proposed depth of the sand layer. The outlet end of the hose is
connected to a 3-in. pipe which is supported on a boat, and the sand
is discharged through this pipe at the point required. Work is first
begun at the far end of the filter, and it is gradually filled by
swinging the boat from side to side and backing it by degrees to the
front end.
At first it was feared that a small quantity of mud would be deposited
on the surface of the old sand, and that this mud would ultimately cause
subsurface clogging. For this reason, when this method was first
adopted, a man was required to rake the sand very thoroughly in front of
the discharge. Later, it was found that by giving the end of the
discharge pipe a slope of about 45 degrees downward from the horizontal,
the force of the current of sand and water could be depended on to cut
the old surface of sand to any required depth, and move it ahead
together with the new sand, thus completely breaking up the possible mud
layer between the old and new sand layers. After having used this method
almost exclusively for 15 months, in which time eleven filters have been
re-sanded, and 24,531 cu. yd. of sand have been replaced, there seems to
be no indication of an increased initial loss of head. The sand is very
compact, and has no apparent tendency to separate into different sizes.
The general appearance is similar to that of very fine sand on the
seashore. The filters re-sanded in this way have been considerably more
efficient than those in which the sand was replaced with carts, and as
yet, no harmful results have been noted. The rate at which the sand is
replaced is shown in Table 12, and the cost of labor for sand handling
is given in detail in Table 14, which shows that quite a perceptible
saving has been effected by the hydraulic method.
The figures showing the cost for sand handling do not include any
charge for the quantity of water used, that item having been carried
on the pumping-station account.
~Table 11--Quantities of Sand Removed.~
==============+=================+================================
|~No. of filters | ~Cubic yards when last
|scraped when last| treatment was:~
|treatment was:~ |
Month. +---------+-------+---------------+----------------
| | | Scraping. | Raking.
|Scraping.|Raking.|----+----+-----+----+----+------
| | |Max.|Min.|Ave. |Max.|Min.|Ave.
--------------+---------+-------+----+----+-----+----+----+------
1909.
--------------+---------+-------+----+----+-----+----+----+------
July | 14 | 0 |338 |121 |190.6| 0 | 0 | 0
August | 8 | 0 |356 |149 |218.5| 0 | 0 | 0
September | 8 | 0 |524 | 97 |178.6| 0 | 0 | 0
October | 9 | 5 |150 | 93 |115.8|301 |121 |169.0
November | 2 | 2 |134 | 88 |111.0|132 | 81 |106.5
December | 0 | 2 | 0 | 0 | 0 |133 |126 |129.5
--------------+---------+-------+----+----+-----+----+----+------
1910.
--------------+---------+-------+----+----+-----+----+----+------
January | 2 | 4 |155 |112 |133.5|195 |121 |147.8
February | 0 | 4 | 0 | 0 | 0 |390 |160 |225.8
March | 1 | 4 |489 |489 |489.0|262 |179 |214.3
April | 4 | 12 |172 | 84 |119.3|230 |146 |178.8
May | 1 | 2 |320 |320 |320.0|249 |241 |245.0
June | 0 | 2 | 0 | 0 | 0 |203 |190 |196.5
--------------+---------+-------+----+----+-----+----+----+------
Year 1909-10 | 49 | 37 |524 | 84 |176.7|390 | 81 |181.0
--------------+---------+-------+----+----+-----+----+----+------
Fiscal Years:
--------------+---------+-------+----+----+-----+----+----+------
1905-06 | 71 | 0 |600 | 71 |250.0| 0 | 0 | 0
1906-07 | 94 | 2 |536 | 52 |259.0|398 |276 |337.0
1907-08 | 81 | 53 |527 | 46 |190.2|411 | 35 |118.4
1908-09 | 92 | 50 |580 | 55 |169.5|472 | 81 |177.5
==============+=========+=======+====+====+=====+====+====+======
==============+================================
| ~Depth, in inches, when
| last treatment was:~
+---------------+----------------
Month. | Scraping. | Raking.
+----+----+-----+----+----+------
|Max.|Min.|Ave. |Max.|Min.|Ave.
--------------+----+----+-----+----+----+------
1909.
--------------+----+----+-----+----+----+------
July |2.51|0.90|1.415|0 |0 |0
August |2.65|1.11|1.631|0 |0 |0
September |3.90|0.72|1.330|0 |0 |0
October |1.12|0.69|0.862|2.24|0.90|1.256
November |1.00|0.65|0.825|0.98|0.60|0.790
December |0 |0 |0 |0.99|0.94|0.965
--------------+----+----+-----+----+----+------
1910.
--------------+----+----+-----+----+----+------
January |1.15|0.83|0.990|1.45|0.90|1.100
February |0 |0 |0 |2.90|1.19|1.678
March |3.64|3.64|3.640|1.95|1.33|1.593
April |1.28|0.62|0.885|1.71|1.09|1.331
May |2.38|2.38|2.380|1.85|1.79|1.820
June |0 |0 |0 |1.51|1.41|1.460
--------------+----+----+-----+----+----+------
Year 1909-10 |3.90|0.62|1.314|2.90|0.60|1.373
--------------+----+----+-----+----+----+------
Fiscal Years:
--------------+----+----+-----+----+----+------
1905-06 |4.47|0.53|1.799|0 |0 |0
1906-07 |4.00|0.56|1.931|2.95|2.05|2.500
1907-08 |3.92|0.21|1.507|3.06|0.21|0.881
1908-09 |4.31|0.41|1.259|3.51|0.60|1.317
==============+====+====+=====+====+====+======
~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.
~Table 13--List of Employees, Rates of Pay, and
Approximate Cost for Supplies.~
========================================================+===========
1 Superintendent | $3,000.00
1 Chief Chemist and Assistant Superintendent | 2,100.00
1 First Assistant Chemist | 1,500.00
1 Second Assistant Chemist | 1,000.00
1 Stenographer and Clerk | 1,200.00
1 Surveyor | 1,200.00
1 Laboratory Helper | 720.00
1 Janitor | 600.00
1 Chief Steam Engineer | 1,800.00
1 First Assistant Steam Engineer | 1,440.00
1 Second Assistant Steam Engineer | 1,080.00
3 Oilers, at $900 each | 2,700.00
3 Firemen, at $900 each | 2,700.00
3 Laborers, at $540 each | 1,620.00
1 Filter Foreman | 1,200.00
2 Foremen, at $900 each | 1,800.00
1 Timekeeper | 900.00
3 Watchmen and Gauge Tenders, at $900 each | 2,700.00
1 Machinist | 1,140.00
1 Blacksmith | 900.00
1 Storekeeper | 900.00
1 Painter | 900.00
1 Mechanic | 900.00
1 Electrician | 900.00
4 Skilled Laborers at $600 each | 2,400.00
1 Watchman and Special Officer | 900.00
1 Recorder | 720.00
27 Laborers, at $1.50 per day for 300 days | 12,150.00
3 Teams, at $2.00 per day for 200 days | 1,200.00
Laboratory and office supplies | 2,700.00
Filter supplies, tools, hose, repair of |
roads, parks, shrubs, etc. | 8,820.00
Pumping station supplies, oil, waste, |
packing, repairs, etc. | 3,570.00
3,600 tons of coal, at $3.15 per ton | 11,340.00
Charges in U. S. Engineer Office, labor | 2,900.00
Charges in U. S. Engineer Office, materials | 400.00
--------------------------------------------------------+-----------
Total | $82,000.00
========================================================+===========
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.
[Illustration: ~Figure 6--Washington Aqueduct, D. C., Experimental
Filters Below Dalecarlia Reservoir Coagulating Basins and Apparatus.~]