inary 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