the end of each run;
Group _C._--Filters raked at the end of each run, until raking
failed to bring back the proper capacity; then they
were scraped light, and at the end of the next run
the raking was resumed;
Group _D._--Light scrapings and rakings alternate at ends of
runs.

The term "deep scraping" means the removal of practically all the
discolored sand, in accordance with the usual practice prior to the
beginning of these experiments; "light scraping" means the removal
of only a thin surface layer of sand. This depth has usually
averaged about 3/8 in. "Raking" means the thorough breaking up of
the clogged surface of the filter by iron-toothed rakes, to a depth
of about 1 or 2 in.

_Results._--A general summary of the results of these experiments is
given in Table 29, which also shows the relative costs of the
different methods per million gallons of water filtered. A normal
period of 9 months just prior to the beginning of these experiments
shows a labor cost (corresponding to that in Table 29) of $0.29-1/4
per million gallons filtered.

~Table 29--Average Results.~

Columns:
A - Group.
B - Number of filters.
C - Number of days of service.
D - Million gallons filtered.
E - Cost of labor per treatment.
F - Sand removed in cubic yards.
G - Sand removed in cubic yards.
H - Cost of labor.
I - Bacteria per cu. cm. in effluent.
J - Turbidity in effluent.

=========================================+===============+==============
| Per Million | |
Per Run: | Gallons | |
| Filtered | I | J
-----+-----+-----+-------+-------+-------+-------+-------+ |
A | B | C | D | E | F | G | H | |
-----+-----+-----+-------+-------+-------+-------+-------+-------+------
_A_ | 5 | 82 | 221.2 |$68.44 | 215 | 1.11 |$0.309 | 13 | 1
_B_ | 9 | 36 | 101.4 | 29.25 | 84 | 0.83 | 0.288 | 16 | 1
_C_ | 5 | 21 | 60.0 | 10.92 | 24 | 0.40 | 0.182 | 18 | 1
_D_ | 10 | 32 | 86.0 | 20.10 | 46 | 0.54 | 0.234 | 22 | 1
=====+=====+=====+=======+=======+=======+=======+=======+=======+======

_Capacity of Filters._--The capacity of the filters under the