Pullman herald. [volume] (Pullman, W.T. [Wash.]) 1888-1989, 1911-03-10

Ter OF coke tor chareoaD axerator
ana then removing the rust by sand
nIteratlon.
II the water is to be filtered. the
cnleL problem is how 10 construct the
most emeIenJ litter. All. oluer 10 con
struct an efficient filter it is neces-
Sary 10 Understand somewhat of the
lheory ON which they operate. Since
several million bacteria could ba
paced III one cubic inch, they are
SO small that it ridiculous tn SIM
pose that they are held mechanical
~ ID the pores Oh sand or any other
ordinary material which will allow
the flow OF water. The explanation
Oh the removal OF bacteria by filter
lies III the fact that there are certain
classes OF harmless bacteria which de-
sIroy the harmful ones that urod,lce
disease. These helpful water bac-
terra propagate rapidly in the filtem
especially Ia the upper layers, col
Iec'lng III the form OF a Shiny deposit
ON the surface. The dangerous bac-
Lerla are attacked as they enter this
part OF the filter ana are rapidly de-
sLroyed. The function Of the lower
layers of the filter is chiefly to re-
move these Shiny surface bacteria
from the water and SO make it more
!
other suspended matter may be re-
moved in the upper layers of the FL
ter, but it is better to let the greater
part OF such material settle out in a
separate basin, because it soon clogs
the filter. The vegetable matter
which causes bad tastes and doors is
usually fairly we'll removed in a sand
filter through oxidation.
In the construction of filter the
sand is the most important element.
since it regulates the rate of flow.
l he rate of flow determines the
| thoroughness OF the bacterial action
| anu this fixes the efficiency OF the
| filter. The rate of liberation in
creases with an increase in the size
! Of the sand and vice versa. 1t has
been found that sand of which about
| 10 per cent is three-fourths to four
tenths of a millimeter, or one-six-
tieth to one-ninetieth of an inch in
size, gives the best results. The
sand should be clean and free from
any large or very small particles, al
though 1T is not absolutely necessary
that it be uniform in size. Another
important feature is the depth, or
distance through which the water fill
| ters. Of course the depth also in
Huences the rate of liberation, and a
small depth does not hold the water
long enough for efficient bacterial &C
ton. The depth of sand should be
at least three feet, and preferably
four or five feet in thickness. There
is no danger OF having too much
sand, except as it lowers the rate of
liberation, and therefore filters are
oolIlSIlliCo lHddC 81 CudC WdIcI Will
run through them horizontally for a
distance of ten or twelve feet.
The remaining factor affecting the
rate of flow is the head, or depth of
water above the sand. As the head
increases the rate of flow increases,
although not proportionately The
I LARGO Gltar r,1antc for, nitv yUofor cum,
| plies usually operate under heads of
| from four to six feet, but a small
plant would operate as efficiently un-
| DEF much less head. For a depth of
| filter of three feet, two feet of head
| should be ample and less might be
used if only a very slow rate of fill
iteration is desirable. 1t is a good
rule to make the head at least as
great as the depth of sand If this
is impracticable, a smaller head may
be used, but the rate of liberation
will be lowered greatly.
The size of the filter, aside from
the depth, depends upon the amount
of water to be filtered. For a given
quantity of water passing through a
filter with a given size of sand, with
fixed depth, operating under a GER
tain head, the amount of the area of
the bed is easily determined. Under
high heads, as much as seight mil
lions of gallons per acre of sand per
day is filtered, but if under the heads
of from four to six feet ordinarily
used three millions Of gallons per
acre per day is considered safe
rate. This latter is equivalent to
about sixty-fye gallons per day per
square foot of area. Knowing the
amount OF water required to be filter
ed per day, the area of filter required
is easily determined by dividing by
the quantity filtered by one square
foot, and from the area the length
and breadth can be arranged. For