11 | ... | 0.16 | 2.37
March | 0.18 | ... | 0.13 | 2.41
April | 0.36 | ... | 0.13 | 2.77
May | 0.55 | ... | 0.18 | 2.70
June | 0.38 | ... | 0.12 | 2.43
-------------+-----------+--------------+----------+----------
Average | 0.36 | ... | 0.14 | 2.56
-------------+-----------+--------------+----------+----------
Fiscal years:
-------------+-----------+--------------+----------+---------
1905-1906 | 0.01 | ... | 0.09 | 1.49
1906-1907 | 0.07 | $0.03 | 0.04 | 2.07
1907-1908 | 0.15 | 0.09 | 0.09 | 2.36
1908-1909 | 0.22 | 0.01 | 0.13 | 2.44
-------------+-----------+--------------+----------+----------

~Table 14~--(_Continued._)
(_B_) ~Materials~.
=============+===========+==========+==================
| Office | | ~Filter~
| and | Pumping | ~operations~:
Month. |laboratory.| station. +------------------
| | | Sand |Repairs
| | |handling.| etc.
-------------+-----------+----------+---------+--------
1909.
-------------+-----------+----------+---------+--------
July | ... | ... | $0.01 | ...
August | $0.01 | ... | ... | ...
September | 0.05 | $0.31 | 0.04 | ...
October | 0.08 | 0.11 | 0.13 | ...
November | 0.13 | 0.78 | 0.10 | ...
December | 0.03 | 0.17 | 0.05 | ...
-------------+-----------+----------+---------+--------
1910.
-------------+-----------+----------+---------+--------
January | 0.12 | 0.74 | 0.14 | ...
February | 0.07 | 1.88 | 0.18 | ...
March | 0.26 | 0.28 | 0.01 | ...
April | 0.18 | 1.22 | 0.10 | ...
May | 0.06 | 0.72 | 0.02 | ...
June | 0.54 | 2.23 | ... |$[2]2.16
-------------+-----------+----------+---------+--------
Average | 0.13 | 0.69 | 0.02 | [3]0.21
-------------+-----------+----------+---------+--------
Fiscal years:
-------------+-----------+----------+---------+--------
1905-1906 | 0.04 | 0.59 | 0.02 | ...
1906-1907 | 0.03 | 0.67 | 0.08 | 0.20
1907-1908 | 0.05 | 0.54 | 0.04 | 0.07
1908-1909 | 0.10 | 0.69 | 0.05 | 0.18
-------------+-----------+----------+---------+--------

=============+===========+==============+==========+=========
| Parking | | |
Month. | (care of | Experimental | Main | Total.
| grounds), | filters. | Office. |
-------------+-----------+--------------+----------+---------
1909.
-------------+-----------+--------------+----------+---------
July | ... | ... | ... | $0.01
August | $0.07 | ... | $0.01 | 0.09
September | 0.01 | ... | 0.03 | 0.44
October | 0.46 | ... | 0.02 | 0.80
November | 0.34 | ... | 0.02 | 1.37
December | 0.01 | ... | 0.05 | 0.31
-------------+-----------+--------------+----------+---------
1910.
-------------+-----------+--------------+----------+---------
January | 0.01 | ... | ... | 1.01
February | 0.01 | ... | 0.01 | 2.15
March | ... | ... | ... | 0.55
April | 0.29 | ... | 0.02 | 1.81
May | 0.11 | ... | 0.02 | 0.98
June | 0.46 | ... | 0.04 | 5.43
-------------+-----------+--------------+----------+----------
Average | 0.17 | ... | 0.02 | 1.24
-------------+-----------+--------------+----------+----------
Fiscal years:
-------------+-----------+--------------+----------+---------
1905-1906 | ... | ... | ... | 0.65
1906-1907 | 0.02 | | ... | 1.00
1907-1908 | 0.06 | ... | 0.01 | 0.77
1908-1909 | 0.18 | ... | 0.02 | 1.22
-------------+-----------+--------------+----------+----------

[Footnote 1: $0.02 for new sand-handling system.]

[Footnote 2: $2.02 for new sand-handling system.]

[Footnote 3: $0.16 for new sand-handling system.]

~Table 14~--(_Continued._)
(_C_) ~Totals.~
=============+===========+==========+===================
| Office | | ~Filter~
| and | Pumping | ~operations~:
Month. |laboratory.| station. +-------------------
| | | Sand |Repairs
| | |handling.| etc.
-------------+-----------+----------+---------+---------
1909.
-------------+-----------+----------+---------+---------
July | $0.73 | $0.57 | $0.87 | ...
August | 0.76 | 0.64 | 0.59 | ...
September | 0.88 | 0.98 | 0.84 | ...
October | 0.80 | 0.77 | 0.86 | ...
November | 1.00 | 1.54 | 0.52 | ...
December | 0.93 | 0.86 | 0.32 | ...
-------------+-----------+----------+---------+---------
1910.
-------------+-----------+----------+---------+---------
January | 0.93 | 1.37 | 0.47 | ...
February | 1.01 | 2.62 | 0.53 | $0.07
March | 1.18 | 1.09 | 0.31 | 0.07
April | 1.11 | 2.05 | 0.59 | 0.03
May | 0.92 | 1.44 | 0.38 | 0.03
June | 1.42 | 2.90 | 0.38 | 2.16
-------------+-----------+----------+---------+---------
Average. | 0.97 | 1.39 | 0.29 | 0.46
-------------+-----------+----------+---------+---------
Fiscal years:
-------------+-----------+----------+---------+---------
1905-1906 | 0.49 | 1.04 | 0.49 | 0.02
1906-1907 | 0.60 | 1.24 | 0.66 | 0.41
1907-1908 | 0.75 | 1.13 | 0.46 | 0.39
1908-1909 | 0.82 | 1.30 | 0.46 | 0.52
=============+===========+==========+=========+=========

=============+===========+==============+==========+=========
| Parking | | |
Month. | (care of | Experimental | Main | Total.
| grounds), | filters. | Office. |
-------------+-----------+--------------+----------+---------
1909.
-------------+-----------+--------------+----------+---------
Jul | $0.31 | ... | $0.15 | $2.63
August | 0.78 | ... | 0.15 | 2.92
September | 0.52 | ... | 0.20 | 3.42
October | 0.80 | ... | 0.10 | 3.33
November | 0.72 | ... | 0.20 | 3.98
December | 0.41 | ... | 0.17 | 2.69
-------------+-----------+--------------+----------+---------
1910.
-------------+-----------+--------------+----------+---------
January | 0.15 | ... | 0.10 | 3.02
February | 0.12 | ... | 0.17 | 4.52
March | 0.18 | ... | 0.13 | 2.96
April | 0.65 | ... | 0.15 | 4.58
May | 0.66 | ... | 0.20 | 3.63
June | 0.84 | ... | 0.16 | 7.86
-------------+-----------+--------------+----------+---------
Average. | 0.58 | ... | 0.16 | 3.80
-------------+-----------+--------------+----------+---------
Fiscal years:
-------------+-----------+--------------+----------+---------
1905-1906 | 0.01 | ... | 0.09 | 2.14
1906-1907 | 0.09 | $0.03 | 0.04 | 3.07
1907-1908 | 0.21 | 0.09 | 0.10 | 3.13
1908-1909 | 0.40 | 0.01 | 0.15 | 3.66
=============+===========+==============+==========+=========

The two preliminary filters were operated at a rate of about 50,000,000 gal. per acre per day, and the three slow sand filters at rates of from 3,000,000 to 4,000,000 gal. per day.

This plant was put in service during the early part of February, 1907, and was kept in practically continuous operation until the end of July, 1908.

~Table 15--Average Cost for Labor for Sand Handling.~
(_A_) ~Per Million Gallons Pumped To Filter.~
=============+=======+=======+=======+=======+=======+========+========
Month. | Scrap-| Eject-| Wash- |Smooth-|Raking.| Re- | Total.
| ing. | ing. | ing. | ing. | |sanding.|
-------------+-------+-------+-------+-------+-------+--------+--------
1909.
-------------+-------+-------+-------+-------+-------+--------+--------
July | $0.10 | $0.21 | $0.03 | $0.02 | ... | $0.21 | $0.57
August | 0.07 | 0.16 | 0.03 | 0.01 | ... | 0.04 | 0.31
September | 0.05 | 0.13 | 0.02 | 0.01 | $0.01 | 0.27 | 0.49
October | 0.06 | 0.15 | 0.03 | 0.01 | 0.02 | 0.12 | 0.39
November | 0.02 | 0.06 | ... | ... | 0.02 | ... | 0.70
December | 0.02 | 0.04 | 0.01 | ... | 0.01 | 0.01 | 0.09
-------------+-------+-------+-------+-------+-------+--------+--------
1910.
-------------+-------+-------+-------+-------+-------+--------+--------
January | 0.04 | 0.07 | ... | 0.01 | 0.02 | ... | 0.14
February | 0.04 | 0.10 | ... | 0.01 | ... | 0.02 | 0.17
March | 0.04 | 0.06 | ... | 0.01 | 0.01 | 0.05 | 0.17
April | 0.10 | 0.15 | 0.04 | 0.01 | 0.02 | 0.06 | 0.38
May | 0.02 | 0.03 | 0.01 | ... | 0.01 | 0.11 | 0.18
June | 0.02 | 0.04 | ... | ... | 0.02 | 0.01 | 0.09
-------------+-------+-------+-------+-------+-------+--------+--------
Average | 0.05 | 0.10 | 0.01 | 0.01 | 0.01 | 0.08 | 0.26
-------------+-------+-------+-------+-------+-------+--------+--------
Fiscal years:| | | | | | |
-------------+-------+-------+-------+-------+-------+--------+--------
1905-06 | 0.06 | 0.29 | 0.02 | 0.06 | ... | 0.04 | 0.47
1906-07 | 0.07 | 0.20 | 0.05 | 0.02 | ... | 0.24 | 0.58
1907-08 | 0.09 | 0.14 | 0.03 | 0.01 | 0.02 | 0.13 | 0.42
1908-09 | 0.07 | 0.15 | 0.03 | 0.01 | 0.01 | 0.14 | 0.41
-------------+-------+-------+-------+-------+-------+--------+--------

~Table 15~--(_Continued._)
(_B_) ~Per Cubic Yard of Sand.~
=============+=======+=======+=======+=======+=======+=======+=========
Month. | Scrap-| Eject-| Wash- |Smooth-|Raking.| Re- | Total.
| ing. | ing. | ing. | ing. | |sanding.|
-------------+-------+-------+-------+-------+-------+--------+--------
1909.
-------------+-------+-------+-------+-------+-------+--------+--------
July | $0.08 | $0.15 | $0.03 | $0.01 | ... | $0.10 | $0.37
August | 0.07 | 0.15 | 0.03 | 0.01 | ... | 0.11 | 0.37
September | 0.07 | 0.17 | 0.03 | 0.01 | ... | 0.17 | 0.45
October | 0.06 | 0.15 | 0.03 | 0.01 | ... | 0.09 | 0.34
November | 0.10 | 0.23 | 0.02 | 0.02 | ... | ... | 0.37
December | 0.12 | 0.25 | 0.04 | 0.02 | ... | 0.08 | 0.51
-------------+-------+-------+-------+-------+-------+--------+--------
1910.
-------------+-------+-------+-------+-------+-------+--------+--------
January | 0.10 | 0.19 | ... | 0.02 | ... | ... | 0.31
February | 0.07 | 0.15 | ... | 0.01 | ... | 0.09 | 0.32
March | 0.06 | 0.11 | ... | 0.02 | ... | 0.08 | 0.27
April | 0.07 | 0.09 | 0.03 | 0.01 | ... | 0.05 | 0.25
May | 0.06 | 0.09 | 0.03 | 0.01 | ... | 0.06 | 0.25
June | 0.06 | 0.12 | ... | 0.01 | ... | 0.10 | 0.29
-------------+-------+-------+-------+-------+-------+--------+--------
Average | 0.07 | 0.14 | 0.02 | 0.01 | ... | 0.10 | 0.34
-------------+-------+-------+-------+-------+-------+--------+--------
Fiscal years:
-------------+-------+-------+-------+-------+-------+--------+--------
1905-06 | 0.07 | 0.35 | 0.04 | 0.07 | ... | 0.14 | 0.67
1906-07 | 0.06 | 0.19 | 0.03 | 0.02 | ... | 0.17 | 0.47
1907-08 | 0.09 | 0.15 | 0.03 | 0.01 | ... | 0.14 | 0.42
1908-09 | 0.06 | 0.14 | 0.03 | 0.01 | ... | 0.13 | 0.37
=============+=======+=======+=======+=======+=======+=======+=========

For convenience in referring to the different systems, the combined rapid and slow sand filter will be designated as Filter Plant No. 1, the combined Maignen scrubber and slow sand filter as Filter Plant No. 2, and the combined coagulating basin and slow sand basin as Filter Plant No. 3.

The length of run of Filter Plant No. 1 was relatively long at first. The rapid rate of filtration, however, tended to carry the clay, which was suspended in the applied water, to a considerable depth in the filtering material, so that the runs gradually decreased in length until they were reduced to about three days. Unfortunately, it was necessary to use unfiltered water for washing, which, together with the great penetration from the applied water, finally made it necessary to remove all the filtering materials, and wash them.

Although this preliminary filter was operated at a high rate, its efficiency was quite satisfactory. In fact, at times when the applied water was comparatively good, very little work was left for the slow sand filter. At times of high turbidity, however, some of the exceedingly fine mud in the applied water passed through this filter, as well as the slow sand filter connected with it, and it proved to be absolutely impossible to produce a clear effluent at all times with this combination.

Filter Plant No. 2 proved more economical and convenient in operation, but somewhat less efficient than Filter Plant No. 1. Neither filter could be depended on to give a clear effluent when the applied water was turbid.

In the operation of Filter Plant No. 3, sulphate of alumina was used when the applied water contained too much turbidity to be treated satisfactorily by slow sand filters.

When the water was comparatively clear, either one of the three systems, or slow sand filtration alone, was entirely satisfactory. At times of high turbidity, however, Filter Plant No. 3 was the only one which could be depended on to produce a clear effluent.

A fair comparison between the results of the three systems when treating turbid water in January, 1908, is given in Table 16.

Table 16 shows very clearly that neither Filter Plant No. 1 nor No. 2 would prove at all satisfactory when treating turbid water, while No. 3 could be depended on under all conditions. The results of operation are shown in detail in Tables 17, 18, and 19. It will be noticed in Table 17, that on March 10th, 1908, Filter Plant No. 1 was put out of service and a Puech system of preliminary filters was substituted for it.

The Puech preliminary filters consisted of five units containing gravel of varying sizes through which the water was filtered successively before it was finally applied to the final slow sand filter. A general idea of this system may be obtained by referring to Figure 8.

~Table 16--Turbidity Results with Experimental Filters,
During Period of High Turbidity, January, 1908.~

Columns:
A - Effluent preliminary filter.
B - Effluent sand filter.
C - Effluent preliminary filter.
D - Effluent sand filter.
E - Effluent coagulant basin.
F - Effluent sand filter.

============+======+=============+=============+==============
| Raw |Filter No. 1.|Filter No. 2.|Filter No. 3.
Date. |water.|------+------+------+------+------+-------
| | A | B | C | D | E | F
------------+------+------+------+------+------+------+-------
January 12th| 40 | 10 | 1 | 12 | 1 | 2 | 0
January 13th| 110 | 45 | 2 | 51 | 2 | 2 | 0
January 14th| 210 | 95 | 3 | 113 | 4 | 2 | 0
January 15th| 325 | 190 | 12 | 222 | 15 | 3 | 0
January 16th| 360 | 210 | 37 | 247 | 42 | 5 | 0
January 17th| 242 | 122 | 24 | 147 | 26 | 6 | 0
January 18th| 137 | ... | ... | 73 | 7 | 6 | 0
January 19th| 117 | 40 | 12 |clean-| ... | 5 | 0
| | | | ing | | |
January 20th| 72 | 31 | 6 | sand | ... |clean-| 0
| | | |filter| | ing |
January 21st| 55 | 20 | 4 | 25 | 4 | sand | ...
January 22d | 49 | 17 | 3 | 21 | 4 |filter| ...
January 23d | 40 | 12 | 3 | 15 | 3 | 3 | 0
January 24th| 40 | 11 | 3 | 13 | 3 | 3 | 0
============+======+======+======+======+======+======+=======

It is unfortunate that this system was not in operation in January, 1908, when the water was cold and turbid. The results, however, indicate that it would be no more successful than either Filter Plant No. 1 or No. 2.

_Experimental Rate Studies._--In September, 1908, an experimental plant consisting of six small filters was put in operation. The object of these experiments was to study the relative efficiencies and cost for the operation of slow sand filters when operated at different rates.

The units of the plant consisted of cylindrical galvanized-iron tanks 4 ft. in diameter and 9 ft. high. The filter sand in these tanks was taken from the supply for the main filters. It was supported on gravel layers and supplied with under-drains of suitable sizes for the proposed rate of flow in each case.

The units of the experimental plant were designated as Nos. 1, 2, 3, 4, 5, and 6, and it was the original intention to operate them at rates of 1,000,000, 3,000,000, 6,000,000, 10,000,000, 30,000,000, and 100,000,000 gal. per acre daily, respectively.

This schedule of rates was carried out in a general way with all the filters, with the exception of Nos. 5 and 6. For these, the rates were found to be higher than could be maintained for any great length of time, owing to the deeper penetration of the mud in the filter sand, which caused high initial losses of head, short runs, and deep scrapings. A rate of about 30,000,000 gal. was maintained in the case of Filter No. 5 from the time it was started on September 9th, 1908, until November 8th, 1909, when it was reduced to about 17,000,000 gal., which rate was maintained thereafter until the filter was shut down in February, 1910.

In the case of Filter No. 6, it was found impossible to maintain a rate of 100,000,000 gal. for more than a very few days at a time. It was started at about this rate, however, at the beginning of each run, and kept as high as possible for the remainder of the time during the first seven runs. At the end of the seventh run, on October 17th, 1908, the filter was given a very deep scraping and re-sanded.

The layer of clean sand restored the original capacity, and the filter was operated as before, but with gradually decreasing rates until December, 1908, when the rate was reduced to about 40,000,000 gal. Even this lower rate was too high to be maintained without removing and replacing a large part of the sand. The rates, therefore, gradually decreased to about 23,000,000 gal. on March 13th, 1909, when the filter was again re-sanded. After this re-sanding the rate was reduced to about 20,000,000 gal., and the filter was operated at approximately that rate until it was again re-sanded on November 13th, 1909, when the rate was again reduced to about 14,000,000 gal., which was maintained until the filter was put out of service on February 28th, 1910.

This experimental plant was in service from September, 1908, to the latter part of February, 1910, or for about 1-1/2 years, and the leading results are summarized in Table 20.

~Table 17--Record of Experimental Filter Plant No. 1.~

Columns:
A - Rate, millions of gallons per acre daily.
B - Loss of head.
C - Rate, millions of gallons per acre daily.
D - Loss of head.
E - Applied water.
F - Effluent preliminary filter.
G - Effluent final filter.
H - Applied water.
I - Effluent preliminary filter.
J - Effluent final filter.

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Transactions of the American Society of Civil Engineers, vol. LXXII, June, 1911Chapter II: Part 2

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